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Weather Station Hardware => Davis Instruments Weather Stations => Topic started by: jgentry on July 04, 2018, 12:00:21 AM

Title: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 04, 2018, 12:00:21 AM
I’m thinking about getting a sling psychrometer to make sure that my RH sensor is within specs. I would like to know which psychrometers are accurate and reliable.  Thanks!
Title: Re: Which sling psychrometer should I buy?
Post by: miraculon on July 04, 2018, 08:17:12 AM
I have used sling psychrometers in the past and broken them as well. I see one that is on Amazon that the thermometers seem to be protected. I don't know if it is good, bad or indifferent though. Most reviews are good, but there are some bad ones too. See: https://www.amazon.com/Bacharach-0012-7012-Spirit-Filled-Psychrometer/dp/B000LDNH8I (https://www.amazon.com/Bacharach-0012-7012-Spirit-Filled-Psychrometer/dp/B000LDNH8I)

I found a used Belfort fan aspirated wet/dry bulb "Psychron" that I use occasionally to confirm the electronic humidity readings. I found it on eBay. There are some listed there currently. At least you aren't flinging breakable thermometers around with the fan, but make sure you getting a good one with functional thermometers and fan.

https://www.ebay.com/sch/12576/i.html?_from=R40&_nkw=psychron (https://www.ebay.com/sch/12576/i.html?_from=R40&_nkw=psychron)

Greg H.
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 04, 2018, 08:54:32 AM
Not a sling, this unit has claimed better specs than most. 3% vs 5% humidity on most including sling types, probably because temperature accuracy is good at (+/-) 1°F.

http://www.extech.com/display/?id=14692

Listed $88 to around $100 depending on seller.


https://www.amazon.com/Extech-RH25-Heat-Index-Psychrometer/dp/B00DWX75QY/ref=sr_1_1?ie=UTF8&qid=1530708749&sr=8-1&keywords=Extech+RH25
Title: Re: Which sling psychrometer should I buy?
Post by: DaleReid on July 04, 2018, 09:39:06 AM
Valentine:
Have you seen one of these little gizmos?
I wonder if they are well built, have reproducible values and so on?

I've seen stuff like this at ham flea markets and some is a real find, other stuff is best bought by handling it for a feel of what it's like.

For this price is is no Vaisala probe, but may function pretty well for our use, especially after we get our stations installed and you want to wander over after a few months or change in season to see what it says compared to the station sensors.

As far as the little units with a fan on them, is that the 'original' FARS concept?

Just teasing.  But it makes sense.  I know our building guy would wonder around the halls with a sling unit and it was a bit daunting to see him whipping the thing around, (mercury) thermometers exposed and going 40 mph in circles. 

I assume it was correct as one could get with distilled water and doing it long enough to be stable.

Didn't Taylor make a twin yellow thermometer unit in a sling configuration, too?

Our lust for accuracy and precision are overwhelming when we get digital and start to see decimal points on a readout.  With a sling you'd need a table and do a little interpolation anyway.

Dale
Title: Re: Which sling psychrometer should I buy?
Post by: miraculon on July 04, 2018, 09:59:49 AM
By the way, the Psychron is (was) like a sling in that it has two mercury thermometers with 1 degree graduations.

I found some specs here: http://www.belfort-inst.com/Model_566.htm (http://www.belfort-inst.com/Model_566.htm)

The fan eliminates the need to "sling". The wet and dry bulb thermometers are terminated in a venturi-like housing and the fan draws the air across the two thermometer bulbs. I use distilled water with mine as recommended.

Greg H.
Title: Re: Which sling psychrometer should I buy?
Post by: DaleReid on July 04, 2018, 10:25:10 AM
I was looking at the search for "Taylor Sling" on eBay and after seeing some gawd awful shoes that women are supposed to wear, mixed in were many nice Taylor units.

Some were used, some said used but obviously were still in the packaging.  Ranged from $24 to way up around $80, with or without shipping.

I have a couple and are super nice units, made back when the thermometers were mercury, had fine engraved markings and such.

I don't know about the Belfort, but looks like a cool instrument and would not endanger anyone around you or the dog if he got too interested in the whirling thing his master was holding.

If you look at the charts and graphs to figure out humidity, it comes within a few % of the Relative Humidity for those values, especially if you use the chart with lines on it, rather than just extrapolating the numbers.

I'm wondering unless you use a Vaisala three salts humidity standard how close you'll get to the single digit accuracy, say nothing of the tenths of a point (and what difference can that be in the long run?) so maybe a sling is close enough.

I'm thinking that the slings were used back in the day when meteorologists were burly fellows who braved even weather that made the Post Office close, standing in the elements twirling their psychrometers not once but three times to get their best readings and then dutifully recording them.

As an aside, for those who haven't read The Children's Blizzard of 1888, there are some descriptions of the weather observer in St. Paul, MN or thereabouts doing his 4 x a day observations with an obsessive attention to detail and telegraphing the numbers on to Washington.  There were some other descriptions of the lax observations (the original dry-labbing of data?) by other observes further west and up towards Canada.

Considering how long ago those observations were being made and what the guy could deduce about the coming storm from the data that was flowing across the telegraph, I was amazed at how intuitive he was.

Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 04, 2018, 10:58:06 AM
Valentine:
Have you seen one of these little gizmos?
I wonder if they are well built, have reproducible values and so on?



No I have a different model same company Extech RH300 Humidity Meter with Dew Point also 3% accuracy claimed. I have the salts (extra) for calibration also. I've had the RH300 for 3 years or so and it's well made. I suggested the other model because it had some different features like heat index.

Extech RH300 Humidity Meter with Dew Point
https://www.amazon.com/Extech-RH300-Humidity-Meter-Point/dp/B000BF5400

These still need a little forced air or breeze for best accuracy I've found.
Title: Re: Which sling psychrometer should I buy?
Post by: klschmidt on July 04, 2018, 12:48:41 PM
I have a Taylor mercury type psychrometer as well as a spirits filled psychrometer (Novalynx model 225-570-A). I have not seen any significant difference in the readings from either. Here in Arizona conditions are often dry, and the correct wet bulb depression is rather large, sometimes forty degrees Fahrenheit. I have found it a challenge to get correct wet bulb depression. I've tried whirling the instrument fast or slow as well as simply placing it in an area with a slight breeze. Too often I simply cannot get the wet bulb temperature as low as expected.

I also own an Assman psychrometer (Novalynx model 225-5230). I really like this instrument. It consistently provides lower wet bulb temperatures than either of the sling type psychrometers, and agrees with my VP2 and other weather data which I trust.

It would seem that something with my method must be the cause of the issues I've had with the sling psychrometers, so I don't want to be too quick to discourage purchasing one. I would however recommend the Assman psychrometer for ease of use and the consistancy of readings.
Title: Re: Which sling psychrometer should I buy?
Post by: jgentry on July 04, 2018, 05:11:08 PM
I’m considering on getting the Fieldpiece PRH 2 Digital Psychromotor
Fieldpiece PRH2 Digital Psychomotor https://www.amazon.com/dp/B0087SD43W/ref=cm_sw_r_cp_api_.wtpBb22MBGDM

Here are the specs: https://www.fieldpiece.com/media/manuals/PRH2-blister-v14.pdf

But I am also considering getting this one when I get some extra money

https://novalynx.com/store/pc/225-566-Battery-Operated-Psychrometer-p646.htm
Title: Re: Which sling psychrometer should I buy?
Post by: klschmidt on July 04, 2018, 05:27:55 PM
I have a Taylor mercury type psychrometer as well as a spirits filled psychrometer (Novalynx model 225-570-A). I have not seen any significant difference in the readings from either. Here in Arizona conditions are often dry, and the correct wet bulb depression is rather large, sometimes forty degrees Fahrenheit. I have found it a challenge to get correct wet bulb depression. I've tried whirling the instrument fast or slow as well as simply placing it in an area with a slight breeze. Too often I simply cannot get the wet bulb temperature as low as expected.

I also own an Assman psychrometer (Novalynx model 225-5230). I really like this instrument. It consistently provides lower wet bulb temperatures than either of the sling type psychrometers, and agrees with my VP2 and other weather data which I trust.

It would seem that something with my method must be the cause of the issues I've had with the sling psychrometers, so I don't want to be too quick to discourage purchasing one. I would however recommend the Assman psychrometer for ease of use and the consistancy of readings.
Q: Do you have accurate TEMP and RH data and is that data in an Excel spreadsheet? There is an Excel "Add-In" software module that can calculate accurate Wet Bulb temperature values.

Accurate temperature and humidity data is what I desire to obtain and verify. I wish I could answer your question with a definite "Yes". Single point psychrometer thermometer calibration can be done with ice water, and the thermometers can be compared before saturating the wet bulb. Other than these methods I simply trust the manufacturer. My Assman psychrometer is giving me a dry bulb temp of 90.5 and a wet bulb temp of 60. Air pressure today is 25.67". Online calculators phone apps and a psychrometric chart for my altitude all give about the same results. Humidity 17%, Dewpoint 39.5 This data is really close to my Davis, NWS current conditions and another nearby station. I would be interested to know if your Excel spreadsheet gives different results. Sling psychrometer dry bulb is 93 and wet bulb 64. I doubt these numbers.

I would gladly discuss sling psychrometer methods and accuracy at length, but I don't want to hijack this thread. Would a new topic on sling psychrometers be in order?

My original post was simply intended to state which of the three psychrometers I own I would trust to check or calibrate the Davis.
Title: Re: Which sling psychrometer should I buy?
Post by: DaleReid on July 04, 2018, 05:35:08 PM
KISCHMIDT:

I would serioiusly doubt anyone would consider further discussion, links to apps or Excel spreadsheet formula, etc. hijacking the thread.

From past experience I have learned a lot about calibration of rain gauges and where to set up stations and all sorts of stuff that comes up during a discussion.

I'm not the owner of this board but I envision the function is to be respectful, to contribute or give one's personal experience and discuss whatever instruments one has had experience with and so on, sort of like sitting around having coffee with friends and learned others.

Unless the board owner says no, I'd encourage you to discuss or offer your resources, links, articles and fun you've had with instruments and we'll all be smarter!  Dale
Title: Re: Which sling psychrometer should I buy?
Post by: DaleReid on July 04, 2018, 05:36:56 PM
Old Tele Man:  I didn't know that, and yet have discovered some real gems (and previously unknown sites) displayed there.

Do they reference their formulae in doing so?  I know from looking at some links to papers on such things as wind chill, these academic types get pretty detailed in their discussions, which always is interesting to read.

Dale
Title: Re: Which sling psychrometer should I buy?
Post by: miraculon on July 04, 2018, 05:49:40 PM
Quote
But I am also considering getting this one when I get some extra money

https://novalynx.com/store/pc/225-566-Battery-Operated-Psychrometer-p646.htm

This NovaLynx seems to be the same (copy/clone?) of the old Belfort 566.

I get a kick out of Amazon's title for the Fieldpiece PRH2 Digital Psychomotor. Sounds like some kind of weird science fiction thing for mind control....  :twisted:

Although the link URL is OK and the photo shows Psychrometer..

Greg H.

Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 04, 2018, 06:19:26 PM
If you google Belfort 566 it comes up with the Novalynx model.

As mentioned using sling psychrometer can bring inconsistent readings if done incorrectly. That's why firefighters have went with the psychrometer for fast readings that are more consistent. The little hand held units that include wind speed is usually what they use in the field but specs only show +/- 5% accuracy on most.

That Novalynx looks interesting.
Title: Re: Which sling psychrometer should I buy?
Post by: klschmidt on July 04, 2018, 06:35:16 PM
Old Tele man,

Thanks for the link. That looks like a site where I'll be spending some time.

DaleReid,

It would appear that that jgentry is considering an electronic psychrometer. I don't want to get too far off topic discussing mechanical psychrometers and how to use them. I suspect much of the reason for the appearantly better accuracy of the Assman psychrometer is due to it's double wall radiation shield and consistent air speed. However, recommending a $1,200 instrument to check the Davis was probably bad advice. Some of us seem to have a bit of affinity for simple mechanical instruments, but that does not make them the best for everyone. I find it most satisfying to note the mark the mercury is nearest when it ceases to move. Much more so than viewing numbers generated on an lcd by some kind of sensor and chip that I don't understand.
Title: Re: Which sling psychrometer should I buy?
Post by: klschmidt on July 04, 2018, 07:52:20 PM
Old Tele man,

This is the table I usually use in the field. https://novalynx.com/manuals/nfes-2140-references-appendix-index.pdf (p. 183-202)

If I need or want more accuracy I use an online calculator or phone app which allows the user to input the exact air pressure and temperatures with decimals.

I have some psychrometric charts, but printed on standard 8.5 x 11 paper, I find it more difficult to follow the small curved lines, especially outside in the sun and wind.
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 05, 2018, 07:47:37 AM
This is the highest precision unit sold by Extech +/- 2% . http://www.extech.com/display/?id=14699

Place that carries some good stuff including NWS sling and newer Belfort Psychro-Dyne - Model 595 which came after the 566. http://www.belfort-inst.com/Model_595.htm

Link to store
http://www.iisusa.com/Psychrometers/products/50/
Title: Re: Which sling psychrometer should I buy?
Post by: jgentry on July 05, 2018, 01:02:56 PM
This is the highest precision unit sold by Extech +/- 2% . http://www.extech.com/display/?id=14699

Place that carries some good stuff including NWS sling and newer Belfort Psychro-Dyne - Model 595 which came after the 566. http://www.belfort-inst.com/Model_595.htm

Link to store
http://www.iisusa.com/Psychrometers/products/50/

Thanks!  Probably will end up getting the Belford 595 model
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 05, 2018, 01:38:53 PM
FYI I think it's the Psychro-Dyne - Model 595 the business I linked carries because the other model is called Psychron - Model 566. Worth calling first because the store hasn't posted to Facebook since 2017 and online blog last post was 2015 just to make sure its in business and have in stock.
Title: Re: Which sling psychrometer should I buy?
Post by: AF6GL on July 05, 2018, 01:58:52 PM
Boy this question sure brought me back to the past. This is what I used some forty years ago to do fire weather forecasts. Still used today.

http://www.forestry-suppliers.com/product_pages/products.php?mi=85191&itemnum=89080

Title: Re: Which sling psychrometer should I buy?
Post by: AF6GL on July 05, 2018, 04:06:53 PM
We had look up tables. A slide rule was provided but we had multiple charts to flip through so it was easier, more consistent, to flip through the tables.

When electronic handheld calculators came out I used to sit in my grandfathers lap and race him while he was using his slide rule. Never did beat him.
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 05, 2018, 07:36:57 PM
This could be a completely new thread, but if issue is SHT31 looks high and you wanted to compare with another instrument I can confirm it does run high.

My findings on the SHT31 are disappointing with humidity. After long term not happy at all.  DP temperature run about 3°F high all the time winter summer doesn't matter but the damn thing still won't make 100% humidity on 4 different units all aspirated.
The SHT-15 I also recently put back in service for comparison reads closer to actual  psychrometer and nearby ASOS but also won't make 100% humidity.
Title: Re: Which sling psychrometer should I buy?
Post by: jgentry on July 05, 2018, 08:00:32 PM
This could be a completely new thread, but if issue is SHT31 looks high and you wanted to compare with another instrument I can confirm it does run high.

My findings on the SHT31 are disappointing with humidity. After long term not happy at all.  DP temperature run about 3°F high all the time winter summer doesn't matter but the damn thing still won't make 100% humidity on 4 different units all aspirated.
The SHT-15 I also recently put back in service for comparison reads closer to actual  psychrometer and nearby ASOS but also won't make 100% humidity.

That has been my concern for a while. Either the SHT 31 having a wet bias or the ASOS has a dry bias during the day. Plus I also hate the fact that the SHT-31 humidity sensor doesn’t reach to 100% like it should. I guess it’s safe to say that the SHT-31 doesn’t operate within specs.

Now I wonder if the SHT-35 sensor is really all that better?

I wonder if Davis would be willing to use a different temp/hum sensor instead of Sensirion sensors?

Title: Re: Which sling psychrometer should I buy?
Post by: CW2274 on July 05, 2018, 08:04:10 PM
SHT31 looks high and you wanted to compare with another instrument I can confirm it does run high.

My findings on the SHT31 are disappointing with humidity. After long term not happy at all. 
Compared to my previous 11 and 15, the 31 is a rock star. I've used two 31's and they're virtually identical, between 1% and 99% this thing is absolute money. When I reach 1%, the dew does get a little cold, but a RAWS down the street does exactly the same thing. I also was W0X0F last winter and it wouldn't pull the 100% trigger, so I get that. I know you've seen some "competitors" PWS's, if you want disappointment, that's where to look. I don't know any sensor that's affordable that can mach the 31, not even close.
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 05, 2018, 08:27:44 PM
It's anything but a rock star, I'm disappointed. Maybe it works better at extremely low humidity but for me they run high.
AP DP was 66 as was my psychrometer.

I can only give snapshot of 3 units but all 3 SHT31's - 69° and SHT15-67°.
Title: Re: Which sling psychrometer should I buy?
Post by: jgentry on July 05, 2018, 08:32:33 PM
SHT31 looks high and you wanted to compare with another instrument I can confirm it does run high.

My findings on the SHT31 are disappointing with humidity. After long term not happy at all. 
Compared to my previous 11 and 15, the 31 is a rock star. I've used two 31's and they're virtually identical, between 1% and 99% this thing is absolute money. When I reach 1%, the dew does get a little cold, but a RAWS down the street does exactly the same thing. I also was W0X0F last winter and it wouldn't pull the 100% trigger, so I get that. I know you've seen some "competitors" PWS's, if you want disappointment, that's where to look. I don't know any sensor that's affordable that can mach the 31, not even close.

I typically don’t compare to other PWS systems except for RainWise units, Texas Instruments etc. I don’t compare it to cheap stations.  My main stations for comparisons are ASOS, AWOS, and RAWS. RAWS stations typically read closer to the Davis stations than the ASOS does. There is one AWOS station around 30-40 miles east of me and their DPs reads very close to mine station or even the same at times.

I’m curious to know how the RainWise and Texas Instruments stations DP readings compare to the ASOS units.

The Earth Networks (used to be known as WeatherBug) stations DPs read closely to ASOS.

I’m just waiting from Weather Metrics and see what the specs are for their units.
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 05, 2018, 08:46:35 PM
What I've done as work around using Cumulus calibrator I can work some of the high humidity out.

Not perfect but does work somewhat, usually dump 2% humidity without hurting scale but DP still runs high daytime unless I do like a straight -7% humidity cut for afternoon.  But then in the morning it's really low, example 90% vs actual 97%.
Title: Re: Which sling psychrometer should I buy?
Post by: CW2274 on July 05, 2018, 08:48:44 PM
It's anything but a rock star, I'm disappointed. Maybe it works better at extremely low humidity but for me they run high.
AP DP was 66 as was my psychrometer.

I can only give snapshot of 3 units but all 3 SHT31's - 69° and SHT15-67°.
Two degrees of dew point is nothing. What's that equate to in humidity, maybe 1/2 of a percent at your temp? If the 15 works for you, go for it. Mine went into the trash long ago.
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 05, 2018, 08:50:52 PM
It's anything but a rock star, I'm disappointed. Maybe it works better at extremely low humidity but for me they run high.
AP DP was 66 as was my psychrometer.

I can only give snapshot of 3 units but all 3 SHT31's - 69° and SHT15-67°.
Two degrees of dew point is nothing. What's that equate to in humidity, maybe 1/2 of a percent at your temp? If the 15 works for you, go for it. Mine went into the trash long ago.

At 80° ambient it's 4%.
It's more than nothing and way out of specs.
Title: Re: Which sling psychrometer should I buy?
Post by: CW2274 on July 05, 2018, 08:57:57 PM
It's anything but a rock star, I'm disappointed. Maybe it works better at extremely low humidity but for me they run high.
AP DP was 66 as was my psychrometer.

I can only give snapshot of 3 units but all 3 SHT31's - 69° and SHT15-67°.
Two degrees of dew point is nothing. What's that equate to in humidity, maybe 1/2 of a percent at your temp? If the 15 works for you, go for it. Mine went into the trash long ago.

At 80° ambient it's 4%.
It's more than nothing and way out of specs.
Yes, I guess I'm used to my temps. What now?
Title: Re: Which sling psychrometer should I buy?
Post by: jgentry on July 05, 2018, 09:08:58 PM
And believe it or not...My grandmother that lives besides me has an Acurite temp/hum/lightning sensor housed in an Acurite radiation shield and is underneath a tree; and it reports DPs that matches well with the ASOSs during the day. At night, it can have a little dry bias at times but it doesn’t take long for it to reach 99% (highest it can go) on a very humid night.
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 05, 2018, 10:05:39 PM

Yes, I guess I'm used to my temps. What now?

Punt I guess, just frustrated with the high humidity issue. When we get in advisory level on high heat index my website needs to be accurate and not just close.
Running around 6% high on humidity, 3° dew point range at 90° makes a big difference with heat index.
I would like to trust the humidity was accurate at least within claimed specs 2%.   ](*,)
Title: Re: Which sling psychrometer should I buy?
Post by: jgentry on July 05, 2018, 10:30:10 PM
I’m in a similar issue here too. I upload data to WeatherLink Website, etc. and I need the DP to be accurate along with the temp. The county I’m in is one of the top produce producers in the state of Alabama. Growers here rely on the data coming from the Davis VP2s. Auburn University is switching to the Davis units for their agricultural Mesonet from the Campbell units.
Title: Re: Which sling psychrometer should I buy?
Post by: CW2274 on July 05, 2018, 10:43:32 PM

Yes, I guess I'm used to my temps. What now?

Running around 6-7% high on humidity
:???: If I were to average out my humidity compared to the two ASOS's and one RAWS around me over a decent span (say a week) I'd bet it'd be within 0 to 1% with the 31. I compare at least a dozen times every single day (night and day), and have done so since the local WFO picked up my PWS on their website about six years ago.
Me thinks there's more afoot....
Title: Re: Which sling psychrometer should I buy?
Post by: jgentry on July 05, 2018, 10:52:58 PM

Yes, I guess I'm used to my temps. What now?

Running around 6-7% high on humidity
:???: If I were to average out my humidity compared to the two ASOS's and one RAWS around me over a decent span (say a week) I'd bet it'd be within 0 to 1% with the 31. I compare at least a dozen times every single day (night and day), and have done so since the local WFO picked up my PWS on their website about six years ago.
Me thinks there's more afoot....

My only guess then (since I have the Davis 24hr FARS and no boost in airflow) that Davis’ bulky filter cap may have a role in this but then again, it may not be the case.

Maybe Randy can take the filter cap off of one of his units and see if it makes any difference.
Title: Re: Which sling psychrometer should I buy?
Post by: CW2274 on July 05, 2018, 11:04:14 PM

Yes, I guess I'm used to my temps. What now?

Running around 6-7% high on humidity
:???: If I were to average out my humidity compared to the two ASOS's and one RAWS around me over a decent span (say a week) I'd bet it'd be within 0 to 1% with the 31. I compare at least a dozen times every single day (night and day), and have done so since the local WFO picked up my PWS on their website about six years ago.
Me thinks there's more afoot....

My only guess then (since I have the Davis 24hr FARS and no boost in airflow) that Davis’ bulky filter cap may have a role in this but then again, it may not be the case.

Maybe Randy can take the filter cap off of one of his units and see if it makes any difference.
Honestly, I think all that would do is make the response time a touch quicker, it'll do nothing for accuracy. However, be sure it'll certainly dirty up the sensor quicker.
Title: Re: Which sling psychrometer should I buy?
Post by: jgentry on July 06, 2018, 05:54:19 AM

Yes, I guess I'm used to my temps. What now?

Running around 6-7% high on humidity
:???: If I were to average out my humidity compared to the two ASOS's and one RAWS around me over a decent span (say a week) I'd bet it'd be within 0 to 1% with the 31. I compare at least a dozen times every single day (night and day), and have done so since the local WFO picked up my PWS on their website about six years ago.
Me thinks there's more afoot....

My only guess then (since I have the Davis 24hr FARS and no boost in airflow) that Davis’ bulky filter cap may have a role in this but then again, it may not be the case.

Maybe Randy can take the filter cap off of one of his units and see if it makes any difference.
Honestly, I think all that would do is make the response time a touch quicker, it'll do nothing for accuracy. However, be sure it'll certainly dirty up the sensor quicker.

You’re probably right. Just a wild guess on my part. Just trying to figure out what’s the cause.
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 06, 2018, 08:04:21 AM
Yeah not interested in removing filter I think it would just cause another issue.

This is sunrise what I'm seeing this morning Extech outside for about 15 minutes, you can see it's dripping with sweat in short period. The SHT15 isn't a solution either for 100% or even close. Remember the SHT's are saturated all night in high humidity and should be reading at least 98%. Airport was 100% at same time.


Title: Re: Which sling psychrometer should I buy?
Post by: jgentry on July 06, 2018, 09:54:51 AM
I wish Davis would ditch the Sensirion sensor and use Vaisala.
Title: Re: Which sling psychrometer should I buy?
Post by: SLOweather on July 06, 2018, 10:30:16 AM
Do you have a model number?
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 06, 2018, 10:43:42 AM
I wish Davis would ditch the Sensirion sensor and use Vaisala.
I'll second that. I would be willing to spend some $$$ for reliable humidity.
 
For sure the claimed 2% accuracy across the entire range is false at least with the units I've received. It's more like +/- 6% from what I see.
At this point I would be willing to try Rainwise but it's not compatible with my software and website so I'll just live with it and hope they upgrade soon. I give grade on sensirion humidity (D+) but the temperature gets solid (A).     
Title: Re: Which sling psychrometer should I buy?
Post by: jgentry on July 06, 2018, 12:13:17 PM
Do you have a model number?

For the SHT-31?
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 06, 2018, 04:53:20 PM
Auburn U. should look at the Dyacon stations. They have web portal capabilities.  https://dyacon.com/
Title: Re: Which sling psychrometer should I buy?
Post by: CW2274 on July 06, 2018, 05:53:56 PM
Auburn U. should look at the Dyacon stations. They have web portal capabilities.  https://dyacon.com/
Merely food for thought reference your complaints about the SHT31, the Daycon specs out as the same for temp accuracy, and the humidity only 0.2% (1.8% vs 2%) more accurate, but that's only between 10 and 80%. They don't even state what it is outside those parameters. I know that is no satisfaction for you, but just sayin'....
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 06, 2018, 06:01:53 PM

They use a capacitive RH sensor...but, I DO like their RAIN gauge leveling and drain tubes approach:


The capacitor type no good I suppose or no better than Davis I assume.
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 06, 2018, 06:02:39 PM
Auburn U. should look at the Dyacon stations. They have web portal capabilities.  https://dyacon.com/
Merely food for thought reference your complaints about the SHT31, the Daycon specs out as the same for temp accuracy, and the humidity only 0.2% (1.8% vs 2%) more accurate, but that's only between 10 and 80%. They don't even state what it is outside those parameters. I know that is no satisfaction for you, but just sayin'....

I saw that.
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 06, 2018, 06:37:25 PM
One day, this is about as close to professional equipment I can afford.  ....http://www.youngusa.com/products/2/29.html
I know it's just a big expensive thermometer but I've always wanted something I know is accurate and will have a good comparison for the Davis SHT series.
 
Wonder if the probe could be retrofitted inside a Davis FARS? 7.25 ".

Title: Re: Which sling psychrometer should I buy?
Post by: jgentry on July 06, 2018, 06:39:37 PM
Auburn U. should look at the Dyacon stations. They have web portal capabilities.  https://dyacon.com/

Looks like a cool system but Auburn has already made the decision to go with the Davis. I think they would’ve stuck with the Campbell units if they were not so expensive to get and to maintain.
Title: Re: Which sling psychrometer should I buy?
Post by: CW2274 on July 06, 2018, 07:04:01 PM
One day, this is about as close to professional equipment I can afford.  ....http://www.youngusa.com/products/2/29.html
I know it's just a big expensive thermometer but I've always wanted something I know is accurate and will have a good comparison for the Davis SHT series.
 
Wonder if the probe could be retrofitted inside a Davis FARS? 7.25 ".
I love RMY. That being said, the only thing that disappoints me about the SHT31 is that it wouldn't pull the 100% trigger, stayed on 99 forever, but that was my old 31, haven't had the opportunity with the new one. Remember, the only difference between 99 and 100% is 0.4F spread between temp and dew point, that's pretty razor thin. Both 31's certainly have gotten to 1%, and have done so a few times this year, hell even the TUS ASOS and a RAWS station hit 1%, I've never seen that before.
Perhaps you have other issues, but maybe not.
Title: Re: Which sling psychrometer should I buy?
Post by: jgentry on July 06, 2018, 07:13:24 PM
Do you have a model number?

In terms of the Vaisala sensor, the HMP 155 would be a good one...

https://www.vaisala.com/sites/default/files/documents/HMP155-Datasheet-B210752EN-F.pdf
Title: Re: Which sling psychrometer should I buy?
Post by: jgentry on July 06, 2018, 07:14:25 PM
But I bet the Davis transmitter couldn’t take it.
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 06, 2018, 07:32:05 PM
[youtube]https://www.youtube.com/watch?v=gbMdYVSAzQg[/youtube]
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 07, 2018, 08:00:37 AM
Good news the Extech was dead on with the Davis this morning at 90% humidity range. Still doesn't change fact 97% humidity is max.



Title: Re: Which sling psychrometer should I buy?
Post by: openvista on July 07, 2018, 09:40:09 AM
Randy, are any of your stations passive?

I say this because I've noticed that my passive station with the stock shield tends to run lower in humidity (~2-3%) than my  aspirated station (both VP2s with SHT31s). By the way, my FARS unit uses the same Mechatronics fan as yours I believe (UF80B12-BTH).

Could the sensor (and the firmware which lacks averaging capabilities) have been designed for passively ventilated enclosures? It seems forcing air onto it causes it to go a bit haywire with rapidly fluctuating dew points. I noticed this behavior previously with my solar powered stock fan too.
 
Take yesterday for example. Almost calm winds all day. The dew point from the FARS was a roller coaster varying 6F between the highs and lows (a complete cycle took as little as 5 minutes).

However, with a stronger, steadier wind (5-15 mph), as exists today, the DP smooths out considerably (<1F variations in same time period). This mimics the behavior of a passive enclosure. And the DPs between the two shields converge (1-2F diff with active still higher) under windier conditions.

Both mornings have been clear and sunny with temps in the upper 60s to lower 70s.

Perhaps humidity and DPs need to have a rolling average applied (last 10 readings?) before comparisons can be made on calmer days?
Title: Re: Which sling psychrometer should I buy?
Post by: openvista on July 07, 2018, 09:50:27 AM
Just thought about it and another explanation for the variations in DPs on days with little to no wind is that the air pockets the active shield is sampling are unmixed. Meaning, they are discrete parcels of air with distinct humidities and temps (which is also why temps fluctuate too (to a lesser degree than DPs). Normally, the wind mixes the air and performs an averaging function in a round about way.

Sampling calm air with an active shield would seem to require some averaging software to get a handle on the true DP. I've been thinking of maybe taking the last 5 DPs and doing a rolling average from that. Of course I'd have to do humidity and heat index too if I went that way.
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 07, 2018, 10:44:13 AM
The SHT's are all aspirated not the handheld extech I was checking against. Today we did have a stiff breeze when they all matched so maybe you are on to something but 100% without using Cumulus calibrator won't happen. 97% is max on all units even dense fog for 8 hours.

FYI you can slow and make less dewpoint/humidity swings by lowering the fan speed. This unit works but has a very narrow range as they all do. Aspiration speed even slowed is still 2x above solar fan.
 
I've tried another speed controllers that failed, would shut fan speed all the way off after few hours but found this unit to keep speed steady and never stops fan.

https://www.amazon.com/gp/product/B00BLOSG7I/ref=oh_aui_search_detailpage?ie=UTF8&psc=1
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 07, 2018, 10:49:53 AM
I only turn fan speed down summer, DP swings aren't band until about 70° dewpoint temperature. Winter I keep it cranked full speed.
Title: Re: Which sling psychrometer should I buy?
Post by: jgentry on July 07, 2018, 11:05:26 AM
The SHT's are all aspirated not the handheld extech I was checking against. Today we did have a stiff breeze when they all matched so maybe you are on to something but 100% without using Cumulus calibrator won't happen. 97% is max on all units even dense fog for 8 hours.

FYI you can slow and make less dewpoint/humidity swings by lowering the fan speed. This unit works but has a very narrow range as they all do. Aspiration speed even slowed is still 2x above solar fan.
 
I've tried another speed controllers that failed, would shut fan speed all the way off after few hours but found this unit to keep speed steady and never stops fan.

https://www.amazon.com/gp/product/B00BLOSG7I/ref=oh_aui_search_detailpage?ie=UTF8&psc=1


Had a similar issue with one of my stations but after installing a new SHT-31 sensor, it will top at 99% fairly often but I’m sure the 100% reading would’ve been correct during some mornings.
Title: Re: Which sling psychrometer should I buy?
Post by: jgentry on July 07, 2018, 11:12:15 AM
To me, it seems like all SHT sensors act just about the same when it comes to humidity. The SHT 31 may do better on the low end of the scale but in the mid and upper range, it basically acts almost the same. In the mid range, it as a wet bias and on the high end, it has a dry bias. IMO.

It would be awesome if someone on here had the smarts and the capabilities of making a home brew sensor for the VP2 using something else besides Sensirion.
Title: Re: Which sling psychrometer should I buy?
Post by: openvista on July 07, 2018, 11:23:28 AM
Thank you for the suggestion on the variable fan speed controller. I'll consider that.

I went back and looked at data from the wee hours of this morning when winds kicked up (10-15mph) and the DP on the aspirated station smoothed out. I'm still seeing 4-6% discrepancies in humidity between passive and active shields. Active ALWAYS runs higher. So something is up.

The air temps were always within the margin of error (0.5F). So no problems there. The shields are only a few feet away from each other.

Something about forcing air against that sensor (SHT31) seems to be in play here. Not exactly sure what. I don't have a psychrometer so I can't test, but I have 2 VP2 stations within about a mile of me and they seem to agree with my active shield (dew points within about 1F). That would suggest my passive station has a sensor running low and that would be odd since that's about a 6 month old sensor.

OR... maybe it's a new revision of the sensor and it's more accurate? Might have to perform surgery and swap the two sensors to find out.


Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 07, 2018, 11:33:23 AM
To me, it seems like all SHT sensors act just about the same when it comes to humidity. The SHT 31 may do better on the low end of the scale but in the mid and upper range, it basically acts almost the same. In the mid range, it as a wet bias and on the high end, it has a dry bias. IMO.

It would be awesome if someone on here had the smarts and the capabilities of making a home brew sensor for the VP2 using something else besides Sensirion.

Agree what I'm seeing too. 
The sensor's are basically really cheap just few $$ when purchased separately and may not have the ability to have linear accuracy everywhere.  Maybe we just expect too much. 

I wouldn't hesitate if there was option for high dollar unit but it won't happen. These Davis sensors already have special needs. Can't image getting a Vaisala to work unless it came with a compatible transmitter that could communicate with the console.   :idea:
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 07, 2018, 11:35:44 AM

OR... maybe it's a new revision of the sensor and it's more accurate? Might have to perform surgery and swap the two sensors to find out.

Interesting thought where did you purchase the sensor :?:
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 07, 2018, 11:43:53 AM
Thank you for the suggestion on the variable fan speed controller. I'll consider that.

I went back and looked at data from the wee hours of this morning when winds kicked up (10-15mph) and the DP on the aspirated station smoothed out. I'm still seeing 4-6% discrepancies in humidity between passive and active shields. Active ALWAYS runs higher. So something is up.



Hmm!  I've never tested passive shield and humidity. I'm wondering if this isn't how sensirion test and calibrates sensor in passive calibration chamber.  Somehow using aspiration knocks sensor calibration off. It is a thought...
Title: Re: Which sling psychrometer should I buy?
Post by: openvista on July 07, 2018, 11:46:51 AM

OR... maybe it's a new revision of the sensor and it's more accurate? Might have to perform surgery and swap the two sensors to find out.

Interesting thought where did you purchase the sensor :?:

Both were purchased from Ryan at SI about 1 year apart.
Title: Re: Which sling psychrometer should I buy?
Post by: DaleReid on July 07, 2018, 12:53:16 PM
What kind of sensors do they use in those electronic hand held devices?

Title: Re: Which sling psychrometer should I buy?
Post by: openvista on July 07, 2018, 01:07:15 PM
Both were purchased from Ryan at SI about 1 year apart.

Strike that. The first sensor was purchased Apr 2017 from Ryan and it's currently in my aspirated shield. The newer sensor was purchased Nov 2017 and it lives in my passive shield. Could Sensirion have pushed out a minor (unpublished?) revision in those 7 months (and whatever lag Ryan has in stock turnover)?

Also in reviewing data from the winter it looks like this was an issue too but much less so. Active station always runs higher but only by a couple percent in cold temps (20F).

Could have a bad sensor that's getting worse over time. The only wrinkle here is that the "bad" sensor might actually be more accurate. And this might be the rare case where others are wrong.

I'd have to get a reliable device to verify that claim and that's a chore and not cheap. See upthread. A margin of error +/- 3% is not what I'd call a reference device. And how do we know that device is accurate without paying hundreds to get it certified? I have an NIST calibrated temp sensor (PRT) that's supposed to be within 0.1C at all times and is 0.5 - 0.7C off at all times. How do I know? Well I tested it against 4 Davis sensors all on the same bench. Either all 4 of those sensors (SHT11, SHT15, SHT31 sensors purchased years apart) were up to 1.2F off all in the same direction (warm) or the certified unit is bad (within 3 months of certification). So... just be aware that anyone can print off a piece of paper or state some specifications. Extech is a mass manufacturer. If you're serious, you need a specialty manufacturer like, say, this: https://www.alphaomega-electronics.com/en/calibration-instruments/2552-cr-4-higrometro-de-espejo-frio.html. I doubt any of us want to drop the briefcase of dough to get our hands on that kind of unit.
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 07, 2018, 01:36:55 PM
I just set up a passive shield with SHT31 using a 24 hr shield core. I've got plenty of spare parts so drilled some ventilation holes on core for added ventilation and extra louver and 2 extra shields on top.  So far with stiff wind the passive is doing ok +.2 warmer than AC fan. Dewpoint 2° lower which is a sign maybe.

I'll test a few days may just leave out as is a see how warm it gets on calm days.  :idea:
Title: Re: Which sling psychrometer should I buy?
Post by: jgentry on July 07, 2018, 02:33:09 PM
I just set up a passive shield with SHT31 using a 24 hr shield core. I've got plenty of spare parts so drilled some ventilation holes on core for added ventilation and extra louver and 2 extra shields on top.  So far with stiff wind the passive is doing ok +.2 warmer than AC fan. Dewpoint 2° lower which is a sign maybe.

I'll test a few days may just leave out as is a see how warm it gets on calm days.  :idea:

Please let us know what your results are. You would think the SHTs  would be designed to take constant airflow from the FARS. There has to be a good temp/hum sensor that Davis could use besides Sensirion.
Title: Re: Which sling psychrometer should I buy?
Post by: jgentry on July 07, 2018, 02:34:28 PM
Here is an interesting journal article on humidity measurements if anyone hasn’t seen it....

https://journals.ametsoc.org/doi/full/10.1175/JTECH-D-12-00232.1
Title: Re: Which sling psychrometer should I buy?
Post by: jgentry on July 07, 2018, 02:45:55 PM
Here is an interesting journal article on humidity measurements if anyone hasn’t seen it....

https://journals.ametsoc.org/doi/full/10.1175/JTECH-D-12-00232.1

Here is an interesting excerpt...

“New capacitive sensors perform well but they usually drift to higher values during deployment (except in arid climates); they perform best at lower humidities and need regular adjustment and recalibration...”

That would explain why CW is having different results with the SHT-31 than Randy & I.


It would also explain why there is a “wet bias” with the SHT-31 for those living in higher humidity environments.
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 07, 2018, 03:02:44 PM
[

Here is an interesting excerpt...

“New capacitive sensors perform well but they usually drift to higher values during deployment (except in arid climates); they perform best at lower humidities and need regular adjustment and recalibration...”

That would explain why CW is having different results with the SHT-31 than Randy & I.


It would also explain why there is a “wet bias” with the SHT-31 for those living in higher humidity environments.

Bingo you just won the lottery.  =D&gt;
Title: Re: Which sling psychrometer should I buy?
Post by: openvista on July 07, 2018, 03:11:40 PM
It would also explain why there is a “wet bias” with the SHT-31 for those living in higher humidity environments.

Which may be worse in actively ventilated enclosures? We'll see what Randy's test produces.

Another takeaway from that paper is how little data exists on measuring humidity on solid state devices. Also, as of 2010, the WMO was saying not to expect better than +/- 3% from ANY solid state humidity sensor.

The study dates back to 2013 using 2012 data. That's a lifetime ago in terms of technology. Perhaps some mass manufacturer has made some improvements? Sensirion, of course, claims that the SHT31 was a breakthrough. But we're finding "discrepancies", let's say.  #-o
Title: Re: Which sling psychrometer should I buy?
Post by: DaleReid on July 07, 2018, 03:19:31 PM
Just like the calculation of humidity with look up tables when you know the two temperatures, if we only knew accurate humidity levels we could construct a look up chart (table) to take the reading and use the correction to make adjustments to know what the real temperature and humidity were.

Old programs for the Campbell Scientific Dataloggers sometimes did this. Your sensor says 4.234 volts?  Hmm, no linear forumula that is good enough, so I'll consult my built in matrix.  Ah yes, it is  93%, based on calibration with a standard.

The Hmmm comes in by having the ability to place a gizmo into an enclosure and really know what the humidity is.  The saturated salts method works for three readings that I know of but it is a royal pain.  Plus it says nothing about the linearity of the readings, which as noted may really start to deviate in certain ranges of the instrument's output.

But then again one might wonder if 94% humidity is too much different from 97?
Title: Re: Which sling psychrometer should I buy?
Post by: CW2274 on July 07, 2018, 04:15:26 PM
I've said this before, I think you guys over do the fan speed thing. Mine runs at 67CFM 24/7 365 and has so for three years now and I don't have y'alls problems, on the contrary for that matter.
I skimmed the article about the dry climate effect on the sensor, maybe that's "helping" mine be more accurate than you's guys, I have no idea.
Good luck with your experiments, my fan will continue to suck at full power. ;)
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 07, 2018, 05:01:41 PM
 I switched the passive to #1 transmitter for now..... Actual dewpoint per ASOS and my Extech is 63°.
All sensors are SHT31 with highest #3 (+5°dp) and the sensor with most airflow with AC fan.
#5 sensor is solar fan (+4°dp).
#1 sensor passive (+3°dp).

The #1 sensor was a spare but had also been used previously with solar fan but seems to be least off +3 dp. 

Not sure this is conclusive extra airflow knocks the humidity out of calibration but it does show for sure humidity drift is apparent. I noticed last year they were running high but over time they are getting worse. 

Title: Re: Which sling psychrometer should I buy?
Post by: CW2274 on July 07, 2018, 05:49:30 PM
Not sure this is conclusive extra airflow knocks the humidity out of calibration
Granted, not conclusive, but if that were the case, I'd probably be having bigger problems than you, right, but I'm not. Three years on the big fan is conclusive to me that it's not degrading the sensor, at least not noticeably. All I can do is state what I see.
 
Title: Re: Which sling psychrometer should I buy?
Post by: openvista on July 07, 2018, 06:00:47 PM
Interesting, Randy. Thanks for doing that.

At some point soon, I will swap the 31s in my two shields. Of course, the active shield humidity reading might start off better and then get wetter over time. I'd have to keep an eye on it.
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 07, 2018, 06:17:50 PM
Not sure this is conclusive extra airflow knocks the humidity out of calibration
Granted, not conclusive, but if that were the case, I'd probably be having bigger problems than you, right, but I'm not. Three years on the big fan is conclusive to me that it's not degrading the sensor, at least not noticeably. All I can do is state what I see.

From what I read in the study the solid state sensors do fine in dry conditions. Adding extra air with humidity may accelerate drifting with extra saturation.
I can't be conclusive without starting out with a couple untouched sensors. One left passive the other with a fan and see how fast they drift apart.
 
My disappointment with humidity side of sensor hasn't changed but at least we have some answers what may be going on.
It's a big thing for my website to be accurate, I owe it to my visitors who trust in it. 
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 07, 2018, 06:20:29 PM
Not sure this is conclusive extra airflow knocks the humidity out of calibration
Granted, not conclusive, but if that were the case, I'd probably be having bigger problems than you, right, but I'm not. Three years on the big fan is conclusive to me that it's not degrading the sensor, at least not noticeably. All I can do is state what I see.
Quoting from page 2041 of that DOI article:

"For naturally ventilated screens, ... , errors may be larger in calm conditions. Relative humidity natural varibility and errors are somewhat larger during daylight hours than at night."

Thanks for pointing out.
Title: Re: Which sling psychrometer should I buy?
Post by: CW2274 on July 07, 2018, 06:28:48 PM
Not sure this is conclusive extra airflow knocks the humidity out of calibration
Granted, not conclusive, but if that were the case, I'd probably be having bigger problems than you, right, but I'm not. Three years on the big fan is conclusive to me that it's not degrading the sensor, at least not noticeably. All I can do is state what I see.
It's a big thing for my website to be accurate, I owe it to my visitors who trust in it.
Oh, completely get that, and I don't even have a web site (well, NWS). Being a perfectionist can be a curse, one that you, I, and more than a few others on here can relate to.
GL with everyone's experiments....hopefully something of true value can be gleaned. UU
Title: Re: Which sling psychrometer should I buy?
Post by: jgentry on July 07, 2018, 07:14:56 PM
I noticed the Davis spec sheet says “Film capacitor element” in describing what kind of humidity sensor they use.

For this RW product, it says that they use a “thin film polymer capacitor.”

http://rainwise.com/products/attachments/6751/20060215113108.pdf

Is there any difference between the two sensors or are they the same?
Title: Re: Which sling psychrometer should I buy?
Post by: openvista on July 07, 2018, 07:39:54 PM
Here's the Sensirion datasheet on the humidity sensor. Sensirion claim drift of <0.25% per year.
Title: Re: Which sling psychrometer should I buy?
Post by: jerryg on July 07, 2018, 10:00:16 PM
 Here is an article about getting to 100% humidity that might hurt your brain to read it  :grin: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4431636/
Title: Re: Which sling psychrometer should I buy?
Post by: openvista on July 07, 2018, 10:47:43 PM
OK, so I swapped out the two SHT31 sensors this evening. After giving about 20 minutes to acclimate, I'm seeing the reverse of what I was seeing previously. Now the newer sensor inside the active shield reads lower dew points (3-4F) than the passive station. I'll monitor it and report back.
Title: Re: Which sling psychrometer should I buy?
Post by: jgentry on July 07, 2018, 11:04:21 PM
What kind of sensors do they use in those electronic hand held devices?

Not sure...

But I think I saw where you stated that you had a RW MK III station? If so, how did the DPs compared to the ASOS? Does RW stations have the “wet bias” like the VP2s?
Title: Re: Which sling psychrometer should I buy?
Post by: jgentry on July 07, 2018, 11:07:13 PM
OK, so I swapped out the two SHT31 sensors this evening. After giving about 20 minutes to acclimate, I'm seeing the reverse of what I was seeing previously. Now the newer sensor inside the active shield reads lower dew points (3-4F) than the passive station. I'll monitor it and report back.

Please do!

I’m thinking it’s all about the  capacitive sensor element that Sensirion uses to measure humidity.
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 07, 2018, 11:12:18 PM



I ordered a fresh sensor also, I'm going to use in passive shield exclusive and do comparisons before I screw it up in FARS shield.
Title: Re: Which sling psychrometer should I buy?
Post by: CW2274 on July 08, 2018, 12:24:39 AM
do comparisons before I screw it up in FARS shield.
Please ....if anything, loses it's virginity..
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 08, 2018, 12:48:22 PM
I know the frustration some are having with the humidity.
My little not so secret secret and why I use Cumulus not the MX version which also uploads to all the favorite weather outlets including CWOP and website.

You can't just remove 5% humidity without hurting the already low upper end. My sensors only max at 97%.

Cumulus has a calibration function that can reduce those mid range humidities but still reach 100%.

The value that works best on all my SHT31's and still reach 100% humidity is this.

-11% humidity (89%) and use multiplier  x 1.156 = 102.79%   Some sensors may require a multiplier of 1.157 for 100%.
The reason my multiplier number goes above 100% is because my sensors stop at 97% so need it for 100%. 


Edit: I should add this works above 35% humidity. If you get below you may need to change multiplier something less -5% (95) multiplier x 1.083 or even remove completely for humidity in the teens. So yes you end up chasing your tail when sensor doesn't work as advertised.
Title: Re: Which sling psychrometer should I buy?
Post by: jerryg on July 08, 2018, 04:06:56 PM
I use weather display and it has a very good humidity calibration setup. I have finally got over the must have 100% reading or go nuts lol. For the sake of this  thread i broke out my wr-25 station from texas weather inst. which is a high dollar set up that was given to me by a friend who worked at a cable vision that went fiber and quit using it, the only thing i had to do was replace the bearings in the wind cup and direction and it works fine. I don't use it because it is wired and you have to use the cables that came with it or the calibrations are way off. I believe it uses analog voltage readings thus the cable can not be messed with. Anyway i was going to compare humidity readings with my two 31's i have up and running but a problem with that is the Davis only senses the humidity about once a minute and the wr-25 is basically real time, reporting every second, so i get almost 60 readings to 1 for the Davis and yes the humidity does go up and down quite a bit in a minute. But just by eye and looking at it my readings are near enough to each other and my dp according to cwop error is 0.0F so it looks like my 31's are doing fine. I sure do like the near real time updates of the wr-25 and it came with the computer interface wps-10 so i can use wd to see the info. I found out along time ago that trying to compare my readings with other stations or metars was a waste of time. I have a metar 10 miles nw of me and one 12 miles se and sometimes they have 20 to 30 percent difference between them. I sometimes wonder about the metar readings, i report one of the metars was showing -40 degrees every time the temp got down in the 30's and they said they would report it to the repair crews and it went all winter without being fixed. I say trust your own gear and no one elses.
Title: Re: Which sling psychrometer should I buy?
Post by: jgentry on July 08, 2018, 04:24:04 PM
I use weather display and it has a very good humidity calibration setup. I have finally got over the must have 100% reading or go nuts lol. For the sake of this  thread i broke out my wr-25 station from texas weather inst. which is a high dollar set up that was given to me by a friend who worked at a cable vision that went fiber and quit using it, the only thing i had to do was replace the bearings in the wind cup and direction and it works fine. I don't use it because it is wired and you have to use the cables that came with it or the calibrations are way off. I believe it uses analog voltage readings thus the cable can not be messed with. Anyway i was going to compare humidity readings with my two 31's i have up and running but a problem with that is the Davis only senses the humidity about once a minute and the wr-25 is basically real time, reporting every second, so i get almost 60 readings to 1 for the Davis and yes the humidity does go up and down quite a bit in a minute. But just by eye and looking at it my readings are near enough to each other and my dp according to cwop error is 0.0F so it looks like my 31's are doing fine. I sure do like the near real time updates of the wr-25 and it came with the computer interface wps-10 so i can use wd to see the info. I found out along time ago that trying to compare my readings with other stations or metars was a waste of time. I have a metar 10 miles nw of me and one 12 miles se and sometimes they have 20 to 30 percent difference between them. I sometimes wonder about the metar readings, i report one of the metars was showing -40 degrees every time the temp got down in the 30's and they said they would report it to the repair crews and it went all winter without being fixed. I say trust your own gear and no one elses.

The NWS in Birmingham, AL does a good job in maintaining the ASOS stations and they do not get wacky readings very often. All the ASOS stations DP reports are sensible and congruent. Sensirion, on the other hand, has a pretty much of a "wet bias" until it reaches in the 90 percentile and then, it starts to have a "dry bias" of sorts.

Shoot, the RW stations performs better when the humidity hits near or at saturation.
Title: Re: Which sling psychrometer should I buy?
Post by: jerryg on July 08, 2018, 04:28:42 PM
Well the primary problem i have is unless you are co-located with the station you are comparing to it is all a guessing game. If your station is properly sited then the readings you get from a quality station like Davis should be pretty good. I am lucky that i live in the country and have my station out in the clear over grass and my anny at 33 feet so i can be as accurate as i can be, i am glad i don't have to compromise on my station like a lot of the guys do because of living in town.
Title: Re: Which sling psychrometer should I buy?
Post by: jerryg on July 08, 2018, 04:36:54 PM
When you look at the specs of the really high dollar sensors nearly all of them are speced between 20 to 40 percent up to 90 percent and above 90 percent they go up in error percentage. The 31 specs at 2% or called nominal and above 90% they show it can go up higher on the error scale. I read a study about sling thermos and how they did a study at a military school where they taught how to use them and found a 5 to 10 percent error in the readings, so many things to consider when using one, how many spings good water supply to wet bulb and reading the chart right to get the humidity. Sure is a lot of places to goof up.
Title: Re: Which sling psychrometer should I buy?
Post by: jgentry on July 08, 2018, 04:49:55 PM
When you look at the specs of the really high dollar sensors nearly all of them are speced between 20 to 40 percent up to 90 percent and above 90 percent they go up in error percentage. The 31 specs at 2% or called nominal and above 90% they show it can go up higher on the error scale. I read a study about sling thermos and how they did a study at a military school where they taught how to use them and found a 5 to 10 percent error in the readings, so many things to consider when using one, how many spings good water supply to wet bulb and reading the chart right to get the humidity. Sure is a lot of places to goof up.

True. But I’m believing the problem is with Sensirion’s humidity sensing element is that it is a capacitive sensing element and it seems to go off calibration in high humidity environments. If Sensirion used something different, we might not have these “wet bias” issues. Then again, who knows...
Title: Re: Which sling psychrometer should I buy?
Post by: openvista on July 08, 2018, 04:53:47 PM
Here are some interesting quotes from The Weather Observer's Handbook by Christopher Burt (2012 Kindle Version; locations 4569-4631):

"Aspirated screens are ideal for accurate temperature measurements, and when fitted with RH sensors will generally give more representative RH values too, but the greater volume of air movement over humidity sensors tends to exacerbate dust and salt ingress problems and probably ultimately shortens their working life." [emphasis mine]

"Calibration drift is a problem with humidity sensors, particularly in less expensive systems where it can exceed 5% per year. Whatever type of equipment is used, regular checking over a range in humidities is essential if reasonably accurate long-period humidity measurements are sought. Calibration checking is best carried out annually, or more frequently if spot checks indicate the sensor is regularly more than about 5% different from independent instruments."

"The errors inherent in the measurement of humidity - whether by dry- and wet-bulb or by electronic sensor - mean that the RH is at best accurate only to about 2-3% in its mid-range.... At high humidities response will be slow, while at low humidities and low temperatures, errors increase and the accuracy falls off further. The same goes for dew point - although often quoted to a precision of 0.1 degC, in reality the measurement is probably no better than +/- 0.5-1 degC when derived from humidity measurements, with still wider error ranges at low temperatures and humidities." [emphasis mine]

"Combined temperature/RH sensors are popular, but can become expensive and inconvenient if the relatively short working lifetime of the humidity component mandates replacement (and recalibration) of the temperature sensor too." [emphasis mine]

"The calculation varies somewhat depending upon the airflow over the sensors, and for accurate work this needs to be taken into account. For this reason there are different psychometric formulae and tables for sensor exposed in a passively ventilated shelter such as a Stevenson screen and for those in a forced airflow, such as an aspirated or whirling psychrometer. There are also slight differences in the method of calculation for temperatures below 0 degC, owing to differences in the saturation vapour pressure over liquid water and ice surfaces."

*************

Essentially what he's been saying, and it's been known for years even before he wrote it, is that, at least in non-arid climates, our sensor packages are disposable since they cannot really be calibrated at multiple points (which is required because they aren't linear). I would add, if you have a fan-aspirated Davis unit, expect to order replacements well before the sensors fail (yearly? every other year?) if you want to be within the ballpark on humidity. Also, the manufacturers specs are marketing, not reality, as we've been discovering.  This isn't a problem limited to Davis. It's just that Davis is one of very few manufacturers selling aspirated stations. The extra air movement exacerbates the wear and tear on the humidity sensor. Over time, they all drift, though.

Humidity checks should only be made against known reference-quality weather stations. One-off comparisons are not advised because even very expensive AWOS systems at airports can sometimes be out of calibration (which is currently the case at my local airport). But if you look at enough NWS/RAWS stations (https://www.wrh.noaa.gov/map/) within about 50 - 100 miles (assuming synoptic weather patterns and relatively flat terrain, no mesoscale influences, etc) a consensus dew point range begins to emerge. Plus you can check the software models for projected dew points in your area (weathernerds.org) and, likewise, perform an average (i.e. HRRR + NAM + Euro divided by 3). Do NOT use other neighborhood weather stations (unless you know them to be accurate) because, at least in my area, all of them run well above the reference stations. Very few station owners properly maintain their stations due to the expense and complexity. So the stations humidities and DPs almost always drift up over time and run wet.

Title: Re: Which sling psychrometer should I buy?
Post by: openvista on July 08, 2018, 05:08:04 PM
Key wording above was the repeated use of "calibration" which is something VERY few CWOP owners do.

Well, when the sensor isn't linear and the consoles and the major software packages only offer one-point calibration, how do you calibrate it? You don't really. You play whack-a-mole. The best you can do is a work-around like Randy advises using a specific piece of software.
Title: Re: Which sling psychrometer should I buy?
Post by: jgentry on July 08, 2018, 05:24:59 PM
I know that Sensirion claims that their humidity sensor drifts around .25% a year but I think that only applies to more arrid climates and not humid climates like Alabama.
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 08, 2018, 05:52:42 PM
Someone here one of the weather techs only occasionally visits warned about year ago the SHT31 chip was losing humidity calibration fast after just a few months of use he retested and it was off substantially enough he posted results.
It was one of the Sensirion stand alone temp/hum gadget with the SHT31 chip.
He was using his own humidity chamber in testing.   
Title: Re: Which sling psychrometer should I buy?
Post by: jgentry on July 08, 2018, 06:02:09 PM
Someone here one of the weather techs only occasionally visits warned about year ago the SHT31 chip was losing humidity calibration fast after just a few months of use he retested and it was off substantially enough he posted results.
It was one of the Sensirion stand alone temp/hum gadget with the SHT31 chip.
He was using his own humidity chamber in testing.

I’ll try to find the thread but if you do, would you post it on here?

I think I’ve seen on the SHT-31 datasheet that if the humidity is really high (mainly in the 90 percentile range) for a while, it will knock the sensor out of calibration.
Title: Re: Which sling psychrometer should I buy?
Post by: jgentry on July 08, 2018, 06:36:01 PM
Would resistive humidity sensors perform better than capacitive sensors?

https://www.sensorsmag.com/components/choosing-a-humidity-sensor-a-review-three-technologies
Title: Re: Which sling psychrometer should I buy?
Post by: jgentry on July 08, 2018, 06:38:04 PM
Surprised that johnd hasn’t chimed in yet on this discussion thread. Wouldn’t mind in hearing his take on the Sensirion humidity sensor issues.
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 08, 2018, 07:00:03 PM
It was kcidwx what he posted about the Gadget with SHT31.

One thing that seriously concerned me with these SHT31 evaluation boards was the long term shift in accuracy on the humidity side. I only test a particular type of sensor in minimum groups of three. When a sensor company is wanting to bid on a government contract it's usually pretty easy for me to secure at least three for testing. Although that's not the case with these evaluation boards, it was more for my own curiosity. The issue is approximately 3 weeks after receiving all three of these evaluation boards, I ran a test at 50% RH. All three were reading low by -1.2%, -1.7% and -1.9%. Almost a year later they were reading +0.5% to +0.8%. I'm at a loss to explain why the large shift over time. I'm not quite ready to call it "drift" because that's an insane amount of drift. I was so concerned, I sent my NIST reference instrument back to Washington D.C. to get it certified again after it was just certified 6 months before. It came back having tested just fine. In the hundreds of sensors I've tested over 20+ years, I never seen this much shift before over the course of a year. I was thinking of getting a fourth one and seeing how it compares right out of the box against these three units that have been running for almost a year. Some sensors do require a conditioning period when first used or what I refer to as exercising the sensor. I know that with some sensor manufacturers when they send you a sample for evaluation, sometimes these are not their best spec'ed sensors. Often they are sensors that are usable but don't quite pass QC 100% for some reason. I wonder if that's what these evaluation boards are. I never did ask Sensirion. The only thing I can think of is since these SHT31 sensors don't have a filter over them, they have become contaminated with dust over time.

https://www.wxforum.net/index.php?topic=32439.msg328209#msg328209
Title: Re: Which sling psychrometer should I buy?
Post by: Old Tele man on July 08, 2018, 07:16:47 PM
YIKES -- IF dust can cause that kind of error, then what the heck will dirty outdoor (and far aspiration) air do?!?
Title: Re: Which sling psychrometer should I buy?
Post by: jgentry on July 08, 2018, 07:37:34 PM
It was kcidwx what he posted about the Gadget with SHT31.

One thing that seriously concerned me with these SHT31 evaluation boards was the long term shift in accuracy on the humidity side. I only test a particular type of sensor in minimum groups of three. When a sensor company is wanting to bid on a government contract it's usually pretty easy for me to secure at least three for testing. Although that's not the case with these evaluation boards, it was more for my own curiosity. The issue is approximately 3 weeks after receiving all three of these evaluation boards, I ran a test at 50% RH. All three were reading low by -1.2%, -1.7% and -1.9%. Almost a year later they were reading +0.5% to +0.8%. I'm at a loss to explain why the large shift over time. I'm not quite ready to call it "drift" because that's an insane amount of drift. I was so concerned, I sent my NIST reference instrument back to Washington D.C. to get it certified again after it was just certified 6 months before. It came back having tested just fine. In the hundreds of sensors I've tested over 20+ years, I never seen this much shift before over the course of a year. I was thinking of getting a fourth one and seeing how it compares right out of the box against these three units that have been running for almost a year. Some sensors do require a conditioning period when first used or what I refer to as exercising the sensor. I know that with some sensor manufacturers when they send you a sample for evaluation, sometimes these are not their best spec'ed sensors. Often they are sensors that are usable but don't quite pass QC 100% for some reason. I wonder if that's what these evaluation boards are. I never did ask Sensirion. The only thing I can think of is since these SHT31 sensors don't have a filter over them, they have become contaminated with dust over time.

https://www.wxforum.net/index.php?topic=32439.msg328209#msg328209

Thanks for posting! Reading through that thread tells me that the SHT-31 isn’t as good of a humidity sensor as the Sensirion specs makes them out to be.
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 08, 2018, 07:58:46 PM
It was kcidwx what he posted about the Gadget with SHT31.

One thing that seriously concerned me with these SHT31 evaluation boards was the long term shift in accuracy on the humidity side. I only test a particular type of sensor in minimum groups of three. When a sensor company is wanting to bid on a government contract it's usually pretty easy for me to secure at least three for testing. Although that's not the case with these evaluation boards, it was more for my own curiosity. The issue is approximately 3 weeks after receiving all three of these evaluation boards, I ran a test at 50% RH. All three were reading low by -1.2%, -1.7% and -1.9%. Almost a year later they were reading +0.5% to +0.8%. I'm at a loss to explain why the large shift over time. I'm not quite ready to call it "drift" because that's an insane amount of drift. I was so concerned, I sent my NIST reference instrument back to Washington D.C. to get it certified again after it was just certified 6 months before. It came back having tested just fine. In the hundreds of sensors I've tested over 20+ years, I never seen this much shift before over the course of a year. I was thinking of getting a fourth one and seeing how it compares right out of the box against these three units that have been running for almost a year. Some sensors do require a conditioning period when first used or what I refer to as exercising the sensor. I know that with some sensor manufacturers when they send you a sample for evaluation, sometimes these are not their best spec'ed sensors. Often they are sensors that are usable but don't quite pass QC 100% for some reason. I wonder if that's what these evaluation boards are. I never did ask Sensirion. The only thing I can think of is since these SHT31 sensors don't have a filter over them, they have become contaminated with dust over time.

https://www.wxforum.net/index.php?topic=32439.msg328209#msg328209

Thanks for posting! Reading through that thread tells me that the SHT-31 isn’t as good of a humidity sensor as the Sensirion specs makes them out to be.

Agree
kcidwx does Government testing and had never seen that kind of drift ever over a short period.
I'm afraid his findings were legit.
Not sure what we can do  :?: Hope the price drops more and replace sensor every 6 months if humidity is important... :evil:
Title: Re: Which sling psychrometer should I buy?
Post by: jgentry on July 08, 2018, 08:24:26 PM
I think he needs to send Davis an email or something and talk them into changing their sensor element.
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 08, 2018, 08:29:30 PM
I think he needs to send Davis an email or something and talk them into changing their sensor element.

Don't think he even owns a Davis his stuff was RM Young if memory serves.
Title: Re: Which sling psychrometer should I buy?
Post by: jgentry on July 08, 2018, 08:36:26 PM
I think he needs to send Davis an email or something and talk them into changing their sensor element.

Don't think he even owns a Davis his stuff was RM Young if memory serves.


Darn...I wished I was born rich instead of “good looking.”
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 08, 2018, 08:42:52 PM
I think he needs to send Davis an email or something and talk them into changing their sensor element.

Don't think he even owns a Davis his stuff was RM Young if memory serves.


Darn...I wished I was born rich instead of “good looking.”

 :lol: ....Well his job has something to do with his toys I'm almost positive. Like sending the SHT31 to Washington for recalibration... ;)
Title: Re: Which sling psychrometer should I buy?
Post by: openvista on July 09, 2018, 10:10:10 AM
It was kcidwx what he posted about the Gadget with SHT31.

One thing that seriously concerned me with these SHT31 evaluation boards was the long term shift in accuracy on the humidity side. I only test a particular type of sensor in minimum groups of three. When a sensor company is wanting to bid on a government contract it's usually pretty easy for me to secure at least three for testing. Although that's not the case with these evaluation boards, it was more for my own curiosity. The issue is approximately 3 weeks after receiving all three of these evaluation boards, I ran a test at 50% RH. All three were reading low by -1.2%, -1.7% and -1.9%. Almost a year later they were reading +0.5% to +0.8%. I'm at a loss to explain why the large shift over time. I'm not quite ready to call it "drift" because that's an insane amount of drift. I was so concerned, I sent my NIST reference instrument back to Washington D.C. to get it certified again after it was just certified 6 months before. It came back having tested just fine. In the hundreds of sensors I've tested over 20+ years, I never seen this much shift before over the course of a year. I was thinking of getting a fourth one and seeing how it compares right out of the box against these three units that have been running for almost a year. Some sensors do require a conditioning period when first used or what I refer to as exercising the sensor. I know that with some sensor manufacturers when they send you a sample for evaluation, sometimes these are not their best spec'ed sensors. Often they are sensors that are usable but don't quite pass QC 100% for some reason. I wonder if that's what these evaluation boards are. I never did ask Sensirion. The only thing I can think of is since these SHT31 sensors don't have a filter over them, they have become contaminated with dust over time.

https://www.wxforum.net/index.php?topic=32439.msg328209#msg328209

My guess on why ALL 3 units ran low initially is that the manufacturer KNOWS that they drift up quickly. So, at least initially and for a period of months after purchase, they'll be within specification. I bet most backyard weather enthusiasts don't even notice that their AWS humidity readings are moving upward because other fellow station owners are reporting similar humidities.
Title: Re: Which sling psychrometer should I buy?
Post by: openvista on July 09, 2018, 10:24:39 AM
I ordered a Kestrel 5000 meter (+/- 2% supposedly) and will be running some tests with it.

Kestrel not only calibrates and certifies every meter they sell, but they promise that if their unit drifts by more than 0.25% per year you can send the unit to them and they will recalibrate it for free within 5 years of purchase. Not sure how most people would determine which instrument is drifting excessively: their kestrel or the sensor they are checking -- but that's another matter.

They also offer a tune-up service for $75. They'll test and calibrate every weather parameter, clean it and install a new wind impeller & battery. Not bad.

I know the military uses these -- they've got a big contract with Kestrel. So do firefighters, athletic orgs, surveyers, shooting clubs and, of course, storm chasers who proudly display them on TV.

I figured it's probably the best you can do under $1000.
Title: Re: Which sling psychrometer should I buy?
Post by: jgentry on July 09, 2018, 10:35:20 AM
I ordered a Kestrel 5000 meter (+/- 2% supposedly) and will be running some tests with it.

Kestrel not only calibrates and certifies every meter they sell, but they promise that if their unit drifts by more than 0.25% per year you can send the unit to them and they will recalibrate it for free within 5 years of purchase. Not sure how most people would determine which instrument is drifting excessively: their kestrel or the sensor they are checking -- but that's another matter.

They also offer a tune-up service for $75. They'll test and calibrate every weather parameter, clean it and install a new wind impeller & battery. Not bad.

I know the military uses these -- they've got a big contract with Kestrel. So do firefighters, athletic orgs, surveyers, shooting clubs and, of course, storm chasers who proudly display them on TV.

I figured it's probably the best you can do under $1000.

Please let us know your results.
Title: Re: Which sling psychrometer should I buy?
Post by: openvista on July 09, 2018, 10:44:10 AM
I ordered a Kestrel 5000 meter (+/- 2% supposedly) and will be running some tests with it.

Kestrel not only calibrates and certifies every meter they sell, but they promise that if their unit drifts by more than 0.25% per year you can send the unit to them and they will recalibrate it for free within 5 years of purchase. Not sure how most people would determine which instrument is drifting excessively: their kestrel or the sensor they are checking -- but that's another matter.

They also offer a tune-up service for $75. They'll test and calibrate every weather parameter, clean it and install a new wind impeller & battery. Not bad.

I know the military uses these -- they've got a big contract with Kestrel. So do firefighters, athletic orgs, surveyers, shooting clubs and, of course, storm chasers who proudly display them on TV.

I figured it's probably the best you can do under $1000.

Please let us know your results.

Will do.

By the way, after swapping my sensors 36 hours ago my results have been consistent. The active shield remains lower in dew point by about 3-4 degrees compared to the passive shield with the older sensor in it.
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 09, 2018, 01:50:24 PM
Proof it's all about age of sensor.
Ron sent me a PM with graphs that demonstrates what's been said here already. His new SHT31 is is running 6% lower humidity than 2 year old sensor only 10 feet distance away. I'll let him comment further if he chooses.

I'm thinking the lifespan on these SHT31 sensors for 2% humidity accuracy is between 6 months and 1 year.
Title: Re: Which sling psychrometer should I buy?
Post by: openvista on July 09, 2018, 02:03:48 PM
I'm thinking the lifespan on these SHT31 sensors for 2% humidity accuracy is between 6 months and 1 year.

I'd concur with that estimate, particularly for aspirated sensors, judging by my own experimentation, analysis of past data and comparisons to nearby reliable sources.

There may be a reason Sensirion uses the prefix "SHT". Looks like they need to be flushed regularly (in non-arid regions). Not enjoying the thought of ordering $50 disposable kits 1-2x per year.
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 09, 2018, 02:17:38 PM


There may be a reason Sensirion uses the prefix "SHT".

 :lol: :lol:
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 09, 2018, 03:02:07 PM
I'll just add to my yearly list "to do" before summer hits. I have two stations online so couple $43 dollar sensors won't break the bank I guess each spring.... ](*,) Make though the summer is what's important, winter it's not as big a deal.
Title: Re: Which sling psychrometer should I buy?
Post by: CW2274 on July 09, 2018, 03:38:31 PM
Proof it's all about age of sensor.
I'm thinking the lifespan on these SHT31 sensors for 2% humidity accuracy is between 6 months and 1 year.
My first 31 had almost two years on it, this current one about five months. I didn't notice the first one getting bad, I just replaced it to see if there was a noticeable difference even after two years of service, and I can honestly say, I did not. Not saying there isn't one, just not noticeable to me, and I hawk this stuff like nobody's business.
Title: Re: Which sling psychrometer should I buy?
Post by: WheatonRon on July 09, 2018, 05:12:54 PM
SHT 31 drifts with age! I have two VP2 stations both have the SHT31 sensor--one station is about 2 months old and it has a 24 hour fan (CWOP FW3075) and the other station is about 2 years old and has a daytime fan (CWOP CW5020). Right now, in a western suburb of Chicago (where I live), the new station reports 91, humidity of 44 and dewpoint of 66 whereas the older station reports 91, humidity of 50 and dewpoint of 70. The stations are about 10 feet apart on the same fence and the sun is making both fans rumble today! The newer station always gets lower humidity and dewpoint readings as demonstrated by the attached QC graphs from CWOP.  I am not a fan (pun intended) of the QC checks on CWOP but this does raise some eyebrows!
Title: Re: Which sling psychrometer should I buy?
Post by: CW2274 on July 09, 2018, 05:48:33 PM
SHT 31 drifts with age!
All sensors drift with age. The question is to what degree (pun intended, or not...) :roll: the 31 does, and does climate/aspiration/rate of aspiration/dust/dirt/pollen/phase of the moon.........
Title: Re: Which sling psychrometer should I buy?
Post by: WheatonRon on July 09, 2018, 06:01:00 PM
Proof it's all about age of sensor.
Ron sent me a PM with graphs that demonstrates what's been said here already. His new SHT31 is is running 6% lower humidity than 2 year old sensor only 10 feet distance away. I'll let him comment further if he chooses.

I'm thinking the lifespan on these SHT31 sensors for 2% humidity accuracy is between 6 months and 1 year.

See my post below.
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 09, 2018, 06:08:08 PM
Thanks Ron, good job... =D&gt; it helps clarify what we mostly had figured out. Until they do something different with the 31 either update or go with a new version I'll just swap sensors in spring provided they don't jack price up.

For those living in arid climates sub 60 dew points you should be good for several years.
Title: Re: Which sling psychrometer should I buy?
Post by: CW2274 on July 09, 2018, 06:11:32 PM
Yes, your two 31's have a big discrepancy, when I pulled my two year old one, they did not, hence the "does climate/aspiration/rate of aspiration/dust/dirt/pollen/phase of the moon........." tongue in cheek response.
Title: Re: Which sling psychrometer should I buy?
Post by: jgentry on July 09, 2018, 06:21:30 PM
Here is some information I received from the good folks at the NWS

Hi Justin! Here is the response below about your weather equipment question which we received from our electronics technicians.  Hope this helps. 🙂                                                                                                         -----------------------------------------------------------------------                                   Our Dew Point Sensor is the Vaisala DTS1.  This sensor uses a HUMICAP® capacitive thin film polymer sensor to determine RH. The measured capacitance of the DTS1 probe will vary depending on the amount of moisture in the air. The resistance of the PT 100  temperature sensor is used to determine the sensor air temperature. During periods of high humidity, the sensor electronics will turn on the sensor heater element to improve sensor humidity resolution. The algorithm then computes the dew point temperature using the measured sensor temperature and capacitance values.

Our calibration procedures include verifying accuracy every 90 days, there are no adjustments.  We also replace the sensor every 18 months as a time change requirement. 

We also use a NWS 1088 hygrothermometer to determine ambient air temperature.  This sensor uses a fan to draw air in and across the temperature probe, thus the temperature is aspirated.  Our calibrations procedures consist of cleaning, adjusting, and verifying accuracy every 90 days.                                                                                                              ----------------------------------------------------------------------------
Title: Re: Which sling psychrometer should I buy?
Post by: WheatonRon on July 09, 2018, 06:24:46 PM
Thanks Ron, good job... =D&gt; it helps clarify what we mostly had figured out. Until they do something different with the 31 either update or go with a new version I'll just swap sensors in spring provided they don't jack price up.

For those living in arid climates sub 60 dew points you should be good for several years.

Good news! Actually, I have 3 VP2s, but only 2 upload to the internet. I just checked my VP2 that doesn't upload to the internet and is 40 feet away from the other stations on the same fence, which also has the daytime fan and SHT31, but is probably 3 years old! The readings are almost identical to my two year old VP2, so the good news? The SHT31 sensor doesn't get much worse after two years! Celebrate and tear down the goal posts! Fortunately, Ryan Wilhour sells these at a reasonable price--but something is clearly wrong--unless you live in Arizona!
Title: Re: Which sling psychrometer should I buy?
Post by: CW2274 on July 09, 2018, 06:28:32 PM
Thanks Ron, good job... =D&gt; it helps clarify what we mostly had figured out. Until they do something different with the 31 either update or go with a new version I'll just swap sensors in spring provided they don't jack price up.

For those living in arid climates sub 60 dew points you should be good for several years.

but something is clearly wrong--unless you live in Arizona!
:-"
Title: Re: Which sling psychrometer should I buy?
Post by: CW2274 on July 09, 2018, 06:36:29 PM
Honestly, I'm half tempted to pull the ISS outta the desert (kinda a PITA) and put in the back yard so I can do instantaneous comparisons between my two 31's, which is exactly what I did when I picked up my second 31. At that time I estimated about 1/2% difference tween the two.
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 09, 2018, 06:54:58 PM
Here is some information I received from the good folks at the NWS

Hi Justin! Here is the response below about your weather equipment question which we received from our electronics technicians.  Hope this helps. 🙂                                                                                                         -----------------------------------------------------------------------                                   Our Dew Point Sensor is the Vaisala DTS1.  This sensor uses a HUMICAP® capacitive thin film polymer sensor to determine RH. The measured capacitance of the DTS1 probe will vary depending on the amount of moisture in the air. The resistance of the PT 100  temperature sensor is used to determine the sensor air temperature. During periods of high humidity, the sensor electronics will turn on the sensor heater element to improve sensor humidity resolution. The algorithm then computes the dew point temperature using the measured sensor temperature and capacitance values.

Our calibration procedures include verifying accuracy every 90 days, there are no adjustments.  We also replace the sensor every 18 months as a time change requirement

We also use a NWS 1088 hygrothermometer to determine ambient air temperature.  This sensor uses a fan to draw air in and across the temperature probe, thus the temperature is aspirated.  Our calibrations procedures consist of cleaning, adjusting, and verifying accuracy every 90 days.                                                                                                              ----------------------------------------------------------------------------

Interesting those run around $800 retail.
And they go bad too, a couple years ago airport completely stopped reporting 100% humidity even after hours of dense fog, this went on for couple years at KVTN. I mentioned it to forecast office and they eventually replaced and now 100% all the time almost too often it seems. 
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 09, 2018, 07:05:16 PM
If you think about it, (I) was expecting too much. 1 year on these sensors especially thoses aspirated isn't really that out of normal if NWS routinely changes there's every 18 months or when needed.
Title: Re: Which sling psychrometer should I buy?
Post by: CW2274 on July 09, 2018, 07:06:59 PM
Well I've decided to pull the ISS and see what I see. The monsoon is in town so I'm not sure when I'll get out to it, I would MUCH prefer cloud cover, possibly tomorrow. At least we can see if the desert hypothesis holds any water with a two year old 31 vs five months.
Think of me when I'm pulling cacti hypo's outta my legs in the interest of science. :-({|=
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 09, 2018, 07:12:22 PM
Anyone want to figure out what the shelf live is if never removed from bag?
Title: Re: Which sling psychrometer should I buy?
Post by: CW2274 on July 09, 2018, 07:20:36 PM
Anyone want to figure out what the shelf live is if never removed from bag?
Purely speculation, but with no current/humidity/dust/dirt/fan/big fan/moon phase......forever. ;)
Title: Re: Which sling psychrometer should I buy?
Post by: jgentry on July 09, 2018, 08:02:12 PM
This might have been posted on another thread but this is some interesting info

“There is a procedure used by the ASOS Technician to check the validity of the ASOS ambient/dew point temperatures by comparing the values with the Psychron Model 566-2 psychrometer.  A measurement is considered valid if the temperature difference is within ±5 F.  dew point validity is determined by values within tolerances based on the temperature/dew point depression and if the temperature falls above or below 32 F.”

https://training.weather.gov/nwstc/DATAACQ/d.ASOShuman/ASOSTemp.htm
Title: Re: Which sling psychrometer should I buy?
Post by: WheatonRon on July 09, 2018, 08:25:23 PM
If you think about it, (I) was expecting too much. 1 year on these sensors especially thoses aspirated isn't really that out of normal if NWS routinely changes there's every 18 months or when needed.

I don't recall the predecessor to the SHT31 having this issue, but I may be wrong. That said, for a semi-professional station like the VP2,  having to replace this sensor every 12-18 months doesn't seem right to me. I think we should contact Davis support and get their views.
Title: Re: Which sling psychrometer should I buy?
Post by: CW2274 on July 09, 2018, 08:48:43 PM
If you think about it, (I) was expecting too much. 1 year on these sensors especially thoses aspirated isn't really that out of normal if NWS routinely changes there's every 18 months or when needed.

I don't recall the predecessor to the SHT31 having this issue, but I may be wrong. That said, for a semi-professional station like the VP2,  having to replace this sensor every 12-18 months doesn't seem right to me.
Ron, the way I look at it, ya gotta pay to play (performance, performance, performance). Perhaps this is just the price of greater accuracy with this sensor, burn out.
My 2 cents.
Title: Re: Which sling psychrometer should I buy?
Post by: WheatonRon on July 09, 2018, 08:57:51 PM
If you think about it, (I) was expecting too much. 1 year on these sensors especially thoses aspirated isn't really that out of normal if NWS routinely changes there's every 18 months or when needed.

I don't recall the predecessor to the SHT31 having this issue, but I may be wrong. That said, for a semi-professional station like the VP2,  having to replace this sensor every 12-18 months doesn't seem right to me.
Ron, the way I look at it, ya gotta pay to play (performance, performance, performance). Perhaps this is just the price of greater accuracy with this sensor, burn out.
My 2 cents.


OR, Davis is testing us to see if we can thread three long screws through the rain base and the rest of the station, remove and install the sensor, and put the station successfully back together again and still love Davis in the morning!
Title: Re: Which sling psychrometer should I buy?
Post by: CW2274 on July 09, 2018, 09:07:55 PM
If you think about it, (I) was expecting too much. 1 year on these sensors especially thoses aspirated isn't really that out of normal if NWS routinely changes there's every 18 months or when needed.

I don't recall the predecessor to the SHT31 having this issue, but I may be wrong. That said, for a semi-professional station like the VP2,  having to replace this sensor every 12-18 months doesn't seem right to me.
Ron, the way I look at it, ya gotta pay to play (performance, performance, performance). Perhaps this is just the price of greater accuracy with this sensor, burn out.
My 2 cents.


OR, Davis is testing us to see if we can thread three long screws through the rain base and the rest of the station, remove and install the sensor, and put the station successfully back together again and still love Davis in the morning!
Ha, easily, blindfolded to boot.
Title: Re: Which sling psychrometer should I buy?
Post by: Dj1225 on July 10, 2018, 02:13:59 AM
Yeah not interested in removing filter I think it would just cause another issue.

This is sunrise what I'm seeing this morning Extech outside for about 15 minutes, you can see it's dripping with sweat in short period. The SHT15 isn't a solution either for 100% or even close. Remember the SHT's are saturated all night in high humidity and should be reading at least 98%. Airport was 100% at same time.

I usually just lurk on this site have for years, and post very rarely. I came across this subject and had to chime in. I live on the Gulf Coast in Gulfport, MS. 8 miles north of the Gulf of Mexico. I've used Davis stations since 1996. The Weather Monitor II, the first VP and the VP2 all recorded 100% humidity. I upgraded my ISS two years ago with the SHT31. As humid as it is down here, the dense fog we see in fall and winter, and going through Hurricane Nate last October since I installed the new ISS with the SHT31 sensor the highest humidity I have recorded is 96% once! At first I thought it was really accurate, but then after awhile I started having my doubts, and now seeing this post it seems like it is more a problem than accurate. Also I agree with Valentine on the higher dp because down here if the sensors dp is high biased the heat index is out of control.
Title: Re: Which sling psychrometer should I buy?
Post by: ValentineWeather on July 10, 2018, 08:47:05 AM
FYI sent an email to Ryan at Scaled Inst. who I purchased all of mine through expressing concern and verifying they are genuine Davis sensors. If you read data sheet about sensor they have different qualities stamped on each one. Also emailed Davis Inst.
 
At least they will know now and maybe Davis will investigate on their own. 

Dj1225 thanks for chiming in. The topic heading probably should be changed since it turned into SHT31 reliability thread.
Title: Re: Which sling psychrometer should I buy?
Post by: jgentry on July 10, 2018, 09:22:01 AM
I’ve noticed the same thing DJ except my SHT-31s Typically top of at 99%...

Let us know what Ryan and Davis has to say...

I’ve noticed that many good stations use a thin film polymer capacitive sensor. According to Davis’ spec sheet, their VP2s use a thin film capacitor element. Is there a difference between the thin film polymer sensor and the thin film capacitive sensor that Davis uses?

Why I ask is that there has to be a reason why the SHT-31 humidity sensor goes off calibration quicker (or least it seems) and for the sensor not reaching 100% most of the time (even in actual 109% humidity).
Title: Re: Which sling psychrometer should I buy?
Post by: jgentry on July 10, 2018, 09:26:14 AM
FYI sent an email to Ryan at Scaled Inst. who I purchased all of mine through expressing concern and verifying they are genuine Davis sensors. If you read data sheet about sensor they have different qualities stamped on each one. Also emailed Davis Inst.
 
At least they will know now and maybe Davis will investigate on their own. 

Dj1225 thanks for chiming in. The topic heading probably should be changed since it turned into SHT31 reliability thread.


I’ve changed the thread title.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Aardvark on July 10, 2018, 09:28:42 AM
I’m thinking about getting a sling psychrometer to make sure that my RH sensor is within specs. I would like to know which psychrometers are accurate and reliable.  Thanks!

So rather than me reading the 163 posts, have you bought your sling yet?  solve your problem?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on July 10, 2018, 09:29:14 AM
I too contacted Davis support (Brett Lane, supervisor) and alerted him to this thread. I also alerted Johnd, a frequent contributor in this Forum as well as a Davis reseller in the UK to this thread as well.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 10, 2018, 09:46:28 AM
I’m thinking about getting a sling psychrometer to make sure that my RH sensor is within specs. I would like to know which psychrometers are accurate and reliable.  Thanks!

So rather than me reading the 163 posts, have you bought your sling yet?  solve your problem?

Not planning on getting a sling...Thought about getting a digital psychrometer but I decided to get this fan-aspirated one whenever I have some extra spending money.

http://www.iisusa.com/mobile/Psychro-Dyne-Battery-Operated-Psychrometer-Fahrenheit-BLACK-ORGANIC/productinfo/22014/
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 10, 2018, 09:56:03 AM
Did noticed that the 1088 hygrothermometer has a very similar specs in terms of ambient temperature as the SHT-31


Looking at the RainWise MK III data from RainWise’s factory location and comparing it to the nearby airport; it appears to me that their humidity sensor has a similar wet bias as the Davis does...

The only difference I’ve noticed is that RW humidity sensor reaches 100% more easily and often than the SHT-31.

I would love to hear what Brett Lane from Davis and others’s take on this issue.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 10, 2018, 10:39:09 AM
Heard back from Ryan he doesn't think there is an issue with Davis covering for replacements.
If you purchased through him send defective units back.
As far as solution which I would rather have than keep replacing nothing yet but hopefully it will now get addressed as they say the ball is rolling with appropriate people notified.


Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 10, 2018, 10:42:57 AM
The question now becomes: how long is the warranty period?

My out-of-spec sensor was purchased April 2017. Does that qualify?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 10, 2018, 04:25:56 PM
The question now becomes: how long is the warranty period?

My out-of-spec sensor was purchased April 2017. Does that qualify?

Ryan will take any 31 back if purchased through others it will be a Davis decision but being an known issue my guess all will be covered.

That's good news! For us at least. I hope it's not bad news for Ryan and that he can convince Davis to step up and Davis can convince Sensirion to stabilize drift or else risk losing a big contract.

Davis might not have the leverage it needs since its sensor platform, Sensibus (?), is so old. What other modern sensor could it run without a major overhaul of the SIM and its power section (solar, battery)? 10 years of hinting at a VP3 with nothing to show for it would now seem to be haunting them.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 10, 2018, 04:27:43 PM
Wow, I'm happily shocked about returns. I figured it would be a "pound sand " approach, nature of the beast thing. Nicely done Randy. =D&gt;
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 10, 2018, 04:29:52 PM
The question now becomes: how long is the warranty period?

My out-of-spec sensor was purchased April 2017. Does that qualify?

Ryan will take any 31 back if purchased through others it will be a Davis decision but being an known issue my guess all will be covered.

 convince Davis to step up and Davis can convince Sensirion to stabilize drift or else risk losing a big contract.

Now that I'll believe when I see it.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 10, 2018, 05:01:17 PM
Wow, I'm happily shocked about returns. I figured it would be a "pound sand " approach, nature of the beast thing. Nicely done Randy. =D&gt;

That is the approach I've had from them before but Ryan is different and reliable for sure.

Still nothing back from Davis Inst.
They may or may not get back I've had it both ways and been flat lied to before when I pointed out an software issue for cup size with Vue console using VP2 wind sensor.

They claimed they ran wind tunnel test which I know wasn't true if they had they would of seen problem immediately.
It wasn't just me but multiple people with VP2 and Vue consoles were seeing different wind speeds.
 
Long story short they did a Vue firmware update, not long after and first one in years (how convenient).
With no mention in update but it fixed issue. I almost didn't bother with it but someone said it fixed the wind speed issue and sure enough it did.
Now both Vue and Vp2 consoles follow each other perfectly.
 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 10, 2018, 05:16:05 PM
Wow, I'm happily shocked about returns. I figured it would be a "pound sand " approach, nature of the beast thing. Nicely done Randy. =D&gt;

That is the approach I've had from them before but Ryan is different and reliable for sure.

Yes, my comment was geared towards Davis, not Ryan. Honestly, not holding my breath with Davis, I think they'll say we did our R&D, take it or leave it, if they comment at all for that matter.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 10, 2018, 06:22:57 PM
Wow, I'm happily shocked about returns. I figured it would be a "pound sand " approach, nature of the beast thing. Nicely done Randy. =D&gt;

That is the approach I've had from them before but Ryan is different and reliable for sure.

Still nothing back from Davis Inst.
They may or may not get back I've had it both ways and been flat lied to before when I pointed out an software issue for cup size with Vue console using VP2 wind sensor.

They claimed they ran wind tunnel test which I know wasn't true if they had they would of seen problem immediately.
It wasn't just me but multiple people with VP2 and Vue consoles were seeing different wind speeds.
 
Long story short they did a Vue firmware update, not long after and first one in years (how convenient).
With no mention in update but it fixed issue. I almost didn't bother with it but someone said it fixed the wind speed issue and sure enough it did.
Now both Vue and Vp2 consoles follow each other perfectly.

Sounds familiar.

Last year I pointed out to them that temperatures could be adjusted in the VP2 console (via offsets) without a corresponding change in relative humidity and they acted like that was news to them (on a 12 year old console). The head of Technical Support had to call in senior engineers and the staff meteorologist to get to the bottom of it!  Ultimately, after over a dozen emails, I requested they simply make a note in the manual in the section with the offset procedures so users are aware of this fact. I didn't even demand they fix it. Yet, they would not commit to even editing a damn PDF and, as far as I know, they have not notified their customers of this issue. I guess they see this as an upgrade opportunity ($$) for anyone who has (what they assume to be without understanding all use cases) an out-of-calibration sensor?

And, of course, on the issue with cold temperature signal drop outs they were nowhere to be found even though several people reached out to them directly and johnd, one of their authorized dealers, was quite involved in that discussion.

So, no, I don't expect them to really acknowledge this publicly either.

Now, having said that, I hope Davis will prove me wrong. Don't care as long as there's a solution.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 10, 2018, 06:32:03 PM
Wow, I'm happily shocked about returns. I figured it would be a "pound sand " approach, nature of the beast thing. Nicely done Randy. =D&gt;

That is the approach I've had from them before but Ryan is different and reliable for sure.

Still nothing back from Davis Inst.
They may or may not get back I've had it both ways and been flat lied to before when I pointed out an software issue for cup size with Vue console using VP2 wind sensor.

They claimed they ran wind tunnel test which I know wasn't true if they had they would of seen problem immediately.
It wasn't just me but multiple people with VP2 and Vue consoles were seeing different wind speeds.
 
Long story short they did a Vue firmware update, not long after and first one in years (how convenient).
With no mention in update but it fixed issue. I almost didn't bother with it but someone said it fixed the wind speed issue and sure enough it did.
Now both Vue and Vp2 consoles follow each other perfectly.
Last year I pointed out to them that temperatures could be adjusted in the VP2 console (via offsets) without a corresponding change in relative humidity and they acted like that was news to them (on a 12 year old console).
I'm confused, changing the temp does effect the RH, why would you not want the offset to effect it?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 10, 2018, 07:06:48 PM
Wow, I'm happily shocked about returns. I figured it would be a "pound sand " approach, nature of the beast thing. Nicely done Randy. =D&gt;

That is the approach I've had from them before but Ryan is different and reliable for sure.

Still nothing back from Davis Inst.
They may or may not get back I've had it both ways and been flat lied to before when I pointed out an software issue for cup size with Vue console using VP2 wind sensor.

They claimed they ran wind tunnel test which I know wasn't true if they had they would of seen problem immediately.
It wasn't just me but multiple people with VP2 and Vue consoles were seeing different wind speeds.
 
Long story short they did a Vue firmware update, not long after and first one in years (how convenient).
With no mention in update but it fixed issue. I almost didn't bother with it but someone said it fixed the wind speed issue and sure enough it did.
Now both Vue and Vp2 consoles follow each other perfectly.
Last year I pointed out to them that temperatures could be adjusted in the VP2 console (via offsets) without a corresponding change in relative humidity and they acted like that was news to them (on a 12 year old console).
I'm confused, changing the temp does effect the RH, why would you not want the offset to effect it?

No, that's what I'm saying. Currently, changing the temperature via an offset in the console does NOT affect the RH. That's not correct. I pointed it out and Keystone Cops ensued over at Davis HQ.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 10, 2018, 07:14:34 PM
Wow, I'm happily shocked about returns. I figured it would be a "pound sand " approach, nature of the beast thing. Nicely done Randy. =D&gt;

That is the approach I've had from them before but Ryan is different and reliable for sure.

Still nothing back from Davis Inst.
They may or may not get back I've had it both ways and been flat lied to before when I pointed out an software issue for cup size with Vue console using VP2 wind sensor.

They claimed they ran wind tunnel test which I know wasn't true if they had they would of seen problem immediately.
It wasn't just me but multiple people with VP2 and Vue consoles were seeing different wind speeds.
 
Long story short they did a Vue firmware update, not long after and first one in years (how convenient).
With no mention in update but it fixed issue. I almost didn't bother with it but someone said it fixed the wind speed issue and sure enough it did.
Now both Vue and Vp2 consoles follow each other perfectly.
Last year I pointed out to them that temperatures could be adjusted in the VP2 console (via offsets) without a corresponding change in relative humidity and they acted like that was news to them (on a 12 year old console).
I'm confused, changing the temp does effect the RH, why would you not want the offset to effect it?

No, that's what I'm saying. Currently, changing the temperature via an offset in the console does NOT affect the RH. That's not correct. I pointed it out and Keystone Cops ensued over at Davis HQ.
You gotta be kidding me........ :shock:   I didn't even check for such a thing, that's an absolute assumable! Talk about wx 101...:roll:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on July 10, 2018, 07:53:50 PM
Brett Lane head of Davis support as it relates to the VP2, just acknowledged my email to him this morning where I pointed out the “gist” of this thread. Next steps? Who knows.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 10, 2018, 08:01:00 PM
This temp offset not changing humidity is interesting. I need to test on my Envoy8x.


Brett Lane head of Davis support as it relates to the VP2, just acknowledged my email to him this morning where I pointed out the “gist” of this thread. Next steps? Who knows.
'
Good deal Ron.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 10, 2018, 08:14:12 PM
Yep you can change temperature and humidity doesn't budge...Very odd.
Cumulus does move however for those using 3rd party software.

Humm.... this just have me an idea. Many of us had to subtract .9 with new SHT31's and older transmitters. I did it using Davis software maybe I need to do it using the Cumulus software and humidity will adjust.
 [ You are not allowed to view attachments ]
Title: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 10, 2018, 09:03:36 PM
Humm.... this just have me an idea. Many of us had to subtract .9 with new SHT31's and older transmitters. I did it using Davis software maybe I need to do it using the Cumulus software and humidity will adjust.
 [ You are not allowed to view attachments ]

Well, in that conversation with Davis it came out that the humidity is calculated by the SIM transmitter, NOT the console. The older SIMs add 0.9F to the temp which is why you need to subtract it out at the console. But the humidity is pinned to the ACTUAL temperature measured by the sensor before the SIM adds the 0.9F. So, in other words, you don't need to worry about the humidity if you only have a temperature offset to compensate for an older SIM. It should be correct.

You can verify that by plugging the adjusted temp and humidity into a dew point calculator and see if it matches the dew point. Or you can take the dew point and temp and find the humidity. Either way. I've done it many times on my older SIM. It always matches up. I also have a newer SIM that doesn't need the offset and that humidity verifies as well.

Now, the problem is that if you have any additional temperature offset (say you discovered the temperature sensor has a bias of some sort) then your humidity and dew point and heat index WILL be off and there's no good way to fix it.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 10, 2018, 09:24:13 PM
Okay makes sense I think.  #-o
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: DaleReid on July 11, 2018, 09:06:37 AM
Why the 0.9 degree 'correction' in the SIM?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 11, 2018, 09:12:59 AM
Heard back from Ryan he doesn't think there is an issue with Davis covering for replacements.
If you purchased through him send defective units back.
As far as solution which I would rather have than keep replacing nothing yet but hopefully it will now get addressed as they say the ball is rolling with appropriate people notified.

This may not be as open closed as expected for replacements. I told Ryan I'm okay with replacing sensor myself before summer hits but would like Davis to dig into issue. Surprised they didn't already see it. I was looking at their station in Hayward Cal. yesterday afternoon and dewpoint was running +2° over airport about what I see all the time here also.

Anyway the questions Davis is asking Ryan and myself. I understand they need to start somewhere but WOW. Had I known I would of kept better track and logged everything which I didn't . Like I previously said I just used Cumulus to correct issue.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 11, 2018, 09:24:41 AM
Why the 0.9 degree 'correction' in the SIM?

If you're asking why does an older SIM (pre 2017?) add 0.9F to the temp, I'm not exactly sure. I just know that it does and Davis instructs owners to subtract it out with an offset in the console.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 11, 2018, 09:36:17 AM
This may not be as open closed as expected for replacements.

I kinda figured. Sounds like Ryan has started having second thoughts? If Davis won't accept the returns then he'd be exposing his business to significant losses potentially.

C'mon Davis. Replacing defective sensors will create a heap of good will.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: kcidwx on July 11, 2018, 11:12:13 AM
So my email has been getting dinged that there's been replies to the SHT-31 thread.  :lol: My thoughts on the SHT-31 have already been expressed. I have not read this entire thread but I want to address the topic of humidity sensor replacement. In the professional world, it is standard practice to replace electronic humidity sensors at a minimum of every 24 months. My standard practice has been every 18 months. For the SHT-31, I would recommend in humid environments, every 9-12 months minimum. Dry environments, every 12-15 months. Extreme dry environments every 15-18 months.

I only approve of two ways for checking humidity sensor accuracy. Using a humidity sensor calibration chamber is my #1 choice. My #2 choice is a sling psychrometer out in the field. It's very important to have a matched set of thermometers in the psychrometer. It's also important to have the psychrometer certification tested so you know precisely what the temperature correction is on the dry bulb and wet bulb thermometers. If you break one of the thermometers, you need to replace both with another matched set. Also, you need to use the psychrometer correctly. I'm amazed when I'm at a station and see the tech using the sling incorrectly. I've seen them sling with the sling in direct sunlight, only 6 inches away from their body, using tap water on the wick, etc.  :lol:

As far as using another electronic humidity sensor to check another one? I wouldn't.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 11, 2018, 11:45:47 AM
So my email has been getting dinged that there's been replies to the SHT-31 thread.  :lol: My thoughts on the SHT-31 have already been expressed. I have not read this entire thread but I want to address the topic of humidity sensor replacement. In the professional world, it is standard practice to replace electronic humidity sensors at a minimum of every 24 months. My standard practice has been every 18 months. For the SHT-31, I would recommend in humid environments, every 9-12 months minimum. Dry environments, every 12-15 months. Extreme dry environments every 15-18 months.

I only approve of two ways for checking humidity sensor accuracy. Using a humidity sensor calibration chamber is my #1 choice. My #2 choice is a sling psychrometer out in the field. It's very important to have a matched set of thermometers in the psychrometer. It's also important to have the psychrometer certification tested so you know precisely what the temperature correction is on the dry bulb and wet bulb thermometers. If you break one of the thermometers, you need to replace both with another matched set. Also, you need to use the psychrometer correctly. I'm amazed when I'm at a station and see the tech using the sling incorrectly. I've seen them sling with the sling in direct sunlight, only 6 inches away from their body, using tap water on the wick, etc.  :lol:

As far as using another electronic humidity sensor to check another one? I wouldn't.

Appreciate your comment kcidwx and yes your old review of the SHT31 and thoughts came up as some are realizing  how fast they are going high. I'm taking the advice on change out and do highly recommend others do the same being you are the expert on the subject.

Now we have Davis Inst. involved not sure this will go anywhere. They are rather defensive of their products as they should be.

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 11, 2018, 12:41:56 PM
So my email has been getting dinged that there's been replies to the SHT-31 thread.  :lol: My thoughts on the SHT-31 have already been expressed. I have not read this entire thread but I want to address the topic of humidity sensor replacement. In the professional world, it is standard practice to replace electronic humidity sensors at a minimum of every 24 months. My standard practice has been every 18 months. For the SHT-31, I would recommend in humid environments, every 9-12 months minimum. Dry environments, every 12-15 months. Extreme dry environments every 15-18 months.

I only approve of two ways for checking humidity sensor accuracy. Using a humidity sensor calibration chamber is my #1 choice. My #2 choice is a sling psychrometer out in the field. It's very important to have a matched set of thermometers in the psychrometer. It's also important to have the psychrometer certification tested so you know precisely what the temperature correction is on the dry bulb and wet bulb thermometers. If you break one of the thermometers, you need to replace both with another matched set. Also, you need to use the psychrometer correctly. I'm amazed when I'm at a station and see the tech using the sling incorrectly. I've seen them sling with the sling in direct sunlight, only 6 inches away from their body, using tap water on the wick, etc.  :lol:

As far as using another electronic humidity sensor to check another one? I wouldn't.

Thank you for your input!  I’m planning on getting one of these since I don’t have a humidity chamber. Lol

http://www.iisusa.com/mobile/Psychro-Dyne-Battery-Operated-Psychrometer-Fahrenheit-BLACK-ORGANIC/productinfo/22014/

The NWS in Alabama tells me that they change out their sensor every 18 months. Then again, it’s dog-gone humid here.

Since we know that the SHT-31 has a wet bias, how does it performs in terms of measuring air temperature compared to a calibrated standard?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 11, 2018, 12:42:46 PM
Unfortunately the Sensirion SHT-31 is (currently) the ONLY sensor compatible with our Davis units...unless you WANT to go BACK to the older SHT-11 or SHT-15 sensors.

Because of the communication change inherent with newer sensors, the SHT-31 is an "end-of-the-line" device, the last of its kind until/if DAVIS comes up with a new ISS.

I’m afraid you’re correct
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on July 11, 2018, 12:50:39 PM
So my email has been getting dinged that there's been replies to the SHT-31 thread.  :lol: My thoughts on the SHT-31 have already been expressed. I have not read this entire thread but I want to address the topic of humidity sensor replacement. In the professional world, it is standard practice to replace electronic humidity sensors at a minimum of every 24 months. My standard practice has been every 18 months. For the SHT-31, I would recommend in humid environments, every 9-12 months minimum. Dry environments, every 12-15 months. Extreme dry environments every 15-18 months.

I only approve of two ways for checking humidity sensor accuracy. Using a humidity sensor calibration chamber is my #1 choice. My #2 choice is a sling psychrometer out in the field. It's very important to have a matched set of thermometers in the psychrometer. It's also important to have the psychrometer certification tested so you know precisely what the temperature correction is on the dry bulb and wet bulb thermometers. If you break one of the thermometers, you need to replace both with another matched set. Also, you need to use the psychrometer correctly. I'm amazed when I'm at a station and see the tech using the sling incorrectly. I've seen them sling with the sling in direct sunlight, only 6 inches away from their body, using tap water on the wick, etc.  :lol:

As far as using another electronic humidity sensor to check another one? I wouldn't.

Appreciate you comment kcidwx and yes your old review of the SHT31 and thoughts came up as some are realizing  how fast they are going high. I'm taking your advice on change out and do highly recommend others do the same being you are the expert on the subject.

Now we have Davis Inst. involved not sure this will go anywhere. They are rather defensive of their products as they should be.

So it appears the life span of a SHT31 is at most 24 months to achieve the desired accuracy. I just reread the User Guide to my Integrated Sensor Suite with the 24 hour fan I just bought from Ryan Wilhour, and nowhere in that document (Rev B, 5/6/15) does it recommend any maintenance associated with Temperature-Humidity sensor. The User Guide talks about cleaning the ISS, the rain collector, changing the 24 hour fan batteries, maintaining the anemometer, etc., but nothing about maintaining or changing this sensor. This is where the problem lies. Davis should have stated in this manual to change the temperature-humidity sensor periodically based on the guidance provided by kcidwx based on where you live, and everybody would be happy, and this thread as it relates to the SHT31, would not exist.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: kcidwx on July 11, 2018, 01:05:48 PM
jgentry, I never tested the temperature side of the SHT-31. So, I can't speak to its accuracy. The one thing that bothers me with the way Davis uses the SHT-31 is the aspiration. We don't aspirate humidity sensors. The ASOS DTS1 is not aspirated. You want to keep that sensor clean or it can go out of calibration. So putting the SHT-31 in the FARS and drawing the dirty outside air across it bothers me. I don't know how good the filtration is on that FARS or how easily dust and dirt can get to it by other means.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 11, 2018, 01:09:30 PM
Please recall earlier in the thread where I quoted from The Weather Observer's Handbook in which Stephen Burt wrote in 2012 that the cheapest capacitive humidity sensors had 5% or more movement per year. Is that what Davis puts in the VP2? The cheapest sensor? I would think such a sensor would be relegated to the <$200 weather stations sold by their competitors.  Here we are all these years later and we should just accept that the humidity will be, what, 8-10% off in 2 years? How does that square with the vendor claiming 0.25% drift per year?
 
Something isn't right here. I'm not saying it's entirely Davis' fault. But the SHT31s have been in production since early 2016 and this is the first anyone's heard of the sensor package being a disposable maintenance item.

Until specs are changed, this is a defective part and should be subject to warranty claims.

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 11, 2018, 01:16:59 PM
jgentry, I never tested the temperature side of the SHT-31. So, I can't speak to its accuracy. The one thing that bothers me with the way Davis uses the SHT-31 is the aspiration. We don't aspirate humidity sensors. The ASOS DTS1 is not aspirated. You want to keep that sensor clean or it can go out of calibration. So putting the SHT-31 in the FARS and drawing the dirty outside air across it bothers me. I don't know how good the filtration is on that FARS or how easily dust and dirt can get to it by other means.

The filter cap on the sensor only filters out the largest particles. Fine dust and VOCs can easily get through that filter. That being said, such deposits should act to negatively bias the sensor, thereby lowering humidity values, should it not? The foreign particles block the adsorption process. Instead, what we're dealing with is a sensor that gets rapidly wetter over time.

When I disassembled my FARS a couple days ago, the filter was dirty. I wouldn't say it was plugged, but still plenty of black particulates on the screen. Regardless, that sensor was running wet by 8%.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 11, 2018, 01:25:12 PM
I should point out that after the screen was cleaned and the offending sensor was re-installed in a passive shelter, the wet bias has not lessened.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 11, 2018, 01:26:30 PM
Please recall earlier in the thread where I quoted from The Weather Observer's Handbook in which Stephen Burt wrote in 2012 that the cheapest capacitive humidity sensors had 5% or more movement per year. Is that what Davis puts in the VP2? The cheapest sensor? I would think such a sensor would be relegated to the <$200 weather stations sold by their competitors.  Here we are all these years later and we should just accept that the humidity will be, what, 8-10% off in 2 years? How does that square with the vendor claiming 0.25% drift per year?
 
Something isn't right here. I'm not saying it's entirely Davis' fault. But the SHT31s have been in production since early 2016 and this is the first anyone's heard of the sensor package being a disposable maintenance item.

Until specs are changed, this is a defective part and should be subject to warranty claims.

I agree, they surely are not within claimed specs of 2% and all should get free replacements every 6 months until a solution is found.
But getting Davis to agree is another thing.. :evil:

The sht31 is only a couple dollars if purchased separately without the hardware included so yes they are a bottom feeder when it comes to price.

Digikey sells them bulk 1000 chips at $3.46 each.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: mcrossley on July 11, 2018, 01:52:01 PM
I haven't read this whole thread, but regarding contamination, Sensirion recommend that their sensors are placed behind a filter and supply chip enclosures that effectively use a Gortex like membrane. Davis chose not to incorporate such a filter.

I find that periodically giving my sht devices a wash in distilled water restores much of their humidity accuracy. I've not seen much drift in the temperature readings.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 11, 2018, 02:03:58 PM
I haven't read this whole thread, but regarding contamination, Sensiron recommend that their sensors are placed behind a filter and supply chip enclosures that effectively use a Gortex membrane. Davis chose not to incorporate such a filter.

I find that periodically giving my sht devices a wash in distilled water restores much of their humidity accuracy. I've not seen much drift in the temperature readings.

So distilled water won't damage a unplugged chip. I have never heard this before but worth a try I've got several that I replaced already. I would like to find the bagging material they recommend also. Any other plastic bags will ruin chip in short order, even sealing bag don't use just any tape.

Found some bags and resealable  =D&gt; https://www.amazon.com/gp/product/B07CX62R99/ref=ox_sc_act_title_1?smid=A3S6XKC1OQNPRR&psc=1
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: kcidwx on July 11, 2018, 02:58:03 PM
A lot of high humidity over the life of the sensor often accelerates aging of the sensor. Here's some info from an engineering doc I have on the SHT-31.

Quote
1.1
Recommended Operating Condition

The sensor shows best performance when operated within recommended normal temperature and humidity range of 5°C–60 °C and 20 %RH–80 %RH, respectively. Long-term exposure to conditions outside normal range, especially at high humidity, may temporarily offset the RH signal (e.g.+3%RH after 60h kept at >80%RH). After returning into the normal temperature and humidity range the sensor will slowly come back to calibration state by itself. Prolonged exposure to extreme conditions may accelerate ageing. To ensure stable operation of the humidity sensor, the conditions described in the document “SHTxx Assembly of SMD Packages”, section “Storage and Handling Instructions” regarding exposure to volatile organic compounds have to be met. Please note as well that this does apply not only to transportation and manufacturing, but also to operation of the SHT3x.

What this tells me is this sensor wasn't really designed for meteorological use. But most humidity sensors aren't.

Most people don't have the means but you can recondition the sensor to try and bring it back into spec.

Quote
Reconditioning Procedure SHT3x
The following reconditioning procedure may bring the sensor back to calibration state:
Baking: 100 – 105°C at < 5%RH for 10h
Re-Hydration: 20 – 30°C at ~ 75%RH for 12h
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 11, 2018, 03:21:06 PM
 [ You are not allowed to view attachments ]

That image is from Stephen Burt's 2009 review of the VP2. This would be relative humidity as measured by the SHT11 (original OEM temp/humidity sensor package) compared against a Vaisala HMP45C capacitive humidity sensor (the dotted black line).

Keep in mind, that measurements on this graph start in January 2009 (red dots) at which point the VP2 is approximately 6 months old as the study commenced in July 2008. The May readings (in blue) are when it's nearly a year old.

Here are his comments on the graph:

Quote
5 minute sampled RH for both instruments for a winter and summer month (January 2009 data in red and May 2009 in blue): in all, 17 780 pairs of observations. The thick dashed line marks the trend line that would be expected with a ‘perfect’ agreement; points above the line indicate where the VP2 indicated higher than the Vaisala sensor, and vice versa below the line.

Look how often the sensor runs high in the middle of the range -- often by 5-10% or more. In fact, I don't see one instance of it running drier than the Vaisala until almost 80% humidity. He said over the whole range it was 2-4% higher on average, but that's balanced out by the dryness on the high end (above 80%).

Those results are why he rated the VP2's humidity measurement only as "adequate" (essentially a "C" letter grade) while giving the temperature portion an excellent rating (or "A"). The humidity sensor was acceptable, I suppose, in England, the testing location, where it's above 80% humidity a great deal of the time. Not so acceptable elsewhere where the middle ranges are more common.

In other words, the Sensirion humidity sensor has run wet in the middle for years at least within 6 months from installation. I'm sure Davis is fully aware of that review as it was widely publicized at the time. 

It doesn't appear that the issue was really "fixed" with the 31 model. Perhaps out of the gate, the readings are better, but not for long.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 11, 2018, 04:23:30 PM
I've stated this here before, but I use this on the sensor every time I service the ISS, which is approximately every six months. I've seen zero effect on my sensors except for the fact they still work. If you've got a bad sensor, whatta ya got to lose in case you're afraid to try it.
http://www.acehardware.com/product/index.jsp?productId=1451535
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on July 11, 2018, 04:36:43 PM
I have prepared a possible response to this debacle from Davis. Maybe this might help Davis and its legal team get started with an actual Davis response.

Davis has learned that the humidity-temperature sensor in our Vantage 2 product line (enter model numbers and date of manufacturing here) is not achieving the advertised accuracy we promised. Specifically, over time the humidity being reported by your sensor will likely be reading too high. We apologize for this deficiency and as a result, are offering to send you, post paid, a replacement every two years since you bought your unit up to a maximum of 4 replacements—the expected life-span (10 years) of your unit. For example, if your unit is two years old now, we will send you a replacement now and another one in two years, and so forth so you will have ten years of service from your Davis unit.

Replacing the sensor is not difficult, but requires you to take down your integrated sensor suite, remove the rain collector and base, remove a few screws, remove your temperature-humidity sensor and replace it with a new sensor and reassemble. Directions for this effort are documented in an instruction manual enclosed with the sensor. If you are not concerned that your humidity readings are likely reading too high, you do not need to do anything. We apologize for any inconvenience we may have caused you.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 11, 2018, 04:55:18 PM
I’m curious if you can change out the Davis’ filter cap with Sensirion’s

https://www.sensirion.com/en/environmental-sensors/humidity-sensors/filter-cap-sf2/
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 11, 2018, 05:07:17 PM
Just found an interesting document from Davis: https://www.davisinstruments.com/product_documents/weather/Doc_Sensor-Maintenance.pdf (https://www.davisinstruments.com/product_documents/weather/Doc_Sensor-Maintenance.pdf).

On page 2 it says it expects that the temperature/humidity sensor will need recalibration every 5 years.

Also, why do they not put such information in product manuals?!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 11, 2018, 05:22:28 PM
Apparently DAVIS is only cautious about "dust" and not actual water condensing upon the RH sensor and thus physically "wetting" the mylar capacitive sensor.

I seem to recall discussion awhile back about the choice of that cap over the sensor. There's some reason they chose it and decided against the manufacturer's recommended cap. But I can't recall it exactly. Not enough air movement, perhaps.

In any event, they should have known that by choosing that cap design (which lets in just about anything), they were sentencing the sensor to a short life.

Now reconsider their doc where they claim you can go 5 years without the sensor needing "recalibration" (whatever that means; replacement probably). That seems like a bad joke.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 11, 2018, 05:33:39 PM
Using the factory filter is not hard to do, i use them on the 31 and the 15. They are different types with the 31 being smaller than the fl1 which fits the 15. I just cut the little mounting tabs off and hold the filter in place with a little sealing putty i have on hand, takes very little. The small filter gives better response time compared to the large one from Davis. I have used them for years without any problems.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 11, 2018, 05:42:43 PM
Using the factory filter is not hard to do, i use them on the 31 and the 15. They are different types with the 31 being smaller than the fl1 which fits the 15. I just cut the little mounting tabs off and hold the filter in place with a little sealing putty i have on hand, takes very little. The small filter gives better response time compared to the large one from Davis. I have used them for years without any problems.

I see them on Amazon: https://www.amazon.com/SENSIRION-SF2-FILTER-CAP-SHT2X/dp/B018CQ2USS (https://www.amazon.com/SENSIRION-SF2-FILTER-CAP-SHT2X/dp/B018CQ2USS). But you say there are different sizes. Where do you obtain the filter for the 31?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 11, 2018, 05:45:04 PM
Apparently DAVIS is only cautious about "dust" and not actual water condensing upon the RH sensor and thus physically "wetting" the mylar capacitive sensor.

I seem to recall discussion awhile back about the choice of that cap over the sensor. There's some reason they chose it and decided against the manufacturer's recommended cap. But I can't recall it exactly. Not enough air movement, perhaps.

In any event, they should have known that by choosing that cap design (which lets in just about anything), they were sentencing the sensor to a short life.

Now reconsider their doc where they claim you can go 5 years without the sensor needing "recalibration" (whatever that means; replacement probably). That seems like a bad joke.

Sensirion’s filter cap worked well with Bellfry Boy’s version of the SHT 15 sensor. I wonder if he can make his version of the SHT-31 that would work with our VP2s?

But I do wish Davis would update their transmitter to where it can take a quality sensor from a different manufacturer. More and more colleges and other professionals are switching to Davis (i.e. WeatherSTEM). It would be wise for them to take in the just criticism and use it for their advantage. Otherwise, some other manufacturers may take advantage of Davis’s lack of action.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 11, 2018, 05:47:12 PM
Using the factory filter is not hard to do, i use them on the 31 and the 15. They are different types with the 31 being smaller than the fl1 which fits the 15. I just cut the little mounting tabs off and hold the filter in place with a little sealing putty i have on hand, takes very little. The small filter gives better response time compared to the large one from Davis. I have used them for years without any problems.

Awesome!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 11, 2018, 05:50:24 PM
Using the factory filter is not hard to do, i use them on the 31 and the 15. They are different types with the 31 being smaller than the fl1 which fits the 15. I just cut the little mounting tabs off and hold the filter in place with a little sealing putty i have on hand, takes very little. The small filter gives better response time compared to the large one from Davis. I have used them for years without any problems.

So the mounting tabs won’t clip into Davis pcb board like their filter caps does?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 11, 2018, 06:00:30 PM
Using the factory filter is not hard to do, i use them on the 31 and the 15. They are different types with the 31 being smaller than the fl1 which fits the 15. I just cut the little mounting tabs off and hold the filter in place with a little sealing putty i have on hand, takes very little. The small filter gives better response time compared to the large one from Davis. I have used them for years without any problems.

I see them on Amazon: https://www.amazon.com/SENSIRION-SF2-FILTER-CAP-SHT2X/dp/B018CQ2USS (https://www.amazon.com/SENSIRION-SF2-FILTER-CAP-SHT2X/dp/B018CQ2USS). But you say there are different sizes. Where do you obtain the filter for the 31?

Get the SF2. It’s for both the SHT 2x and 3x series of sensors.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 11, 2018, 06:26:29 PM
No it won't fit in the holes, the filter just barely covers the sensor itself.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 11, 2018, 06:31:43 PM
Here is link to Mouser the filter is 1.06 plus shipping.https://www.mouser.com/Sensirion/Sensors/Sensor-Hardware-Accessories/_/N-11ei3?P=1ytt2dn  It is the sf2 filter.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 11, 2018, 06:45:14 PM
For those that read my last post, I deleted it, I had the wrong cover. :oops:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 11, 2018, 06:50:38 PM
For those that read my last post, I deleted it, I had the wrong cover. :oops:

Too late  :grin: But you brought up a good point even if by mistake.  Davis made the decision not to include the polyimide foil dedicated to protect the sensor opening from pollution.  Cheaper but may have solved the problem of contamination if it's the problem.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 11, 2018, 06:54:12 PM
For those that read my last post, I deleted it, I had the wrong cover. :oops:

Too late  :grin: But you brought up a good point even if by mistake.  Davis made the decision not to include the polyimide foil dedicated to protect the sensor opening from pollution.  Cheaper but may have solved the problem of contamination if it's the problem.
Poo... :-) Ya know what? If we go in on a group buy of 10, we can save one cent per filter!! It's our lucky day guys!!! \:D/
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 11, 2018, 07:11:48 PM
I bought 10 of them because of paying shipping only once and when i change one out i put it up and put a new one on and don't have to clean any until i get them all used. I had one on for a year and it never showed any signs of clogging up.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 11, 2018, 07:39:41 PM
I bought 10 of them because of paying shipping only once and when i change one out i put it up and put a new one on and don't have to clean any until i get them all used. I had one on for a year and it never showed any signs of clogging up.

Nice Jerry thanks for info.. I can always use a little quicker reaction time.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 11, 2018, 08:01:24 PM
Davis made the decision not to include the polyimide foil dedicated to protect the sensor opening from pollution.  Cheaper but may have solved the problem of contamination if it's the problem.

Not just pollution but WATER. As many have pointed out, it would seem water ingress is the main culprit here.

For those ordering the cap, please post pics and procedures so we know what works. I'm not sure a putty like substance is going to be enough to keep this cap in place in a FARS where it is installed perpendicular to the ground (rather than parallel like in the passive shield) and will have air blowing on it constantly. Super glue would make it impossible to remove when it gets dirty. Not sure if there's a substance in between these two that will secure it but not permanently. Ideas?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 11, 2018, 08:14:40 PM
The way I look at it, since it's basically an "impenetrable" shield when sealed, you might as well super glue it into place. When said sensor gets unreliable, chuck it. That pretty much the way electronics are anymore, use and lose.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 11, 2018, 08:25:26 PM
I was thinking about what kcidwx said about how abnormal it is to aspirate the humidity sensor. There must be reason for it...Possibly they wear out much faster...Anyway 4 days ago I started testing the Davis 24 hr shield in passive mode. 
We've only had 1 day with light enough winds I considered a good test day. It's the best passive shield I've ever tested. The sensor is well protected inside from thermal heat and painted inside flat black, radiation and reflective heat is reduced also.
Peak temperature was only 1 degree warmer than the AC fan and 1.5° warmer than the dc solar fan. The DC fan actually runs cooler than the AC on average .5F.

What I did to modify was removed fan, painted interior where sensor mounts all the way to bottom flat black added a couple more top shields, but those with rain bucket not necessary and drilled some holes on outer shield core...This may not be necessary also.  Final step I used same 5/8" bit and drilled a bottom center hole through plastic for a little more upward airflow and removed the small pyramid shaped black shield on bottom.

This is graph of the one day with wind speed light enough to consider a valid test under clear cloudless sky with high summer sun angle 43° longitude.
#3 orange is passive, #1 AC fan #5 Dc fan

Added wind speed and dewpoint
 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 11, 2018, 08:37:39 PM
The putty is more than enough to hold the filter on, it is so small and light it was in place for a year and i had to force it loose from the sensor. The only reason i changed it was i thought after a year it might be getting clogged but it looked ok and the new one did not show any change in performance. I used the end of a toothpick to apply a small amount to each side and mold the filter to the sensor.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 11, 2018, 08:44:11 PM
Nobody's mentioned it, but most probably know it, but there IS a possibility that DAVIS uses the combined TEMP/RH sensor because internally within the ISS they need the TEMP to compensate the RH sensor's temperature non-linearity. Anybody know for sure *IF* the SHT-31 needs temp-compensation?

It was mentioned they are totally different circuits yesterday I believe the big debate with Davis techs why you can change temperature and RH doesn't move. 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 11, 2018, 09:00:46 PM
I was just thinking how all this discussion is about a 45 dollar sensor. When the cost is considered it is a very good deal for home use. I priced the Vaisala HMP155 sensor which is one of the better ones out there and the price was 823.00 dollars, wow. The spec for the top end stopped at 97% and it was + or -  1.8% not far from the cheap 31 spec. at 100%. I am just glad that there is a sensor that works really well at a price i can afford. I can buy a whole lot of replacement sensors for the price of the better sensor lol.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 11, 2018, 09:02:42 PM
Nobody's mentioned it, but most probably know it, but there IS a possibility that DAVIS uses the combined TEMP/RH sensor because internally within the ISS they need the TEMP to compensate the RH sensor's temperature non-linearity. Anybody know for sure *IF* the SHT-31 needs temp-compensation?

It was mentioned they are totally different circuits yesterday I believe the big debate with Davis techs why you can change temperature and RH doesn't move.

I believe I'm the one who referenced that "debate" which was really more of an act more than anything in retrospect. Seems unlikely Davis engineers had no idea that temperature had no influence on relative humidity in the console they designed.

Sure, from the console's perspective temperature and humidity are two separate values that seem unrelated. But as for the SIM, I'd assume it uses temperature for no other reason than to calculate saturation vapor pressure as a denominator for vapor pressure (the ratio that produces humidity). Maybe it has a lookup table instead? As to whether the SIM also uses temperature as a parameter for a calibration routine, I would not know.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 11, 2018, 09:10:28 PM
I was just thinking how all this discussion is about a 45 dollar sensor. When the cost is considered it is a very good deal for home use. I priced the Vaisala HMP155 sensor which is one of the better ones out there and the price was 823.00 dollars, wow. The spec for the top end stopped at 97% and it was + or -  1.8% not far from the cheap 31 spec. at 100%. I am just glad that there is a sensor that works really well at a price i can afford. I can buy a whole lot of replacement sensors for the price of the better sensor lol.

Perhaps the reason you've had good luck with these sensors is you knew to put the Sensirion cap on them. Did you notice a difference in their long-term accuracy once you started doing that?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on July 11, 2018, 09:38:09 PM
I was just thinking how all this discussion is about a 45 dollar sensor. When the cost is considered it is a very good deal for home use. I priced the Vaisala HMP155 sensor which is one of the better ones out there and the price was 823.00 dollars, wow. The spec for the top end stopped at 97% and it was + or -  1.8% not far from the cheap 31 spec. at 100%. I am just glad that there is a sensor that works really well at a price i can afford. I can buy a whole lot of replacement sensors for the price of the better sensor lol.

Jerry,

I agree with you, but I wish Davis informed us of the requirement to change sensors every 12 to 24 months depending on where you live (climate dryness) to ensure accuracy. This would have done two things—made me feel better about Davis and would have forced me to do a job I hate—cleaning my ISS. It ranks right below sanitizing my reverse osmosis drinking system as the worst home maintenace job I have.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 11, 2018, 09:38:17 PM
Here is link to Mouser the filter is 1.06 plus shipping.https://www.mouser.com/Sensirion/Sensors/Sensor-Hardware-Accessories/_/N-11ei3?P=1ytt2dn  It is the sf2 filter.

Jerry is this the filter you cut the legs off and glue on?
When you say sealing putty would any glue work?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 11, 2018, 10:12:55 PM
I was just thinking how all this discussion is about a 45 dollar sensor. When the cost is considered it is a very good deal for home use. I priced the Vaisala HMP155 sensor which is one of the better ones out there and the price was 823.00 dollars, wow. The spec for the top end stopped at 97% and it was + or -  1.8% not far from the cheap 31 spec. at 100%. I am just glad that there is a sensor that works really well at a price i can afford. I can buy a whole lot of replacement sensors for the price of the better sensor lol.

TBH, I think the high prices of these sensors like Vaisala is due to more of supply/demand than in terms of the quality of the sensors. Sure, they will be higher due to the components and their quality of materials but the supply/demand plays a bigger role. I’m sure mass production of chips is way cheaper than making one Vaisala sensor. That’s basically my opinion & I could be wrong.

Now I’m curious if the SHT-35 is any better than the 31?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: kcidwx on July 11, 2018, 10:40:13 PM
As far as why you shouldn't aspirate a humidity sensor goes.... Two things that will kill a humidity sensor prematurely, contaminants/pollution and condensation. With the SHT-31 in the Davis FARS you are throwing both at it. At or near saturation, there's no heater to keep the sensor temperature a degree or two above the dew point so condensation doesn't form on the sensor and, you are blowing dirty air across it 24/7. Definitely not best practice for sensor longevity. It comes down to how much abuse the SHT-31 can handle. I have my Vaisala HMT337 humidity sensor in a passive shield and the filter never gets very dirty. On my RM Young FARS for my temperature sensor it gets incredibly dirty from pulling air through it 24/7. If Sensirion offers their own filter that's superior over the Davis filter, then I would go that route. Ideally for temperature compensation you can use the onboard temperature sensor for the humidity sensor temperature in a passive shield. Then use a separate temperature probe in a FARS for ambient air temperature. That's best practice.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 11, 2018, 10:47:38 PM
As far as why you shouldn't aspirate a humidity sensor goes.... Two things that will kill a humidity sensor prematurely, contaminants/pollution and condensation.
Well, according to Senserion, this filter, when properly sealed around the sensor should eliminate all these factors.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on July 11, 2018, 10:55:50 PM
As far as why you shouldn't aspirate a humidity sensor goes.... Two things that will kill a humidity sensor prematurely, contaminants/pollution and condensation.
Well, according to Senserion, this filter, when properly sealed around the sensor should eliminate all these factors.

So does the Davis 24 hour FARS seal it like this to eliminate “these factors”?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 11, 2018, 10:57:18 PM
As far as why you shouldn't aspirate a humidity sensor goes.... Two things that will kill a humidity sensor prematurely, contaminants/pollution and condensation.
Well, according to Senserion, this filter, when properly sealed around the sensor should eliminate all these factors.

So does the Davis 24 hour FARS seal it like this to eliminate “these factors”?
No.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 11, 2018, 11:03:54 PM
Well i am no expert on what to use to secure the filter to the sensor i used the putty because i had it on hand and it was sticky and water proof plus it didn't have any chemical spell to it that might be bad for the sensor. I have some stuff that is used to seal electrical connections that is non corrosive that i could have used but wanted to be able to remove the filter for cleaning.
What a mess, need fars for good temp reports but bad for humidity, no fars good for humidity but bad for temp with no wind.  ](*,)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: klschmidt on July 11, 2018, 11:06:03 PM
Would switching the aspirator fan off when wind exceeds a certain speed (2MPH) be advantageous for sensor life?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 11, 2018, 11:08:17 PM
but wanted to be able to remove the filter for cleaning.
Why? If the inside is sealed, why would removal be necessary?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 11, 2018, 11:19:33 PM
Because i had no info as to how well the filter would perform until i tried one and with the seal all the way around the outside of the filter the only way the air can be sampled is through the membrane. When you compare the big Davis shield to the really small fl2 filter and its little opening it looks like it would plug up in no time but even out here in the country surrounded by farm land that is plowed and harvested all the time with crop dusting a surface spraying it has held up real well. My humidity runs over 90% nearly every night unless in a major drought lol. Two of my 3 sensors are over 2 years old in this bad, humid and dusty area and are still tracking with the new sensor i just put in. As far as i can tell i have not had any wet bias.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 11, 2018, 11:32:29 PM
Because i had no info as to how well the filter would perform until i tried one and with the seal all the way around the outside of the filter the only way the air can be sampled is through the membrane. When you compare the big Davis shield to the really small fl2 filter and its little opening it looks like it would plug up in no time but even out here in the country surrounded by farm land that is plowed and harvested all the time with crop dusting a surface spraying it has held up real well. My humidity runs over 90% nearly every night unless in a major drought lol. Two of my 3 sensors are over 2 years old in this bad, humid and dusty area and are still tracking with the new sensor i just put in. As far as i can tell i have not had any wet bias.

You might have solved our problems! Lol  Does your sensors struggle to reach 100%?

I hope the filter cap is the answer. I would’ve love to see the DP readings match up well with the Airport stations. Only difference I want to see is due to transpiration and not from a wet biased sensor.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 11, 2018, 11:55:35 PM
The problem with maintaining two sensors, one in passive for humidity and one aspirated for temp, is that there isn't a Davis console (I'm aware of) that can feed temperature and humidity on separate IDs (what some folks call "channels"). You could add a 2nd console to receive the 2nd SIM signal, but that doesn't solve how you feed your weather software both consoles' data simultaneously.  While it may be possible to run two different, incompatible weather apps concurrently and somehow feed humidity from one and temperature from the other to your website (custom job but do-able), I can't see how you would be able to send both to the same statistics database, let alone online services like CWOP & WU, without some serious custom programming.

At this point, I'd have to side with jerryg. Does it make sense to spend hundreds in initial and ongoing expenses and invite untold headaches all in an effort to save a $45 sensor from annual or semi-annual replacement?

Food for thot. Old Teleman provided a link several pages ago in this thread to an academic study which concluded that naturally aspirated humidity readings are, in fact, less accurate, on the whole, than fan aspirated readings. Why? Same reason as with temp readings: on low wind days an artificial environment can build up within the shield. I remember reading that elsewhere too.

If the cap solution work out, then maybe we can have our cake and eat it too?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: kcidwx on July 12, 2018, 12:03:02 AM
Here's the spec on that Filter Cap SF2.

Filtration efficiency 99.99% 
0.1μm particles at 0.05m/s air flow.

I'm not sure what the Davis fan pulls for air flow. Keep in mind as you increase air flow through the chamber the filter becomes less efficient but there's no data table to show by how much. I think this filter is your best bet.

Anyway, that's why I don't like FARS on humidity sensors. You are constantly ramming air through the chamber 24/7 and there's always some contaminants/pollution that's getting through the filter to the sensor.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 12, 2018, 12:31:05 AM
I think I'll still use the stock filter as well considering my strong fan and dusty climate, my sensor chamber gets filthy. I'm sure it'll slow response slightly, but I'm okay with that for the added protection.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: kcidwx on July 12, 2018, 12:52:20 AM
Food for thot. Old Teleman provided a link several pages ago in this thread to an academic study which concluded that naturally aspirated humidity readings are, in fact, less accurate, on the whole, than fan aspirated readings. Why? Same reason as with temp readings: on low wind days an artificial environment can build up within the shield. I remember reading that elsewhere too.

That depends on the design of the passive shield. The open air passive shields such as the Vaisala one used on the DTS1 for ASOS doesn't suffer from that. I always bring that up because PWS owners are always comparing their humidity readings to ASOS and ASOS doesn't use a fan aspirated shield with the humidity sensor. It isn't needed. I have an open air shield as well for my humidity sensor and love it. Since the Davis FARS is an enclosed chamber, then I can see why a fan is needed. The open air shields are not the same as the gill plated passive shields that are notorious for trapping stagnate air.

At this point, I'd have to side with jerryg. Does it make sense to spend hundreds in initial and ongoing expenses and invite untold headaches all in an effort to save a $45 sensor from annual or semi-annual replacement?

If $45 is all it costs then yes. Buy the sensor, put the filter on it, install it and replace as needed.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: johnd on July 12, 2018, 02:43:21 AM
The problem with maintaining two sensors, one in passive for humidity and one aspirated for temp, is that there isn't a Davis console (I'm aware of) that can feed temperature and humidity on separate IDs (what some folks call "channels").

If you can still find an Envoy8X then that would be one option of course. A Meteobridge Pro Red (or whatever Ed calls it in the US) or Meteostick-based station would be another.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 12, 2018, 07:30:55 AM
I would like to see the SHT31 with some side by side temperature data against vaisala temperature sensor.
The ASOS here has been running around 2° warmer than my instruments. I've taken nearby data with portable FARS and Davis console nearby and come up 1.7° lower vs what the ASOS 5 minute average shows.  I've cross checked the SHT31 with NIST certified and they are very close within .1-.2F.
Does anyone know of a study where this has been conducted?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: kcidwx on July 12, 2018, 08:05:47 AM
ASOS still uses the 1088 system from TSL for ambient air temperature. The chilled mirror sensor in the 1088 is still used as a backup to the DTS1 humidity sensor.

https://tslinc.com/systems/instruments-hygrothermometer/ (https://tslinc.com/systems/instruments-hygrothermometer/)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 12, 2018, 08:38:42 AM
What i have decided to try now with my test setup that has the Davis filter on it is to put a sf2 on it with the Davis shield and see what happens to the response time. Sure would make for some really good shielding, the Davis for the big stuff and the sf2 for the fine stuff. Should be interesting if nothing else. The last info i saw on the air flow was 24 cfm daytime and 11 cfm at night. Also read where the shield is designed for low air flow and in air intake is designed to increase the air flow by 2.7 times by narrowing the input.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 12, 2018, 10:23:42 AM
What i have decided to try now with my test setup that has the Davis filter on it is to put a sf2 on it with the Davis shield and see what happens to the response time. Sure would make for some really good shielding, the Davis for the big stuff and the sf2 for the fine stuff. Should be interesting if nothing else. The last info i saw on the air flow was 24 cfm daytime and 11 cfm at night. Also read where the shield is designed for low air flow and in air intake is designed to increase the air flow by 2.7 times by narrowing the input.

Let us know what your test results show.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: mcrossley on July 12, 2018, 11:08:05 AM
Also read where the shield is designed for low air flow
I think the low figure for the shield (0.1µm particles at 0.05m/s air flow.) is related to the particles getting through the membrane at the stated flow rate *through* the membrane, not the max air flow you can subject it to.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: mcrossley on July 12, 2018, 11:41:37 AM
But there is no air flow though (to a good approximation) through the Sensirion filter. The moisture diffuses through it. Airflow past the sensor keeps it at ambient temp, and keeps 'fresh' air ready to diffuse in/out.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 12, 2018, 12:34:22 PM
Since we are talking about air flow i have wondered what the fan deflector does. I thought it was used to mix the air in the sensor chamber but really don't know and i can not find any info on it. :?:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: kcidwx on July 12, 2018, 12:49:59 PM
But there is no air flow though (to a good approximation) through the Sensirion filter. The moisture diffuses through it. Airflow past the sensor keeps it at ambient temp, and keeps 'fresh' air ready to diffuse in/out.

Correct, you are not passing air through the filter like one would think like an air filtration system. However, even air moving past the sensor filter, you will still get particles that end up on the sensor. The FARS just makes the problem worse by the volume of air it moves through the radiation shield chamber. A really good sensor filter can help but the trade-off can be reduced response time.

I'd like to see an up-close photo of an SHT-31 sensor that's been in FARS service for 24 months that's had the stock filter on it to see how clean it looks. It should look clean to the untrained eye but further up-close magnified inspection might show contamination.

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 12, 2018, 01:22:48 PM
I'm going to replace sensor for sure each spring and depending how humidity looks going into fall again on x2 stations so 2 to 4 new units yearly.

The passive shield idea will reduce sensor contamination 10 maybe 50x depending on aspiration rate but averaging 2° warmer over long term really changes data when it comes to averages.  In truth however the data from a good passive shield is probably closer to what the old Cotton Region (Stevenson) type shelters provided.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: kcidwx on July 12, 2018, 02:00:38 PM
I'm testing three of these evaluation boards right now. RH spec: 1.5%

https://www.idt.com/products/sensor-products/humidity-sensors (https://www.idt.com/products/sensor-products/humidity-sensors)

I'll run them against my three SHT-31 evaluation boards as well.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 12, 2018, 02:48:52 PM
I'm testing three of these evaluation boards right now. RH spec: 1.5%

https://www.idt.com/products/sensor-products/humidity-sensors (https://www.idt.com/products/sensor-products/humidity-sensors)

I'll run them against my three SHT-31 evaluation boards as well.

Nice! It would be interesting to see how they compare.

Just reading the spec sheet, Davis would do well to get their transmitter to take I2c and switch over to IDT’s HS 3001. That’s IMO.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: kcidwx on July 12, 2018, 02:50:59 PM
I'm testing three of these evaluation boards right now. RH spec: 1.5%

https://www.idt.com/products/sensor-products/humidity-sensors (https://www.idt.com/products/sensor-products/humidity-sensors)

I'll run them against my three SHT-31 evaluation boards as well.
Which kit includes the "data logging" chip & coin battery?

SDAH02
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 12, 2018, 02:53:30 PM
I'm testing three of these evaluation boards right now. RH spec: 1.5%

https://www.idt.com/products/sensor-products/humidity-sensors (https://www.idt.com/products/sensor-products/humidity-sensors)

I'll run them against my three SHT-31 evaluation boards as well.
Which kit includes the "data logging" chip & coin battery?

The first kit was this one. Looks like others are now out.
https://www.sensirion.com/en/environmental-sensors/humidity-sensors/development-kit/
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: johnd on July 12, 2018, 02:59:59 PM
Just reading the spec sheet, Davis would do well to get their transmitter to take I2c and switch over to IDT’s HS 3001. That’s IMO.

But aren't SHT35 and HS3001 pretty much on a par? Actually SHT35 looks better in the 90-100% region. Where would HS3001 score over SHT35?

Not saying there aren't differences - they're just not too obvious from the specs, other than details like 0.1% difference in nominal tolerance which may well get lost anyway in slightly different interpretation of the specs - eg what is the difference between tolerance and accuracy or nominal vs max - and differences in eg binning. And yes Davis would need to move away from Sensibus to modern I2C if these were to be relevant to use in VP2 units.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 12, 2018, 03:02:37 PM
I'm testing three of these evaluation boards right now. RH spec: 1.5%

https://www.idt.com/products/sensor-products/humidity-sensors (https://www.idt.com/products/sensor-products/humidity-sensors)

I'll run them against my three SHT-31 evaluation boards as well.

Nice! It would be interesting to see how they compare.

Just reading the spec sheet, Davis would do well to get their transmitter to take I2c and switch over to IDT’s HS 3001. That’s IMO.

But I just saw that it can do either digital or analog output.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 12, 2018, 03:04:41 PM
Just reading the spec sheet, Davis would do well to get their transmitter to take I2c and switch over to IDT’s HS 3001. That’s IMO.

But aren't SHT35 and HS3001 pretty much on a par? Actually SHT35 looks better in the 90-100% region. Where would HS3001 score over SHT35?

Not saying there aren't differences - they're just not too obvious from the specs, other than details like 0.1% difference in nominal tolerance which may well get lost anyway in slightly different interpretation of the specs - eg what is the difference between tolerance and accuracy or nominal vs max - and differences in eg binning. And yes Davis would need to move away from Sensibus to modern I2C if these were to be relevant to use in VP2 units.

True, the 35 does looks better but will it act like the 31 and not perform within specs under certain conditions? It also appears according to the specs sheet that the HS 3001 seems more durable sensor. I could be wrong however
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: kcidwx on July 12, 2018, 03:19:35 PM
I'd like to test the 35 but it doesn't work with my Evaluation Kit.  :sad:

https://www.sensirion.com/en/environmental-sensors/humidity-sensors/evaluation-kit-ek-h4/ (https://www.sensirion.com/en/environmental-sensors/humidity-sensors/evaluation-kit-ek-h4/)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: johnd on July 12, 2018, 04:29:36 PM
To me, that (end-of-life) indicates that Sensirion is moving on to the I2C interface, necessitating a whole new "development kit."

As I read it, it's just that particular evaluation kit that's EOL, not the SHT3x series.

And isn't the SHT3x series mainly I2C already, albeit with some other options available like the legacy Sensibus variant that Davis uses? For anything that would be compatible with a VP2 you need a sensor that has a Sensibus interface rather than the mainstream I2C.


Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: johnd on July 12, 2018, 04:50:05 PM
You are correct, the whole SHT-3X line are I2C and/or analog. Don't know what Sensibus is.

I'm no expert but AIUI Sensibus is a kind of older variant of I2C which is what Davis SIM boards use but which is incompatible with 'proper' I2C. Any sensor that wants to talk to a VP2 must use Sensibus and not I2C is how I understand it. If you look in detail I think you'll see that SHT31 is available with a Sensibus interface, but SHT35 isn't (was the last I heard). Here's one explanation:

https://www.i2cchip.com/humidity_sensors.html (https://www.i2cchip.com/humidity_sensors.html)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 12, 2018, 05:02:53 PM
The way I understood it, Davis had to collaborate with Sensirion to devise the Sensibus solution specifically for them. I don't think anyone else uses it besides Davis. Hence, their leverage is somewhat limited when it comes to squawking about humidity performance.

I'm sure Sensirion's first defense would be: you chose to forego our OEM cap. Davis' cap reminds me of a pool filter meant to keep only the largest objects out rather than a serious attempt at protecting the sensor from corruption. The reasoning might have been airflow related but at least in FARS units those sensors are SUPER sensitive and could use some delay.

I've put in an order for 4 of the OEM caps. I'll apply 1 first to my passive shield and see how it performs. Then I'll do the active shield. That leaves 2 spares for the future (or breakage).
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 12, 2018, 05:06:34 PM
2 spares for the future (or breakage).
That's why I just picked up five. #-o
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 13, 2018, 05:44:33 AM
The Good The Bad The Ugly.

First the good, replaced one of my sensors yesterday and now easily hits 99% this is my first sensor ever to do this. So my other sensors were probably worn out when I received them.

The bad sensor was received in same bubble wrap Davis ships to retailers because retailer happened to be out of  anti static ESD plastic bags he normally ships in so unknowing used the same bubble wrap made of gaseous polyethylene which is absolute sin and contaminates sensor. Not only that Davis is shipping sensors open air so we are receiving used sensors from the get-go. 

The ugly Davis Instruments is negligent and going against all Sensirion protocol of handling humidity sensors.
 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 13, 2018, 06:24:12 AM
Wow.  SMH.

Could that be a part of the reasons why Sensirion humidity sensors have issues?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 13, 2018, 06:34:27 AM
Yep and I bet Sensirion won't be all too happy either. It's their product getting a bad reputation from the mishandling.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 13, 2018, 08:50:28 AM
So, here's the document with handling procedures: https://www.sensirion.com/fileadmin/user_upload/customers/sensirion/Dokumente/2_Humidity_Sensors/Sensirion_Humidity_Sensors_Handling_Instructions.pdf (https://www.sensirion.com/fileadmin/user_upload/customers/sensirion/Dokumente/2_Humidity_Sensors/Sensirion_Humidity_Sensors_Handling_Instructions.pdf). It gives explicit instructions about wrapping the sensor in an anti-static, sealed bag and not exposing it to volatile compounds. Bubble wrapping the sensor violates ALL these instructions. Thus, Davis has been sending everyone contaminated sensors.

Furthermore, by choosing to use their own cap which lets in nearly every particle instead of the manufacturer's cap which, supposedly, filters out 99.99% of particles, Davis dooms the sensor to inaccuracy in pretty short order.

Inexcusable doesn't even begin to describe the situation.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 13, 2018, 09:30:15 AM
I’m curious if Davis used the HS3001 sensor, would their actions have the same effect on the sensor as Sensirion’s?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: johnd on July 13, 2018, 10:25:33 AM
I’m curious if Davis used the HS3001 sensor, would their actions have the same effect on the sensor as Sensirion’s?

You're asking 'if'? But obviously they can't use it (the HS3001) without a significant redesign of the SIM board and firmware, which is I suspect pretty unlikely at this stage in its product cycle. So it's presumably just a hypothetical question that can't really be answered?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: kcidwx on July 13, 2018, 12:22:39 PM
Sensor mishandling according to the manufacturers handling document is not uncommon. If you inquire they will just say the handling guidelines are overkill. It's like every time I step foot into a weather office and the computer tech has computer memory modules just thrown around on his work bench. If you inquire about why don't you keep the memory in an ESD bag, use an anti-static wrist strap with an anti-static work mat, you will get told, "Been doing it this way for 20 years. Never had a problem." You'll get the same response with sensor handling. Unless they see a high failure rate, it's business as usual.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 13, 2018, 01:56:34 PM
Sensor mishandling according to the manufacturers handling document is not uncommon. If you inquire they will just say the handling guidelines are overkill. It's like every time I step foot into a weather office and the computer tech has computer memory modules just thrown around on his work bench. If you inquire about why don't you keep the memory in an ESD bag, use an anti-static wrist strap with an anti-static work mat, you will get told, "Been doing it this way for 20 years. Never had a problem." You'll get the same response with sensor handling. Unless they see a high failure rate, it's business as usual. [emphasis mine]

Except in this instance, they ARE having a problem. We have documented it pretty thoroughly in this thread. It was you, in fact, who stated you had never seen "drift" like what exists in these sensors.

I'd like someone to explain, for instance, how putting a pool or drain filter style cap over a sensor that needs microscopic protection won't predictably destroy that sensor's ability to give accurate readings in relatively short order. Then go ahead and square that with Davis' literature stating you shouldn't need recalibration for humidity within 5 years.

You can say "well that's just business as usual". And I can say you get what you expect. It works both ways.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: kcidwx on July 13, 2018, 02:48:00 PM
Just for fun, I still have my three SHT-31 eval boards. In the humidity chamber this morning they were reading:

Stability Time: 42 minutes
Ref Humidity: 45.0%
SHT-31-1: 46.31%
SHT-31-2: 46.20%
SHT-31-3: 46.97%

Number 3 went outside for 6 hours in the shade without a filter on it. And now......

Stability Time: 48 minutes
Ref Humidity: 45.0%
SHT-31-1: 47.12%
SHT-31-2: 47.03%
SHT-31-3: 50.02%

Number 3 is not happy.  :lol:


 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: kcidwx on July 13, 2018, 03:05:56 PM
Just for fun, I still have my three SHT-31 eval boards. In the humidity chamber this morning they were reading:

Stability Time: 42 minutes
Ref Humidity: 45.0%
SHT-31-1: 46.31%
SHT-31-2: 46.20%
SHT-31-3: 46.97%

Number 3 went outside for 6 hours in the shade without a filter on it. And now......

Stability Time: 48 minutes
Ref Humidity: 45.0%
SHT-31-1: 47.12%
SHT-31-2: 47.03%
SHT-31-3: 50.02%

Number 3 is not happy.  :lol:
Two "points" I see: (1) 48 minutes to stabilize means "diffusion" or extremely SLOW response time; and (2) the OEM filter probably is inhibiting contaminates.

Stability time just means the amount of time they were in the chamber. I always give the sensors a minimum of 30 minutes in the chamber before I read them. I provide that because people will always ask, "How long did you leave them in there before you took a reading?"
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 13, 2018, 03:20:37 PM
As promised, here are my initial measurements with my new Kestrel 5000 meter calibrated at the factory for +/- 2% and +/- 0.9F Temp.  All readings are compared against an 8 month old SHT31 installed in the field in a Davis 24hr FARS shield (about half it's life was spent in a passive shield though). The SHT31 sensor has the Davis filter over it.

I took a few readings between 80 and 90% humidity and, as expected, it runs cool (dry) in the dew point but only between 0.5 - 1.0 degF. That's 1-2% in the lower to mid 70s F. Cannot compare humidities directly because of solar error in the air temp during the day.

Around 75% it seems to be accurate (0.3 - 0.5F within Kestrel DP). So this must be near the crossover point in the curve if we were to draw one modeling the non-linearities of the sensor.

In the 60 percentiles is when the Davis sensor starts running noticeably wet. For example, at 82.5F and 61% humidity it ran 1.4 degrees warmer (wetter) in dew point compared to the Kestrel. That's 3% high in humidity which is out of spec. By comparison, my 15 month old SHT31 was running 4.8 degF warmer (wetter) in DP than the Kestrel with a humidity error above 8% at 61% reference humidity.

I will continue testing and post back if anything interesting comes up.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 13, 2018, 03:39:30 PM
Alright testing. More the merrier  =D&gt;

I'm in the frustration mode after finding out my sensors aren't really new just not aspirated yet.  ](*,)


 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: kcidwx on July 13, 2018, 03:59:47 PM
When I requested my eval boards from Sensirion, I requested them with the filter membrane.

https://www.sensirion.com/fileadmin/user_upload/customers/sensirion/Dokumente/0_Datasheets/Humidity/Sensirion_Humidity_Sensors_SHT3x_Datasheet_Filter_Membran.pdf (https://www.sensirion.com/fileadmin/user_upload/customers/sensirion/Dokumente/0_Datasheets/Humidity/Sensirion_Humidity_Sensors_SHT3x_Datasheet_Filter_Membran.pdf)

So on the Davis sensor PCB's, there's absolutely nothing but the Davis cap and filter?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 13, 2018, 04:15:03 PM
So on the Davis sensor PCB's, there's absolutely nothing but the Davis cap and filter?
Yes.
I wish I had seen this filter option earlier, putting on that tiny cap is going to be fun.... ](*,)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 13, 2018, 04:18:08 PM
Just for fun, I still have my three SHT-31 eval boards. In the humidity chamber this morning they were reading:

Stability Time: 42 minutes
Ref Humidity: 45.0%
SHT-31-1: 46.31%
SHT-31-2: 46.20%
SHT-31-3: 46.97%

Number 3 went outside for 6 hours in the shade without a filter on it. And now......

Stability Time: 48 minutes
Ref Humidity: 45.0%
SHT-31-1: 47.12%
SHT-31-2: 47.03%
SHT-31-3: 50.02%

Number 3 is not happy.
  :lol:

Seems to get worse as you get toward the middle (50% humidity). It doesn't take much exposure to knock them out of calibration. This is appalling, though.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 13, 2018, 04:21:22 PM
Wouldn't it be wonderful if this was our sensor   \:D/
 
The SHT3x membrane is a PTFE foil, dedicated to
protect the sensor opening from water and dust
according to IP67. It will thus allow for sensor use under
harsh environmental conditions, where spray water and
high exposure to dust is challenging for accurate sensor
performance
. The membrane cover is attached to the
sensor package by a double faced adhesive tape, which
is designed to stay on the package over the whole
sensor life time. Due to the minimum enclosed air
volume, the response time of the RH signal is the same
as the value achieved by the uncovered sensor


Oh this probably cost too much maybe extra .50 cents per sensor. Our sensors don't even come with a .20 cent bag to keep fresh and protected.   #-o What was I thinking.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: JudinNorman on July 13, 2018, 04:29:41 PM
What is Davis reply so far if any of you has contacted them?

A little over a year since installing the Sht31. Last summer my dewpoints were tracking the nearest weather service station but this year my dewpoints are usually a few degrees higher.

Looks like my sensor is out of calibration to.

Not that I can't afford a new one, it's the principle that the sensor didn't last or perform per their specs.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 13, 2018, 04:55:51 PM
They admit nothing.. 
I know how they roll from experience when I contacted them about firmware issue on Vue console and VP2 anemometer calibration they denied any issue existed even when I made a video.

Then they come out with console update a few months later, first update in years and issue was fixed. Still no acknowledgement...
They admit nothing...And won't here either.

Our best hope is they start packaging and handling humidity sensors correctly, but I wouldn't count on it.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on July 13, 2018, 04:56:34 PM
What is Davis reply so far if any of you has contacted them?

...

As stated previously, I contacted the Davis support person (Brett Lane) and alerted him to the discussion in this thread and provided him a link hereto. He acknowledged receipt of my email but nothing more. If I were Davis I would be very careful posting anything in this Forum, rather I would post something on the Davis website, then post in this thread a link to the Davis website. My guess, Davis is talking internally whether or how to address these issues on its website. If I was a betting man, they will not publish anything; rather just consider our comments should they decide to replace the horribly aging VP-2!

Unrelated to the above, I have been reading this Forum for about 5 years and lately contributing, and there has been considerable fluff in many threads. Not this one. I just spent several hours re reading this thread and identified several useful takeaways. When I finish my list, I will seek editing review by contributors to this thread then post so folks don't have to spend hours reading this thread--I will call it the Cliffs Notes of SHT 31 issues. Are Cliffs Notes still published? Setting the record straight by a very knowledgeable poster, kcidwx, has been most refreshing.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: JudinNorman on July 13, 2018, 05:04:47 PM
They admit nothing.. 
I know how they roll from experience when I contacted them about firmware issue on Vue console and VP2 anemometer calibration they denied any issue existed even when I made a video.

Then they come out with console update a few months later, first update in years and issue was fixed. Still no acknowledgement...
They admit nothing...And won't here either.

Our best hope is they start packaging and handling humidity sensors correctly, but I wouldn't count on it.


Most consumers don't test like you guys do on here. If we call Davis they just would say weather is not same even a mile away and get away with it as the consumer would agree the weather in my backyard is just that, my weather.

Why couldn't Davis recall the sensors and fix or replace them like auto industry does when parts are found defective ??
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 13, 2018, 05:08:51 PM
They admit nothing.. 
I know how they roll from experience when I contacted them about firmware issue on Vue console and VP2 anemometer calibration they denied any issue existed even when I made a video.

Then they come out with console update a few months later, first update in years and issue was fixed. Still no acknowledgement...
They admit nothing...And won't here either.

Our best hope is they start packaging and handling humidity sensors correctly, but I wouldn't count on it.
Why couldn't Davis recall the sensors and fix or replace them like auto industry does when parts are found defective ??
That's when it's a safety issue, the DOT makes them under US law.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 13, 2018, 05:19:58 PM
What is Davis reply so far if any of you has contacted them?

...

As stated previously, I contacted the Davis support person (Brett Lane) and alerted him to the discussion in this thread and provided him a link hereto. He acknowledged receipt of my email but nothing more. If I were Davis I would be very careful posting anything in this Forum, rather I would post something on the Davis website, then post in this thread a link to the Davis website. My guess, Davis is talking internally whether or how to address these issues on its website. If I was a betting man, they will not publish anything; rather just consider our comments should they decide to replace the horribly aging VP-2!

Unrelated to the above, I have been reading this Forum for about 5 years and lately contributing, and there has been considerable fluff in many threads. Not this one. I just spent several hours re reading this thread and identified 11 useful takeaways. When I finish my list, I will seek editing review by contributors to this thread then post. Setting the record straight by a very knowledgeable poster, kcidwx, has been most refreshing.

Thanks Ron! This forum has been very educational for me. Hopefully Davis will take consideration to the information on this thread and make the necessary changes. Thanks gang for all your input!  Please keep them going.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 13, 2018, 05:45:19 PM
I should add just because they (Davis Inst) probably aren't going to admit any wrong doing something good may  still come from thread.
It brought to everyone's attention humidity has issues and proper handling of these sensitive sensors is important. Davis did fix the problem with Vue just didn't admit the problem existed.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 13, 2018, 07:44:09 PM
I'm testing three of these evaluation boards right now. RH spec: 1.5%

https://www.idt.com/products/sensor-products/humidity-sensors (https://www.idt.com/products/sensor-products/humidity-sensors)

I'll run them against my three SHT-31 evaluation boards as well.

How did those sensors perform?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 13, 2018, 08:01:38 PM
Just reading the spec sheet, Davis would do well to get their transmitter to take I2c and switch over to IDT’s HS 3001. That’s IMO.

But aren't SHT35 and HS3001 pretty much on a par? Actually SHT35 looks better in the 90-100% region. Where would HS3001 score over SHT35?

Not saying there aren't differences - they're just not too obvious from the specs, other than details like 0.1% difference in nominal tolerance which may well get lost anyway in slightly different interpretation of the specs - eg what is the difference between tolerance and accuracy or nominal vs max - and differences in eg binning. And yes Davis would need to move away from Sensibus to modern I2C if these were to be relevant to use in VP2 units.

johnd- After carefully reading through the datasheet of the HS3001, I think you’re correct in your overall thinking. At first, I thought the HS3001 was a more “durable sensor” and it maybe is (when it comes to performance while the sensor is contaminated with dust or debris) but that’s about it. Other than that, it behaves basically the same as the Sensirion sensors. Here is what the HS 3001 datasheet says:

High humidity conditions:

“Important note: The HS300x series sensors are optimized to perform best in the more common temperature and humidity ranges of 10°C to 50°C and 20% RH to 80% RH, respectively. If operated outside of these conditions for extended periods, especially at high humidity levels, the sensors may exhibit an offset. In most cases, this offset is temporary and will gradually disappear once the sensor is returned to normal temperature and humidity conditions. The amount of the shift and the duration of the offset vary depending on the duration of exposure and the severity of the relative humidity and temperature conditions. The time needed for the offset to disappear can also be decreased by using the procedures described in sections 10 and 11.”


“Storage and Handling
Recommendation: Once the sensors are removed from their original packaging, store them in metal-in antistatic bags.
Avoid using polyethylene antistatic bags as they may affect sensor accuracy.
The nominal storage conditions are 10 to 50°C and humidity levels within 20% to 60%RH. If stored outside of these conditions for extended periods of time, the sensor readings may exhibit an offset. The sensor can be reconditioned and brought back to its calibration state by applying the following procedure:
1. Bake at a temperature of 100°C with a humidity < 10%RH for 10 to 12 hours.
2. Rehydrate the sensor at a humidity of 75%RH and a temperature between 20 to 30°C for 12 to 14 hours.”


Soo.... I think the best thing for Davis is to update their transmitter board to take pure I2c and switch to the SHT 35. But even more importantly, be sure that all sensors are packaged correctly.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 13, 2018, 08:42:02 PM
We really could use the special coating to protect from contamination.
As mentioned humidity sensors are not designed to be continually aspirated.
Not taking these special precautions will surely end sensor life sooner than later.
And yes handling and storage and not open air is very important. Open air starts the end of live for sensor prematurely who knows how old they are when we get them. 12 months 24?  Who knows. 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 13, 2018, 09:06:01 PM
I’m curious to know why Belfry Boy hasn’t make his version of the SHT-31 sensor like he did the 15?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: kcidwx on July 13, 2018, 09:11:32 PM
I'm testing three of these evaluation boards right now. RH spec: 1.5%

https://www.idt.com/products/sensor-products/humidity-sensors (https://www.idt.com/products/sensor-products/humidity-sensors)

I'll run them against my three SHT-31 evaluation boards as well.

How did those sensors perform?

The LCD screen died on one of the eval boards so the testing is on hold until I get a replacement.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 13, 2018, 09:14:13 PM
We really could use the special coating to protect from contamination.
As mentioned humidity sensors are not designed to be continually aspirated.
Not taking these special precautions will surely end sensor life sooner than later.
And yes handling and storage and not open air is very important. Open air starts the end of live for sensor prematurely who knows how old they are when we get them. 12 months 24?  Who knows.

But man, it’s definitely frustrating to get a “Aged” sensor when you paid for a fresh new one
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 13, 2018, 09:33:19 PM
But just thinking about it, I bet this is the reason why RainWise hasn’t produced a FARS version of the MK III.

But I do believe that the RW stations have a similar problem in terms of humidity as the Davis except for in the 90 percentile humidities.

Just looking at some of RW’s data on the company’s website, their temperature readings have a warm bias (most likely due to the shield they use).

In response to the ASOS being warmer than the Davis: I think the ASOS stations may have a warm bias. There is a MMTS at the Extension office in my county and they also have the Davis VP2 locates inside the same fence. The Davis (24hr FARS might add) reads sometimes warmer or colder than the MMTS. Typically by 1°
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 13, 2018, 09:53:18 PM
But just thinking about it, I bet this is the reason why RainWise hasn’t produced a FARS version of the MK III.

But I do believe that the RW stations have a similar problem in terms of humidity as the Davis except for in the 90 percentile humidities.

Just looking at some of RW’s data on the company’s website, their temperature readings have a warm bias (most likely due to the shield they use).

In response to the ASOS being warmer than the Davis: I think the ASOS stations may have a warm bias. There is a MMTS at the Extension office in my county and they also have the Davis VP2 locates inside the same fence. The Davis (24hr FARS might add) reads sometimes warmer or colder than the MMTS. Typically by 1°

The ASOS here for sure plus bias +2° on average. Don't see that with nearby AWOS or any of the Mesonet stations in area. All follow real close with aspirated Davis. 

You know who believes the ASOS are those with thermometers on the porches.   :lol:

I said this in jest my neighbor lady says the airport matches her porch thermometer...I told her airport thermometer is in middle of open field nothing but grass and hay no heat source at all and your porch thermometer is 2" off siding of house on southside of all places. What's wrong here? 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 13, 2018, 10:18:27 PM
But just thinking about it, I bet this is the reason why RainWise hasn’t produced a FARS version of the MK III.
Even if that's true, which I highly doubt, I don't care if fans are bad for the humidity portion of the sensor, I'll never quit using one til something comes along that's superior.
The benefits a FARS to me far outweighs the potential negative.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on July 13, 2018, 10:26:33 PM
But just thinking about it, I bet this is the reason why RainWise hasn’t produced a FARS version of the MK III.
Even if that's true, which I highly doubt, I don't care if fans are bad for the humidity portion of the sensor, I'll never quit using one til something comes along that's superior.
The benefits a FARS to me far outweighs the potential negative.

Having a FARS setup results in better temperature accuracy and not so accurate humidity readings—so pick your poison. Which is more important to you?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 13, 2018, 10:31:40 PM
But just thinking about it, I bet this is the reason why RainWise hasn’t produced a FARS version of the MK III.
Even if that's true, which I highly doubt, I don't care if fans are bad for the humidity portion of the sensor, I'll never quit using one til something comes along that's superior.
The benefits a FARS to me far outweighs the potential negative.

I was thinking about that also. But then I remembered the RM Young dual sensor temp/hum they run in aspirated shields. Maybe we took the fact because ASOS doesn't aspirate humidity it wasn't done.
Just look at sensor on the RMY temperature tracker and shields they offer both passive and aspirated for the dual sensor probe.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 13, 2018, 10:32:18 PM
But just thinking about it, I bet this is the reason why RainWise hasn’t produced a FARS version of the MK III.
Even if that's true, which I highly doubt, I don't care if fans are bad for the humidity portion of the sensor, I'll never quit using one til something comes along that's superior.
The benefits a FARS to me far outweighs the potential negative.

Having a FARS setup results in better temperature accuracy and not so accurate humidity readings—so pick your poison. Which is more important to you?
Look at it this way, what happens when your passive shield is running 4-5F warmer than ambient? What's that do to your humidity.... #-o
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on July 13, 2018, 10:35:19 PM
But just thinking about it, I bet this is the reason why RainWise hasn’t produced a FARS version of the MK III.
Even if that's true, which I highly doubt, I don't care if fans are bad for the humidity portion of the sensor, I'll never quit using one til something comes along that's superior.
The benefits a FARS to me far outweighs the potential negative.

Having a FARS setup results in better temperature accuracy and not so accurate humidity readings—so pick your poison. Which is more important to you?
Look at it this way, what happens when your passive shield is running 4-5F warmer than ambient? What's that do to your humidity.... #-o

As many PWS owners would conclude, “you show me a happy moose, and I will find him an acorn!”

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 13, 2018, 10:36:39 PM
But just thinking about it, I bet this is the reason why RainWise hasn’t produced a FARS version of the MK III.
Even if that's true, which I highly doubt, I don't care if fans are bad for the humidity portion of the sensor, I'll never quit using one til something comes along that's superior.
The benefits a FARS to me far outweighs the potential negative.

Having a FARS setup results in better temperature accuracy and not so accurate humidity readings—so pick your poison. Which is more important to you?

Personally temperature, but being in a county that’s one of the top fruit crop producers in the state of Alabama (especially peaches) accurate temperature and dew point reports are a must for growers. But I have noticed that a Davis passive shield with a constant breeze performs very well IMO.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 13, 2018, 10:42:22 PM
But just thinking about it, I bet this is the reason why RainWise hasn’t produced a FARS version of the MK III.
Even if that's true, which I highly doubt, I don't care if fans are bad for the humidity portion of the sensor, I'll never quit using one til something comes along that's superior.
The benefits a FARS to me far outweighs the potential negative.

I was thinking about that also. But then I remembered the RM Young dual sensor temp/hum they run in aspirated shields. Maybe we took the fact because ASOS doesn't aspirate humidity it wasn't done.
Just look at sensor on the RMY temperature tracker and shields they offer both passive and aspirated for the dual sensor probe.

If we are able to get a good SHT-31 (purely new one) with the Sensirion filter cap, I think we would be in good shape, even with FARS. The only thing is that we will need to switch them out around 9-12 months due to constant high humidities aging the sensor.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 13, 2018, 10:47:09 PM
RM YOUNG Temperature tracker they suggest the passive shield with the dual sensor.  ](*,)
Welp maybe I was thinking wrong.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 13, 2018, 10:55:36 PM
RM YOUNG Temperature tracker they suggest the passive shield with the dual sensor.  ](*,)
Welp maybe I was thinking wrong.

Probably the most idealistic set-up to keep contaminates out of the sensor, and slowing down the aging process of the sensor.

But according to jerryg, he doesn’t have any problems since he has Sensirion’s filter cap protecting the sensor.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: kcidwx on July 14, 2018, 01:15:10 AM
So my daughter was on my computer reading this thread and saw the complaints about the SHT-31 not going to 100%. It started raining here a little while ago and she wanted me to share that her weather station goes to 100%. She said maybe you guys should buy one like hers.  :lol:

Hers is a Weather Monitor II.  :lol:  She thinks it's pretty neat she's the only one around here that gets to 100% on WU.  :lol:

(https://farm2.staticflickr.com/1803/42679352794_6300e920b4_o.png)

(https://farm2.staticflickr.com/1827/42679370914_a7a1a0e393_o.png)

My Vaisala HMT337 a few feet away is showing 99.5%. Typically, hers is running about +3% over my HMT337.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 14, 2018, 07:28:52 AM
So my daughter was on my computer reading this thread and saw the complaints about the SHT-31 not going to 100%. It started raining here a little while ago and she wanted me to share that her weather station goes to 100%. She said maybe you guys should buy one like hers.  :lol:

Hers is a Weather Monitor II.  :lol:  She thinks it's pretty neat she's the only one around here that gets to 100% on WU.  :lol:

(https://farm2.staticflickr.com/1803/42679352794_6300e920b4_o.png)

(https://farm2.staticflickr.com/1827/42679370914_a7a1a0e393_o.png)

My Vaisala HMT337 a few feet away is showing 99.5%. Typically, hers is running about +3% over my HMT337.


Hahaha.  Tell her that they don’t make em’ like they used too. Shoot, that’s not bad considering how old the WM II is.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 14, 2018, 08:02:01 AM
So my daughter was on my computer reading this thread and saw the complaints about the SHT-31 not going to 100%. It started raining here a little while ago and she wanted me to share that her weather station goes to 100%. She said maybe you guys should buy one like hers.  :lol:

Hers is a Weather Monitor II.  :lol:  She thinks it's pretty neat she's the only one around here that gets to 100% on WU.  :lol:

(https://farm2.staticflickr.com/1803/42679352794_6300e920b4_o.png)

(https://farm2.staticflickr.com/1827/42679370914_a7a1a0e393_o.png)

My Vaisala HMT337 a few feet away is showing 99.5%. Typically, hers is running about +3% over my HMT337.

One of my VP2s (SHT-31 in passive shield) reached 100% for the first time this morning. Will note that it’s a new sensor (only around a month old).
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 14, 2018, 10:25:24 AM
Update on sf2 and Davis filter at same time. When i put new sensor in use on my main station i used the sf2 filter, the same has been in use on the old sensor for a year, i removed the sf2 from the old and put the Davis in place of it for comparison with the new sensor. Well the sf2 filter specs say the sensor performance is the same with or without the filter so i put the sf2 back on the old sensor with the Davis filter on also. I can not see any difference in performance of the old sensor readings with both filters in use. It looks like it could give added protection in high dust and humidity areas. I figured the manufacturer says the sf2 does not have any effect on the sensor readings that maybe it would be worth a try even though the bigger filter does add some lag time on the readings. It is hard to measure lag time using the Davis set up because of the high sampling rates of the temp and humidity readings. Since i started using the sf2 i have not had any humidity related problems like i did when using the big Davis filter. I will never run a sensor without the sf2 in place. I am going to let the test sensor run for some time with both filters in place to see if anything shows up down the road.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: kcidwx on July 14, 2018, 10:43:26 AM
So my daughter was on my computer reading this thread and saw the complaints about the SHT-31 not going to 100%. It started raining here a little while ago and she wanted me to share that her weather station goes to 100%. She said maybe you guys should buy one like hers.  :lol:

Hers is a Weather Monitor II.  :lol:  She thinks it's pretty neat she's the only one around here that gets to 100% on WU.  :lol:

(https://farm2.staticflickr.com/1803/42679352794_6300e920b4_o.png)

(https://farm2.staticflickr.com/1827/42679370914_a7a1a0e393_o.png)

My Vaisala HMT337 a few feet away is showing 99.5%. Typically, hers is running about +3% over my HMT337.


Hahaha.  Tell her that they don’t make em’ like they used too. Shoot, that’s not bad considering how old the WM II is.

I picked it up on eBay a couple years ago new in the box with three extra temp/humidity sensors still sealed in the package for $125. I thought it would be a good starter weather station for her. I'm very impressed with it. She wants a VP2 6163 for Xmas because she wants (as she calls it) the "sun" sensor. I told her she needs to have a better reason than that. So I told her she needs to justify getting a VP2 in a technical paper presented to me on how the VP2 6163 will enhance her weather watching. She also needs to explain the pros and cons of switching from the WM II to the VP2 including outlining the sensor specs. She also needs to identify any common problems people are having with the VP2 and if there is a solution to those problems. If no solution is available, then how does she plan to live with that problem. I know the SHT-31 not going to 100% will drive her crazy  :lol: So she's been doing a lot of research on here. I gave her until October 1st to finish the paper. Then we'll see if she still wants a VP2. Yes, I expect a lot out of my 4th grader.  :lol:

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 14, 2018, 11:01:21 AM
Update on sf2 and Davis filter at same time. When i put new sensor in use on my main station i used the sf2 filter, the same has been in use on the old sensor for a year, i removed the sf2 from the old and put the Davis in place of it for comparison with the new sensor. Well the sf2 filter specs say the sensor performance is the same with or without the filter so i put the sf2 back on the old sensor with the Davis filter on also. I can not see any difference in performance of the old sensor readings with both filters in use. It looks like it could give added protection in high dust and humidity areas. I figured the manufacturer says the sf2 does not have any effect on the sensor readings that maybe it would be worth a try even though the bigger filter does add some lag time on the readings. It is hard to measure lag time using the Davis set up because of the high sampling rates of the temp and humidity readings. Since i started using the sf2 i have not had any humidity related problems like i did when using the big Davis filter. I will never run a sensor without the sf2 in place. I am going to let the test sensor run for some time with both filters in place to see if anything shows up down the road.

Might be on to something Jerry. Aspiration helps sensor go out fast so the dual filter makes sense.

Maybe doubling up on the Davis fiter material would also be a huge benefit. Instead of 1 have 2 or 3 of the filters inside the filter housing.  :idea: Much easier than gluing Sf2 on I would think and safer not knowing what toxic fumes could be produced with whatever glue or sealer you use. I'm not worried about a little lag time it's still going to be much faster than any passive setup.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 14, 2018, 11:21:27 AM
True about glue out gassing but the putty i used is several years old an came in a coil separated by paper to keep it from sticking to itself, so any gassing has long passed. I really like the fact that the sf2 has no effect on the sensor performance. The Davis filter is so large it takes up a lot of room in the sensor chamber an collects a lot of moisture on it so the humidity readings are slow to come down from high readings but once the chamber has dried out along with the filter then the filter becomes less of a problem.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 14, 2018, 11:33:43 AM
Would doubling the Davis filter increase the filtration? I really believe these sensors are losing calibration because of dust contamination. High humidity plays its role also but seems more likely dust is main issue with the aspiration.

I realize it may slow response time down some but for many here could be better option vs taking a chance with gaseous chemical damaging sensor.
Just thoughts  :idea:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 14, 2018, 12:11:05 PM
The only way to know is try it and see how much lag it might add. Here is a putty like mine that should work keeping in mind that it takes a teeny bit to mount the filter. https://www.truevalue.com/plumbers-putty?ctplacement=176385-43411605579&cid=gooshop  Should be enough to mount thousands of filters lol. My main concern was that in long periods of high humidity the sensor was getting wet with the Davis filter and causing the sensor to go bonkers with the readings until it dried out. With the sf2 and getting rid of the high velocity fan and going back to the Davis fan running on ac i have not had that problem anymore.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 14, 2018, 12:21:37 PM
With the sf2 and getting rid of the high velocity fan and going back to the Davis fan running on ac i have not had that problem anymore.

The Davis fan sucks, to be blunt. They break constantly and are way overpriced. You can always cut the velocity of a much more durable fan intended for the elements (the Davis fan is an indoor fan!) using a variable fan controller like Randy linked to upthread.

I wonder if anyone has considered drilling two tiny holes in the board and mounting the SF2 with the provided clips rather than cutting them off. Is there a way to avoid traces in the PCB?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: JudinNorman on July 14, 2018, 12:36:26 PM
After this latest fiasco with VP2 are any of you thinking about going to another maker? Or do you all have to much invested in Davis products to just abandon?

Seems like a lot of us got lured to Davis for its price. We humans don't like the cheapest option nor the expensive so we went in so called middle with Davis who promotes a professional station

I'm considering a Rain wise.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 14, 2018, 12:41:35 PM
I think RW humidity has same issue and the temperature has no aspiration. As much as we bitch Davis is still best product out there and they know it.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: kcidwx on July 14, 2018, 01:41:40 PM
Before I put eval board #3 outside and ruined it, I did another 100% humidity test in the chamber. Your best bet for hitting 100% is right out of the box on day one because it just goes downhill from there.

Temperatures are very consistent across all three but did go up some over time.

Chamber Conditions
Humidity: 100.0%
Temperature: 83.0°F


(https://farm1.staticflickr.com/918/28537058247_c762c7a500_b.jpg)


Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 14, 2018, 02:16:22 PM
Yesterday I hit 99% on my new sensor today max 98%...Tomorrow?
I did just pull it out and double filtered with the Davis filters.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 14, 2018, 03:43:32 PM
Before I put eval board #3 outside and ruined it, I did another 100% humidity test in the chamber. Your best bet for hitting 100% is right out of the box on day one because it just goes downhill from there.

Temperatures are very consistent across all three but did go up some over time.

Chamber Conditions
Humidity: 100.0%
Temperature: 83.0°F


(https://farm1.staticflickr.com/918/28537058247_c762c7a500_b.jpg)

Wait a minute. Are these drifting UP in temperature over time? Looks like they started with a cold bias and now have a warm bias. Sensirion says they should be within 0.36F so these sensors would be out of spec. That's assuming the chamber temperature is exactly accurate and not drifting itself.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 14, 2018, 04:50:17 PM
I saw that humidity and temperature are out of specs. 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 14, 2018, 05:47:14 PM
Really the readings are in spec, temp is +- .5f and humidity is typically +-2% but can run up higher to 3% or so.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 14, 2018, 05:55:24 PM
Really the readings are in spec, temp is +- .5f and humidity is typically +-2% but can run up higher to 3% or so.

No, Sensirion's specs are tighter than Davis'. He's testing OEM sensors, not sensors run through Davis electronics (which introduce their own uncertainties, apparently).

The key question here: is the chamber drifting upwards in temperature or the sensors? We need to know.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 14, 2018, 07:04:18 PM
Those specs are from the sht 31 data sheet not from Davis.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 14, 2018, 07:31:04 PM
Chamber I'm guessing would be dead on since its used for calibration. Looks like 31's ran about -.3 low new and +.5 high after two years.

But after further review I'm comparing a brand new sensor 1 day old against 26 month old and see no difference at night. The two sensors swing from  0-.3 delta T with neither sensor biased one direction so I'm thinking temperature is solid.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 14, 2018, 07:41:28 PM
Those specs are from the sht 31 data sheet not from Davis.

Correct. And because he's testing Sensirion equipment with no Davis connection, +/- 0.36F is the correct specification.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: kcidwx on July 14, 2018, 07:58:18 PM
The chamber is accurate. The problem is repeatability with the sensors. Repeatability means subjecting the sensor to the exact same conditions multiple times to see if it produces the same reading. If I were to do this test ten times with these, I'm thinking I would end up with a maximum repeatability window of 0.7°F and a standard window of 0.5°F. The average temperature between all tests would probably come out around 83.1°F.

I did the test one more time and got different but better results on the temperature side.

Chamber Conditions
Temperature: 83.0°F
Humidity: 100.0%

(https://farm1.staticflickr.com/925/28542558267_0f3febf986_b.jpg)

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 14, 2018, 08:11:10 PM
Whew. Yes, of course, repeatability. Forgot about that bugaboo.

Thanks for repeating the test!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: kcidwx on July 14, 2018, 09:05:33 PM
Hitting 100% on the SHT-31 is not really part of the spec. At 100% humidity it's considered in calibration at >96.5%. If you really want to see 100% when it's 100% humidity then you will have to get into the pro level sensors and they start around $400 each.

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Dj1225 on July 14, 2018, 11:45:30 PM
Yesterday I hit 99% on my new sensor today max 98%...Tomorrow?
I did just pull it out and double filtered with the Davis filters.


My new sensor maxed out at 97% First time I've seen 97% in a year and a half. The old sensor was barely making it over 94% I was reading in this thread about the sensor being shipped in bubble wrap and it causing issues to the sensor. Mine was wrapped up in bubble wrap, should that be a cause for concern?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: kcidwx on July 15, 2018, 12:12:00 AM
They do make ESD bubble wrap bags but because of what they're typically made out of, it usually violates the sensor handling/packaging requirements.

My 31's came in this.

(https://farm2.staticflickr.com/1808/28545591227_23045eeaf7_b.jpg)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Dj1225 on July 15, 2018, 02:16:34 AM
They do make ESD bubble wrap bags but because of what they're typically made out of, it usually violates the sensor handling/packaging requirements.

My 31's came in this.

(https://farm2.staticflickr.com/1808/28545591227_23045eeaf7_b.jpg)


Are you talking about the actual sensor from Sensirion, or the temp/hum sensor for the VP2?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 15, 2018, 07:39:33 AM
Hitting 100% on the SHT-31 is not really part of the spec. At 100% humidity it's considered in calibration at >96.5%. If you really want to see 100% when it's 100% humidity then you will have to get into the pro level sensors and they start around $400 each.

Hmm... That hurts my feelings. Lol

The one I have at my friend’s farm has reported 100% for the second time in a row. This is after he received 2” of rain yesterday. My main station with FARS is currently reporting 97%
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: kcidwx on July 15, 2018, 09:16:15 AM
They do make ESD bubble wrap bags but because of what they're typically made out of, it usually violates the sensor handling/packaging requirements.

My 31's came in this.



Are you talking about the actual sensor from Sensirion, or the temp/hum sensor for the VP2?

My sensors came PCB mounted from Sensirion. The VP2 sensor should be shipped in similar qualifying packaging according to Sensirion's guidelines.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: kcidwx on July 15, 2018, 11:03:56 AM
Hitting 100% on the SHT-31 is not really part of the spec. At 100% humidity it's considered in calibration at >96.5%. If you really want to see 100% when it's 100% humidity then you will have to get into the pro level sensors and they start around $400 each.

Hmm... That hurts my feelings. Lol

The one I have at my friend’s farm has reported 100% for the second time in a row. This is after he received 2” of rain yesterday. My main station with FARS is currently reporting 97%

Sure. You will find some of the SHT31's will go to 100% humidity, however most of them will not. That doesn't mean there is something wrong with the sensors that don't reach 100%. Of course they should reach at least 96.5% to be in spec. What you're seeing with the different maximum values is normal behavior from a budget sensor. You are pushing the operational limits of the sensor once you get above 95% and you will get varying results. That's why they relax the spec above 90%. If you want an SHT31 that goes to 100% then you have to play the lottery. Buy a pack of 10 sensors, test and hope you win by getting one sensor that will reach 100%. Actually, I've seen really picky guys do this with budget sensors. They will buy ten, test them all at 100% and keep the one that shows the highest humidity and send the rest back.  :lol:

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: DaleReid on July 15, 2018, 11:22:46 AM
Weather Display, for example, has a tweak that says IF the humidity is 95% or greater, display as 100%

Clearly this has been a situation that has been around for awhile.

I have read this discussion and yet still am wondering what the technology is that the high priced, more-likely-to-read-100%, type of sensors use?  I assume we're talking RM Young, Vaisala, Campbell Scientific and perhaps a few others. 

Is it an entirely different technology?  do they actually measure dew point (doubt it from looking at most of the sensors on-line) and then do a conversion?

Perhaps we're all arguing how many angels can dance on the head of a pin.  It just "feels" uncomfortable when the humidity is that high.  I wonder what my built in sensor is that tells me that, other than sweating in Florida this time of year? 

I know the local talking heads on the weather portion of the news always talk about 'how it will feel' especially with the heat waves and incredible tropical humidity we have been having in the MidWest.  How does my sense of oppressive environment get sensed by my body?  And the wonderful feeling of low humidity and clear air and you can actually breath more easily after a front goes through?  Or the 'fall like' feeling for those of us up north here in October.  I even remember  doing a ride at Disney where they were showing how energy came to be with a trip through dino land with noises, calling birds, sort of a wet smell piped over the little cars carrying us along, but also a high humidity mist being sprayed along side to keep the feel of being in the tropics along that whole segment.

I'm just wondering about sensing things, just with our bodies, as a side question about experiencing humidity.

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: kcidwx on July 15, 2018, 12:12:21 PM

I have read this discussion and yet still am wondering what the technology is that the high priced, more-likely-to-read-100%, type of sensors use?  I assume we're talking RM Young, Vaisala, Campbell Scientific and perhaps a few others. 

Is it an entirely different technology?  do they actually measure dew point (doubt it from looking at most of the sensors on-line) and then do a conversion?


It's the same technology just different manufacturing process with tighter tolerances. Of course the tighter the manufacturing tolerance the higher the manufacturing cost but you will get a better spec'ed sensor and better performance above 90%. Apples to apples, the SHT31 chip itself runs about $5. My Vaisala Humicap chip runs $80. SHT31 spec is 2%, Vaisala Humicap is 1%. An engineer once told me that once you get down around 2% spec, it costs a lot more to shave off another 0.5% in manufacturing.

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 15, 2018, 12:19:31 PM
I ran across this cheap indoor sensor that i just had to order, it is spec out as an sht31 sensor. The sensor being used is the sht 3x on the picture but says .5 +/- temp and 2% humidity. It would make a nice cheap unit to use for testing purposes. Here is the link https://www.amazon.com/gp/product/B01HDW58GS/ref=od_aui_detailpages00?ie=UTF8&psc=1
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 15, 2018, 01:15:47 PM
It's due here Tuesday and i will find out keep in mind the sensor determines the accuracy and it is  an sht sensor so i got my fingers crossed.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dalecoy on July 15, 2018, 01:35:07 PM
Measures indoor temperature and humidity from -4 to 158°F (-20 to 70°C) and 1-99% RH

Professional accuracy: +/- 0.5°F, +/- 2% RH with manual calibration option
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 15, 2018, 02:01:05 PM
I ran across this cheap indoor sensor that i just had to order, it is spec out as an sht31 sensor. The sensor being used is the sht 3x on the picture but says .5 +/- temp and 2% humidity. It would make a nice cheap unit to use for testing purposes. Here is the link https://www.amazon.com/gp/product/B01HDW58GS/ref=od_aui_detailpages00?ie=UTF8&psc=1

If it's the same as our 31's nice little unit. Easy to read.

Weather Display, for example, has a tweak that says IF the humidity is 95% or greater, display as 100%



Cumulus has a similar but it's 98% I didn't have SHT31 that would make 98% until this new one and yes I have 98% = 100% on Cumulus for website. I can watch actual humidity on my Envoy8x display or console.


For anyone wondering about using double filters I started and don't see noticeable response time differences, accuracy was unchanged comparing with another sht31 side by side so recommend on new sensors to double up filter on new units. I just washed old and installed with the new. This at least doubles the filtration I'm thinking.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: kcidwx on July 15, 2018, 02:03:21 PM
I ordered this one since it reads out in tenths. I just need that extra resolution for my satisfaction.  :lol: These will be great for monitoring humidity in garage, basement etc.

https://www.amazon.com/gp/product/B000XTJRRA (https://www.amazon.com/gp/product/B000XTJRRA)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 15, 2018, 02:25:49 PM
I ordered this one since it reads out in tenths. I just need that extra resolution for my satisfaction.  :lol: These will be great for monitoring humidity in garage, basement etc.

https://www.amazon.com/gp/product/B000XTJRRA (https://www.amazon.com/gp/product/B000XTJRRA)

X2  :-)
Title: Re: SHT31
Post by: WheatonRon on July 15, 2018, 02:45:57 PM
SHT 31 drifts with age! I have two VP2 stations both have the SHT31 sensor--one station is about 2 months old and it has a 24 hour fan (CWOP FW3075) and the other station is about 2 years old and has a daytime fan (CWOP CW5020). Right now, in a western suburb of Chicago (where I live), the new station reports 91, humidity of 44 and dewpoint of 66 whereas the older station reports 91, humidity of 50 and dewpoint of 70. The stations are about 10 feet apart on the same fence and the sun is making both fans rumble today! The newer station always gets lower humidity and dewpoint readings as demonstrated by the attached QC graphs from CWOP.  I am not a fan (pun intended) of the QC checks on CWOP but this does raise some eyebrows!

My post above, a few days old, is no longer accurate. My new VP2 with the 24 hour fan is now showing higher humidity than my older 2 year old VP2 with a daytime fan. In short, the accuracy of the SHT31 is any person’s guess. One day it looks correct, the next day, not so correct. If I could be assured I could get a replacement SHT31 (properly packaged) from Davis for my new VP2 with the 24 hour fan, I would do that but I guess I will accept the inaccuracies in my humidity readings (all three VP2s have the SHT31 and report consistent temperatures) knowing at least my temperatures are solid. What a shame—we spend good money on what we believe is “semi-professional” weather gear only to get something less.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on July 15, 2018, 02:55:06 PM
I ordered this one since it reads out in tenths. I just need that extra resolution for my satisfaction.  :lol: These will be great for monitoring humidity in garage, basement etc.

https://www.amazon.com/gp/product/B000XTJRRA (https://www.amazon.com/gp/product/B000XTJRRA)

X2  :-)

X3  👌
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 15, 2018, 04:00:18 PM
Hitting 100% on the SHT-31 is not really part of the spec. At 100% humidity it's considered in calibration at >96.5%. If you really want to see 100% when it's 100% humidity then you will have to get into the pro level sensors and they start around $400 each.

Hmm... That hurts my feelings. Lol

The one I have at my friend’s farm has reported 100% for the second time in a row. This is after he received 2” of rain yesterday. My main station with FARS is currently reporting 97%
If you want an SHT31 that goes to 100% then you have to play the lottery. Buy a pack of 10 sensors, test and hope you win by getting one sensor that will reach 100%.
That's fine and dandy if you're just buying the sensor, but then you've got to put it on the PCB. Don't know about you guys, but mounting that tiny, tiny sensor is waaay above my pay grade. And buying 10 from Ryan ready to go, as much as he'd like it, I ain't shelling out that kind of money for a crap shoot.
Title: Re: Which sling psychrometer should I buy?
Post by: CW2274 on July 15, 2018, 06:27:30 PM
Well I've decided to pull the ISS and see what I see. The monsoon is in town so I'm not sure when I'll get out to it, I would MUCH prefer cloud cover, possibly tomorrow. At least we can see if the desert hypothesis holds any water with a two year old 31 vs five months.
Think of me when I'm pulling cacti hypo's outta my legs in the interest of science. :-({|=
Weather cooperated with overcast skies so I pulled the ISS and did my comparisons, which are at ideal conditions now. Simply put, I found absolutely no noticeable difference between the 31 that has 2 years of service vs the 5 month old one. Temp, dew, humidity were basically exactly the same. Now, one could say they've drifted the exact same amount, perhaps.
This certainly lends credence to the "dry climate syndrome" making the sensors last longer, but I think hurts the notion that aspiration degrades the sensor considering the very dusty climate here, and that one 31 has almost 500% more service time than the other.
I"m sure not the most scientific method, but I got some results, take it for what it's worth.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 15, 2018, 07:20:37 PM
Issue isn't linear at all.  Everything is hunky dory just a nice typical summer day mid 88 and DP 67° on average with heat index only 91° around here. 
It's always near high heat index advisor levels above 100° the issue arises for me.

Once temps get into 90's and dewpoint 70-80.
 
Example Actual T93°,  DP 75, Hum 56% ,Heat Index 103°

SHT31 will show T93°, DP 78, Hum 62% Heat index 109°

That's the issue.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 15, 2018, 07:46:02 PM
Issue isn't linear at all.
That's the issue.
I get that. I supplied info on what's going on with my conditions now, and regarding issues, baseless or not, that have been brought up with this sensor, simple as that. Not saying there isn't a problem, merely sharing what I observed. My ISS will be accessible til I decide to put it back out to pasture, and I'll continue testing, if nothing else but for myself. If I find something that may be relevant, I'll share.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Dj1225 on July 15, 2018, 07:55:10 PM
Issue isn't linear at all.  Everything is hunky dory just a nice typical summer day mid 88 and DP 67° on average with heat index only 91° around here. 
It's always near high heat index advisor levels above 100° the issue arises for me.

Once temps get into 90's and dewpoint 70-80.
 
Example Actual T93°,  DP 75, Hum 56% ,Heat Index 103°

SHT31 will show T93°, DP 78, Hum 62% Heat index 109°

That's the issue.

Yes! That was the issue I was having and like I said in an earlier post the humidity down on the Gulf Coast is unbearable but when you start noticing the heat index becoming outrageous and laughable (close to 120°) when around the area it's between 100°-108° I knew that something wasn't right, then I came across this topic. My new sensor I received Friday showed better results than my 2+ year old sensor, instead of mid 60s-low 70s humidity percentage while in the low to mid 90s it has been showing mid 50s to low 60s percent range. As far as the 100% issue in 2 days it's topped out at 97% which I hadn't seen with the old sensor in almost a year. It's just very frustrating.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 15, 2018, 08:09:18 PM
Issue isn't linear at all.  Everything is hunky dory just a nice typical summer day mid 88 and DP 67° on average with heat index only 91° around here. 
It's always near high heat index advisor levels above 100° the issue arises for me.

Once temps get into 90's and dewpoint 70-80.
 
Example Actual T93°,  DP 75, Hum 56% ,Heat Index 103°

SHT31 will show T93°, DP 78, Hum 62% Heat index 109°

That's the issue.

Yes! That was the issue I was having and like I said in an earlier post the humidity down on the Gulf Coast is unbearable but when you start noticing the heat index becoming outrageous and laughable (close to 120°) when around the area it's between 100°-108° I knew that something wasn't right, then I came across this topic. My new sensor I received Friday showed better results than my 2+ year old sensor, instead of mid 60s-low 70s humidity percentage while in the low to mid 90s it has been showing mid 50s to low 60s percent range. As far as the 100% issue in 2 days it's topped out at 97% which I hadn't seen with the old sensor in almost a year. It's just very frustrating.

Glad you found the thread,
Now you know you aren't alone.  :sad:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 15, 2018, 08:15:30 PM
I get that. I supplied info on what's going on with my conditions now, and regarding issues, baseless or not, that have been brought up with this sensor, simple as that. Not saying there isn't a problem, merely sharing what I observed. My ISS will be accessible til I decide to put it back out to pasture, and I'll continue testing, if nothing else but for myself. If I find something that may be relevant, I'll share.

Thanks, yes please do.
Keep those sensors even here they work good enough maybe average 2-3% high humidity winter but come summer and high dew points, watchout...
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 15, 2018, 08:34:34 PM
I get that. I supplied info on what's going on with my conditions now, and regarding issues, baseless or not, that have been brought up with this sensor, simple as that. Not saying there isn't a problem, merely sharing what I observed. My ISS will be accessible til I decide to put it back out to pasture, and I'll continue testing, if nothing else but for myself. If I find something that may be relevant, I'll share.
but come summer and high dew points, watchout...
Well, considering the monsoon is here..... As I look at this very moment comparing my dews with the "local gang" there's two 64F's and two 66F's. BTW, I'm using the 2 year old sensor at the moment. ;)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 15, 2018, 09:34:14 PM
Issue isn't linear at all.  Everything is hunky dory just a nice typical summer day mid 88 and DP 67° on average with heat index only 91° around here. 
It's always near high heat index advisor levels above 100° the issue arises for me.

Once temps get into 90's and dewpoint 70-80.
 
Example Actual T93°,  DP 75, Hum 56% ,Heat Index 103°

SHT31 will show T93°, DP 78, Hum 62% Heat index 109°

That's the issue.


I deal with the same issues and it’s frustrating!!!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 15, 2018, 10:57:00 PM
Seems like jerryg sensor with the sf2 filter cap is performing well and the DPs appear to aligns with the Airport reports
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 16, 2018, 07:04:17 AM
I'm looking at ASOS stations and they are doing the same. Skipping over 98 and 99% and jumping to 100%. Looks like they have same option using software like Cumulus where if sensor hits 98 jumps to 100%.

Omaha sensor can't make 98% like many SHT31's peaking at 97% with dense fog and mist.

Some images OMAHA,
North platte and Valentine




Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 16, 2018, 07:28:38 AM
I noticed the same thing with the Airport that’s 15 milesnorth of me. Think I only seen 1 Airport station ever report 99% humidity.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 16, 2018, 07:40:06 AM
I noticed the same thing with the Airport that’s 15 milesnorth of me. Think I only seen 1 Airport station ever report 99% humidity.

Even the vaisala. I realize these are 5 minute averages but still, no 98 and 99 very suspect. I went back and downloaded 60 days of KVTN data and nothing.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 16, 2018, 08:06:32 AM
I noticed the same thing with the Airport that’s 15 milesnorth of me. Think I only seen 1 Airport station ever report 99% humidity.

Even the vaisala. I realize these are 5 minute averages but still, no 98 and 99 very suspect. I went back and downloaded 60 days of KVTN data and nothing.

I think the accuracy of it in the 90 percentile is around +/-1.7%. SHT-31 is +/-2%
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 16, 2018, 08:08:08 AM
My SHT-31 at my friend’s place has reached 100% for the third time in a row. It’s basically a new sensor but it does have the Davis filter cap.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Intheswamp on July 16, 2018, 09:48:00 AM
Well, not to be one to rush things... 

I bought a new Sensiron SHT31 sensor from Ryan back in April of 2016.  It's been enjoying a dark, cool existence in a closet ever since. :roll:

I'm getting ready to replace the old sensor (and the rain gauge) and have been following this thread.  I've seen my heat indexes go out of sight for a while now, just not sure how long it's really been doing that.  My records only go back a year and supposedly the latest readings are the highest so maybe(?) this started since last year(?).  It seems I don't really remember the super-high heat index numbers (120'ish) until this year.

Anyhow, it will be interesting to see how things compare with the new(er) sensor installed...when I get it installed.  #-o

A question here (yeah, I need someone to hold my hand)...  What is the "Davis filter cap"???

Thanks!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on July 16, 2018, 09:57:04 AM
The “white house” (filter cap) covering the sensor on the PC Board—see photo attached.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: hwcorder on July 16, 2018, 10:01:14 AM
Hi guys. Don't post here a lot but have been following this thread closely the last day or two since I have been a loyal Davis customer since 1993 when I got a WMII. Still miss that thing sometimes!

I am also a Contract Weather Observer at KCLT so I have a good bit of experience with ASOS. The reason you do not see 98 or 99 percent readings with ASOS is that it directly measures the DP then calculates RH values using ambient temp where Davis directly measures RH then calculates DP.  For example if ASOS measures 70/69 it calculates a RH of 97%. IF the DP moves up to 70 then of course the calculated RH is 100%. You've got to take the differences in the method of measurement into account when comparing your readings to airports.  As for the sensors reading up to 100% even the expensive ones like the HMP155 can have issues. Back a few years ago our ASOS would rarely hit 100% even in fog with visibilities of 1/4mi. Then they changed the sensor and it hits 100% with relative ease.  I used to obsess about getting to 100% too but don't anymore especially when I can adjust the higher readings using Weather Display. 

As for the SHT31 is was initially excited about its improved accuracy but quickly learned it suffered from the same amount if not more drift towards a wet bias as its predecessors.  I think its more of a moisture problem than a dust problem.  I bought several sensors and when I first installed them I would track  about 1-2 degrees higher than surrounding ASOS stations two if which are within 5-7 miles of me (KCLT, KAKH). I do have a lot of vegetation around my house so I take that into account when comparing DP readings to other places. I also have a Viasala HM46 probe I do spot checks with. RH values would be generally the same < 3% difference.  However I would notice after the first few very humid spells where we would have a day or two of RH values remain 90% or above the sensor would never quite come back to its reading prior to that. Differences in RH between the SHT31 and the Vaisala would be anywhere from 4-8% with the SHT31 higher.  I would buy another sensor and the same thing. 

Doing a little research I learned this is really a technological problem.  Electronics and weather specifically moisture do not mix. Therefore when using electronics to measure moisture there are a lot of problems to overcome to get accurate readings over the long term of say more than two years.  The DTS1 ASOS uses to measure DP comes with features to increase sensor longevity.  Its heated in high RH environments to protect it from condensation.  It also undergoes a high heat cycle I believe once a day to cleanse any chemical vapors that it may encounter. Even so, as has been mentioned prior in this thread the NWS replaces this very expensive sensor every 18 months.  I have recently come across a memo that NWS is looking to upgrade a few sensors for ASOS and the dew point sensor is on that list.

Anyway, bottom line is that if you really want to keep accurate readings in humid climates you need to replace these sensors every year to 18months.  I actually went back to the sht75 as they are very easy to switch out and actually seem to drift slightly less than the board mounted sensors. I wired in a female connector then mounted it through a slot I cut into the sensor plate in the FARS unit. Now when I change out the sensors I simply pull out the old one and replace it with the new about every 18 months.  I still use the Davis filter and replace that every year.  Senserion is phasing out the SHT75 however so I think I'm going to use Jerryg's idea and buy a 31 and replace the Davis filter with the SF2 using plumbers putty to see how that works out.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 16, 2018, 10:05:33 AM
Well, not to be one to rush things... 

I bought a new Sensiron SHT31 sensor from Ryan back in April of 2016.  It's been enjoying a dark, cool existence in a closet ever since. :roll:

I'm getting ready to replace the old sensor (and the rain gauge) and have been following this thread.  I've seen my heat indexes go out of sight for a while now, just not sure how long it's really been doing that.  My records only go back a year and supposedly the latest readings are the highest so maybe(?) this started since last year(?).  It seems I don't really remember the super-high heat index numbers (120'ish) until this year.

Anyhow, it will be interesting to see how things compare with the new(er) sensor installed...when I get it installed.  #-o

A question here (yeah, I need someone to hold my hand)...  What is the "Davis filter cap"???

Thanks!

It's the stock filter Davis uses. The thinking is because of aspiration the sensor gets dirty and makes sensor less accurate. Sensirion has its own filter with better filtration but isn't designed for Davis sensor board.

What I've done is double up on stock Davis filter material and looks good in response time, no difference I see. All new sensors come with filter so just added old filter after washing along with new filter. This doubles filtration and hoping sensor will last longer.
I'll probably change sensor each spring anyway as long as price doesn't take off. Don't think it will the actual sensor is $4 and like electronics may actually trend down in price.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 16, 2018, 10:12:41 AM
Hi guys. Don't post here a lot but have been following this thread closely the last day or two since I have been a loyal Davis customer since 1993 when I got a WMII. Still miss that thing sometimes!

I am also a Contract Weather Observer at KCLT so I have a good bit of experience with ASOS. The reason you do not see 98 or 99 percent readings with ASOS is that it directly measures the DP then calculates RH values using ambient temp where Davis directly measures RH then calculates DP.  For example if ASOS measures 70/69 it calculates a RH of 97%. IF the DP moves up to 70 then of course the calculated RH is 100%. You've got to take the differences in the method of measurement into account when comparing your readings to airports.  As for the sensors reading up to 100% even the expensive ones like the HMP155 can have issues. Back a few years ago our ASOS would rarely hit 100% even in fog with visibilities of 1/4mi. Then they changed the sensor and it hits 100% with relative ease.  I used to obsess about getting to 100% too but don't anymore especially when I can adjust the higher readings using Weather Display. 

As for the SHT31 is was initially excited about its improved accuracy but quickly learned it suffered from the same amount if not more drift towards a wet bias as its predecessors.  I think its more of a moisture problem than a dust problem.  I bought several sensors and when I first installed them I would track  about 1-2 degrees higher than surrounding ASOS stations two if which are within 5-7 miles of me (KCLT, KAKH). I do have a lot of vegetation around my house so I take that into account when comparing DP readings to other places. I also have a Viasala HM46 probe I do spot checks with. RH values would be generally the same < 3% difference.  However I would notice after the first few very humid spells where we would have a day or two of RH values remain 90% or above the sensor would never quite come back to its reading prior to that. Differences in RH between the SHT31 and the Vaisala would be anywhere from 4-8% with the SHT31 higher.  I would buy another sensor and the same thing. 

Doing a little research I learned this is really a technological problem.  Electronics and weather specifically moisture do not mix. Therefore when using electronics to measure moisture there are a lot of problems to overcome to get accurate readings over the long term of say more than two years.  The DTS1 ASOS uses to measure DP comes with features to increase sensor longevity.  Its heated in high RH environments to protect it from condensation.  It also undergoes a high heat cycle I believe once a day to cleanse any chemical vapors that it may encounter. Even so, as has been mentioned prior in this thread the NWS replaces this very expensive sensor every 18 months.  I have recently come across a memo that NWS is looking to upgrade a few sensors for ASOS and the dew point sensor is on that list.

Anyway, bottom line is that if you really want to keep accurate readings in humid climates you need to replace these sensors every year to 18months.  I actually went back to the sht75 as they are very easy to switch out and actually seem to drift slightly less than the board mounted sensors. I wired in a female connector then mounted it through a slot I cut into the sensor plate in the FARS unit. Now when I change out the sensors I simply pull out the old one and replace it with the new about every 18 months.  I still use the Davis filter and replace that every year.  Senserion is phasing out the SHT75 however so I think I'm going to use Jerryg's idea and buy a 31 and replace the Davis filter with the SF2 using plumbers putty to see how that works out.

Thanks for this information very good.
You might consider just doubling the Davis filter to avoid any chemical fumes that can damage sensor. SF2  filters are very small so whatever putty or glue used will surely get into sensor while drying.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: kcidwx on July 16, 2018, 01:37:29 PM
Daughters WMII pegged out all night long at 100% humidity in the fog. I really like this weather station. Too bad you can't get parts for it. I'd really like to know what brand the humidity sensor is. It tracks pretty well with my Vaisala HMT337. Either that or she just got lucky with this one.

(https://farm2.staticflickr.com/1802/42734326814_a7153b661d_o.png)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 16, 2018, 01:53:15 PM
You might consider just doubling the Davis filter to avoid any chemical fumes that can damage sensor. SF2  filters are very small so whatever putty or glue used will surely get into sensor while drying.

I don't really think doubling up that filter material will keep water off the sensor. That'd be like installing overlapping bug screens with 1-inch holes hoping that will keep the skeeters out. We're talking about tiny coagulates of water vapor that form when the air is near or at saturation.

I will investigate the possibility of mounting it with the clips when my caps arrive by drilling two very small holes in the board. That would be the ideal solution, but probably not possible. If not, then jerryg's solution might be the only one that's been field tested to work.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 16, 2018, 01:53:41 PM
Hi guys. Don't post here a lot but have been following this thread closely the last day or two since I have been a loyal Davis customer since 1993 when I got a WMII. Still miss that thing sometimes!

I am also a Contract Weather Observer at KCLT so I have a good bit of experience with ASOS. The reason you do not see 98 or 99 percent readings with ASOS is that it directly measures the DP then calculates RH values using ambient temp where Davis directly measures RH then calculates DP.  For example if ASOS measures 70/69 it calculates a RH of 97%. IF the DP moves up to 70 then of course the calculated RH is 100%. You've got to take the differences in the method of measurement into account when comparing your readings to airports.  As for the sensors reading up to 100% even the expensive ones like the HMP155 can have issues. Back a few years ago our ASOS would rarely hit 100% even in fog with visibilities of 1/4mi. Then they changed the sensor and it hits 100% with relative ease.  I used to obsess about getting to 100% too but don't anymore especially when I can adjust the higher readings using Weather Display. 

As for the SHT31 is was initially excited about its improved accuracy but quickly learned it suffered from the same amount if not more drift towards a wet bias as its predecessors.  I think its more of a moisture problem than a dust problem.  I bought several sensors and when I first installed them I would track  about 1-2 degrees higher than surrounding ASOS stations two if which are within 5-7 miles of me (KCLT, KAKH). I do have a lot of vegetation around my house so I take that into account when comparing DP readings to other places. I also have a Viasala HM46 probe I do spot checks with. RH values would be generally the same < 3% difference.  However I would notice after the first few very humid spells where we would have a day or two of RH values remain 90% or above the sensor would never quite come back to its reading prior to that. Differences in RH between the SHT31 and the Vaisala would be anywhere from 4-8% with the SHT31 higher.  I would buy another sensor and the same thing. 

Doing a little research I learned this is really a technological problem.  Electronics and weather specifically moisture do not mix. Therefore when using electronics to measure moisture there are a lot of problems to overcome to get accurate readings over the long term of say more than two years.  The DTS1 ASOS uses to measure DP comes with features to increase sensor longevity.  Its heated in high RH environments to protect it from condensation.  It also undergoes a high heat cycle I believe once a day to cleanse any chemical vapors that it may encounter. Even so, as has been mentioned prior in this thread the NWS replaces this very expensive sensor every 18 months.  I have recently come across a memo that NWS is looking to upgrade a few sensors for ASOS and the dew point sensor is on that list.

Anyway, bottom line is that if you really want to keep accurate readings in humid climates you need to replace these sensors every year to 18months.  I actually went back to the sht75 as they are very easy to switch out and actually seem to drift slightly less than the board mounted sensors. I wired in a female connector then mounted it through a slot I cut into the sensor plate in the FARS unit. Now when I change out the sensors I simply pull out the old one and replace it with the new about every 18 months.  I still use the Davis filter and replace that every year.  Senserion is phasing out the SHT75 however so I think I'm going to use Jerryg's idea and buy a 31 and replace the Davis filter with the SF2 using plumbers putty to see how that works out.

Do you know what sensors the NWS will upgrade too?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 16, 2018, 02:44:52 PM
You might consider just doubling the Davis filter to avoid any chemical fumes that can damage sensor. SF2  filters are very small so whatever putty or glue used will surely get into sensor while drying.

I don't really think doubling up that filter material will keep water off the sensor. That'd be like installing overlapping bug screens with 1-inch holes hoping that will keep the skeeters out. We're talking about tiny coagulates of water vapor that form when the air is near or at saturation.

I will investigate the possibility of mounting it with the clips when my caps arrive by drilling two very small holes in the board. That would be the ideal solution, but probably not possible. If not, then jerryg's solution might be the only one that's been field tested to work.

You shouldn't be getting water on sensor anyway, jerryg was because he was sucking water in with overpowered fan. Not sure if he mentioned that on this thread.

My fars is slowed down so much I need to be within 3 feet to hear with the speed controller. Still faster than solar but not by much. I no longer seal around top of AC fan reducing air intake even more. Most of this was done to reduce the humidity swings. 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 16, 2018, 03:06:18 PM
You shouldn't be getting water on sensor anyway...

No, I'm not talking about sucking rain into the shield. I'm talking about some combination of very high humidity events that seem to knock the sensor out of spec. That moist air WILL enter the shield either passively or actively. Air nearing saturation becomes like a cloud where tiny water vapor droplets collide, combine and grow in size, even inside the cap. It's got nowhere to go. The SF2 rejects anything over a certain size from getting to the sensor and building up on it. Doubled up, oversized screens aren't going to prevent that situation.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: kcidwx on July 16, 2018, 03:09:51 PM
You shouldn't be getting water on sensor anyway...

No, I'm not talking about sucking rain into the shield. I'm talking about some combination of very high humidity events that seem to knock the sensor out of spec. That moist air WILL enter the shield either passively or actively. Air nearing saturation becomes like a cloud where tiny water vapor droplets collide, combine and grow in size, even inside the cap. It's got nowhere to go. The SF2 rejects anything over a certain size from getting to the sensor and building up on it. Doubled up, oversized screens aren't going to prevent that situation.

Correct! The only solution I'm aware of for that is the heated humidity sensor. I'm not aware of a filter that will prevent that or how it even could.

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 16, 2018, 03:15:46 PM
I think a problem we have with any kind of solution is that the temp/humidity is in one unit so no way to heat humidity without raising temp. I wish the sensors were separate so the temp could get max airflow and the sensor would be located away from the air flow. Oh well i guess it is more cost effective for Davis and the others to have it that way, cost wise that is.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 16, 2018, 04:10:16 PM
I think its more of a moisture problem than a dust problem.
Concur.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 16, 2018, 04:23:54 PM
Well, not to be one to rush things... 

I bought a new Sensiron SHT31 sensor from Ryan back in April of 2016.  It's been enjoying a dark, cool existence in a closet ever since. :roll:

I'm getting ready to replace the old sensor (and the rain gauge) and have been following this thread.  I've seen my heat indexes go out of sight for a while now, just not sure how long it's really been doing that.  My records only go back a year and supposedly the latest readings are the highest so maybe(?) this started since last year(?).  It seems I don't really remember the super-high heat index numbers (120'ish) until this year.

Anyhow, it will be interesting to see how things compare with the new(er) sensor installed...when I get it installed.  #-o

A question here (yeah, I need someone to hold my hand)...  What is the "Davis filter cap"???

Thanks!

It's the stock filter Davis uses. The thinking is because of aspiration the sensor gets dirty and makes sensor less accurate.
If that were the case my strongly aspirated ISS and dirty two year old 31 would not have hit and stayed at both 99% and 1%. I'm sure the filter is there to help protect from dirt/dust, but it's becoming much more evident moisture is the culprit in sensor degradation, not aspiration. As a matter of fact, I'm using the two year old 31 now and it's just as good as the newer one.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 16, 2018, 04:37:35 PM
Well, not to be one to rush things... 

I bought a new Sensiron SHT31 sensor from Ryan back in April of 2016.  It's been enjoying a dark, cool existence in a closet ever since. :roll:

I'm getting ready to replace the old sensor (and the rain gauge) and have been following this thread.  I've seen my heat indexes go out of sight for a while now, just not sure how long it's really been doing that.  My records only go back a year and supposedly the latest readings are the highest so maybe(?) this started since last year(?).  It seems I don't really remember the super-high heat index numbers (120'ish) until this year.

Anyhow, it will be interesting to see how things compare with the new(er) sensor installed...when I get it installed.  #-o

A question here (yeah, I need someone to hold my hand)...  What is the "Davis filter cap"???

Thanks!

It's the stock filter Davis uses. The thinking is because of aspiration the sensor gets dirty and makes sensor less accurate.
If that were the case my strongly aspirated ISS and dirty two year old 31 would not have hit and stayed at both 99% and 1%. I'm sure the filter is there to help protect from dirt/dust, but it's becoming much more evident moisture is the culprit in sensor degradation, not aspiration. As a matter of fact, I'm using the two year old 31 now and it's just as good as the newer one.

I never said moisture wasn't the issue just the opposite but its been brought up aspiration may also be an issue because normally humidity sensor isn't aspirated, but if you can still hit 99% after two years that can't be a problem, I agree. The amount of moisture we are talking about however can't be stopped with any filter.  If it did the sensor wouldn't work.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: hwcorder on July 16, 2018, 04:48:48 PM
Daughters WMII pegged out all night long at 100% humidity in the fog. I really like this weather station. Too bad you can't get parts for it. I'd really like to know what brand the humidity sensor is. It tracks pretty well with my Vaisala HMT337. Either that or she just got lucky with this one.

I loved mine too. The humidity sensor itself is of similar design to the Vaisala but have no idea of manufacture. If you open up the guts of the Temp/Hum sensor you will see two small boxes with tiny adjustment screws one marked low the other hi. You could actually perform multipoint humidity calibration using these if you have an accurate field instrument.

Do you know what sensors the NWS will upgrade too?

No I don't. This is in the early stages so I would would imagine the NWS is accepting bids. Wouldn't think they would go back to the  chilled mirror technology, perhaps another brand of capacitance sensor.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: kcidwx on July 16, 2018, 04:58:10 PM
Humidity sensor aspiration has been shown to accelerate sensor aging. Is this guaranteed to happen 100% of the time? No! However, you have to consider it when troubleshooting your issue. If you have eliminated aspiration as the cause of your sensor problem, then move on to the next thing.

I think you are probably dealing with a moisture problem unrelated to aspiration. As Sensirion even states in their documentation, the sensor will show a wet bias after a high humidity event. However, they also say the sensor should slowly recover from it over time. I don't think the recovery is happening as it should.

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 16, 2018, 05:18:45 PM
Humidity sensor aspiration has been shown to accelerate sensor aging. Is this guaranteed to happen 100% of the time? No! However, you have to consider it when troubleshooting your issue. If you have eliminated aspiration as the cause of your sensor problem, then move on to the next thing.

I think you are probably dealing with a moisture problem unrelated to aspiration. As Sensirion even states in their documentation, the sensor will show a wet bias after a high humidity event. However, they also say the sensor should slowly recover from it over time. I don't think the recovery is happening as it should.
Not doubting this. However, considering my ISS setup and climate and apparently still good 2 year old sensor, the fan and dirt ain't the problem (at least for me), being in a wet climate is. That being said, I think it more likely a fan in a wet climate would exacerbate the aging process.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 16, 2018, 05:25:49 PM
I might have been wrong with the RW MK III stations. There is one in Victoria, Texas I’m tracking and it seems to line up well with the Airport

As of 4:21p CDT

Victoria Emergency Operations Center 91/72

Victoria Regional 91/70 at 4:15p

Victoria (jerryg) 91/76
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 16, 2018, 05:28:26 PM
The RW MK III specs are +/-2% from 10-90% and +/-4% from 90-100% if my memory serves me correctly.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 16, 2018, 05:29:26 PM
You shouldn't be getting water on sensor anyway...
The SF2 rejects anything over a certain size from getting to the sensor and building up on it.
Mine just came in the mail. Now the fun part..... #-o
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 16, 2018, 05:32:02 PM
Ok lets put this in locations, the station at at the 911 center is on the roof of a brick building surrounded by a parking lot and road, the metar at the air port is located off a runway and they put gravel around the site so as not to get stuck, heck with the good readings and mine is located over grass and ground that just got through with 15 inches of rain in the last 4 weeks.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 16, 2018, 05:35:34 PM
Just for info i just put out a sht31 in a passive shield to do some comparisons and right now with the wind running around 6 to 12 mph the temp shows about .7f high, not bad for 93 degree heat lol. Should be interesting to see how things shake out.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 16, 2018, 05:55:58 PM


Victoria Emergency Operations Center 91/72

Victoria Regional 91/70 at 4:15p

Victoria (jerryg) 91/76

Nice thing about Cumulus you can get the dewpoint down some average 2°

I'm at no adjustment 88/68
Cumulus adjustment 88/66
Airport ASOS 1.3 miles away 89.6/64
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 16, 2018, 06:14:17 PM
Ok lets put this in locations, the station at at the 911 center is on the roof of a brick building surrounded by a parking lot and road, the metar at the air port is located off a runway and they put gravel around the site so as not to get stuck, heck with the good readings and mine is located over grass and ground that just got through with 15 inches of rain in the last 4 weeks.

Thank you for the info. It now puts things into perspective.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 16, 2018, 06:15:55 PM
BTW: What’s your station’s ID on WU jerryg?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 16, 2018, 06:34:20 PM
It sure would be nice if we had a picture of every location reporting weather info so we could weed out the really poor sited stations and cheap stations that are never even close to the correct readings. It is not just the amateur weather stations but a lot of the "official" stations are poorly sited and maintained also. How can a asos station put gravel around all the gear? My goodness don't they follow their own instructions lol.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: kcidwx on July 16, 2018, 06:43:06 PM
Humidity sensor aspiration has been shown to accelerate sensor aging. Is this guaranteed to happen 100% of the time? No! However, you have to consider it when troubleshooting your issue. If you have eliminated aspiration as the cause of your sensor problem, then move on to the next thing.

I think you are probably dealing with a moisture problem unrelated to aspiration. As Sensirion even states in their documentation, the sensor will show a wet bias after a high humidity event. However, they also say the sensor should slowly recover from it over time. I don't think the recovery is happening as it should.
Not doubting this. However, considering my ISS setup and climate and apparently still good 2 year old sensor, the fan and dirt ain't the problem (at least for me), being in a wet climate is. That being said, I think it more likely a fan in a wet climate would exacerbate the aging process.

Correct but there are two things that can exacerbate humidity sensor aging when under aspiration. Contamination and/or moisture. I went down the contamination path in this thread because people were talking about the Davis filter being crappy and that was the only filter on the sensor. Raised a red flag.

Now with more information it appears the sensor just doesn't recover completely after a high humidity event which results in a slight wet bias. At this point, I think it's an issue for Sensirion to explain. I don't think there's anything anybody can do themselves to correct it.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on July 16, 2018, 06:43:20 PM
You shouldn't be getting water on sensor anyway...
The SF2 rejects anything over a certain size from getting to the sensor and building up on it.
Mine just came in the mail. Now the fun part..... #-o

Let us know how the “fun” was so we can determine whether to join you in SF2 land. Where did you order them from and how much are they?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 16, 2018, 06:47:13 PM
It sure would be nice if we had a picture of every location reporting weather info so we could weed out the really poor sited stations and cheap stations that are never even close to the correct readings. It is not just the amateur weather stations but a lot of the "official" stations are poorly sited and maintained also. How can a asos station put gravel around all the gear? My goodness don't they follow their own instructions lol.

I think they did that because it would help the DP sensor to report actual DP instead of the DP influenced by transpiration from the grass.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 16, 2018, 06:50:05 PM
You shouldn't be getting water on sensor anyway...
The SF2 rejects anything over a certain size from getting to the sensor and building up on it.
Mine just came in the mail. Now the fun part..... #-o

Let us know how the “fun” was so we can determine whether to join you in SF2 land. Where did you order them from and how much are they?
I bought five, just in case..... #-o
https://www.mouser.com/ProductDetail/Sensirion/SF2?qs=wWC4CIiyLaNCAejbFUK5dw%3D%3D
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 16, 2018, 06:51:40 PM
It sure would be nice if we had a picture of every location reporting weather info so we could weed out the really poor sited stations and cheap stations that are never even close to the correct readings. It is not just the amateur weather stations but a lot of the "official" stations are poorly sited and maintained also. How can a asos station put gravel around all the gear? My goodness don't they follow their own instructions lol.

I think they did that because it would help the DP sensor to report actual DP instead of the DP influenced by transpiration from the grass.

That would be a violation of NWS siting guidelines that state temperature and humidity are to be measured over grass or whatever is natural for the area (which would be desert in Tucson, for example). Gravel is not natural terrain.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 16, 2018, 07:00:01 PM
It sure would be nice if we had a picture of every location reporting weather info so we could weed out the really poor sited stations and cheap stations that are never even close to the correct readings. It is not just the amateur weather stations but a lot of the "official" stations are poorly sited and maintained also. How can a asos station put gravel around all the gear? My goodness don't they follow their own instructions lol.

I think they did that because it would help the DP sensor to report actual DP instead of the DP influenced by transpiration from the grass.

(which would be desert in Tucson, for example).
Which explains why I come back with battle scars every time I go fetch the ISS. All in the name of science.  :grin:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 16, 2018, 07:00:44 PM
It sure would be nice if we had a picture of every location reporting weather info so we could weed out the really poor sited stations and cheap stations that are never even close to the correct readings. It is not just the amateur weather stations but a lot of the "official" stations are poorly sited and maintained also. How can a asos station put gravel around all the gear? My goodness don't they follow their own instructions lol.

This has been done, at least to some degree, and the report filed at http://surfacestations.org/ (http://surfacestations.org/). If you examine their findings you'll realize that only a minority of official stations don't have significant siting issues. If you haven't visited that website before, buckle up.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 16, 2018, 07:11:20 PM
It sure would be nice if we had a picture of every location reporting weather info so we could weed out the really poor sited stations and cheap stations that are never even close to the correct readings. It is not just the amateur weather stations but a lot of the "official" stations are poorly sited and maintained also. How can a asos station put gravel around all the gear? My goodness don't they follow their own instructions lol.

This has been done, at least to some degree, and the report filed at http://surfacestations.org/ (http://surfacestations.org/). If you examine their findings you'll realize that only a minority of official stations don't have significant siting issues. If you haven't visited that website before, buckle up.
That web site's well sited Stevenson screen is over gravel.....
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 16, 2018, 07:18:16 PM
That web site's well sited Stevenson screen is over gravel.....

As far as I know, that website doesn't have a weather station attached to it. It's merely a depository of various field reports and presents findings based on those reports.

Maybe you're referring to one of the pictures (upper right of home page, for instance) showing examples of what NOT to do when siting a weather station?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 16, 2018, 07:27:56 PM
That web site's well sited Stevenson screen is over gravel.....
Maybe you're referring to one of the pictures (upper right of home page, for instance) showing examples of what NOT to do when siting a weather station?
No sir, bottom left, two pics, one good siting (according to them), one bad. Frankly, the gravel is white and I'd think of minimal effect.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 16, 2018, 07:34:37 PM
No sir, bottom left, two pics, one good siting (according to them), one bad. Frankly, the gravel is white and I'd think of minimal effect.

Ah, I see now. Yes, that is a violation of the guidelines. I'm guessing they list that as "good" relative to the other horror shows that are catalogued there.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 16, 2018, 07:38:09 PM
Aight gang, I’m going to try something on both of my stations. One being a passive shield with a month old SHT 31 and another one with a year old SHT-31 inside of a 24hr FARS. Both stations are around 6miles apart from each other. I decided to go on to the WL software and calibrated the humidity sensor by putting in the correct value based off of the current air temperature reported by the station itself and the DP temp from the nearby airport. Going to see how that goes and especially when the humidities reach near 100%
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 16, 2018, 07:41:06 PM
wu id  is KTXVICTO6
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 16, 2018, 07:42:29 PM
wu id  is KTXVICTO6

Thanks!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 16, 2018, 07:47:16 PM
Aight gang, I’m going to try something on both of my stations. One being a passive shield with a month old SHT 31 and another one with a year old SHT-31 inside of a 24hr FARS. Both stations are around 6miles apart from each other.
With that distance, you'll need to study obs for quite a while to get a "feel" for what and how you're comparing, probably days at least.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Dj1225 on July 16, 2018, 08:58:17 PM
Well after 2.96" of rain today here just north of Gulfport with rain rates 7.5-9.5" at certain points my three day old SHT31 made it to 99%  \:D/  I haven't seen 99% in almost 2 years! I wonder if I'll get the elusive 100% overnight or early in the morning?  :-k Do you guys think the sf2 filter along with the davis filter might calm down the wet bias the sensor has? My heat indices have come down from the 120+ range to a more respectable 110, but that still seems a tad bit to high.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 17, 2018, 08:35:39 AM

If that were the case my strongly aspirated ISS and dirty two year old 31 would not have hit and stayed at both 99% and 1%. I'm sure the filter is there to help protect from dirt/dust, but it's becoming much more evident moisture is the culprit in sensor degradation, not aspiration. As a matter of fact, I'm using the two year old 31 now and it's just as good as the newer one.

I'm having a hard time finding 99% when did this occur?  Last 30 days which includes the humid season monson highest is low 90's.

 [ You are not allowed to view attachments ]
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Aardvark on July 17, 2018, 09:30:30 AM
420 opinions so far,  One question?   Have you determined the reliability of the humidity sensor and have you the psychometer in possession for that?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 17, 2018, 10:51:03 AM
420 opinions so far,  One question?   Have you determined the reliability of the humidity sensor and have you the psychometer in possession for that?

A drive by question deserves a drive by answer. The humidity sensor is unreliable and there are only a few decent psychrometers.  If you want to know more than that you're going to have to EXPEND EFFORT like the rest of us did.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Aardvark on July 17, 2018, 12:09:59 PM
420 opinions so far,  One question?   Have you determined the reliability of the humidity sensor and have you the psychometer in possession for that?

A drive by question deserves a drive by answer. The humidity sensor is unreliable and there are only a few decent psychrometers.  If you want to know more than that you're going to have to EXPEND EFFORT like the rest of us did.

And a snarky answer deserves a  better answer.  I would assume then, if one sensor is fer-cocked,  then the whole unit might be similar.  I'd then send the whole dang thing back and buy something more professional and accurate.

However...  I must have been blessed, born under a happy cloud, lived right, prayed harder , danced the Wautse better than anyone else in my prime, because my VP2 is spot on.   I check it often and it is spot on.

 Yup, I'd save my pennies and buy something around the quality of a  WeatherHawk 620 Wireless Weather Station.  Now for the price, I'd assume it has to be accurate.

There are many variables that can cause fluctuations in temperature and humidity readings on a moment to moment situation.  the  consoles, Envoys,  can be off and have to be set in the software or on the logger, and in 10 minutes they are off again.

I just wanted to know if the person who started this thread had got the whole thing solved yet.  Sheesh.. You remind me of a certain world leader
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 17, 2018, 12:50:42 PM
I would assume then, if one sensor is fer-cocked,  then the whole unit might be similar.  I'd then send the whole dang thing back and buy something more professional and accurate.

However...  I must have been blessed, born under a happy cloud, lived right, prayed harder , danced the Wautse better than anyone else in my prime, because my VP2 is spot on.   I check it often and it is spot on.

 Yup, I'd save my pennies and buy something around the quality of a  WeatherHawk 620 Wireless Weather Station.  Now for the price, I'd assume it has to be accurate.

If only we could trade your assumptions for something valuable!

No, people CAN'T just send it back for a refund. That's not how it works anywhere except for a short period after purchase.

And, no, just because something is expensive doesn't make it better. Spec sheets don't always reflect reality as this thread demonstrates.

Got any more cheap wise cracks, or are you ready to contribute to the thread? Perhaps start by telling us how you verify your VP2 is accurate. Maybe you have a psychrometer or hygrometer you trust? And, if so, how do you know that device is accurate?

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Aardvark on July 17, 2018, 01:50:26 PM
I would assume then, if one sensor is fer-cocked,  then the whole unit might be similar.  I'd then send the whole dang thing back and buy something more professional and accurate.

However...  I must have been blessed, born under a happy cloud, lived right, prayed harder , danced the Wautse better than anyone else in my prime, because my VP2 is spot on.   I check it often and it is spot on.

 Yup, I'd save my pennies and buy something around the quality of a  WeatherHawk 620 Wireless Weather Station.  Now for the price, I'd assume it has to be accurate.

If only we could trade your assumptions for something valuable!

No, people CAN'T just send it back for a refund. That's not how it works anywhere except for a short period after purchase.

And, no, just because something is expensive doesn't make it better. Spec sheets don't always reflect reality as this thread demonstrates.

Got any more cheap wise cracks, or are you ready to contribute to the thread? Perhaps start by telling us how you verify your VP2 is accurate. Maybe you have a psychrometer or hygrometer you trust? And, if so, how do you know that device is accurate?
  I did.  I suggested a sling psychrometer. Inexpensive and works fine .   

One could use something like a Kestrel, if they have one or access  and measure the temperature variance as well, assuming that the Kestrel is calibrated.  But a sling psychrometer, as ancient as they are , is fairly accurate.    I used one years ago and I mean years ago.   Wet the wick with distilled water, and spin away.  then consult tables for the humidity at that point.

Yes, I suggested that early on. 


How do I know it is correct.  Well, I have a couple of Kestrels that are calibrated  (which isn't fun to do).  I compared them with  the sling psychrometer and it was fairly within limits. I mean the Sling doesn't do decimals, but it worked for me.

 Now as far a cheap wise cracks,  there are none.  I am deeply offended by your insinuations and hope the moderator gets after you.

No,  my question was simple,  has the issue been resolved?  How far off was the SHT-31 unit from whatever was used?  And you got your panties in a knot after well beyond 400 responses?   My lord.    I am done offering any suggestion.

OH   I have had a weather station since 1990.  I originally and a WMII,  then the VP1 and now the VP2.   I have had the VP2 since 1993 and other than having to replace the ISS transmitter board once,  replace the super cap once, and have a fully populated station,soil station and so on,  I think I speak with some experience,   OH and the anemometer on the top of my mast is the original vp1 which is mysteriously working for over 20 years.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 17, 2018, 02:34:59 PM
Ok just to put some info out there for those who want some accuracy in their sensor i asked Ryan over at Scaled Inst. about a calibrated sensor and he got word back from Davis they offer an nist traceable with paperwork for 100 dollars plus the price of a sensor. That would be the best way to have a known reference to work with.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 17, 2018, 02:55:44 PM
Yep that's what i would do, use the sensor for cal. purposes only. When you think of the price for some of the humidity gear to check a sensor for humidity accuracy it really isn't that much.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 17, 2018, 03:09:44 PM
  I did.  I suggested a sling psychrometer. Inexpensive and works fine .   

...

Yes, I suggested that early on. 

Where? I only show one other contribution to this thread under your screen name. In it you stated that you'd rather not have to read the proceeding 163 posts and asked the O.P. whether he solved his problem. And he answered you.

Then you return hundreds of posts later and imply that you're too important to be bothered reading a bunch of "opinions" and that, once again, someone should furnish you the information you desire upon request?

Should we just put the report on your desk when we're *finally* done or would you like it emailed?

I am done offering any suggestion.

Indeed, there is no reason for you to be here since your station is always "spot on". I mean the rest of us can't even comprehend that the weather is constantly changing and, therefore, how difficult it is to calibrate our stations properly. Only YOU understand that, Aardvark.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 17, 2018, 03:28:13 PM

If that were the case my strongly aspirated ISS and dirty two year old 31 would not have hit and stayed at both 99% and 1%. I'm sure the filter is there to help protect from dirt/dust, but it's becoming much more evident moisture is the culprit in sensor degradation, not aspiration. As a matter of fact, I'm using the two year old 31 now and it's just as good as the newer one.

I'm having a hard time finding 99% when did this occur?  Last 30 days which includes the humid season monson highest is low 90's.
This was back on 2/16, we were WOXOF (super rare here), I hit 99% starting at 10pm and stayed there til, wait for it.......
10:30am the next morning. Yes, 12 and half hours, the spread between temp and dew stayed at 0.3F the whole time. The service time on the 31 then was 22 months. Two months later I put in the other 31 because I was getting gigged for having too low of a dew when I was at 1%. The TUS ASOS and a RAWS both hit 1% also, just didn't stay there as long as I did. Obviously at that time the 31 was a full two years old.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 17, 2018, 03:40:03 PM
I just ordered a nist sensor so i will have a calibrated reference to check my working sensor. Ryan over at Scaled Inst. now has the sensor with calibration pricing on his site.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Aardvark on July 17, 2018, 03:45:38 PM
I just ordered a nist sensor so i will have a calibrated reference to check my working sensor. Ryan over at Scaled Inst. now has the sensor with calibration pricing on his site.
   Ryan has good things at a reasonable price.   
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 17, 2018, 04:01:11 PM
I'm testing three of these evaluation boards right now. RH spec: 1.5%

https://www.idt.com/products/sensor-products/humidity-sensors (https://www.idt.com/products/sensor-products/humidity-sensors)

I'll run them against my three SHT-31 evaluation boards as well.

Were you able to test those IDT sensors out?  How did they do?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 17, 2018, 04:21:31 PM
Ok just to put some info out there for those who want some accuracy in their sensor i asked Ryan over at Scaled Inst. about a calibrated sensor and he got word back from Davis they offer an nist traceable with paperwork for 100 dollars plus the price of a sensor. That would be the best way to have a known reference to work with.

If you check Davis' NIST reference publications (https://www.davisinstruments.com/product_documents/weather/spec_sheets/NIST_Traceable_Sensors.pdf (https://www.davisinstruments.com/product_documents/weather/spec_sheets/NIST_Traceable_Sensors.pdf)), you'll see they only guarantee +/- 3% even after calibration. That's worse then the stated specs between 10 and 90%!

If you need a reference device, you can get the psychrometer that jgentry mentioned (http://www.iisusa.com/mobile/Psychro-Dyne-Battery-Operated-Psychrometer-Fahrenheit-BLACK-ORGANIC/productinfo/22014/ (http://www.iisusa.com/mobile/Psychro-Dyne-Battery-Operated-Psychrometer-Fahrenheit-BLACK-ORGANIC/productinfo/22014/)) that's accurate to <1% for $175. That's only $25 more than Davis sensor + calibration, and is much more accurate. If you want 2% accuracy, you can get a Kestrel 5000 for $260 from Ryan. Those are pre-calibrated at the factory and have a 5 year warranty, including the calibration, and they measure more than just temp/humidity.

Please don't reward Davis for not standing behind their products. Davis could easily have cut this thread short by stepping up and promising to better protect the sensors in the future (installing the SF2 cap) and offering warranty replacements for anyone experiencing abnormally high humidity readings. They chose not to do that and, instead, put their retailers, like Ryan, in the line of fire.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 17, 2018, 04:23:45 PM
Jerryg,

I’ve been keeping an eye on your station, the RW station at the EOC, and two nearby airport stations. I do know that the RW is installed on the roof with pavement and concrete around the general area. Airport has a similar setup except it isn’t on the rooftop. Lol.

Even though your station is sited over grass, I’m beginning to think that it shouldn’t cause your station to report DPs that are 4-5° higher than the Airport & RW during the daytime hours. It would be more sensible if it was 1-3° higher at times during transpiration but that should be it I would think. The RW & your station matches up well along with the airport at night when it comes to DP. During the daytime, the RW seems to read the same or 1° higher or lower than the airport where you are reading 2-4° higher consistently. The RW air temps during the dayare very representative of your area, even though it’s instalked on top of a roof.

I’m getting the feeling that it doesn’t matter if you have the SF2 filter or not, the SHT-31 is going to have a wet bias during the daytime hours.

Here are some currents at 3:18p

Jerryg 94/77

RW station: 93/73

ASOS 91/72

The only other idea I have is to calibrate the humidity on the console. That will work fairly well until the humidities get closer to 100% (late 80s & 90s).

My thinking might be wrong here but it doesn’t make sense to me when a RW (for example) humidity sensor would perform better than the Davis.

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 17, 2018, 05:24:54 PM
After 4 days with 2 new sensors, my conclusion they aren't much better than the old still running high on DP at those critical levels when its important to be most accurate (high heat index).

So basically I have 2 new sensors both now reach 98%.. Whoopie!  I'm beginning to think these sensors are just off. They surely don't come close to meeting the claimed specs when it counts. 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 17, 2018, 05:38:05 PM
I’m getting the feeling that it doesn’t matter if you have the SF2 filter or not, the SHT-31 is going to have a wet bias during the daytime hours.

I'm starting to get that impression too. jerryg initally stated with such confidence that the SF2 had kept his sensors accurate. The readings you posted would suggest otherwise.

I also recall Ron's (kcidwx) statement about heat ultimately being the only way to prevent too much moisture sitting against the sensor.

However, that makes one wonder why it is Rainwise sensors don't fall victim to this same problem?

The only other idea I have is to calibrate the humidity on the console. That will work fairly well until the humidities get closer to 100% (late 80s & 90s).

My tests, purely from the field involving 2 sensors of differing age, indicate the sensor's bias point crosses over somewhere in the 70 percentiles. Meaning, the 70s are the range where the sensor is least positively or negatively biased. Below that, once a sensor is out of specification, I've observed a wet bias (at least down to the 30% range - haven't been able to test below that). Above that starting around 80%, there's a dry bias that worsens as you approach 100% (although technically within spec).

The departures both in DP and humidity are worse when it's warm (above 80), but shows up when it's cooler too. Currently, my good sensor reads 63/47 and my bad sensor reads 64/50. That's a 3% humidity difference in the mid 50 percentiles. Still out of spec on the older sensor, but not nearly as bad as when it gets warmer. When it's in the 80s/90s and midrange humidities, I'm seeing 5-8% and 4-5 deg DP departures between the two sensors.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 17, 2018, 05:50:10 PM
I know IDT HS 3001 spec sheet says that it can go out of spec if it’s in high humidity conditions for a long time but I’m curious if it would behave the same way as the Sensirion sensors would or not?

I do know that the VP2 can’t take IDT’s sensor but I’m curious to how it would perform compared to Sensirion’s sensors.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 17, 2018, 05:52:42 PM
After 4 days with 2 new sensors, my conclusion they aren't much better than the old still running high on DP at those critical levels when its important to be most accurate (high heat index).

So basically I have 2 new sensors both now reach 98%.. Whoopie!  I'm beginning to think these sensors are just off. They surely don't come close to meeting the claimed specs when it counts.

I guess a key question is the temp at which Sensirion tests the sensors' humidity spectrum. My guess would be 25C (77F) since I see mention of it along with 63% in the notes on their datasheet. If they aren't testing beyond that, then, indeed, there could be discrepancies elsewhere.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 17, 2018, 06:01:32 PM
Here is Earth Networks (formerly WeatherBug) spec sheet for their stations. Will note that their DPs matches up well with ASOS stations
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 17, 2018, 06:04:17 PM
After 4 days with 2 new sensors, my conclusion they aren't much better than the old still running high on DP at those critical levels when its important to be most accurate (high heat index).

So basically I have 2 new sensors both now reach 98%.. Whoopie!  I'm beginning to think these sensors are just off. They surely don't come close to meeting the claimed specs when it counts.

I guess a key question is the temp at which Sensirion tests the sensors' humidity spectrum. My guess would be 25C (77F) since I see mention of it along with 63% in the notes on their datasheet. If they aren't testing beyond that, then, indeed, there could be discrepancies elsewhere.

I guess we should’ve paid attention to the “maximum accuracy” on Sensirion’s spec sheet. Lol
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Aardvark on July 17, 2018, 06:10:43 PM
perhaps bringing in the station to test it in a controlled environment to determine the difference between the sensor and the standard.

Davis has been moving the location of that sensor.   Initially it was fastened to the bottom of one of the radiation plates, but they moved it to the top of the next plate up because water was  collecting just enough that it saturated the sensor. Now if you have a FARS, as I do, the unit is mounted on its side in the chamber. 

How much of a difference if the sensor is removed from the chamber and tested vs.  one of the current 3 prime locations in the chamber.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 17, 2018, 06:11:22 PM
After 4 days with 2 new sensors, my conclusion they aren't much better than the old still running high on DP at those critical levels when its important to be most accurate (high heat index).

So basically I have 2 new sensors both now reach 98%.. Whoopie!  I'm beginning to think these sensors are just off. They surely don't come close to meeting the claimed specs when it counts.

I guess a key question is the temp at which Sensirion tests the sensors' humidity spectrum. My guess would be 25C (77F) since I see mention of it along with 63% in the notes on their datasheet. If they aren't testing beyond that, then, indeed, there could be discrepancies elsewhere.

I guess we should’ve paid attention to the “maximum accuracy” on Sensirion’s spec sheet. Lol

No kidding.. :sad:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 17, 2018, 06:15:04 PM
They surely don't come close to meeting the claimed specs when it counts.

Just looking again at Davis' NIST traceable doc (link posted above) and it says they verify humidity to be within +/- 3% between 33% and 90% with the checkpoints being 33%, 80% and 90%. Please notice they skip testing the most common middle ranges.

Once again, the question arises: at what temperature(s) are they testing humidity? They don't say. You could assume they use the temperature checkpoints, but I don't like assuming.

So even if you pay to have the humidity certified, we still don't know if the device is accurate in the most problematic middle ranges.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 17, 2018, 06:26:40 PM
perhaps bringing in the station to test it in a controlled environment to determine the difference between the sensor and the standard.

How much of a difference if the sensor is removed from the chamber and tested vs.  one of the current 3 prime locations in the chamber.

This is why I bark at you for parachuting into the thread and not bothering to do the least bit of reading. If you had read page 14, for instance, you'd see that Ron had run extensive tests in his humidity chamber and reproduced the wet bias. Then again, Ron's results from his laboratory-grade instrument are just another of 420 "opinions" right?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Aardvark on July 17, 2018, 06:42:45 PM
perhaps bringing in the station to test it in a controlled environment to determine the difference between the sensor and the standard.

How much of a difference if the sensor is removed from the chamber and tested vs.  one of the current 3 prime locations in the chamber.

This is why I bark at you for parachuting into the thread and not bothering to do the least bit of reading. If you had read page 14, for instance, you'd see that Ron had run extensive tests in his humidity chamber and reproduced the wet bias. Then again, Ron's results from his laboratory-grade instrument are just another of 420 "opinions" right?
Would you please leave me alone.  Why do you seem to think you are in charge here.   I have reported you to the moderators and  definitely they need to step in and quell your hatred toward me and anything I ask .  Just leave me alone. Stop harassing me.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 17, 2018, 08:19:33 PM
Look, lots of people have given freely of their time, expertise and money to try to understand and solve a problem that's not their fault. Some of us have thousands invested in a company that, apparently, thinks it can bait and switch us with impunity.

Given the circumstances, it's not a straight line to the answer. We don't have the same testing facilities as the manufacturer. We have to do our best with what we have. The situation, unfortunately, is still evolving on a trial and error basis.

So I find it awfully insulting when someone drops by every couple hundred posts and ask us if we're done yet. That's why I gave you the response I did. Had it been a lighter thread, then I'm sure I would have responded differently or not at all. My apologies.

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: hwcorder on July 17, 2018, 09:07:59 PM
Look, lots of people have given freely of their time, expertise and money to try to understand and solve a problem that's not their fault. Some of us have thousands invested in a company that, apparently, thinks it can bait and switch us with impunity.

Given the circumstances, it's not a straight line to the answer. We don't have the same testing facilities as the manufacturer. We have to do our best with what we have. The situation, unfortunately, is still evolving on a trial and error basis.

Yep can't even calculate the time and money I've put in and continue to put in to try to figure it out.  Seems like until Davis ditches this Digital Sbus protocol for the I2C they have us by the you know what. Unless someone here wants to try to build there own ISS transmitter that has a I2C microcontroller I don't see a whole lot of other options other than switching these sensors out every year or so.

Would love to try these sensors out as they look promising but alas, wrong protocol.
https://www.ist-ag.com/en-us/products/hyt-221-calibrated-and-temperature-compensated
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 17, 2018, 09:27:11 PM
I'll duplicate this, forum member "ct" where are you....?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 17, 2018, 09:27:30 PM
Look, lots of people have given freely of their time, expertise and money to try to understand and solve a problem that's not their fault. Some of us have thousands invested in a company that, apparently, thinks it can bait and switch us with impunity.

Given the circumstances, it's not a straight line to the answer. We don't have the same testing facilities as the manufacturer. We have to do our best with what we have. The situation, unfortunately, is still evolving on a trial and error basis.

Yep can't even calculate the time and money I've put in and continue to put in to try to figure it out.  Seems like until Davis ditches this Digital Sbus protocol for the I2C they have us by the you know what. Unless someone here wants to try to build there own ISS transmitter that has a I2C microcontroller I don't see a whole lot of other options other than switching these sensors out every year or so.

Would love to try these sensors out as they look promising but alas, wrong protocol.
https://www.ist-ag.com/en-us/products/hyt-221-calibrated-and-temperature-compensated

Looks like this one might work best in high humidity environments.

Here is the data sheet- https://www.ist-ag.com/sites/default/files/DHHYT221_E.pdf

Application notes- https://www.ist-ag.com/sites/default/files/AHHYTM_E.pdf
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: hwcorder on July 17, 2018, 09:52:25 PM
Look, lots of people have given freely of their time, expertise and money to try to understand and solve a problem that's not their fault. Some of us have thousands invested in a company that, apparently, thinks it can bait and switch us with impunity.

Given the circumstances, it's not a straight line to the answer. We don't have the same testing facilities as the manufacturer. We have to do our best with what we have. The situation, unfortunately, is still evolving on a trial and error basis.

Yep can't even calculate the time and money I've put in and continue to put in to try to figure it out.  Seems like until Davis ditches this Digital Sbus protocol for the I2C they have us by the you know what. Unless someone here wants to try to build there own ISS transmitter that has a I2C microcontroller I don't see a whole lot of other options other than switching these sensors out every year or so.

Would love to try these sensors out as they look promising but alas, wrong protocol.
https://www.ist-ag.com/en-us/products/hyt-221-calibrated-and-temperature-compensated

Looks like this one might work best in high humidity environments.

Here is the data sheet- https://www.ist-ag.com/sites/default/files/DHHYT221_E.pdf

Application notes- https://www.ist-ag.com/sites/default/files/AHHYTM_E.pdf

Actually mentions one of the typical applications for the HYT221 for measuring humidity in saunas lol! Wonder how it recovers afterwards though?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 17, 2018, 10:02:58 PM
Look, lots of people have given freely of their time, expertise and money to try to understand and solve a problem that's not their fault. Some of us have thousands invested in a company that, apparently, thinks it can bait and switch us with impunity.

Given the circumstances, it's not a straight line to the answer. We don't have the same testing facilities as the manufacturer. We have to do our best with what we have. The situation, unfortunately, is still evolving on a trial and error basis.

Yep can't even calculate the time and money I've put in and continue to put in to try to figure it out.  Seems like until Davis ditches this Digital Sbus protocol for the I2C they have us by the you know what. Unless someone here wants to try to build there own ISS transmitter that has a I2C microcontroller I don't see a whole lot of other options other than switching these sensors out every year or so.

Would love to try these sensors out as they look promising but alas, wrong protocol.
https://www.ist-ag.com/en-us/products/hyt-221-calibrated-and-temperature-compensated

Looks like this one might work best in high humidity environments.

Here is the data sheet- https://www.ist-ag.com/sites/default/files/DHHYT221_E.pdf

Application notes- https://www.ist-ag.com/sites/default/files/AHHYTM_E.pdf

Actually mentions one of the typical applications for the HYT221 for measuring humidity in saunas lol! Wonder how it recovers afterwards though?

Maybe kcidwx will get one of those sensors and put it in his humidity chamber and see how it does.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 17, 2018, 10:51:34 PM
Looks like max is 90% humidity if I read the data sheet correctly.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 17, 2018, 11:01:05 PM
It goes up to 100%. I think the accuracy is +/-3% 90-100
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 18, 2018, 09:37:22 AM
Interesting to note that the RainWise MK III uses the Sensirion SHT 15 as their temperature/humidity sensor.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 18, 2018, 05:19:43 PM
Ok new info on the nist calibration check on the 31, that previous info was old stuff and Ryan got an email from Davis and the new info is a 3 point check of +/- 1% at 33% and 80% and +/-2% at 90%. Looks a little better but i think they should have included one around 97%. When you look at a lot of the sensors most of the really good readings are confined to 30 to 90%, not many give much in the way of a good shot of accuracy about 90%.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 18, 2018, 06:07:47 PM
FWIW the published literature shows most capacitive sensors go WET above about 90ºF DP...so, asking for "exactness" might be futile...for now.
And when is the last time you saw a dew point AOA 90F? :shock: I know I sure as hell wouldn't live there.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: JudinNorman on July 18, 2018, 06:14:03 PM
Ok new info on the nist calibration check on the 31, that previous info was old stuff and Ryan got an email from Davis and the new info is a 3 point check of +/- 1% at 33% and 80% and +/-2% at 90%. Looks a little better but i think they should have included one around 97%. When you look at a lot of the sensors most of the really good readings are confined to 30 to 90%, not many give much in the way of a good shot of accuracy about 90%.

Is a correction chart included like I have with my old max and min thermometers ?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 18, 2018, 06:29:53 PM
FWIW the published literature shows most capacitive sensors go WET above about 90ºF DP...so, asking for "exactness" might be futile...for now.
And when is the last time you saw a dew point AOB 90F? :shock: I know I sure as hell wouldn't live there.
Ooops, (typo) that should've been 90%RH!  :oops:
Ahhh, that sounds better! I guess I should feel fortunate that I have a sensor that'll hit 1% as well as 99.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 18, 2018, 06:31:18 PM
FWIW the published literature shows most capacitive sensors go WET above about 90ºF DP...so, asking for "exactness" might be futile...for now.
And when is the last time you saw a dew point AOB 90F? :shock: I know I sure as hell wouldn't live there.

You mean Houston  :shock:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 18, 2018, 06:33:29 PM
FWIW the published literature shows most capacitive sensors go WET above about 90ºF DP...so, asking for "exactness" might be futile...for now.
And when is the last time you saw a dew point AOB 90F? :shock: I know I sure as hell wouldn't live there.

You mean Houston  :shock:
No doubt! I have no idea how those people deal with that stuff all-summer-long.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 18, 2018, 06:47:18 PM
Got 2 new SHT31's in handled by a different company lets see if they do any better. I just happen to have passive shield I may try. Getting data could be an issue however they do record high and lows however.
 
Good point was brought up why rainwise sensors seem better using the SHT15 on mid to high dp temps in 60-80 range.

Something was brought to my attention by someone not on this thread the molding process Davis uses to mount sensor could be contamination source dooming sensor from getgo. Maybe that's why the reckless handling.

The sensor looks embedded in rubber or plastic which would be against all handling procedures. 

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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 18, 2018, 06:55:09 PM
Got 2 new SHT31's in handled by a different company lets see if they do any better. I just happen to have passive shield I may try. Getting data could be an issue however they do record high and lows however.
 
Good point was brought up why rainwise sensors seem better using the SHT15 on mid to high dp temps in 60-80 range.

Something was brought to my attention by someone not on this thread the molding process Davis uses to mount sensor could be contamination source dooming sensor from getgo. Maybe that's why the reckless handling.

The sensor looks embedded in rubber or plastic which would be against all handling procedures. 

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Randy if I were you, I'd just look for some shade, or make your own.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 18, 2018, 06:56:29 PM
Got 2 new SHT31's in handled by a different company lets see if they do any better. I just happen to have passive shield I may try. Getting data could be an issue however they do record high and lows however.
 
Good point was brought up why rainwise sensors seem better using the SHT15 on mid to high dp temps in 60-80 range.

Something was brought to my attention by someone not on this thread the molding process Davis uses to mount sensor could be contamination source dooming sensor from getgo. Maybe that's why the reckless handling.

The sensor looks embedded in rubber or plastic which would be against all handling procedures. 

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Randy if I were you, I'd just look for some shade, or make your own.

Good idea easier to see for sure.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: drew1021 on July 18, 2018, 07:01:12 PM
Got 2 new SHT31's in handled by a different company lets see if they do any better. I just happen to have passive shield I may try. Getting data could be an issue however they do record high and lows however.
 
Good point was brought up why rainwise sensors seem better using the SHT15 on mid to high dp temps in 60-80 range.

Something was brought to my attention by someone not on this thread the molding process Davis uses to mount sensor could be contamination source dooming sensor from getgo. Maybe that's why the reckless handling.

The sensor looks embedded in rubber or plastic which would be against all handling procedures. 

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Or perhaps RW manipulates the raw readings with software??
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 18, 2018, 07:04:23 PM
Got 2 new SHT31's in handled by a different company lets see if they do any better. I just happen to have passive shield I may try. Getting data could be an issue however they do record high and lows however.
 
Good point was brought up why rainwise sensors seem better using the SHT15 on mid to high dp temps in 60-80 range.

Something was brought to my attention by someone not on this thread the molding process Davis uses to mount sensor could be contamination source dooming sensor from getgo. Maybe that's why the reckless handling.

The sensor looks embedded in rubber or plastic which would be against all handling procedures. 

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Or perhaps RW manipulates the raw readings with software??
Doubtful.
The RW shield is passive, and as has been discussed, perhaps a difference maker.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 18, 2018, 07:18:51 PM
Okay 15 minutes in shade.
Snapshot of station at same time I took photo.  Still watching running 5% higher on Davis sensor.
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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 18, 2018, 07:28:03 PM
But less than a 2F in the dew. Not perfect.......
http://www.dpcalc.org/
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 18, 2018, 07:34:12 PM
Yes thought specs were +/- 2 % humidity :?:  Test wasn't really great because with clouds now the station has cooled below patio +3° on temperature so test was only 20 minutes during that period averaged 5% difference.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 18, 2018, 08:06:34 PM
Rainwise sensors ain't gonna be cheap. Look at these "Government" (GSA) prices!!!

https://www.gsaadvantage.gov/ref_text/GS07F5342R/GS07F5342R_online.htm (https://www.gsaadvantage.gov/ref_text/GS07F5342R/GS07F5342R_online.htm)
At least the feds got a 29.25% discount. What's there to whine about? :roll:

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 18, 2018, 08:50:21 PM
Rainwise sensors ain't gonna be cheap. Look at these "Government" (GSA) prices!!!

https://www.gsaadvantage.gov/ref_text/GS07F5342R/GS07F5342R_online.htm (https://www.gsaadvantage.gov/ref_text/GS07F5342R/GS07F5342R_online.htm)
At least the feds got a 29.25% discount. What's there to whine about? :roll:
We ain't the Feds (nor the Federale's either).  :roll:
Even with the discount, the prices are ludicrous. 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 18, 2018, 10:27:25 PM
Longer test this evening with the 2-Acurite SHT31's sitting on top of Davis Shield. Took snapshot of Davis running 8% higher humidity. Not exactly scientific because Davis is aspirated.

So this is 2 brand new SHT31's different company and a 5 day old Davis sensor VP2 aspirated.
 

Now down to 71° as I type still running 8% high. 74% vs 83% on Davis vs 2 different Acurite's .

Maybe this is why the Rainwise sensors look normal, non aspirated and probably don't destroy sensor mounting on board.


Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 18, 2018, 10:32:38 PM
Randy, my newest SHT-31 sensor is housed in a passive shield and it basically performs the same as if it’s in the FARS. Could just be the board itself. If David used a probe-type sensor, we might not have the problems we are currently dealing with.  Who knows...
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 18, 2018, 10:52:15 PM
Acurite don't use probes these are the real Mccoys.

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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 19, 2018, 07:44:38 AM
This morning with humidity up the new acurite with sht31's both peaked at 95% same as the new Davis aspirated. The 2 year old Davis peaked at 93% while the handheld Extech 96% this morning just to complete how these sensors compare.

They do appear to run substantially lower 8% daytime and much closer with airport ASOS yesterday afternoon and evening, along with reaching the upper end of humidity scale 95%, airport also was 94% today. This makes everything really frustrating knowing the 31's can perform just as data sheet says but the Davis Units don't.

Why the Davis sensors run so high I can only conjecture molding into board process contamination.

This is very aggravating after seeing the Acurite $15 sensor work so much better. This probably goes to why the rainwise sensors are much closer and don't seem to suffer like the Davis sensors. They don't destroy sensor before consumer gets them.

I'll let you guys continue but I've seen enough to know the Davis units are junk.

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 19, 2018, 08:07:32 AM
Well that ticks me off too. Either David just doesn’t realize the their board is screwing up the humidity sensor, or they do but don’t care.  Thank you for doing these tests. I think we now got into the culprit of our problems.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Dj1225 on July 19, 2018, 08:31:11 AM
This morning with humidity up the new acurite with sht31's both peaked at 95% same as the new Davis aspirated. The 2 year old Davis peaked at 93% while the handheld Extech 96% this morning just to complete how these sensors compare.

They do appear to run substantially lower 8% daytime and much closer with airport ASOS yesterday afternoon and evening, along with reaching the upper end of humidity scale 95%, airport also was 94% today. This makes everything really frustrating knowing the 31's can perform just as data sheet says but the Davis Units don't.

Why the Davis sensors run so high I can only conjecture molding into board process contamination.

This is very aggravating after seeing the Acurite $15 sensor work so much better. This probably goes to why the rainwise sensors are much closer and don't seem to suffer like the Davis sensors. They don't destroy sensor before consumer gets them.

I'll let you guys continue but I've seen enough to know the Davis units are junk.

Randy would you consider switching back to the SHT15?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 19, 2018, 08:43:24 AM
This morning with humidity up the new acurite with sht31's both peaked at 95% same as the new Davis aspirated. The 2 year old Davis peaked at 93% while the handheld Extech 96% this morning just to complete how these sensors compare.

They do appear to run substantially lower 8% daytime and much closer with airport ASOS yesterday afternoon and evening, along with reaching the upper end of humidity scale 95%, airport also was 94% today. This makes everything really frustrating knowing the 31's can perform just as data sheet says but the Davis Units don't.

Why the Davis sensors run so high I can only conjecture molding into board process contamination.

This is very aggravating after seeing the Acurite $15 sensor work so much better. This probably goes to why the rainwise sensors are much closer and don't seem to suffer like the Davis sensors. They don't destroy sensor before consumer gets them.

I'll let you guys continue but I've seen enough to know the Davis units are junk.

Randy would you consider switching back to the SHT15?

No the high humidity started about when the 15 came out, plus 15 temperature accuracy suffers below 14°f and I live way below that in winter and would rather have accurate temperature over humidity.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dport on July 19, 2018, 09:35:36 AM
This is highly annoying as I just bought a VP2 with 24/hr FARS and the WifiLogger last month.  That's a substantial investment for me.  I was under the impression that they SHT31 was one of the best prosumer sensors out there.  My humidity has hit 99% since I have installed, but I have also noticed that humidity reads high more often than not.

So, how can we engage Davis at a higher level?  Other options available to us?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 19, 2018, 10:18:04 AM
I still believe the 31 is a great sensor just not after Davis gets done with them.
As a consumer and spending oodles of money on Davis products I'll now look at other options. Meanwhile I'm hoping some smart electronic people will come to the rescue with a new transmitter so we can install our own sensors not pre trashed before we get them.

 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 19, 2018, 10:28:27 AM
dport look at Cumulus also I run the desktop version you can run some of that high bias out with the humidity multiplier and still reach 100%. It's kind of a pain however once you get below the 35% range the high bias is gone so you need to keep a watch all the time. Frustrating.. ](*,) Other softwares also may have this ability.
 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on July 19, 2018, 10:35:53 AM
This morning with humidity up the new acurite with sht31's both peaked at 95% same as the new Davis aspirated. The 2 year old Davis peaked at 93% while the handheld Extech 96% this morning just to complete how these sensors compare.

They do appear to run substantially lower 8% daytime and much closer with airport ASOS yesterday afternoon and evening, along with reaching the upper end of humidity scale 95%, airport also was 94% today. This makes everything really frustrating knowing the 31's can perform just as data sheet says but the Davis Units don't.

Why the Davis sensors run so high I can only conjecture molding into board process contamination.

This is very aggravating after seeing the Acurite $15 sensor work so much better. This probably goes to why the rainwise sensors are much closer and don't seem to suffer like the Davis sensors. They don't destroy sensor before consumer gets them.

I'll let you guys continue but I've seen enough to know the Davis units are junk.
Try a 75...tiny thermal mass, preinstalled to a board by Sensirion, and plug and play after a little soldering or 1.27mm 4-pin connector.

As for your evening experiment. I would expect the Davis to run higher RH with fan aspiration and dew beginning to form at the surface. If you're seeing it in the afternoon as well then maybe it's a different issue. I don't like comparing my humidity to ASOS since I'm in an area with lots of trees and transpiration. The airports have wide open areas with great mixing which cuts down on those pooling dewpoints from the vegetation. You're in NE so maybe this doesn't apply since you may have mostly plains and fields too. But out here, it's lots of hills and trees.

I feel like we've been discussing this for years now. Personally, 98% vs 100% isn't a big deal to me. Your local ASOS miles may vary, but most of the ones out here never reach 100% when viewing the 1-min data...even with M1/4SM or 1/4SM visibility. I just logged in yesterday for the first time in awhile so maybe you've addressed this, but have you tried a NIST certified sensor?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: johnd on July 19, 2018, 11:18:06 AM
I just logged in yesterday for the first time in awhile so maybe you've addressed this, but have you tried a NIST certified sensor?

I hesitate to intrude again on this long-running thread, but isn't the issue here more one of drift from the initial specification over a period of time, whether shorter or longer?

All a NIST sensor from Davis is going to tell you is that when brand new (unless it's a subsequent retest) the RH is confirmed as within ±3% of the reading of some NIST-traceable reference instrument (apparently a General Eastern M4-RH Dew Point Monitor but presumably with some intrinsic error of its own) at 33%; 80% and 90% RH only. Does that really help a lot?

More details in attached PDF. This is the document currently downloadable from Davis but I can't see its original date. Conceivably the tests specs have been tightened since the introduction of SHT31 but there's nothing to indicate that.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on July 19, 2018, 11:36:00 AM
I just logged in yesterday for the first time in awhile so maybe you've addressed this, but have you tried a NIST certified sensor?

I hesitate to intrude again on this long-running thread, but isn't the issue here more one of drift from the initial specification over a period of time, whether shorter or longer?

All a NIST sensor from Davis is going to tell you is that when brand new (unless it's a subsequent retest) the RH is confirmed as within ±3% of the reading of some NIST-traceable reference instrument (apparently a General Eastern M4-RH Dew Point Monitor but presumably with some intrinsic error of its own) at 33%; 80% and 90% RH only. Does that really help a lot?

More details in attached PDF. This is the document currently downloadable from Davis but I can't see its original date. Conceivably the tests specs have been tightened since the introduction of SHT31 but there's nothing to indicate that.
I'm pretty sure in his recent posts he was doing experiments with 2 newer acurite sensors and a 5 day old SHT31.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: johnd on July 19, 2018, 11:41:01 AM
I'm pretty sure in his recent posts he was doing experiments with 2 newer acurite sensors and a 5 day old SHT31.

That would be the shorter period then.  :grin: But obviously there may be issues over short-term and then long-term drift, though that's a separate issue. The main point I was aiming at was that NIST certification probably doesn't add much over a random unchecked SHT31, other than to rule out one that was way out of spec, which really shouldn't happen.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 19, 2018, 11:53:50 AM
I'm pretty sure in his recent posts he was doing experiments with 2 newer acurite sensors and a 5 day old SHT31.

That would be the shorter period then.  :grin: But obviously there may be issues over short-term and then long-term drift, though that's a separate issue. The main point I was aiming at was that NIST certification probably doesn't add much over a random unchecked SHT31, other than to rule out one that was way out of spec, which really shouldn't happen.

The thread keeps evolving. Yes, some of us are experiencing drift over a long period. Others of us are experiencing bad sensors from the start (Randy got two bad ones in row recently). All of us reporting issues have wet dew points/humidities in common.

As for NIST certification, apparently now Davis is claiming in an email conversation their certified sensors are good to +/- 1% at 33% and 80% and +/- 2% at 90%. This was mentioned a couple pages back by jerryg. But I would again point out that they don't mention what temperature(s) they test. This is relative humidity after all. Also, there is no checkpoint in the middle range (40-70%) which is where most of the problems lie. So, presumably, the sensor could pass all those tests and someone could hook it up and see that at a temp/dew point of 90F/70F humidity is running high by 5% or more.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on July 19, 2018, 12:05:17 PM
What is most irritating is I, and many other folks posting in this Forum, thought by purchasing Davis equipment we were buying the best available in the consumer market. Now I have second thoughts and I am on my third VP2, the latest being the 24hour FARS I bought from Ryan about 2 months ago. The latest appears to be better rainwise than the prior two I have owned but I haven’t had a huge downpour yet to see if the tipper does better than the prior two. However, the humidity and temperature readings have been inconsistent over the past 2 months—sometimes high other times low—when compared to what I believe are good comparables nearby. I informed Ryan that I won’t be asking for a replacement of the SHT31 yet, as I am not convinced I will be gaining anything. As I posted earlier, I thought my 2 year old VP2 was reading humidity high when compared to my new one but the next day—the results flipped and continue to be variable—when compared with my other VP2s on the same fence 10-50 feet apart. And as others have noted, I don’t recall ever seeing a 100% humidity reading recorded by my stations since I bought my first one in 2005.

In short, I wish Davis would address these issues or a decent competitor would arrive on the scene. Sorry Davis, but adding bird spikes, creating an aerocone, adding the Davis logo to the anenometer, replacing the RH sensor in both the ISS and console with the SHT31 that hasn’t lived up to expectations (poor packaging and bad caps?), dropping the USB Weatherlink software that frequently drops the connection with my console (not once by the new WiFiLogger), doesn’t make me feel better.

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: johnd on July 19, 2018, 12:38:15 PM
What is most irritating is I, and many other folks posting in this Forum, thought by purchasing Davis equipment we were buying the best available in the consumer market.
....
In short, I wish Davis would address these issues or a decent competitor would arrive on the scene.

But what is the evidence that a VP2 is not 'the best available in the consumer market'? (At least in the sense that there's no better all-round station for performance and capability even if, arguably, some other device may offer better accuracy or whatever in some specific respect - though even there I think hard evidence is lacking, except possibly in respect of rainfall accuracy.)

I'm not particularly looking to defend Davis here, but more making the - fairly obvious - point that it's surely unrealistic to expect higher-end drift-free accuracy at the price-point of a VP2. If a potential competitor felt that there was money to be made from introducing a new range of stations with better all-round performance but at the VP2 price point then surely they would have done so.

But to date this simply hasn't happened, which tends to suggest that it's just not an attractive commercial proposition. VP2-type weather station volumes are in the several thousands per year, not the millions which make innovative mass production much more appealing.

Davis manufactures stations at VP2 prices because it makes it makes some initial design decisions and then sticks with them over extended production periods, with only limited design evolution along the way until a complete new model is released. Anything else would push up the price further.

Don't get me wrong. I think a strong competitor to Davis would be excellent news to drive innovative features and and overall performance of VP2-class station onwards at a faster pace, but equally I can understand the commercial reasons as to why there's no real sign of it happening at present. (Maybe there'll be an announcement of some new wonder station tomorrow even, just to prove me wrong, but somehow I doubt it.) For now IMO, the VP2 represents what is commercially possible at its price point. For users willing to pay substantially more there are other options.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: klschmidt on July 19, 2018, 01:26:04 PM
What is most irritating is I, and many other folks posting in this Forum, thought by purchasing Davis equipment we were buying the best available in the consumer market.
....
In short, I wish Davis would address these issues or a decent competitor would arrive on the scene.

But what is the evidence that a VP2 is not 'the best available in the consumer market'? (At least in the sense that there's no better all-round station for performance and capability even if, arguably, some other device may offer better accuracy or whatever in some specific respect - though even there I think hard evidence is lacking, except possibly in respect of rainfall accuracy.)

I'm not particularly looking to defend Davis here, but more making the - fairly obvious - point that it's surely unrealistic to expect higher-end drift-free accuracy at the price-point of a VP2. If a potential competitor felt that there was money to be made from introducing a new range of stations with better all-round performance but at the VP2 price point then surely they would have done so.

But to date this simply hasn't happened, which tends to suggest that it's just not an attractive commercial proposition. VP2-type weather station volumes are in the several thousands per year, not the millions which make innovative mass production much more appealing.

Davis manufactures stations at VP2 prices because it makes it makes some initial design decisions and then sticks with them over extended production periods, with only limited design evolution along the way until a complete new model is released. Anything else would push up the price further.

Don't get me wrong. I think a strong competitor to Davis would be excellent news to drive innovative features and and overall performance of VP2-class station onwards at a faster pace, but equally I can understand the commercial reasons as to why there's no real sign of it happening at present. (Maybe there'll be an announcement of some new wonder station tomorrow even, but I somehow doubt it.) For now, the VP2 IMO represents what is commercially possible at its price point. For users willing to pay substantially more there are other options.
Thank-you for the well written post johnd, I agree with you. In most manufactured products, the price increase vs quality increase is non-linear. (A station that is .5% more accurate may cost 20 times as much.) When providing feedback to Davis I think we should convey one of the following points: I as a customer/potential customer
(A) would pay more for a higher quality station.
(B) am happy with the price and quality of the current stations.
(C) would accept lower quality if the price could be lowered.
Complaining about the quality AND high price probably won't accomplish much.

I would like to provide highly accurate data to NWS and others. However, after spending a lot of time and effort trying to tweak my station to perfection, will forecasters be aware of the fact that my station is the best in the county? I stopped by the NWS office one day to mention my really good station siting. I got the impression that they would rather use data from a station that had been online longer than mine, and therefore had more history. They were not aware of the fact that this was a Vantage Vue mounted about two feet above a metal roof and surrounded by grain bins. After this visit it was very clear to me that any efforts that I put into high accuracy are for my own benefit only; other's don't know or care. If I'm the only one who cares about my station, I find it easier to say "hmmm... dense fog is more humid than the 96% my Davis is showing".
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 19, 2018, 01:47:59 PM
I have a question, this thread has been about the vp2 and i was just wondering if the vue has this problem.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 19, 2018, 01:55:48 PM
johnd, I don't believe the issue is whether Davis is longer than the other short straws in its segment. It's that they market the station with tight specs. When it's demonstrated through a variety of field and lab tests that those specs aren't accurate or even close, they refuse to run any of their own tests or to stand behind their product. Instead, they explicitly put the onus on the customer and put their retailers (you) into the direct line of fire.

Since Davis lets its marketing department publish fantasy specs, this thread exists to investigate and publicize the real specs granting prospective and existing customers better information. Perhaps, through some ingenuity, hard work and luck, we can cobble together a viable workaround that produces better results. Perhaps not.

In any event, it is not unreasonable to expect a device to live up to its published specifications (+/- 2% 10-90% humidity) and calibration longevity (5 years). If we all passively accept that it's impossible to get even somewhat accurate technical data on scientific equipment, then WE become the problem.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 19, 2018, 01:58:31 PM
Last update on the Acurite sensor passive in shade still running 8% lower humidity. 
Acurite:82-45%
Davis 82-53%

I agree with the above on price point but for anyone living in high heat and high humidity regions getting heat indexes in excess of 120° when it's actually much lower doesn't matter what the price point without reliability.

For me it's more about accuracy that I can afford. In this case the sensor itself is just fine.. The problem may very likely be the way Davis molds chip onto board.
I wasn't prepared to come forward with this information I received a few days ago from unnamed source until now after seeing the Acurite performance. I believe it's a strong possibility sensor is being damaged in this process. 
 
And for many this issue may never arise like arid regions and much of the western USA when its hot it's normally dry no problem found.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 19, 2018, 02:18:34 PM
Last update on the Acurite sensor passive in shade still running 8% lower humidity. 
Acurite:82-45%
Davis 82-53%

I agree with the above on price point but for anyone living in high heat and high humidity regions getting heat indexes in excess of 120° when it's actually much lower doesn't matter what the price point without reliability.

This is really worth repeating. We're not talking about a small discrepancy here. No one is quibbling that the sensor runs +/- 3 or 4% at times rather than 2%. It's WAY out of spec in certain ranges. I've seen errors approaching 9% with one of my sensors that's less than 2 years old.

You add in Davis' careless handling of the sensor and this would seem to be a preventable problem, not one that exists because of its price point.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: johnd on July 19, 2018, 02:50:34 PM
I don't believe the issue is whether Davis is longer than the other short straws in its segment. It's that they market the station with tight specs.

OK that's fine - by all means call Davis out for specs that aren't accurate - I don't have a problem with that at all and I genuinely wish you success in persuading Davis to improve their product or revise their specs. That would be an excellent result and, to be clear, I couldn't be more in favour of accurate specs.

But in reality I suspect that the only likely outcome is that the RH specs could get downgraded and the product will stay exactly the same and at the same price and users will still have exactly the same buying decision to make over price/performance - it's just that the RH specs in the background will be slightly different.

A few other random comments, since I've intruded on the thread this far:

* What actually is the Sensirion spec for RH drift over time eg 2 years with the SHT31?

* I can totally understand your concern with the RH spec in hot/humid parts of the world. Here in the UK and probably much of the world north of say 45N or something it's really a non-problem. Of course it's good to have data as accurate as possible - not suggesting otherwise for a moment, but here few users pay too much daily attention to RH or dew point. OTOH we feel about rainfall inaccuracy probably much as you feel about RH inaccuracy and that's our #1 issue with the VP2.

* The whole specs issue on these mid-price stations is a minefield. Davis do provide quite a bit more detailed information than some competitors, but it's still not good enough. Everyone pretty much quotes intrinsic sensor specs and not operational specs that take into account things like radiation shield performance, exposure, wind speed vs wind direction, rainfall vs rain rate vs wind strength etc, nor do they typically address expected drift with age. Specifically on temp and RH, Davis have taken the easy option AFAICS of simply requoting the Sensirion spec rather than spend quite a bit of time, effort and money generating their own data-set over eg 2-3 years in a wide variety of climatic conditions. I wish they had done that, but I can see why they haven't.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 19, 2018, 04:01:26 PM
* What actually is the Sensirion spec for RH drift over time eg 2 years with the SHT31?

Sensirion claims less than 0.25% per year in drift. Therefore, we shouldn't be seeing the kind of issues we are given these sensors having only been for sale for a couple years now.

Davis wouldn't have to spend a lot of resources at this point to begin investigating the problem. They could do what many of us have done and locate stations that they know contain the SHT31 sensor and that also happen to be within, say, 5 miles of an official station that measures humidity/dew point. Their staff meteorologist should be able to identify days and locations with synoptic rather than mesoscale patterns and make meaningful comparisons. Put the results in a spreadsheet. That would provide them at least prima facie evidence. Beyond that they could ask select trusted clients (universities, agricultural networks, weather service offices -- any place that possesses reference equipment) to send them data. They wouldn't even have to tell them why. They could just say they are committed to accuracy in the field and are aggregating data to verify the performance of their instruments... something to that effect.

I can tell you, I've pulled up as many VP2 stations as I could find within a couple hundred miles of my house and I haven't found one yet that doesn't have a significant wet bias compared to known, good NWS stations nearby. That's not a good sign. I've also got a device that's calibrated and guaranteed to be accurate within +/- 2% so I can verify my station's accuracy and the accuracy of stations in town.

I'm north of the 46th parallel. You don't have to get beyond 25C to really notice the issue. I was seeing it yesterday when it was less than 20C outside, with RHs in the 40-50% range. It's just not as obvious (3-5% departure vs 8+% departure when its north of 30C and above about 40% humidity).

The thing is, John, Davis doesn't seem at all interested in the truth. They only seem interested in ducking responsibility. Their public relations department must be on life support because they've handled this in about the worst way imaginable. I highly doubt they'll even bother to change their spec sheet because... ignorance is bliss.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: johnd on July 19, 2018, 04:36:54 PM
I'm north of the 46th parallel. You don't have to get beyond 25C to really notice the issue.

I think I've said pretty much all I have to say here at present. But just to clarify on this one specific point: I wasn't suggesting that any RH shortfall wasn't noticeable hereabouts, it was more that the RH reading, accurate or not, isn't a major priority for our UK users in general. If I had to rank the standard VP2 parameters by importance (ie by the extent to which minor errors trigger support calls)  I'd probably say:

1 Air temperature
2= Rainfall
2= Wind speed
4 Wind Direction
5 Pressure
6 RH/Dew point

I do understand that those in different regions may have other priorities.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: johnd on July 19, 2018, 04:52:08 PM
England itself doesn't have deserts, but the UK et.al. does for sure...in Australia!

Probably best stand back while one of the Aussies answers that one  :grin: .

But actually we're not far off desert here at present over much of England - longest drought since 1976 and maybe further back. Surprisingly enough maybe, we don't get a lot of rain in Eastern England - maybe 20-25"/year but for crops, gardens etc we do rely on it being reasonably distributed throughout the year at roughly 2"/month with no marked seasonal variation. It varies randomly of course, some months might be 1" and others up to 4", but 2"/month is a pretty standard average. Right now though it's been 6-8 weeks with no significant rain and so everywhere is unusually parched. 30% chance of thunderstorms tomorrow though, but back to 30° next week apparently.

Apologies for the OT weather report!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on July 19, 2018, 05:33:47 PM
JohnD, you made some excellent points today (as you always do) in this thread, particularly regarding the economics of why a competitor would take on Davis in view of the size of the PWS market—not likely. The thing most troubling to me is Davis is apparently selling products that aren’t up to spec and part of this problem, based on what I read in this thread (that makes it the Gospel, correct?), is a poorly designed cap around the SHT31 sensor and (or) how Davis ships SHT31 replacements to resellers like you and Mr. Wilhour, and eventually to me, the end user, is packaging that does not meet its supplier’s recommendation—that I find very disturbing.

But you are right. There isn’t a good reasonably priced comparable product out there, so we complain, but still buy more Davis gear, and yes, if a VP-3 ever comes out, I probably will buy it too, because the PWS market won’t have any similar toys for me to buy!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 19, 2018, 07:43:36 PM
This is very good news Davis is going to take a serious look.
From Davis tech support: 

Hello Randy,

 

This is of serious concern to us and our engineering team is actively looking in to this.  I really appreciate you pushing so hard on this issue.  It’s only going to make us better.

 

I need to give them time to quantify the problem/drift, determine root cause and come up with a solution.

 


Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 19, 2018, 07:44:37 PM
Granted, completely different issues, but I imagine y'all have noticed that WU is at least trying to get things straighten out. With that, just as I and others have predicted, crickets from Davis. Would it be too difficult to acknowledge there's possibly, just possibly, an issue to your devoted clientele?? Throw us a bone, say you're culpable or not. Say something.
Look boys (Davis), you're rapidly losing credibility on this forum, if nothing else but because of your silence. Granted, we're only one tiny segment of your sales, but comparatively speaking, we're a damned educated one and want answers.
Give us something, like at least you're "looking into it".
Balls in your court.....
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: DaleReid on July 19, 2018, 07:45:06 PM
I guess I don't live around 96+% humidity much, so I really don't care.

I know Davis stations are expensive, I'm still not upgrading until the 3 comes out or I die, and then my wife will put on on the marker when they do.

But face it, for all the functions they provide, at a premium, there are few alternatives that are integrated and are wireless and last up to 14+ years (like my original  VP) and still be darned close, what do you expect?

Texas Weather has some old stuff that keeps on cranking since they were built very well and tough and their temperature/humidity sensors are pretty close to every thing else.  The only thing that is very consistent and as far as I an tell repeatable but then again I don't have a humidity cabinet, are the Vaisala WXT500 series.  Their rain sensor is a bit weird and they caution that it isn't for research grade observations but I have a (used) 510 and a 520 that cork along putting out temperatures within a 1/2 degree of each other and the humidity is within a percent or two.  The highest I've had was 99% on one and 98.7 % on the other one.  The barometers track within a few 0.001 in pressure as often as I cross check them.  Get out a checkbook for a (new) one in the range of two grand. They are by far the most consistent tracking with themselves or the TWI or the RM Young stuff.  There are some lower teir professional stuff that runs into the two to five grand figures, with a temp/humidity HMP333 from Vaisala as part of that for a couple grand with the goodies on board.

I think that Davis is expensive.  But buying a couple hundred dollar grade school system a few times adds up, too.  My $1200 Rainwise MarkIII is now showing me 100% and when it is lower humidity, it is pretty close to the other stations, all located on my 5 acres of property.

I learned long ago that the rain measurements were a bit of a variable, too.  The (used) tipping buckets that run $300 to $500 new are never equivalent, even though within a few feet of each other on the same surface and height.  And compared to the open collector (without the wind dress around the outside to shield it, which aren't at all the airports, either) there is always a few hundreds of variation.

I guess that while seeing this is one of the most active conversations on this thread, and with some really cool comparisons by some dedicated observers, I can't see faulting Davis for using the best they can get at a competitive price and doing quite well.  If they have packaged or shipped their new replacement sensors incorrectly, then folks like Scaled and Ambient need to complain that their customers have discovered a problem with Davis' packing and shipping, along with notes from the dedicated users here who have done the testing.

But after I read the specs for RM Young, Vaisala and other pro grade devices, they all say there is a known drift per year, and give graphs showing how much it might be.  And they come (new) calibrated with a sticker saying when the next calibration is due, if you can afford it.  The Vaisala WXT510 and 520 I have would run about $500 each to be tuned up.  My Met One ultrasonic wind sensor would be a $150 charge to just ship it in, and then more on top of it.  So when someone tells me that a $20 chip won't stay in calibration for a year or so, I'm not too put out.

I wish they'd say how to calibrate them rather than sending them back, with which screw to turn to adjust the slope and which to adjust the intercept, but then you'd need a closed chamber with a large bath of various salts to jump back and forth between to do it right.  I don't want to have the wife mad at me for spending the equivalent of a brand new Davis or two a year just to get my stuff re-calibrated.

I do note that one of my Vaisala's is no longer in certification (by about a decade) and it is within a few percent most of the time with all the other humidity sniffers I have. Even though it isn't perfect, it remains a stable workhorse for me.

But I'm not a university or have a government grant so I'm thinking anything from Rainwise, to Davis to Texas Weather is very cool to have, heck even the Peet Brothers that I have compare far closer than I thought based on what I paid for them.

Sorry to seem a bit touchy, but you buy a $400 cam corder don't expect to get Spielberg or Stanley Kubrik production quality videos, but the kids and grandkids still look so much better on the HD from the cheap camera compared to the really expensive prosumer stuff.  And the prop blades on airplanes or helicopters still are bent.

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 19, 2018, 07:47:47 PM
Holly crap! Talk about timing!! :shock:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: SnowHiker on July 19, 2018, 08:37:43 PM
... I'm still not upgrading until the 3 comes out or I die, ...
When/if the 3 comes out I would still wait some time until it has a proven track record. Just look at how a few days or so ago the SHT 31 was being pushed and praised as if it was one of the greatest things ever by the same people who are complaining most vocally now.  Even if the 3 comes out, why upgrade if the equipment you have is still working fine?  I'm not one who sees every change as being a must-have upgrade.

I guess I don't live around 96+% humidity much, so I really don't care.
My view may also be "fogged" by my cool, dry climate.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Intheswamp on July 20, 2018, 10:52:24 AM
Hmmm.  Ok, seems the Davis implementation of the Sht31 sensor isn't optimal and has it's deficiencies.  Rather disappointing to find out.  :-|  As I've mentioned elsewhere, I'm the king of procrastinators and have a 2-year-old Sht31 sensor still in the bubble wrap that I intended to install soon.  Now I'm wondering if I should just wait and see what Davis comes up with.  Over the last year or so I've noticed the heat indexes have "seemed" elevated more than I recalled them being when the station was newer.

Yesterday I measured (old circa 2013 sensor still in place) a high temperature of 91.5F, looking at a graph the humidity appeared to be 55%...this calculates to a heat index of ~100F.  My Davis station reported a heat index of 111.4F.  So, even going with the discredited SHT31 assembly, I suppose I should see "some" improvement regarding these numbers...??? 

My station is now a touch over five years old, so I guess it's expected that the temp/humidity sensor is questionable.  I'm one of those living in a high-temperature/high humidity area as are most folks in the southeast part of the USA where the heat index can be fairly important.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: graculus on July 20, 2018, 11:53:14 AM
This is very good news Davis is going to take a serious look.
From Davis tech support: 

Hello Randy,

 

This is of serious concern to us and our engineering team is actively looking in to this.  I really appreciate you pushing so hard on this issue.  It’s only going to make us better.

 

I need to give them time to quantify the problem/drift, determine root cause and come up with a solution.

 


Strange that in all the years Davis have been using Sensiron they would not have seen this problem with the stations they have on test at Hayward and out in the field  :?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 20, 2018, 12:18:28 PM
Hmmm.  Ok, seems the Davis implementation of the Sht31 sensor isn't optimal and has it's deficiencies.  Rather disappointing to find out.  :-|  As I've mentioned elsewhere, I'm the king of procrastinators and have a 2-year-old Sht31 sensor still in the bubble wrap that I intended to install soon.  Now I'm wondering if I should just wait and see what Davis comes up with.  Over the last year or so I've noticed the heat indexes have "seemed" elevated more than I recalled them being when the station was newer.

Yesterday I measured (old circa 2013 sensor still in place) a high temperature of 91.5F, looking at a graph the humidity appeared to be 55%...this calculates to a heat index of ~100F.  My Davis station reported a heat index of 111.4F.  So, even going with the discredited SHT31 assembly, I suppose I should see "some" improvement regarding these numbers...??? 

My station is now a touch over five years old, so I guess it's expected that the temp/humidity sensor is questionable.  I'm one of those living in a high-temperature/high humidity area as are most folks in the southeast part of the USA where the heat index can be fairly important.

If I were you, I’ll go ahead and replace it with your new sensor but replace the filter cap to Sensirion’s SF2 cap
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 20, 2018, 12:35:22 PM




Strange that in all the years Davis have been using Sensiron they would not have seen this problem with the stations they have on test at Hayward and out in the field  :?
 

Maybe they just don't look, these aren't necessarily all weather geeks like us watching closely especially me and many on this forum do.
They were running about +3 DP last time I looked over airport and currently same temperature and + 10% humidity from nearby airport. Really classic example without the high heat. I really don't know exactly where both are located other than Davis is near the water.

Put this in high heat index content and they may have noticed.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: SLOweather on July 20, 2018, 01:05:51 PM
I don't recall if all of this has been mentioned... Sensirion has some specific info in their literature and datasheets that is pertinent.

From the datasheet for my analog SHT3x

Quote
The sensor shows best performance when operated within recommended normal temperature and humidity range of 5 –
60 °C and 20 – 80 %RH, respectively. Long term exposure to conditions outside normal range, especially at high humidity,
may temporarily offset the RH signal (e.g. +3%RH after 60h at >80%RH). After returning into the normal temperature and
humidity range the sensor will slowly come back to calibration state by itself. Prolonged exposure to extreme conditions
may accelerate ageing. To ensure stable operation of the humidity sensor, the conditions described in the document
“SHTxx Assembly of SMD Packages”, section “Storage and Handling Instructions” regarding exposure to volatile organic
compounds have to be met. Please note as well that this does apply not only to transportation and manufacturing, but also
to operation of the SHT3x-ARP.

From their handling instructions PDF:

Quote
For exposure to extreme conditions with regards to humidity and temperature please consult the datasheet of respective product. Please make sure that exposure time of the sensor to maximum range of operating conditions is limited as given in datasheet. Exposure to volatile organic compounds at high concentration and long exposure time is critical not only in assembly but also in the field. Such application needs to be carefully tested and qualified.

From the same PDF re: reconditioning:

Quote
Reconditioning Procedure
As stated above extreme conditions or exposure to solvent vapors may offset the sensor. The following reconditioning procedure may bring the sensor back to calibration state:
Baking: 100 – 105°C at < 5%RH for 10h
Re-Hydration: 20 – 30°C at ~ 75%RH for 12h 1.

Their storage, handling, mounting, and cleaning instructions are also sufficiently strict that it's likely that not everything is is being adhered to during manufacturing. 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on July 20, 2018, 01:12:04 PM




Strange that in all the years Davis have been using Sensiron they would not have seen this problem with the stations they have on test at Hayward and out in the field  :?
 
Maybe they just don't look, these aren't necessarily all weather geeks like us watching closely especially me and many on this forum do.
They were running about +3 DP last time I looked over airport and currently same temperature and + 10% humidity from nearby airport. Really classic example without the high heat. I really don't know exactly where both are located other than Davis is near the water.

Put this in high heat index content and they may have noticed.
Here's the Gladstone for KHWD.

https://weather.gladstonefamily.net/site/KHWD

If you zoom out on the map you will see the cluster of about 5 stations to the SSW that are all at Davis HQ. It may be interesting to compare all of those. Of course they probably have some at 2m and some up on their roof.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 20, 2018, 01:24:42 PM
I don't recall if all of this has been mentioned... Sensirion has some specific info in their literature and datasheets that is pertinent.

From the datasheet for my analog SHT3x


From their handling instructions PDF:

Quote
For exposure to extreme conditions with regards to humidity and temperature please consult the datasheet of respective product. Please make sure that exposure time of the sensor to maximum range of operating conditions is limited as given in datasheet. Exposure to volatile organic compounds at high concentration and long exposure time is critical not only in assembly but also in the field. Such application needs to be carefully tested and qualified.



Think it has been but worth mentioning again. The assembly may be key, I just don't know how they do it and not expose chip to bad stuff when molded onto board.

Fingers crossed they find issue.   
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on July 20, 2018, 01:25:05 PM
For some reason only E4718 is showing on the map for me now. Anyway...

Gladstone has the avg daytime error for HWD running at -1.5F. The Davis E4718 station is +1.9F. Maybe Davis experiences more marine taint? Just throwing out possibilities. Of course it could simply just be 3% too high too. ;)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 20, 2018, 02:05:34 PM
My last Acurite check early this morning max humidity at 97%, Davis 96% aspirated so both looking normal.

At 12:40 CT I did a passive shade check against new Davis sensor with 5mph breeze running 7-9% lower humidity so really nothing changed from yesterday.

Not keeping acurite saturated in high humidity outside 24/7 so can't really determine how much drift if any is taking place toward wet side.  Max exposure time has been around 2 hours in early morning allowing it to record highest humidity.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 20, 2018, 10:18:42 PM
My last Acurite check early this morning max humidity at 97%, Davis 96% aspirated so both looking normal.

At 12:40 CT I did a passive shade check against new Davis sensor with 5mph breeze running 7-9% lower humidity so really nothing changed from yesterday.

Not keeping acurite saturated in high humidity outside 24/7 so can't really determine how much drift if any is taking place toward wet side.  Max exposure time has been around 2 hours in early morning allowing it to record highest humidity.

Im guessing you’re like me. DPs are 3-4 degrees off being wet in bias
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 20, 2018, 10:33:50 PM
My last Acurite check early this morning max humidity at 97%, Davis 96% aspirated so both looking normal.

At 12:40 CT I did a passive shade check against new Davis sensor with 5mph breeze running 7-9% lower humidity so really nothing changed from yesterday.

Not keeping acurite saturated in high humidity outside 24/7 so can't really determine how much drift if any is taking place toward wet side.  Max exposure time has been around 2 hours in early morning allowing it to record highest humidity.

Im guessing you’re like me. DPs are 3-4 degrees off being wet in bias

+4° dp when I did the afternoon check.  Thread has went about as far as it can go unless anyone has something we didn't cover. Let Davis do their thing and see what they come up with.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: SLOweather on July 20, 2018, 10:40:13 PM
Thread has went about as far as it can go unless anyone has something we didn't cover. Let Davis do their thing and see what they come up with.

Unless someone wants to tackle the reconditioning procedure and report back.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 20, 2018, 10:55:18 PM
I would be interested in how to recondition but removing chip for baking I wouldn't know where to start.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: SLOweather on July 20, 2018, 11:33:50 PM
I inferred from the docs that baking could be done with a mounted sensor. It's only at ~100°c for 10 hours, so it shouldn't hurt anything, board or cable. The tough part would be holding that temp pretty close. Hmmm. Maybe in a crock pot on low with a thermometer to monitor the temp. The thermal mass should be great enough that you could manually control it.

Interesting... The reconditioning for the SHT1x series is slightly different:

Quote
Conditions outside the recommended range may
temporarily offset the RH signal up to ±3%RH. After return
to normal conditions it will slowly return close to calibration
state by itself.
To accelerate this process we recommend the following
reconditioning procedure:
90°C at <5%RH for 24h followed by
20-30°C at >74%RH for 48h

 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 20, 2018, 11:49:11 PM
Only 212F sounds like a plan. I have some of the correct bags non gaseous for Re-Hydration and wet rag.

Reconditioning Procedure
As stated above extreme conditions or exposure to solvent vapors may offset the sensor. The following reconditioning procedure may bring the sensor back to calibration state:
Baking: 100 – 105°C at < 5%RH for 10h
Re-Hydration: 20 – 30°C at ~ 75%RH for 12h 1.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 21, 2018, 08:06:14 AM
Would it be ok to leave filter cap on or take it off before baking? Secondly, would it be ok to put the sensor on a metal baking pan?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 21, 2018, 08:30:31 AM
Take it off for sure. I'm doing one now I'll let you know how it comes out .  :?:  If this fixes sensor its something everyone can try. I'll be done around about 9pm tonight including hydration.
I'm using a small crockpot and it doesn't have temperature control and gets to about 120c if lid is on so have cracked open about 1/2" and sensor sitting on top of foil tent I built.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Intheswamp on July 21, 2018, 08:42:22 AM
Excellent, Randy!!! 

This sounds much like renewing silica desiccant, though at a higher temperature.  :-k  It will be interesting to see whether this improves performance.  If it does then it poses the question, for me, of whether I should do this before I install my new/old SHT31.

Thanks for taking the initiative!

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 21, 2018, 09:22:49 AM
Excellent, Randy!!! 

This sounds much like renewing silica desiccant, though at a higher temperature.  :-k  It will be interesting to see whether this improves performance.  If it does then it poses the question, for me, of whether I should do this before I install my new/old SHT31.

Thanks for taking the initiative!

If big IF  I'm just not sure how perfect baking temps must be. I've went over recommended and been under also being I don't have a controlled environment. I would hope Davis would do all sensors and ship fresh restored units so you don't have to do it.  My week old sensor has the high bias only difference from older sensors it will reach 98% vs 97 and 96%.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 21, 2018, 09:23:17 AM
Yes, Randy, thank you for running this experiment.

However, the handling and reconditioning instructions prompt me to ask two questions:

1) Why would 2 sensors less than a couple weeks old need "reconditioning"?

2) Why would Davis choose a sensor in which normal environmental conditions for most of its customers would knock it out of calibration not long after purchase (within a year) and require a complicated procedure to remain accurate?

Based on Sensirion's literature, it would appear these sensors are designed for indoor use.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on July 21, 2018, 09:43:32 AM
As discussed in several posts before, it is believed Davis has not done a very good job packaging the SHT31 when it leaves the Davis facility and secondly, the white cap that encases the sensor is not very effective in keeping dirt, etc. away from the sensor. Both reasons, if accurate, may give rise to a short life of the SHT31 requiring recalibration.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 21, 2018, 09:52:41 AM
Yes, Randy, thank you for running this experiment.

However, the handling and reconditioning instructions prompt me to ask two questions:

1) Why would 2 sensors less than a couple weeks old need "reconditioning"?

2) Why would Davis choose a sensor in which normal environmental conditions for most of its customers would knock it out of calibration not long after purchase (within a year) and require a complicated procedure to remain accurate?

Based on Sensirion's literature, it would appear these sensors are designed for indoor use.



Well my thinking is because new sensors already have the high bias as stated but the Acurite units don't they are being contaminated in production process to board. By doing the baking we get rid of the production contaminates and sensor will work as advertised. This is wishful thinking I'm sure.  :lol:
 
And yes after some reading it does look like these aren't designed for continuous high humidity.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 21, 2018, 09:57:12 AM
As discussed in several posts before, it is believed Davis has not done a very good job packaging the SHT31 when it leaves the Davis facility and secondly, the white cap that encases the sensor is not very effective in keeping dirt, etc. away from the sensor. Both reasons, if accurate, may give rise to a short life of the SHT31 requiring recalibration.

Not disputing that at all, but look again at the recommendations for the sensor.

Quote
The sensor shows best performance when operated within recommended normal temperature and humidity range of 5 –
60 °C and 20 – 80 %RH
, respectively.

Quote
Prolonged exposure to extreme conditions may accelerate ageing.

So operating the sensor below 41F or in humid/foggy conditions will knock it out of calibration. Repeatedly exposing it to these temperatures and humidities will "age" the sensor (which I take to mean degrade).
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 21, 2018, 09:58:51 AM
Only 212F sounds like a plan. I have some of the correct bags non gaseous for Re-Hydration and wet rag.

Reconditioning Procedure
As stated above extreme conditions or exposure to solvent vapors may offset the sensor. The following reconditioning procedure may bring the sensor back to calibration state:
Baking: 100 – 105°C at < 5%RH for 10h
Re-Hydration: 20 – 30°C at ~ 75%RH for 12h 1.

Can you pick which option you want to do or do you have to do both for recalibration to take place (if it does)?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 21, 2018, 10:08:08 AM
Quote
After returning into the normal temperature and humidity range the sensor will slowly come back to calibration state by itself.

Many of us, including Ron, have noted that, at some point, these sensors don't return to calibration "slowly" as Sensirion claims. Perhaps that's why they include the caveat about sensor aging. If a year and a half old sensor is "aged" due only to the fact that it wasn't kept above 40F and under 80% humidity, the most logical conclusion would be it shouldn't be installed outdoors in most climates.

Now, perhaps the sensor is damaged by Davis' handling. If so, then Randy's reconditioning procedure should not help.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: SLOweather on July 21, 2018, 10:21:34 AM
Can you pick which option you want to do or do you have to do both for recalibration to take place (if it does)?

In closely reading the procedure, it seems to me that the 2 steps are required, and that they must be done in that order.  Baking at low humidity would dry out the sensor, and the the rehydrating part would recondition it to a "normal" operating condition.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 21, 2018, 10:30:53 AM
Ugh  ](*,)  So if these aren't even the correct sensor for outdoors applications as suggested this process may only work until over saturated with moisture again.  I'm in the rehydration phase now maybe I should skip, it may not be necessary because once outside it will get all the moisture it needs plus.
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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 21, 2018, 10:37:41 AM
I'm in the rehydration phase now maybe I should skip, it may not be necessary because once outside it will get all the moisture it needs plus.

No, please complete the procedure. It's helpful information to have either way!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 21, 2018, 10:47:55 AM
Question: does Acurite have any outdoor weather stations with the SHT31 installed?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 21, 2018, 10:59:23 AM
Question: does Acurite have any outdoor weather stations with the SHT31 installed?

Don't think so.. The unit that comes with multiple stations specs different.

Temperature Range   -40 to 158 degrees Fahrenheit; -40 to 70 degrees Celsius
Temperature Accuracy   +/- 2 degrees Fahrenheit; Learn more
Humidity Range   1% to 99% Relative Humidity
Humidity Accuracy   +/- 5% from 0% to 10% Relative Humidity
+/- 4% from 10% to 20% Relative Humidity
+/- 3% from 20% to 80% Relative Humidity
+/- 4% from 80% to 90% Relative Humidity
+/- 5% from 90% to 100% Relative Humidity
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 21, 2018, 11:05:09 AM
Indeed, according to a forum posting 7 months ago by an Acurite Rep (https://support.acurite.com/acurite/topics/are-you-planning-to-release-a-wireless-external-measuring-instrument-or-sensor-using-the-sht31-sensor (https://support.acurite.com/acurite/topics/are-you-planning-to-release-a-wireless-external-measuring-instrument-or-sensor-using-the-sht31-sensor)), the answer is no.

Wonder why they would put the sensor in their new indoor unit (the one Randy's been testing), but not an outdoor unit like the Atlas?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 21, 2018, 11:13:57 AM
Now why would Acurite recently release the Atlas to compete against the VP2 and have a less accurate temperature sensor? It's billed as a "professional" station. Specs I've seen claim +/- 1F. Davis, of course, is +/- 0.5F. Humidity specs are, supposedly, the same. See: https://www.acurite.com/media/documents/AcuRite-Atlas-Elite-Specifications.pdf (https://www.acurite.com/media/documents/AcuRite-Atlas-Elite-Specifications.pdf)

They obviously know about the Sensirion sensor. They put it in their much cheaper indoor-only product. So awareness or cost cannot be the issue.

I would think they'd want to be at least as accurate as their competitor(s) in a certain product segment. They are not. But maybe they decided they didn't want all the warranty headaches that come from putting a sensor best suited for indoor conditions in an outdoor unit. I'm sure there are other considerations as well.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: DoctorKnow on July 21, 2018, 11:23:04 AM
I have the atlas. I also have the 5n1. The humidity is spot on with the Atlas. The temperature is within 1 degree for sure. I hope you guys will try it when it starts being sold. I think you will be very happy with the hardware. I hope the software will continue to offer more, like CWOP uploads because everything is much improved, compared to the 5in1.

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 21, 2018, 11:29:44 AM
Ok here is some test info mainly for people in high humidity areas. I have been running 3 vp2 shields with Davis fans on ac power, two test shields running 2.3vots on fan all the time and my main station runs 2.3 volts during the day and with the use of a photocell and relay drops down to 1 volt at night. I put in sht11 sensors in all three that match pretty well in performance. What i found is the amount of airflow at night when the humidity rises is important. When the sun came up the main station humidity reading started to drop pretty soon after the sun was up and the temp began to rise. The two test sensors did not show a drop until almost 30 minutes later and the main reading dropped steadily while the two test lagged behind around 4% higher and did not get even with the main reading until the sun had dried off the grass shields. The higher fan speed at night is causing the sensor chamber to get a lot more moisture in the shield and takes longer to dry out. Now is put the new 31 in one test shield and a two year old 31 in the other and the results were the same except it took longer for the 31 to start dropping than it did the sht11 and at one point the main station was running 7% drier than the 31's. It looks like the fans at night are putting a lot of moisture inside the shields at night and the 31's don't handle it as well as the 11's. I ordered two of the sht15 from
Ryan and they will be here Monday and i will repeat the test using the new 31, the new 15 and the two year old 31, should be interesting to find out what shows up.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dport on July 21, 2018, 11:34:27 AM
Now why would Acurite recently release the Atlas to compete against the VP2 and have a less accurate temperature sensor? It's billed as a "professional" station. Specs I've seen claim +/- 1F. Davis, of course, is +/- 0.5F. Humidity specs are, supposedly, the same. See: https://www.acurite.com/media/documents/AcuRite-Atlas-Elite-Specifications.pdf (https://www.acurite.com/media/documents/AcuRite-Atlas-Elite-Specifications.pdf)

They obviously know about the Sensirion sensor. They put it in their much cheaper indoor-only product. So awareness or cost cannot be the issue.

I would think they'd want to be at least as accurate as their competitor(s) in a certain product segment. They are not. But maybe they decided they didn't want all the warranty headaches that come from putting a sensor best suited for indoor conditions in an outdoor unit. I'm sure there are other considerations as well.

Keep in mind acurite plans to introduce two new stations.  The atlas and the atlas elite.  The atlas, which some folks are testing now, is your all-in-one unit.  Atlas Elite looks more like a Davis with the separate of the anemometer.  Not even sure that one exists in the field today though, at least I haven't seen any real pictures yet.  Guessing it could be quite some time before we see it.  This is why I bought a VP2.  I'm very happy with the unit overall.  Doubtful acurite will offer the replaceable hardware the way Davis does.

Let's hope we find some solutions to the SHT31 though.  Not sure baking is in my future :)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 21, 2018, 11:57:04 AM
Just to add to my testing results i am now using the sht11 instead of the new 31 because my humidity and dp readings are so much better with the 11, in every why the basic readings are better with the sht11 and i am glad that i have a few to spare because the sht1x line is no longer being sold. That is why i ordered to 15's from Ryan before they are gone for good. In my opinion the 31's have good on paper performance but poor field performance, I wonder if they even test these sensors in the home weather station settings.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: miraculon on July 21, 2018, 12:10:01 PM
For what it's worth, I just took my Belfort 566 psychrometer outside. We are showing 97% RH on my Davis ISS and 95% on the Davis Temp/Hum station. I believe that these are both SHT-11 (or -15) and definitely not SHT-31.

Using distilled water I got 65°F dry bulb and 64°F wet bulb.

The slide rule shows about 94% RH a web calculator gives 94.88% RH. http://linricsoftw.web701.discountasp.net/webpsycalc.aspx (http://linricsoftw.web701.discountasp.net/webpsycalc.aspx)

I ordered one of those Sensirion SHT-31 eval boards from Mouser.

Greg H.
 

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 21, 2018, 12:11:44 PM
I have the atlas. I also have the 5n1. The humidity is spot on with the Atlas. The temperature is within 1 degree for sure. I hope you guys will try it when it starts being sold. I think you will be very happy with the hardware. I hope the software will continue to offer more, like CWOP uploads because everything is much improved, compared to the 5in1.

Ah, okay, so the Atlas has not been released yet. Sorry about that. They have released preliminary specs though.

I want to back up and say that I'm not suggesting that certain compromises don't have to be made in consumer (prosumer) weather gear. Ron (kcidwx) suggested earlier that all stations in this price range are going to have sensors that operate optimally from, say, 30-90% and to get true linear performance you have to spend at least 10x more for the sensor due to tighter tolerances and production inefficiencies (no mass production discount).

As johnd pointed out humidity is pretty low on many people's priority lists. Maybe Davis thought they could compromise on humidity and that most people wouldn't care. I don't know how you interpret their own weather station on WeatherLink being 3 degrees high in dew point at last check yesterday. Maybe they've never noticed or maybe they thought we'd never notice?

In any event, I agree with jerryg on under delivering field performance. Claiming +/- 2% from 10-90% for 5 years after purchase and delivering +10% mid range within a year (sometimes sooner and sometimes longer) is not exactly what I'd consider a reasonable compromise. That's especially true since we know agricultural customers buy these stations to measure evapotranspiration.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 21, 2018, 12:15:53 PM
For what it's worth, I just took my Belfort 566 psychrometer outside. We are showing 97% RH on my Davis ISS and 95% on the Davis Temp/Hum station. I believe that these are both SHT-11 (or -15) and definitely not SHT-31.

Using distilled water I got 65°F dry bulb and 64°F wet bulb.

The slide rule shows about 94% RH a web calculator gives 94.88% RH. http://linricsoftw.web701.discountasp.net/webpsycalc.aspx (http://linricsoftw.web701.discountasp.net/webpsycalc.aspx)

I ordered one of those Sensirion SHT-31 eval boards from Mouser.

Greg H.

Hmm. Interesting. Let us know the results using the 31
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 21, 2018, 01:21:58 PM
Reconditioning I'm seeing no change so I'm scrapping that idea.

Davis units still running +9% humidity and +4dp vs Acurite.

The directions on Acurite say you can manually calibrate but pulling batteries and reinstall resets calibration. I've never messed with any calibration. I don't know if Acurite has asked for different calibration vs the Davis  from factory or if it's even possible, but dp temps on Acurite run very close to airport.

test results after recondition dp temps were as follows

all 3-sht31's -69°
Acurite  -65°
ASOS 1.3 miles away -65°

I just don't know  #-o.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 21, 2018, 01:55:55 PM
Reconditioning I'm seeing no change so I'm scrapping that idea.

Davis units still running +9% humidity and +4dp vs Acurite.

The directions on Acurite say you can manually calibrate but pulling batteries and reinstall resets calibration. I've never messed with any calibration. I don't know if Acurite has asked for different calibration vs the Davis  from factory or if it's even possible, but dp temps on Acurite run very close to airport.

test results after recondition dp temps were as follows

all 3-sht31's -69°
Acurite  -65°
ASOS 1.3 miles away -65°

I just don't know  #-o.

Bummer. Thank you for trying it out and letting us know. But if you take the rehydration part out of the equation and just install the unit back outside, would it make any difference?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 21, 2018, 02:20:36 PM
I have replaced the errant, older SHT31 in my passive shield with a used SHT15. This was in my solar-powered FARS unit for well over a year. So it should be a good test. It's now sitting about 6 feet from my main station, a 24 hr FARS with an AC fan (~20 cfm), containing a 7 month old still accurate SHT31. After letting it acclimate for half an hour, I can report both stations are running extremely close to each other (within a few tenths of a degree F in dew point). I took a reading from my Kestrel 5000 psychrometer and all 3 sensors are within 0.5F dew point.

To be fair, the humidity is currently running around 90-91% here after some rain. That's not where the Davis sensors tend to be problematic. The true test in performance will be once humidity drops below 70%. The SHT15 has been sitting in a basement in the package it came in for 18 months or so. That basement doesn't get above 65% in humidity (currently measuring 55% during the wettest part of the year), though, so there shouldn't be an issue with it exposed to wet air.

We've got rain in the forecast tonight and tomorrow AM but then clearing in the PM Sunday so I should get to see how the older sensor recovers from extended periods of near saturated air while measuring more common humidities.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 21, 2018, 03:55:34 PM
To be fair, the humidity is currently running around 90-91% here after some rain. That's not where the Davis sensors tend to be problematic. The true test in performance will be once humidity drops below 70%.
Maybe Davis should market the 31 as an arid climate only sensor as both my 31's have no issues here, or really anywhere for that matter.
Obviously shouldn't be that way, but certainly looking it.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 21, 2018, 04:39:04 PM
Reconditioning I'm seeing no change so I'm scrapping that idea.

Davis units still running +9% humidity and +4dp vs Acurite.

The directions on Acurite say you can manually calibrate but pulling batteries and reinstall resets calibration. I've never messed with any calibration. I don't know if Acurite has asked for different calibration vs the Davis  from factory or if it's even possible, but dp temps on Acurite run very close to airport.

test results after recondition dp temps were as follows

all 3-sht31's -69°
Acurite  -65°
ASOS 1.3 miles away -65°

I just don't know  #-o.

Bummer. Thank you for trying it out and letting us know. But if you take the rehydration part out of the equation and just install the unit back outside, would it make any difference?

It went 2 hours in bag 75% then realized outdoor humidity and temperatures were just right. So it's back in aspirated shield and received few more of 75% and tonight it will get 10 plus hours of 75%. I think these sensor were meant for indoors thus the hydration procedure is necessary where it's rarely above 40%.

It acts exactly as before will be interesting where it peaks on humidity. 97% was max even when new.

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 21, 2018, 04:50:45 PM
Why would Davis use something that was designed for indoor use as their temp/hum sensor for the VP2?  That doesn’t make sense if that’s the case. Let me know tomorrow morning what your results are in upper 90% humidities. Now I’m curious to what would happen if you just bake the sensor and afterwards put it in the shield? I would guess it may have a different result but not for long.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 21, 2018, 04:54:36 PM
I posted this earlier and I'm sure I got a :roll: from every member here, but I use this to clean my sensors and always have. Just did both of them a few days ago. Probably won't help, but who knows? Whatta ya got to lose, an already bad sensor??
http://www.acehardware.com/product/index.jsp?productId=1451535
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Old Tele man on July 21, 2018, 05:19:18 PM
I posted this earlier and I'm sure I got a :roll: from every member here, but I use this to clean my sensors and always have. Just did both of them a few days ago. Probably won't help, but who knows? Whatta ya got to lose, an already bad sensor??
http://www.acehardware.com/product/index.jsp?productId=1451535

Wonder if Sensirion approves? Here's it's MSDS: http://s7d9.scene7.com/is/content/GenuinePartsCompany/683483pdf?$PDF$ (http://s7d9.scene7.com/is/content/GenuinePartsCompany/683483pdf?$PDF$)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 21, 2018, 05:22:14 PM
I posted this earlier and I'm sure I got a :roll: from every member here, but I use this to clean my sensors and always have. Just did both of them a few days ago. Probably won't help, but who knows? Whatta ya got to lose, an already bad sensor??
http://www.acehardware.com/product/index.jsp?productId=1451535


WE can always just subtract 6-9% humidity and be normal. But will read really low in the morning when it's near 100% almost daily. Or  :idea: kill the sensor with chemical spray.. Let's see which one... :-(
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 21, 2018, 05:24:17 PM
I posted this earlier and I'm sure I got a :roll: from every member here, but I use this to clean my sensors and always have. Just did both of them a few days ago. Probably won't help, but who knows? Whatta ya got to lose, an already bad sensor??
http://www.acehardware.com/product/index.jsp?productId=1451535

Wonder if Sensirion approves? Here's it's MSDS: http://s7d9.scene7.com/is/content/GenuinePartsCompany/683483pdf?$PDF$ (http://s7d9.scene7.com/is/content/GenuinePartsCompany/683483pdf?$PDF$)

I have a sneaking suspicion they don't.  ;)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 21, 2018, 05:25:28 PM
I posted this earlier and I'm sure I got a :roll: from every member here, but I use this to clean my sensors and always have. Just did both of them a few days ago. Probably won't help, but who knows? Whatta ya got to lose, an already bad sensor??
http://www.acehardware.com/product/index.jsp?productId=1451535

Wonder if Sensirion approves? Here's it's MSDS: http://s7d9.scene7.com/is/content/GenuinePartsCompany/683483pdf?$PDF$ (http://s7d9.scene7.com/is/content/GenuinePartsCompany/683483pdf?$PDF$)
Don't care.....
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 21, 2018, 05:27:33 PM
I posted this earlier and I'm sure I got a :roll: from every member here, but I use this to clean my sensors and always have. Just did both of them a few days ago. Probably won't help, but who knows? Whatta ya got to lose, an already bad sensor??
http://www.acehardware.com/product/index.jsp?productId=1451535


WE can always just subtract 6-9% humidity and be normal. But will read really low in the morning when it's near 100% almost daily. Or  :idea: kill the sensor with chemical spray.. Let's see which one... :-(
Yeah, that's how I killed my SHT11, 15 and two 31's that work, that all have been getting the treatment for 10 years.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 21, 2018, 06:00:48 PM
I had to remove a post I just made. Sorry about that if you came here to read it. It contained some incorrect information.

I just found that the SHT1x datasheet has the same language about optimal operating conditions and premature aging as the SHT3x. Although, that might explain the anomalous results posted by Stephen Burt in his 2009 review of the humidity sensor for the VP2 (running the SHT11). Looks like his unit, that was less than a year old, was running high by 5-6% in the middle ranges pretty consistently. I've got the 15 set up now and I'm just waiting for humidity to come down so I can see if it's still in calibration after over a year of wear & tear not to mention 18 months in storage.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Old Tele man on July 21, 2018, 07:02:02 PM
I had to remove a post I just made. Sorry about that if you came here to read it. It contained some incorrect information.

I just found that the SHT1x datasheet has the same language about optimal operating conditions and premature aging as the SHT3x. Although, that might explain the anomalous results posted by Stephen Burt in his 2009 review of the humidity sensor for the VP2 (running the SHT11). Looks like his unit, that was less than a year old, was running high by 5-6% in the middle ranges pretty consistently. I've got the 15 set up now and I'm just waiting for humidity to come down so I can see if it's still in calibration after over a year of wear & tear not to mention 18 months in storage.
I'm gonna guess that the ONLY differences between SHT-11, -15, and -31 are tighter & better production and quality control during fabrication because the basic "science" of mylar polyester capacitive sensors has NOT changed much since mylar was invented back in the 1950's.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 21, 2018, 07:08:06 PM
I had to remove a post I just made. Sorry about that if you came here to read it. It contained some incorrect information.

I just found that the SHT1x datasheet has the same language about optimal operating conditions and premature aging as the SHT3x. Although, that might explain the anomalous results posted by Stephen Burt in his 2009 review of the humidity sensor for the VP2 (running the SHT11). Looks like his unit, that was less than a year old, was running high by 5-6% in the middle ranges pretty consistently. I've got the 15 set up now and I'm just waiting for humidity to come down so I can see if it's still in calibration after over a year of wear & tear not to mention 18 months in storage.
I'm gonna guess that the ONLY differences between SHT-11, -15, and -31 are tighter & better production and quality control during fabrication...the basic "science" of mylar polyester capacitive sensors has NOT changed much since mylar was invented back in the 1950's.

That could be the major culprit of our problems.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 21, 2018, 07:19:48 PM
Here is the current temp/DP as of 6:19p at RW Headquarters compared to nearby Airport station

71/52 at RW

70/52 at Bar Harbor Airport

Max Air Temp at RW 80.5°

Max Air Temp at Airport 78.8°

Max DP at RW 70.4°. Minimum 51°

Max DP at Airport 60.8. Minimum 51.8°

Max Humidity at RW 100% at 2:59a. Minimum 42% at 2:38p

Max Humidity at Airport 100% at 7:05a. Minimum 44% at 1:05p

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: kcidwx on July 21, 2018, 08:45:55 PM
The weather station market is so small that companies are not going to manufacture a humidity sensor with specs just for weather stations. Most humidity sensors are designed for indoor use. One of their biggest uses is for HVAC duct testing. That includes the Sensirion sensors.

As I said earlier in the thread, the SHT-31 was not designed for meteorological use and that should be obvious by its limited normal operating temperature and humidity ranges. But those limited operating ranges are normal for humidity sensors because again, they are not designed for meteorological applications. Davis must have felt that this was the best sensor to do the job even though it was not designed for meteorological applications.

I only know of one humidity sensor that is designed for meteorological applications. That is the one I use at home, the Vaisala HMT337. From the Vaisala literature it states:

HMT337 Configurations

Vaisala HUMICAP Humidity and Temperature Transmitter

HMT337 is ideal for the most demanding process and meteorological measurements in high-humidity condensing environments.


It is a rock solid sensor across the board. If Davis were to use it in the VP2 the cost of the station would probably double.

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on July 21, 2018, 08:59:57 PM
As stated before, variablilty is the norm with the SHT31. Currently (8pm):

New station (24 hour fan)—T 71, H 84
2 year old station (daytime fan)—T 72, H 79
2.5 year old station (daytime fan)—T 72, H 82.

Again, all on the same fence, same height, at most 50 feet apart.

Temperature is right on, humidity, well pick a number. That said, they are “directionally” the same. But many users are correct, the humidity reading is low in importance to most folks. 

The good news today, we had a rain. All three VP2 stations reported .50 give or take which matched my CoCo gauge! It has been many moons since that happened!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Old Tele man on July 21, 2018, 09:46:01 PM
The weather station market is so small that companies are not going to manufacture a humidity sensor with specs just for weather stations. Most humidity sensors are designed for indoor use. One of their biggest uses is for HVAC duct testing. That includes the Sensirion sensors.

As I said earlier in the thread, the SHT-31 was not designed for meteorological use and that should be obvious by its limited normal operating temperature and humidity ranges. But those limited operating ranges are normal for humidity sensors because again, they are not designed for meteorological applications. Davis must have felt that this was the best sensor to do the job even though it was not designed for meteorological applications.

I only know of one humidity sensor that is designed for meteorological applications. That is the one I use at home, the Vaisala HMT337. From the Vaisala literature it states:

HMT337 Configurations

Vaisala HUMICAP Humidity and Temperature Transmitter

HMT337 is ideal for the most demanding process and meteorological measurements in high-humidity condensing environments.


It is a rock solid sensor across the board. If Davis were to use it in the VP2 the cost of the station would probably double.
Let me edit that: "If Davis were to use it in the VP2 the cost accuracy/value of the station would probably double."
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 21, 2018, 09:59:37 PM
The weather station market is so small that companies are not going to manufacture a humidity sensor with specs just for weather stations. Most humidity sensors are designed for indoor use. One of their biggest uses is for HVAC duct testing. That includes the Sensirion sensors.

As I said earlier in the thread, the SHT-31 was not designed for meteorological use and that should be obvious by its limited normal operating temperature and humidity ranges. But those limited operating ranges are normal for humidity sensors because again, they are not designed for meteorological applications. Davis must have felt that this was the best sensor to do the job even though it was not designed for meteorological applications.

I only know of one humidity sensor that is designed for meteorological applications. That is the one I use at home, the Vaisala HMT337. From the Vaisala literature it states:

HMT337 Configurations

Vaisala HUMICAP Humidity and Temperature Transmitter

HMT337 is ideal for the most demanding process and meteorological measurements in high-humidity condensing environments.


It is a rock solid sensor across the board. If Davis were to use it in the VP2 the cost of the station would probably double.


You’re probably right but it’s interesting that IDT makes the claim that their sensor chips are designed for weather stations but their chips pretty much operates in a similar fashion as Sensirion’s

https://www.idt.com/products/sensor-products/humidity-sensors

The only other option that Davis has (in terms of using a cheap chip) is this sensor that was mentioned by hwcorder (which is supposedly designed for high humidities).

https://www.ist-ag.com/en-us/products/hyt-221-calibrated-and-temperature-compensated
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 21, 2018, 11:06:09 PM
The only other option that Davis has (in terms of using a cheap chip) is this sensor that was mentioned by hwcorder (which is supposedly designed for high humidities).

https://www.ist-ag.com/en-us/products/hyt-221-calibrated-and-temperature-compensated

How would they marry that to Sensibus? Also, it's listed as "currently not in stock" by the manufacturer.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 21, 2018, 11:32:38 PM
The only other option that Davis has (in terms of using a cheap chip) is this sensor that was mentioned by hwcorder (which is supposedly designed for high humidities).

https://www.ist-ag.com/en-us/products/hyt-221-calibrated-and-temperature-compensated

How would they marry that to Sensibus? Also, it's listed as "currently not in stock" by the manufacturer.

The only thing I know is to redo the transmitter board to make it compatible with I2C.

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 21, 2018, 11:34:55 PM
The weather station market is so small that companies are not going to manufacture a humidity sensor with specs just for weather stations. Most humidity sensors are designed for indoor use. One of their biggest uses is for HVAC duct testing. That includes the Sensirion sensors.

As I said earlier in the thread, the SHT-31 was not designed for meteorological use and that should be obvious by its limited normal operating temperature and humidity ranges. But those limited operating ranges are normal for humidity sensors because again, they are not designed for meteorological applications. Davis must have felt that this was the best sensor to do the job even though it was not designed for meteorological applications.

I only know of one humidity sensor that is designed for meteorological applications. That is the one I use at home, the Vaisala HMT337. From the Vaisala literature it states:

HMT337 Configurations

Vaisala HUMICAP Humidity and Temperature Transmitter

HMT337 is ideal for the most demanding process and meteorological measurements in high-humidity condensing environments.


It is a rock solid sensor across the board. If Davis were to use it in the VP2 the cost of the station would probably double.

Another note, the RainWise MK III stations use the Sensirion SHT-15 sensor and as far as I know, they do not have the same issues as Davis does.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 22, 2018, 03:48:13 AM
Saw this on Sparkfun today.

Hygrometers: A Cautionary Tale
Our (kinda) epic journey about learning the proper way to handle humidity sensors.

https://www.sparkfun.com/news/1090


Humidity Sensors make use of a conductive polymer to measure relative humidity. If that polymer
gets too dry (or over-saturated) the sensor won't function properly, but that can be reversed.
Whenever we use one of these sensors on our designs, we put them through a re-conditioning
procedure to ensure that they keep their factory calibration
. If you expose your sensor to a really dry
environment for a prolonged period of time (or saturate it with water) you may have to run it through
the same process.

https://cdn.sparkfun.com/datasheets/Sensors/Weather/Hygrometer_ReCon.pdf
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Old Tele man on July 22, 2018, 08:52:09 AM
Saw this on Sparkfun today.

Hygrometers: A Cautionary Tale
Our (kinda) epic journey about learning the proper way to handle humidity sensors.

https://www.sparkfun.com/news/1090


Humidity Sensors make use of a conductive polymer to measure relative humidity. If that polymer
gets too dry (or over-saturated) the sensor won't function properly, but that can be reversed.
Whenever we use one of these sensors on our designs, we put them through a re-conditioning
procedure to ensure that they keep their factory calibration
. If you expose your sensor to a really dry
environment for a prolonged period of time (or saturate it with water) you may have to run it through
the same process.

https://cdn.sparkfun.com/datasheets/Sensors/Weather/Hygrometer_ReCon.pdf

...and, that was written FIVE (5) YEARS ago.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 22, 2018, 08:54:26 AM
How did your reconditioned humidity sensor do this morning?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 22, 2018, 10:46:24 AM
How did your reconditioned humidity sensor do this morning?

We had rain so didn't hit 98% for any.. The newest now over week old SHT31 hit 97% multiple times the reconditioned unit ran lower as it did before even new made only 96%.  What I did notice even though the reconditioned didn't peak well it averaged about 1° dewpoint lower yesterday so I'm baking 3 more units today after reading Sparks Fun conditions all sensors in designs for top performance.

They use to skip the hydration at 75% 20 hours and said it was a mistake in the link above. They bake their units at 125c, any higher I'm thinking things start melting.
 
 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: kcidwx on July 22, 2018, 11:06:25 AM
The weather station market is so small that companies are not going to manufacture a humidity sensor with specs just for weather stations. Most humidity sensors are designed for indoor use. One of their biggest uses is for HVAC duct testing. That includes the Sensirion sensors.

As I said earlier in the thread, the SHT-31 was not designed for meteorological use and that should be obvious by its limited normal operating temperature and humidity ranges. But those limited operating ranges are normal for humidity sensors because again, they are not designed for meteorological applications. Davis must have felt that this was the best sensor to do the job even though it was not designed for meteorological applications.

I only know of one humidity sensor that is designed for meteorological applications. That is the one I use at home, the Vaisala HMT337. From the Vaisala literature it states:

HMT337 Configurations

Vaisala HUMICAP Humidity and Temperature Transmitter

HMT337 is ideal for the most demanding process and meteorological measurements in high-humidity condensing environments.


It is a rock solid sensor across the board. If Davis were to use it in the VP2 the cost of the station would probably double.

I got some PM's about this comment so let me clarify. Manufacturers don't design humidity sensors specifically for weather stations. However, they will often say their sensor is ideal for weather station use. Then they slap you with the fine print about the normal operating temperature and humidity range and what you can expect if the sensor is subjected to conditions outside those ranges. They are simply saying you can use our sensor in a weather station but don't expect optimal performance outside the normal operating temperature and humidity range. I'm sure Davis tested these sensors thoroughly before shipping them in the VP and concluded that the sensor had acceptable performance outside the sensors normal operating temperature and humidity range. Now what Davis thought was acceptable versus what you are expecting may be two different things  :lol: Also, I'm not saying this is the cause of the Davis sensor issue as the problem seems to occur even within the normal operating temperature and humidity range.

If you have high expectations of a particular weather station then before you buy that weather station, you should read the entire sensor datasheet from the sensor manufacturer. Read them for every single sensor on the weather station to see if it's acceptable to you. Just reading the sensors specifications doesn't tell the whole story.

I like to use the analogy of the webcam, security camera. It's funny how when I shop for a camera and it says "Outdoor Security Camera" (emphasis on outdoor) and then I check the operating specifications and it says operating temperature 4°F to 120°F. Outdoor? Really? Not in a Midwest winter! If you call the manufacturer, they will say, we're not saying you can't use it outside that temperature range. We just don't guarantee optimal performance outside that range and you'll have to try it outside the operating temperature range and see if it's acceptable to you.

Again, I'm not saying this has anything to do with the Davis sensor issue. I'm also not saying there is nothing wrong with the Davis sensor. It's more of a general comment on humidity sensors and why they don't have a wide operating temperature and humidity range when the manufacturer lists "weather stations" as one of their sensors uses.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 22, 2018, 11:30:30 AM
As stated before, variablilty is the norm with the SHT31. Currently (8pm):

New station (24 hour fan)—T 71, H 84
2 year old station (daytime fan)—T 72, H 79
2.5 year old station (daytime fan)—T 72, H 82.

So your new station runs the wettest?

If so, then Randy's 2 new out-of-spec sensors are not a fluke. That would begin to point toward a raft of defective sensors currently for sale.

Or was the point that all 3 sensors trade places in terms of accuracy?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on July 22, 2018, 11:37:15 AM
As stated before, variablilty is the norm with the SHT31. Currently (8pm):

New station (24 hour fan)—T 71, H 84
2 year old station (daytime fan)—T 72, H 79
2.5 year old station (daytime fan)—T 72, H 82.

So your new station runs the wettest?

If so, then Randy's 2 new out-of-spec sensors are not a fluke. That would begin to point toward a raft of defective sensors currently for sale.

Or was the point that all 3 sensors trade places in terms of accuracy?

Yes, my newest station is running wet compared to my others. Originally wasn’t doing that but this past week it shifted! Go figure.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 22, 2018, 12:26:06 PM
The Atlas uses a Sensirion SHT31 temp/humidity chip.  This is an upgrade from the SHT21 used in the 5n1.

OK, this is interesting. That's from the Acurite Atlas thread (https://www.wxforum.net/index.php?topic=33456.msg348911#msg348911).  Can anyone else confirm this?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 22, 2018, 12:35:42 PM
Found an Atlas station online: https://www.wunderground.com/personal-weather-station/dashboard?ID=KCADUBLI16#history/tgraphs/s20180722/e20180722/mdaily (https://www.wunderground.com/personal-weather-station/dashboard?ID=KCADUBLI16#history/tgraphs/s20180722/e20180722/mdaily). It's reporting 69/60 (temp/dp). Nearby airports are mostly reporting 57 dp, so it would appear to be running high as well (by over 6% humidity).
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 22, 2018, 12:46:35 PM
Found another Atlas (https://www.wunderground.com/personal-weather-station/dashboard?ID=KOHBEDFO15 (https://www.wunderground.com/personal-weather-station/dashboard?ID=KOHBEDFO15)). It's reporting dew point of 69 and nearby airports are reporting 60-63.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 22, 2018, 01:14:11 PM
Maybe using the SHT31?
Someone needs to find a better sensor that's accurate, I'm not sold on any of the Sensirion sensors for meteorological use now even the 15 had high bias just not as bad but also never went above 95%.

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 22, 2018, 01:33:08 PM
Maybe using the SHT31?

Looks like it based on the report from nincehelser. Because of NDAs signed by beta testers, it's going to be hard to confirm. Remember this is for a product that hasn't even been released and the prototypes are less than 1 year old.

I found exactly two of these units online and both had an out of spec, wet humidity bias. One of them was high by approx 15%! Anyone still salivating to get an Atlas?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: DoctorKnow on July 22, 2018, 01:34:39 PM
Found another Atlas (https://www.wunderground.com/personal-weather-station/dashboard?ID=KOHBEDFO15 (https://www.wunderground.com/personal-weather-station/dashboard?ID=KOHBEDFO15)). It's reporting dew point of 69 and nearby airports are reporting 60-63.

That is a Bloomsky. Atlas does not show UV
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ocala on July 22, 2018, 01:36:38 PM
Maybe using the SHT31?
Someone needs to find a better sensor that's accurate, I'm not sold on any of the Sensirion sensors for meteorological use now even the 15 had high bias just not as bad but also never went above 95%.
Would the Vaisala sensor work with a Davis?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Old Tele man on July 22, 2018, 01:37:16 PM
Maybe using the SHT31?
Someone needs to find a better sensor that's accurate, I'm not sold on any of the Sensirion sensors for meteorological use now even the 15 had high bias just not as bad but also never went above 95%.

So...MAYBE...we're gonna have to chose between:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Old Tele man on July 22, 2018, 01:38:36 PM
Maybe using the SHT31?
Someone needs to find a better sensor that's accurate, I'm not sold on any of the Sensirion sensors for meteorological use now even the 15 had high bias just not as bad but also never went above 95%.
Would the Vaisala sensor work with a Davis?

Not currently...they use different communication protocols.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 22, 2018, 01:57:47 PM
Maybe using the SHT31?
Someone needs to find a better sensor that's accurate, I'm not sold on any of the Sensirion sensors for meteorological use now even the 15 had high bias just not as bad but also never went above 95%.

So...MAYBE...we're gonna have to chose between:
  • top-end limited, somewhat accurate, SHT-15
  • top-end capable, somewhat accurate, SHT-31

If you're referring to humidity readings between 95-100% I don't think the 31s are any better in that respect than the 15s. Besides, that's not the issue. Technically, if the 31 hits 96.5% or more during saturated conditions, it's within spec. That's a separate issue.

The main issue is the range from approx 40 - 70% where most humidity readings occur during the day in temperate, non-arid climates. Those readings, especially during the summer, are not "somewhat accurate" unless you think having a 10-15 degree heat index error is acceptable. Of course, being in the desert, that's not an issue for you generally.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 22, 2018, 02:13:20 PM
I'm wondering if some software can't fix this issue.
I don't know enough about how they put code in sensor but I've got 2-SHT31's by Acurite that doesn't display the high bias just the opposite almost right with the airport all the time. 

What did they do different with these $15 dollar units vs their new Atlas running high also?  Maybe they didn't realize there was an issue. Looks like they are following Davis even having aspiration by fan, maybe they thought everything was good.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: DoctorKnow on July 22, 2018, 02:20:43 PM
Currently...

Fine offset... 89 F DP 78 H 72
Acurite 5n1 91 82 76
Atlas 89 79 73
Acurite tower 89 76 67
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 22, 2018, 02:23:25 PM
I'm wondering if some software can't fix this issue.
I don't know enough about how they put code in sensor but I've got 2-SHT31's by Acurite that doesn't display the high bias just the opposite almost right with the airport all the time. 

What did they do different with these $15 dollar units vs their new Atlas running high also?  Maybe they didn't realize there was an issue. Looks like they are following Davis even having aspiration by fan, maybe they thought everything was good.

Maybe that’s what RW did. They probably pregammed the console to correct the inaccuracies of the SHT-15 in certain humidity ranges.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: drew1021 on July 22, 2018, 02:35:31 PM
Yes, I was thinking the same thing. Maybe a software update would correct the wet bias.
Another thought. I wonder how the Visalia probe would perform in the 24 hour FARS?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 22, 2018, 02:44:11 PM
Yes, I was thinking the same thing. Maybe a software update would correct the wet bias.
Another thought. I wonder how the Visalia probe would perform in the 24 hour FARS?

I’m no expert on Vaisala but I would think the same. It might age the sensor quicker in high humidity environments. Might have to switch out every 11-12 months instead of 18 months.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 22, 2018, 02:44:55 PM
I'm wondering if some software can't fix this issue.
I don't know enough about how they put code in sensor but I've got 2-SHT31's by Acurite that doesn't display the high bias just the opposite almost right with the airport all the time. 

What did they do different with these $15 dollar units vs their new Atlas running high also?  Maybe they didn't realize there was an issue. Looks like they are following Davis even having aspiration by fan, maybe they thought everything was good.

Maybe that’s what RW did. They probably pregammed the console to correct the inaccuracies of the SHT-15 in certain humidity ranges.

 I'll look at WD software and see what the calibrator can do. Cumulus can only really trim 1° dp and still reach 100%. That's about 3-4% humidity in area of issue but isn't a full fix.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 22, 2018, 02:49:06 PM
Maybe that’s what RW did. They probably pregammed the console to correct the inaccuracies of the SHT-15 in certain humidity ranges.

I don't know how RW would do that, as the errors don't seem consistent. Since everyone's siting these in different environments, each sensor will degrade differently. Some may stay within spec for months or years. Others arrive out of spec. Seems likely most go out within a year, but we don't have enough data yet to say for sure what the timeline and exact triggering mechanism (how many times must it encounter saturated air and for how long?).

For instance, currently, my 15 is reporting 66/56 and my 31 is reporting 65/56. That's around 70% humidity for both. Both are within spec according to my Kestrel and nearby reference stations.

See WheatonRon's report about his three different 31s all having different humidity readings (5% apart).

I have two 31s, one of them is right on and the other is way off. So if Davis tried to perform a calibration curve on both, it wouldn't result in two accurate sensors. I'd still have 1 accurate and 1 inaccurate sensor and, quite possibly, two inaccurate sensors.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 22, 2018, 02:50:33 PM
I should add that all these sensors come with a calibration curve of some sort based on inherent non-linearities the manufacturer identified during initial testing. But that's something quite apart from solving an inconsistent wet bias discovered in the field.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 22, 2018, 03:17:27 PM
This is what WD... Weather Display humidity calibration looks like.
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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on July 22, 2018, 04:22:59 PM
Randy...

I brought it up before, but have you ever tried an SHT75? If you're really suspicious about Davis' handling of the sensors it may be worth giving a sensor soldered at the Sensirion factory a try. I'm sure you have an old Davis SHTxx that you can sacrifice to solder directly to the 75 pins or insert into an acceptable connector.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 22, 2018, 04:52:22 PM
Randy...

I brought it up before, but have you ever tried an SHT75? If you're really suspicious about Davis' handling of the sensors it may be worth giving a sensor soldered at the Sensirion factory a try. I'm sure you have an old Davis SHTxx that you can sacrifice to solder directly to the 75 pins or insert into an acceptable connector.

Correct me if I'm wrong, Randy, but didn't you say that your 15s were fried too when it came to humidity?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 22, 2018, 04:53:08 PM
Randy...

I brought it up before, but have you ever tried an SHT75? If you're really suspicious about Davis' handling of the sensors it may be worth giving a sensor soldered at the Sensirion factory a try. I'm sure you have an old Davis SHTxx that you can sacrifice to solder directly to the 75 pins or insert into an acceptable connector.

So remove sht15 chip and solder SHT75 in its place? How is the 75 with humidity? Does it have the same high bias even after reconditioning the 31 nothing really changed.


Yes
Randy...

I brought it up before, but have you ever tried an SHT75? If you're really suspicious about Davis' handling of the sensors it may be worth giving a sensor soldered at the Sensirion factory a try. I'm sure you have an old Davis SHTxx that you can sacrifice to solder directly to the 75 pins or insert into an acceptable connector.

Correct me if I'm wrong, Randy, but didn't you say that your 15s were fried too when it came to humidity?

Yes it's not much better.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: drew1021 on July 22, 2018, 04:58:44 PM
Here's a thought, maybe Davis could eliminate soldering the sensors directly to the board by installing sockets directly on the pcb. It may not be physically possible though, I have not seen the shtxx pinout.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 22, 2018, 05:15:52 PM
This is interesting...drier air moved into central Alabama today.

Airport that is 15 miles N of me. 88/64

Acurite Temp/Hum sensor with lightning detector 89/65.5

Davis VP 2 with -8% offset 88/67.7
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 22, 2018, 05:33:41 PM
This is interesting...drier air moved into central Alabama today.

Airport that is 15 miles N of me. 88/64

Acurite Temp/Hum sensor with lightning detector 89/65.5

Davis VP 2 with -8% offset 88/67.7

Yikes! By my calculations that's a >20% absolute humidity error, assuming the dew point of 67.7 is actually 75.7 without the offset.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 22, 2018, 05:36:18 PM
This is interesting...drier air moved into central Alabama today.

Airport that is 15 miles N of me. 88/64

Acurite Temp/Hum sensor with lightning detector 89/65.5

Davis VP 2 with -8% offset 88/67.7

So chasing humidity too.... :sad:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Old Tele man on July 22, 2018, 05:46:31 PM
This is interesting...drier air moved into central Alabama today.

Airport that is 15 miles N of me. 88/64

Acurite Temp/Hum sensor with lightning detector 89/65.5

Davis VP 2 with -8% offset 88/67.7

So chasing humidity too.... :sad:

Dewpoint (DP) is calculated from the TEMP and RH values, so if either one is off, so too will be the calculated DP value. However, TEMP is usually far more accurate than RH, so DP errors can usually be traced back to errant RH values more than TEMP values.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on July 22, 2018, 05:58:12 PM
Randy...

I brought it up before, but have you ever tried an SHT75? If you're really suspicious about Davis' handling of the sensors it may be worth giving a sensor soldered at the Sensirion factory a try. I'm sure you have an old Davis SHTxx that you can sacrifice to solder directly to the 75 pins or insert into an acceptable connector.

So remove sht15 chip and solder SHT75 in its place? How is the 75 with humidity? Does it have the same high bias even after reconditioning the 31 nothing really changed.


Yes
Randy...

I brought it up before, but have you ever tried an SHT75? If you're really suspicious about Davis' handling of the sensors it may be worth giving a sensor soldered at the Sensirion factory a try. I'm sure you have an old Davis SHTxx that you can sacrifice to solder directly to the 75 pins or insert into an acceptable connector.

Correct me if I'm wrong, Randy, but didn't you say that your 15s were fried too when it came to humidity?

Yes it's not much better.
The 75 is basically a 15 with slightly better RH specs between 10-90% (1.8% versus 2.0%). The difference is that it is presoldered by Sensirion. Solder the correct wires to an old 6-conductor Davis temp/hum and it becomes plug and play. With your FARS the temp response will be through the roof because of the low thermal mass...especially if you run it with the filter off.

http://www.wxforum.net/index.php?topic=14175.msg189484#msg189484

http://www.mouser.com/ds/2/682/Sensirion_Humidity_SHT7x_Datasheet_V5-469726.pdf
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 22, 2018, 06:03:14 PM
This is interesting...drier air moved into central Alabama today.

Airport that is 15 miles N of me. 88/64

Acurite Temp/Hum sensor with lightning detector 89/65.5

Davis VP 2 with -8% offset 88/67.7

So chasing humidity too.... :sad:

Dewpoint (DP) is calculated from the TEMP and RH values, so if either one is off, so too will be the calculated DP value. However, TEMP is usually far more accurate than RH, so DP errors can usually be traced back to the errant RH values more than TEMP values.

I'm talking about his -8% HUM offset. You either do that or have outrageous heat index readings. 120°+ for some.
Move it down 8-9% during heat of day and  then back up in the evening so morning humidity doesn't peak at 91% when it's 100% (Gets real old)  ](*,)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 22, 2018, 06:14:56 PM
This is interesting...drier air moved into central Alabama today.

Airport that is 15 miles N of me. 88/64

Acurite Temp/Hum sensor with lightning detector 89/65.5

Davis VP 2 with -8% offset 88/67.7

Yikes! By my calculations that's a >20% absolute humidity error, assuming the dew point of 67.7 is actually 75.7 without the offset.

Probably needed to note that the VP2 is sited at a location where it doesn’t get great air mixing if the wind is from the North.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 22, 2018, 06:15:59 PM
Interesting comparison with new sht31 and three about 2years old and an old sht11. New showed 86.8/69 old 1 86.8/72 old 2 86.8/71 old 3 86.9/71 and sht11 86.8/69  the temps are really something.  Interesting how the sht11 was right on with the new 31.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 22, 2018, 06:20:42 PM
I forgot to put in my little acurite is showing 86/64 humidity is way off for a 31 sensor chip.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 22, 2018, 06:53:31 PM
I forgot to put in my little acurite is showing 86/64 humidity is way off for a 31 sensor chip.

Because it's not reading high? My 2 acurite units are normal on humidity. 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 22, 2018, 07:10:25 PM
No because it is reading lower than all the other sensors i have, by 5% or more. I guess i will have to put it in the bag and see how close it comes to 75%.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 22, 2018, 07:15:09 PM
I forgot to put in my little acurite is showing 86/64 humidity is way off for a 31 sensor chip.

Because it's not reading high? My 2 acurite units are normal on humidity.

Have you thought about, perhaps, leaving one of them outside in the shade during days without precipitation? I wonder how long it would stay in calibration?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 22, 2018, 07:25:35 PM
No because it is reading lower than all the other sensors i have, by 5% or more. I guess i will have to put it in the bag and see how close it comes to 75%.

From my testing the Davis sensors all read high. I didn't use a bag but going from my other instruments and ASOS the acurite is correct.
That was my point how can I have 2 acurite sht31's read normal with instruments and ASOS and then have 3 Davis sensors all high.
That's where the idea of contamination came in with 2 different companies mounting differently onto boards.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 22, 2018, 07:30:26 PM
I forgot to put in my little acurite is showing 86/64 humidity is way off for a 31 sensor chip.

Because it's not reading high? My 2 acurite units are normal on humidity.

Have you thought about, perhaps, leaving one of them outside in the shade during days without precipitation? I wonder how long it would stay in calibration?

I'll try it and see if they drift apart.

Also Jerry FYI both these acurites have hit 95% in morning when Davis only 1 of 3 managed.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 22, 2018, 08:01:23 PM
I put mine out when light fog was expected and it pegged out at 99 and main station was reporting 97 and that was the general high for around the area. Not too impressed with my cheap meter, i might have to take it apart and see if it does have a 31 and how it is mounted.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 22, 2018, 08:14:46 PM
I put mine out when light fog was expected and it pegged out at 99 and main station was reporting 97 and that was the general high for around the area. Not too impressed with my cheap meter, i might have to take it apart and see if it does have a 31 and how it is mounted.

Wait so it reads lower when it should and normal 99% in fog and you think it's wrong because it doesn't match what? Your Davis sensor? I'm very confused unless for some odd reason you don't believe in the high bias Davis sensors that are junk. 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 22, 2018, 08:28:45 PM
From my testing the Davis sensors all read high. I didn't use a bag but going from my other instruments and ASOS the acurite is correct.
That was my point how can I have 2 acurite sht31's read normal with instruments and ASOS and then have 3 Davis sensors all high.
That's where the idea of contamination came in with 2 different companies mounting differently onto boards.

Well, keep in mind that Ron observed some weird behavior in the PCB-mounted sensors he obtained from Sensirion. Remember when he put one outside in the shade for a mere 6 hours without a cap and after that it read 5% high? Those were brand new sensors. See: http://www.wxforum.net/index.php?topic=34658.msg353147#msg353147 (http://www.wxforum.net/index.php?topic=34658.msg353147#msg353147)

Your indoor Acurite sensors might not have a cap which would make for a good test against the Davis sensors which are, effectively, uncapped as well.

Just trying to get a handle on whether exposure itself will cause issues. If so, how much does it take to knock it out of calibration permanently?

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 22, 2018, 08:35:49 PM
Well i am going to wait and see how it looks in the am after being in the bag overnight. I have two new sht15's due in the am and i also ordered a sht75 which comes mounted and sealed from the manufacturer to do some testing, it has a little better humidity spec than the 31.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 22, 2018, 08:38:55 PM
I've had my acurite out for couple hours now cloudy day matching in temperature and humidity is spot on. All 3  Davis units continue to run the normal high bias.
I see nothing that says the acurite is off just the opposite.

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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: hwcorder on July 22, 2018, 10:48:49 PM
Randy...

I brought it up before, but have you ever tried an SHT75? If you're really suspicious about Davis' handling of the sensors it may be worth giving a sensor soldered at the Sensirion factory a try. I'm sure you have an old Davis SHTxx that you can sacrifice to solder directly to the 75 pins or insert into an acceptable connector.

So remove sht15 chip and solder SHT75 in its place? How is the 75 with humidity? Does it have the same high bias even after reconditioning the 31 nothing really changed.




Yes
Randy...

I brought it up before, but have you ever tried an SHT75? If you're really suspicious about Davis' handling of the sensors it may be worth giving a sensor soldered at the Sensirion factory a try. I'm sure you have an old Davis SHTxx that you can sacrifice to solder directly to the 75 pins or insert into an acceptable connector.

Correct me if I'm wrong, Randy, but didn't you say that your 15s were fried too when it came to humidity?

Yes it's not much better.
The 75 is basically a 15 with slightly better RH specs between 10-90% (1.8% versus 2.0%). The difference is that it is presoldered by Sensirion. Solder the correct wires to an old 6-conductor Davis temp/hum and it becomes plug and play. With your FARS the temp response will be through the roof because of the low thermal mass...especially if you run it with the filter off.

http://www.wxforum.net/index.php?topic=14175.msg189484#msg189484

http://www.mouser.com/ds/2/682/Sensirion_Humidity_SHT7x_Datasheet_V5-469726.pdf

I havent done any actual testing on humidity drift but just from what I can remember the sht75s I order directly from Newark14 have less drift than the two or three sht31s I bought.
 I recently bought another sht31 and used the sf2 filter like jerryg had done and when I compared the results it immediately ran 2-3% higher than the sht75 that is almost two years old.  SHT75 was reading right along with the ASOS 5 miles away.  Got my Vaisala HMP46 out to spot check and it was reading 2% lower than the sht75 and about 4-5% lower than the brand new 31. 
I can switch out my 75s directly since I soldered a connector making them plug and play.  Really think it may be something happening in the manufacturing process possibly making these sensor defective or perhaps its a flaw in the design of the sht31 itself, who knows. All I know is I have no plans of switching to anything else anytime soon until something changes, protocol, or Davis manufacturing process.  Thinking about making a diy temp/hum probe using some old magic marker housing and some epoxy then topping it off with a better filter from Vaisala or Cambell Scientific.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on July 22, 2018, 11:17:52 PM
I have a few of these and a new 75 coming in tomorrow from Newark. They should work. We'll find out tomorrow. No soldering required.

http://www.newark.com/harting/14310410301000/receptacle-4way-1row-1-27mm-har/dp/53X2803
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 22, 2018, 11:56:58 PM
I have a few of these and a new 75 coming in tomorrow from Newark. They should work. We'll find out tomorrow. No soldering required.

http://www.newark.com/harting/14310410301000/receptacle-4way-1row-1-27mm-har/dp/53X2803

I’m not very knowledgeable with this stuff so here is my question.

1. If I order a SHT-75, what all exactly I need to do to put it together so that I can use it in my FARS?

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 23, 2018, 12:16:29 AM
Will note that Sensirion says that the SHT-75 is designed for weather stations
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 23, 2018, 12:29:36 AM
I know this may be getting off subject a little but for those interested in switching software from WL or Cumulus over to Weather Display.... WD has a nice slope adjustment that would work. I just downloaded and impressed, my biggest problem I have 4 years of data on Cumulus and website data all using Cumulus lot's of work to change over.

Anyway the slope adjustment at -3% looks like it would work, here's the PDF. I tried it out and works.
There is a setting for decreasing slope as humidity drops lower end of scale also which I have checked.  It doesn't show on PDF however.



Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 23, 2018, 12:46:15 AM
I know this may be getting off subject a little but for those interested in switching software from WL or Cumulus over to Weather Display.... WD has a nice slope adjustment that would work. I just downloaded and impressed, my biggest problem I have 4 years of data on Cumulus and website data all using Cumulus lot's of work to change over.

Anyway the slope adjustment at -3% looks like it would work, here's the PDF. I tried it out and works.
There is a setting for decreasing slope as humidity drops lower end of scale also which I have checked.  It doesn't show on PDF however.

Nice! Personally don’t care for WD. I personally don’t want to keep doing calibration offsets etc. I’m thinking about giving the 75 a try if I can set everything up without soldering. I’m hoping someone on here can tell me how to install the SHT-75. And I’m also curious too if the 75 needs a filter cap?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on July 23, 2018, 05:55:47 AM
I know this may be getting off subject a little but for those interested in switching software from WL or Cumulus over to Weather Display.... WD has a nice slope adjustment that would work. I just downloaded and impressed, my biggest problem I have 4 years of data on Cumulus and website data all using Cumulus lot's of work to change over.

Anyway the slope adjustment at -3% looks like it would work, here's the PDF. I tried it out and works.
There is a setting for decreasing slope as humidity drops lower end of scale also which I have checked.  It doesn't show on PDF however.

Nice! Personally don’t care for WD. I personally don’t want to keep doing calibration offsets etc. I’m thinking about giving the 75 a try if I can set everything up without soldering. I’m hoping someone on here can tell me how to install the SHT-75. And I’m also curious too if the 75 needs a filter cap?
Read the first few pages of this thread...specifically this post.

http://www.wxforum.net/index.php?topic=14175.msg138241#msg138241

Sensirion makes a filter for it, but you need a way to make it fit and stay secure. I have run mine no filter, but that will obviously degrade performance with time. I don't have much time this morning...maybe more later.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 23, 2018, 07:27:30 AM
I know this may be getting off subject a little but for those interested in switching software from WL or Cumulus over to Weather Display.... WD has a nice slope adjustment that would work. I just downloaded and impressed, my biggest problem I have 4 years of data on Cumulus and website data all using Cumulus lot's of work to change over.

Anyway the slope adjustment at -3% looks like it would work, here's the PDF. I tried it out and works.
There is a setting for decreasing slope as humidity drops lower end of scale also which I have checked.  It doesn't show on PDF however.

Nice! Personally don’t care for WD. I personally don’t want to keep doing calibration offsets etc. I’m thinking about giving the 75 a try if I can set everything up without soldering. I’m hoping someone on here can tell me how to install the SHT-75. And I’m also curious too if the 75 needs a filter cap?
Read the first few pages of this thread...specifically this post.

http://www.wxforum.net/index.php?topic=14175.msg138241#msg138241

Sensirion makes a filter for it, but you need a way to make it fit and stay secure. I have run mine no filter, but that will obviously degrade performance with time. I don't have much time this morning...maybe more later.

Ok. Thanks!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 23, 2018, 09:28:22 AM
Ok got to try make this short lol. After 12 hrs in the bag the acurite, as expected, showed 71% which is 4 low. I calibrated to 75 and put it outside in the light fog at the time. My main station was showing 98% the acurite went up slowly to 98 and stayed there for some time before going to 99. Looks pretty good. Now i put the acurite in unused room and let it settle in to 62% and steady and put the 3 old 31 and the new 31 and let them settle in. One read 1% low, two of them read 3% high and one was right on. So i took the one that was showing 1 low and bagged it for a while and it showed to be 73.7% which is right at 1 low and right now is showing 61% next to acurite which still shows 62%. I bagged the one right on an it showed 75.3% and it shows the same as the acurite. I have not check the two that have 3% high but probably will check one though it sure looks like they will be high on the bag test. 3% high is not within specs.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 23, 2018, 09:48:27 AM
Well the acurite went up to 64 and the one went to 64 and the other to 63 so tracking as tested. Now the sad news i bagged one of the two that was running 3 high and it is the new one i just got and sure enough it showed 78.1, out of spec. I guess i could send it back to Ryan but not sure if it's worth the effort. I am waiting for the new 15's to get here in the mail this am so i can do some comparing while it is fresh on my feeble brain lol.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 23, 2018, 09:50:55 AM
Ok got to try make this short lol. After 12 hrs in the bag the acurite, as expected, showed 71% which is 4 low. I calibrated to 75 and put it outside in the light fog at the time. My main station was showing 98% the acurite went up slowly to 98 and stayed there for some time before going to 99. Looks pretty good. Now i put the acurite in unused room and let it settle in to 62% and steady and put the 3 old 31 and the new 31 and let them settle in. One read 1% low, two of them read 3% high and one was right on. So i took the one that was showing 1 low and bagged it for a while and it showed to be 73.7% which is right at 1 low and right now is showing 61% next to acurite which still shows 62%. I bagged the one right on an it showed 75.3% and it shows the same as the acurite. I have not check the two that have 3% high but probably will check one though it sure looks like they will be high on the bag test. 3% high is not within specs.

The problem with testing at 75% humidity on the 31s is that's where they are the MOST accurate, even the ones that are out of spec. 75% is the crossover point from wet to slight dry bias (assuming that particular sensor contains either bias). So if you're looking for drift, you need to test below 70%. I know they make 62% humidity kits, for instance.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 23, 2018, 10:02:37 AM
Right now i am looking at comparing readings at a given calibration point. I have a 33% packet for testing and i also have potassium sulfate for a 97% reference. My real pain is that the brand new 31 is 3% high to start.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 23, 2018, 10:16:04 AM
Right now i am looking at comparing readings at a given calibration point. I have a 33% packet for testing and i also have potassium sulfate for a 97% reference. My real pain is that the brand new 31 is 3% high to start.

33% might be a better test, I don't know. I've seen notable bias around 35%. But I haven't been able to replicate bias consistently at 75%. Of course, the other problem is that those kits need to be run at room temperature, no? The highest biases start north of 25C (77F).

Also, weren't you one of the original proponents of the SHT75? Could you test one (or more) that's been in use for awhile and see if it's truly more accurate with humidity than the 31s?

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 23, 2018, 10:34:50 AM
Well heck, i tried a couple 75 but alas they both fell victims of too much humidity and not enough knowledge of how humidity kills the sensor in long periods of high humidity with a fan running. I have one on order and am going to do some comparisons when i get it in.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on July 23, 2018, 12:27:42 PM
Ok. Thanks!
Just remember that you don't need the capacitor for the SHT75 since it's preinstalled. That post I referenced was for the SHT15. You just need to worry about the proper wiring.

I don't have a huge gripe with the RH values of any of the SHT sensors so I really couldn't tell anyone, in any scientific way, how it compares to the 31 or 15. I do know I was regularly hitting 98-99% with it before I took it out. I think the temp geeks with the AC FARS are going to really like the rapid response though.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on July 23, 2018, 01:56:01 PM
And one more thing...

If you go from the 31 to the 75 make sure you adjust your temp cal settings. I have an older ISS so I had to go back to a 0 cal, but you guys with the newer ones will need to adjust the 0.5C.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: JudinNorman on July 23, 2018, 02:58:51 PM
I dug my old Weather Monitor II station out few days ago to hook up the temp/hum sensor to compare to my sht31 vp2. 
Even after all these years the dewpoints last few days were constantly close to those of nearby weather service station than the vp2.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 23, 2018, 06:01:02 PM
For those who are getting the SHT-75 and installing it soon, please report back your findings. I’m curious if the DP readings are better. I’m also curious to how the temps compare with the 31. Thanks!

Also, if you have experience using the SHT-75 when temps got below 32°?  How did the sensor perform under those conditions? Spec sheet suggests that the temp will start drifting under 32°
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 23, 2018, 06:02:10 PM
I dug my old Weather Monitor II station out few days ago to hook up the temp/hum sensor to compare to my sht31 vp2. 
Even after all these years the dewpoints last few days were constantly close to those of nearby weather service station than the vp2.

And that’s not a good thing for the VP2...
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 23, 2018, 06:50:25 PM
Finally today the humidity fell below 70% and I was able to test the midrange on the SHT15. Not good. I'm seeing an average dew point error of about 2-3 degrees F (54F on my good 31 vs 57F on my bad 15). That's a 5-7% humidity difference. So, it appears my 15 has succumbed to the same problem as other Davis sensors. It reads wetter with time (it's about 3 years old now). My guess, based on past experience with this problem, is that as humidity descends toward 50% and/or the air temp goes up the errors will worsen.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 23, 2018, 08:30:40 PM
Finally today the humidity fell below 70% and I was able to test the midrange on the SHT15. Not good. I'm seeing an average dew point error of about 2-3 degrees F (54F on my good 31 vs 57F on my bad 15). That's a 5-7% humidity difference. So, it appears my 15 has succumbed to the same problem as other Davis sensors. It reads wetter with time (it's about 3 years old now). My guess, based on past experience with this problem, is that as humidity descends toward 50% and/or the air temp goes up the errors will worsen.

I’m now curious how a new SHT-75 will perform overall. Are you thinking about getting the 75?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 23, 2018, 08:33:12 PM
I gave up on WD was having so many weird issues, spent all day on it. Back to old reliable. Wish Cumulus had the humidity slope it's an easy fix.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 23, 2018, 08:57:42 PM
Finally today the humidity fell below 70% and I was able to test the midrange on the SHT15. Not good. I'm seeing an average dew point error of about 2-3 degrees F (54F on my good 31 vs 57F on my bad 15). That's a 5-7% humidity difference. So, it appears my 15 has succumbed to the same problem as other Davis sensors. It reads wetter with time (it's about 3 years old now). My guess, based on past experience with this problem, is that as humidity descends toward 50% and/or the air temp goes up the errors will worsen.

I’m now curious how a new SHT-75 will perform overall. Are you thinking about getting the 75?

No. I don't really have any more confidence in that sensor than a 15 at this point. I realize it's cheaper. But it has the same issues, ultimately, as the 15. It's obsolete, no longer manufactured and, most likely it will eventually develop a wet bias. jerryg had two 75s and both had issues with humidity.

Hoping against hope that Davis can come up with a solution. I'm lucky because I have 1 sensor (out of 4) that's still within spec for humidity.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WxLover16 on July 23, 2018, 09:05:15 PM
I know this may be getting off subject a little but for those interested in switching software from WL or Cumulus over to Weather Display.... WD has a nice slope adjustment that would work. I just downloaded and impressed, my biggest problem I have 4 years of data on Cumulus and website data all using Cumulus lot's of work to change over.

Anyway the slope adjustment at -3% looks like it would work, here's the PDF. I tried it out and works.
There is a setting for decreasing slope as humidity drops lower end of scale also which I have checked.  It doesn't show on PDF however.

Nice! Personally don’t care for WD. I personally don’t want to keep doing calibration offsets etc. I’m thinking about giving the 75 a try if I can set everything up without soldering. I’m hoping someone on here can tell me how to install the SHT-75. And I’m also curious too if the 75 needs a filter cap?
Read the first few pages of this thread...specifically this post.

http://www.wxforum.net/index.php?topic=14175.msg138241#msg138241

Sensirion makes a filter for it, but you need a way to make it fit and stay secure. I have run mine no filter, but that will obviously degrade performance with time. I don't have much time this morning...maybe more later.
     

Just wondering cause I'm all about accuracy, how would you really know the sensor is degrading, slower response times? Also, I know you run a 40 cfm fan, but with no filter that has really got to be doing a huge number on your sensor without the filter on. You probably have to change out your sensor about every month, no?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on July 23, 2018, 09:11:23 PM
I know this may be getting off subject a little but for those interested in switching software from WL or Cumulus over to Weather Display.... WD has a nice slope adjustment that would work. I just downloaded and impressed, my biggest problem I have 4 years of data on Cumulus and website data all using Cumulus lot's of work to change over.

Anyway the slope adjustment at -3% looks like it would work, here's the PDF. I tried it out and works.
There is a setting for decreasing slope as humidity drops lower end of scale also which I have checked.  It doesn't show on PDF however.

Nice! Personally don’t care for WD. I personally don’t want to keep doing calibration offsets etc. I’m thinking about giving the 75 a try if I can set everything up without soldering. I’m hoping someone on here can tell me how to install the SHT-75. And I’m also curious too if the 75 needs a filter cap?
Read the first few pages of this thread...specifically this post.

http://www.wxforum.net/index.php?topic=14175.msg138241#msg138241

Sensirion makes a filter for it, but you need a way to make it fit and stay secure. I have run mine no filter, but that will obviously degrade performance with time. I don't have much time this morning...maybe more later.
     

Just wondering cause I'm all about accuracy, how would you really know the sensor is degrading, slower response times? Also, I know you run a 40 cfm fan, but with no filter that has really got to be doing a huge number on your sensor without the filter on. You probably have to change out your sensor about every month, no?
I'm still doing fine. Just throwing the disclaimer out there.

https://weather.gladstonefamily.net/site/C7324
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ct on July 23, 2018, 11:03:17 PM
I don't own any Davis SHT31 sensors, but have a few I2C SHT3x sensors in operation in various fan aspirated shields.  I haven't done any scientific comparisons, but I am not seeing any significant difference in the humidity readings.

Yesterday the lowest humidity for the day was around 17:40.

SHT31, 22 months old, no protective cover:  41.90%
SHT35, 15 months old, Davis cap:  41.97%

3kms away I have another station:

SHT35, 5 months old, SF2 cap:  41.26%

At this time the closest meteorology bureau station was reading 41%.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 23, 2018, 11:35:07 PM
I don't own any Davis SHT31 sensors, but have a few I2C SHT3x sensors in operation in various fan aspirated shields.  I haven't done any scientific comparisons, but I am not seeing any significant difference in the humidity readings.

Yesterday the lowest humidity for the day was around 17:40.

SHT31, 22 months old, no protective cover:  41.90%
SHT35, 15 months old, Davis cap:  41.97%

3kms away I have another station:

SHT35, 5 months old, SF2 cap:  41.26%

At this time the closest meteorology bureau station was reading 41%.

So maybe the analog versions of the sensors are the bad ones?

How does the 35 & 31 compare when it comes to humidities in the upper 90 percentile? How do they compare in terms of air temp?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 23, 2018, 11:45:55 PM
I don't see it unless it's hot. Like tonight running lower than airport on dp.


66.0   58.2   73%    SHT31

66.2   62.6    88%  ASOS 1.3 miles away
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 23, 2018, 11:49:20 PM
So maybe the analog versions of the sensors are the bad ones?

I don't *believe* that the Davis sensor is analog. I've inquired about this before because I, too, assumed it must be using one of Sensirion's analog solutions and was told by someone knowledgeable that Sensibus is a digital-only platform. johnd should know.

** edited post and deleted dumb question **
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ct on July 24, 2018, 12:08:15 AM
How does the 35 & 31 compare when it comes to humidities in the upper 90 percentile? How do they compare in terms of air temp?

In my observations with a limited number of sensors, the SHT35 reaches 100% humidity more often than the SHT31.   For temperature, the SHT35 is usually about 0.3 °C lower than the SHT31.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on July 24, 2018, 12:41:34 AM
I have followed this forum for years, but never registered or even really considered posting. I have learned a great deal from following all of your comments and experiences over the years. So let me start by saying thank you. This particular topic, however, has brought me out of the shadows …

I have owned Davis equipment since 2005. I bought a VP2. In 2006, I moved to south Texas and upgraded to a 24hr FARS and this is where my frustration with the dew point readings of the VP2 began. My new sensor would never hit 100% humidity and the rest of the scale had a wet bias, but my VP2 from 2005 never had these issues. My 2006 VP2 with FARS had an SHT11 and my 2005 had the old "analog" sensor.

I am no expert obviously, just another guy with a borderline unhealthy obsession with this stuff, but bottom line: my opinion and experience strongly supports that despite the inherent limitation of the Sensirion sensors, there is something about Davis' manufacturing process that causes an offset toward a wet bias.

I have been obsessed with trying to fix and find the source of this bias as well as trying to find a sensor that consistently hits 100% RH. The SHT11 had these issues and disappointingly so did the SHT31. I have moved around a bunch, so I have tested these things from anywhere from Texas to Maine to DC and to the Pacific Northwest. The result has always been the same. I have spent hours over the years with my sensors within 10 feet from the HO-1088 and DTS1 of an ASOS. I have purchased a psychro-dyne for quality checking. I have had the sensors in the -20sF in Maine and Vermont and over 100F in DC and Texas. Here are the main points of what I have learned:

-Outgassing of silicon and glue offsets the Sensirion sensor. The offset seems irreversible. Silicon offsets to the high side. Some glues to the low side. (I discovered this trying to use Sensirion's own filters instead of Davis' filter.)
-The Davis filter may increase response time, but is not responsible for the wet bias.
-The Sensirion sensors do seem to have a slight wet bias as has been documented in some studies, but this wet bias is some how exacerbated by how Davis mounts the sensor.
-The wet bias for Sensirion sensors in prolonged periods near saturation as mentioned in their datasheet is a separate issue from the wet bias created by the Davis mounting.
-The SHT75 straight from the manufacturer without any filter is superior to the Davis mounted SHT31 in terms of dew point and humidity.
-The SHT75 will hit up to 99%, with random very short lived jumps to 100%.
-I miss the old analog sensor

I currently run my station with an SHT75 with no filter at all. I've been doing it for about 6mos. The first 5 of those months were in a stock Davis 24hr FARS. Now it is in the Davis 7714, which is a surprisingly good passive shield (much better than the VP2 stock passive shield) (sometimes a degree or two cooler than the FARS, sometimes a degree or two warmer, at least in the north latitudes of the Seattle area...) I am concerned with contamination and dust, but after trying to put a filter on it with silicon and other glues, I have ruined a fair number of SHT75s with seemingly irreversible offsets. So far I have not seen any evidence of poor performance for lack of a filter.

I run a calibration in Cumulus for the SHT75 (1.041x-3) and field check it with the psychro-dyne. It generally is very close. The psychro-dyne is surprisingly accurate compared side by side to ASOS. I set Cumulus to report 98% as 100% and the calibration I set ensures 99% is reported as 100% too.

This is the best I have been able to do given the limitations of these sensors. I really hope the efforts here will force Davis to look at their process as I'd be happy to have an accurate Davis stock sensor that is engineered to last.

Hope this helps...
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Dj1225 on July 24, 2018, 01:49:39 AM
I have followed this forum for years, but never registered or even really considered posting. I have learned a great deal from following all of your comments and experiences over the years. So let me start by saying thank you. This particular topic, however, has brought me out of the shadows …

I have owned Davis equipment since 2005. I bought a VP2. In 2006, I moved to south Texas and upgraded to a 24hr FARS and this is where my frustration with the dew point readings of the VP2 began. My new sensor would never hit 100% humidity and the rest of the scale had a wet bias, but my VP2 from 2005 never had these issues. My 2006 VP2 with FARS had an SHT11 and my 2005 had the old "analog" sensor.

I am no expert obviously, just another guy with a borderline unhealthy obsession with this stuff, but bottom line: my opinion and experience strongly supports that despite the inherent limitation of the Sensirion sensors, there is something about Davis' manufacturing process that causes an offset toward a wet bias.

I have been obsessed with trying to fix and find the source of this bias as well as trying to find a sensor that consistently hits 100% RH. The SHT11 had these issues and disappointingly so did the SHT31. I have moved around a bunch, so I have tested these things from anywhere from Texas to Maine to DC and to the Pacific Northwest. The result has always been the same. I have spent hours over the years with my sensors within 10 feet from the HO-1088 and DTS1 of an ASOS. I have purchased a psychro-dyne for quality checking. I have had the sensors in the -20sF in Maine and Vermont and over 100F in DC and Texas. Here are the main points of what I have learned:

-Outgassing of silicon and glue offsets the Sensirion sensor. The offset seems irreversible. Silicon offsets to the high side. Some glues to the low side. (I discovered this trying to use Sensirion's own filters instead of Davis' filter.)
-The Davis filter may increase response time, but is not responsible for the wet bias.
-The Sensirion sensors do seem to have a slight wet bias as has been documented in some studies, but this wet bias is some how exacerbated by how Davis mounts the sensor.
-The wet bias for Sensirion sensors in prolonged periods near saturation as mentioned in their datasheet is a separate issue from the wet bias created by the Davis mounting.
-The SHT75 straight from the manufacturer without any filter is superior to the Davis mounted SHT31 in terms of dew point and humidity.
-The SHT75 will hit up to 99%, with random very short lived jumps to 100%.
-I miss the old analog sensor

I currently run my station with an SHT75 with no filter at all. I've been doing it for about 6mos. The first 5 of those months were in a stock Davis 24hr FARS. Now it is in the Davis 7714, which is a surprisingly good passive shield (much better than the VP2 stock passive shield) (sometimes a degree or two cooler than the FARS, sometimes a degree or two warmer, at least in the north latitudes of the Seattle area...) I am concerned with contamination and dust, but after trying to put a filter on it with silicon and other glues, I have ruined a fair number of SHT75s with seemingly irreversible offsets. So far I have not seen any evidence of poor performance for lack of a filter.

I run a calibration in Cumulus for the SHT75 (1.041x-3) and field check it with the psychro-dyne. It generally is very close. The psychro-dyne is surprisingly accurate compared side by side to ASOS. I set Cumulus to report 98% as 100% and the calibration I set ensures 99% is reported as 100% too.

This is the best I have been able to do given the limitations of these sensors. I really hope the efforts here will force Davis to look at their process as I'd be happy to have an accurate Davis stock sensor that is engineered to last.

Hope this helps...

I agree 100% with you. I've said on this thread that I don't remember having this issue with the Weather Monitor II that I used from 1999-2002 (which is backed up by WMII users on this thread) the VP1 from 2002-2005 or even my VP2 I got in Oct. 2005 that I used until May 2016 until I upgraded and bought a new ISS with the SHT31. I'm thinking about looking for a VP1 and using it along side the VP2 with the SHT31 sensor and see what the results are. I miss the analog sensors also.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: johnd on July 24, 2018, 04:30:19 AM
I don't *believe* that the Davis sensor is analog. I've inquired about this before because I, too, assumed it must be using one of Sensirion's analog solutions and was told by someone knowledgeable that Sensibus is a digital-only platform. johnd should know.

No, I don't know with 100% certainty I'm afraid - that level of detail would only be known within Davis Engineering.

But to the absolute best of my knowledge, Davis have only ever used (what seems to be referred to as) Sensibus or Sbus - ie digital - versions of the SHT series of T/H sensors in eg the post 2006 VP2 stations. So any thought of analogue would be - I strongly suspect - completely wide of the mark.

I've never been bothered to look into it in sufficient detail, but don't the markings on the sensor reveal its type/subtype etc? They're tiny and you probably need to take a good photo and enlarge appropriately and then cross-check with the detailed Sensirion documentation.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on July 24, 2018, 07:53:14 AM
I have followed this forum for years, but never registered or even really considered posting. I have learned a great deal from following all of your comments and experiences over the years. So let me start by saying thank you. This particular topic, however, has brought me out of the shadows …

I have owned Davis equipment since 2005. I bought a VP2. In 2006, I moved to south Texas and upgraded to a 24hr FARS and this is where my frustration with the dew point readings of the VP2 began. My new sensor would never hit 100% humidity and the rest of the scale had a wet bias, but my VP2 from 2005 never had these issues. My 2006 VP2 with FARS had an SHT11 and my 2005 had the old "analog" sensor.

I am no expert obviously, just another guy with a borderline unhealthy obsession with this stuff, but bottom line: my opinion and experience strongly supports that despite the inherent limitation of the Sensirion sensors, there is something about Davis' manufacturing process that causes an offset toward a wet bias.

I have been obsessed with trying to fix and find the source of this bias as well as trying to find a sensor that consistently hits 100% RH. The SHT11 had these issues and disappointingly so did the SHT31. I have moved around a bunch, so I have tested these things from anywhere from Texas to Maine to DC and to the Pacific Northwest. The result has always been the same. I have spent hours over the years with my sensors within 10 feet from the HO-1088 and DTS1 of an ASOS. I have purchased a psychro-dyne for quality checking. I have had the sensors in the -20sF in Maine and Vermont and over 100F in DC and Texas. Here are the main points of what I have learned:

-Outgassing of silicon and glue offsets the Sensirion sensor. The offset seems irreversible. Silicon offsets to the high side. Some glues to the low side. (I discovered this trying to use Sensirion's own filters instead of Davis' filter.)
-The Davis filter may increase response time, but is not responsible for the wet bias.
-The Sensirion sensors do seem to have a slight wet bias as has been documented in some studies, but this wet bias is some how exacerbated by how Davis mounts the sensor.
-The wet bias for Sensirion sensors in prolonged periods near saturation as mentioned in their datasheet is a separate issue from the wet bias created by the Davis mounting.
-The SHT75 straight from the manufacturer without any filter is superior to the Davis mounted SHT31 in terms of dew point and humidity.
-The SHT75 will hit up to 99%, with random very short lived jumps to 100%.
-I miss the old analog sensor

I currently run my station with an SHT75 with no filter at all. I've been doing it for about 6mos. The first 5 of those months were in a stock Davis 24hr FARS. Now it is in the Davis 7714, which is a surprisingly good passive shield (much better than the VP2 stock passive shield) (sometimes a degree or two cooler than the FARS, sometimes a degree or two warmer, at least in the north latitudes of the Seattle area...) I am concerned with contamination and dust, but after trying to put a filter on it with silicon and other glues, I have ruined a fair number of SHT75s with seemingly irreversible offsets. So far I have not seen any evidence of poor performance for lack of a filter.

I run a calibration in Cumulus for the SHT75 (1.041x-3) and field check it with the psychro-dyne. It generally is very close. The psychro-dyne is surprisingly accurate compared side by side to ASOS. I set Cumulus to report 98% as 100% and the calibration I set ensures 99% is reported as 100% too.

This is the best I have been able to do given the limitations of these sensors. I really hope the efforts here will force Davis to look at their process as I'd be happy to have an accurate Davis stock sensor that is engineered to last.

Hope this helps...
Nice first post. My observations tend to align with yours. We have fairly close CWOP IDs so we both got our VP2s when the Davis analog sensor was nearing its final days. That thing hit 100% so often that I was afraid it was hitting it too much. On a radiational cooling night I'd hit 100% around 9pm and it would flatline there until about 8-9am.

I have always ran my 75 with no filter as well and never noticed a problem, but the consensus here and from the documentation is that it will degrade performance over time so I don't want to be responsible for people ruining sensors following my filterless practices. Interesting that you noticed degraded performance with filters versus no filters though.

The SHT35 will be out in a pin form later this year. It'd be nice if someone could find a simple way to adapt it to a Davis ISS.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 24, 2018, 09:44:52 AM
Nice post, CW7491! This is why the thread exists - so people can share their experiences and knowledge.

Couple questions:

1) How do you mount the 75 to the shield if you don't use glue or silicon?
2) Approximately how long did your previous 75s last before they went off the rails?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 24, 2018, 09:54:25 AM
Well my first check with new sht15 humidity max 93, sht11 max 94 and new sht31 max96 all temps were within .1f of each other. I compared the two new 15's and they showed the same humidity and temp was within .2f of each other. Now then i have another test going starting this am and that is i put the other new 15 in a Davis dfars that i will run fan during the day and off at night. I put a high volume fan in place of the standard fan which is set up for 24 hr fars use and is a low volume fan with the shield being designed to use effectively but i figure the dfars should have a larger volume because of the open plate design. I am hoping to get decent daytime temps with the ability to shut the fan off at night and use the normal open plate air flow at night to get accurate readings without puting the sht to excessive moisture input. Of course you might know after weeks of clear skies and high heat the day i start my test the skies are cloudy lol. It is only supposed to last a day or so then back to clear and hot. :lol:


Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: miraculon on July 24, 2018, 10:28:34 AM
I was considering making some kind of "electronic wet/dry bulb" setup, and I found that SLOWeather already did it.

In California, dealing with the water source for the wet bulb sock would be better for more of the year than here in northern Michigan. I would have to take it down for winter. (or at least empty it)

Has anyone tried this setup?

It is an old thread but is still a valid and timely idea WRT this SHT-31 subject.

https://www.wxforum.net/index.php?topic=4844.0 (https://www.wxforum.net/index.php?topic=4844.0)

Greg H.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on July 24, 2018, 10:48:41 AM
Nice post, CW7491! This is why the thread exists - so people can share their experiences and knowledge.

Couple questions:

1) How do you mount the 75 to the shield if you don't use glue or silicon?
2) Approximately how long did your previous 75s last before they went off the rails?
I just wiretie the cord to the temp/hum mounting plate.

https://www.scaledinstruments.com/shop/davis-instruments/davis-7353-065-24-hour-spars-temperature-humidity-sensor-mounting-plate/

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 24, 2018, 10:53:57 AM
I don't *believe* that the Davis sensor is analog. I've inquired about this before because I, too, assumed it must be using one of Sensirion's analog solutions and was told by someone knowledgeable that Sensibus is a digital-only platform. johnd should know.

No, I don't know with 100% certainty I'm afraid - that level of detail would only be known within Davis Engineering.

But to the absolute best of my knowledge, Davis have only ever used (what seems to be referred to as) Sensibus or Sbus - ie digital - versions of the SHT series of T/H sensors in eg the post 2006 VP2 stations. So any thought of analogue would be - I strongly suspect - completely wide of the mark.

I've never been bothered to look into it in sufficient detail, but don't the markings on the sensor reveal its type/subtype etc? They're tiny and you probably need to take a good photo and enlarge appropriately and then cross-check with the detailed Sensirion documentation.

I’ve heard that the version of the SHT-31 that Davis uses is the “analog.” That’s why I said it but I’m probably not correct in calling it analog.

If someone on here knows how alter the Davis transmitter so that it can take a SHT-35 would be awesome! I also might add that I would be willing to buy a SHT-35 from someone who knows how to make the SHT-35 sensor that will fit into the Davis 24hr FARS & passive shields.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 24, 2018, 11:00:27 AM
I've been thinking about the issue of humidity reading too low on clear nights with radiational cooling resulting in dew and/or fog. Those of us with aspirated (FARS) shields tend to read warmer in those conditions because we are less subject to longwave radiation than passive shields. That's good and bad. It's good because that's technically closer to the guidelines for temperature measurement thereby achieving greater "accuracy". It's bad because the ground, the grass, and living things outdoors within a few feet of the ground ARE experiencing long wave radiation which drops the temperature until it meets the dew point. If you are outdoors during that time, your uncovered skin will feel the long-wave induced temperature, not the one reported by your weather station.

Also, there's a height difference to deal with on all shields that are at the standard 5-6ft off the ground. The further off the ground you are, the more likely there is an inversion (temperature increasing with height).

I'm not saying that the SHTxx sensors don't have a dry bias. They do. You can confirm this on any day or night with thick fog regardless of cloud cover. I'm just saying there are certain conditions that may prevent the temperature inside the shield from meeting the dew point. Yet another reason the ASOS/AWOS systems separate temperature measurement from humidity measurement and expose the humidity sensor with minimal shielding.

EDIT: I should add that even passive shielding will keep the temperature a bit higher than it would be otherwise, suppressing humidity.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on July 24, 2018, 11:05:16 AM
I don't *believe* that the Davis sensor is analog. I've inquired about this before because I, too, assumed it must be using one of Sensirion's analog solutions and was told by someone knowledgeable that Sensibus is a digital-only platform. johnd should know.

No, I don't know with 100% certainty I'm afraid - that level of detail would only be known within Davis Engineering.

But to the absolute best of my knowledge, Davis have only ever used (what seems to be referred to as) Sensibus or Sbus - ie digital - versions of the SHT series of T/H sensors in eg the post 2006 VP2 stations. So any thought of analogue would be - I strongly suspect - completely wide of the mark.

I've never been bothered to look into it in sufficient detail, but don't the markings on the sensor reveal its type/subtype etc? They're tiny and you probably need to take a good photo and enlarge appropriately and then cross-check with the detailed Sensirion documentation.

I’ve heard that the version of the SHT-31 that Davis uses is the “analog.” That’s why I said it but I’m probably not correct in calling it analog.

If someone on here knows how alter the Davis transmitter so that it can take a SHT-35 would be awesome! I also might add that I would be willing to buy a SHT-35 from someone who knows how to make the SHT-35 sensor that will fit into the Davis 24hr FARS & passive shields.
SHT3x-LSS datasheet

https://www.glynshop.com/erp/owweb/Daten/Datenblaetter/Sensirion/SHT3x_LSS_V0.94.pdf
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: hwcorder on July 24, 2018, 11:24:28 AM
So here is a rudimentary drawing of my setup. Apologies and the next chance I have I will try to take pics.  Anyway, I cut a small slot in the sensor shelf that comes with the 24hr FARS unit and inserted a 4 in connector soldering the wiring to the correct solder points.  Sorry I don't remember which color wires connect to which. I will have to take the sensor out of the shield and look at some point.  I protected and mounted the wires to the sensor shelf using electronic potting epoxy. Be careful not to use too much as to block the pins from being able to go through the 4 pin connector but enough to protect the solder joints. 
It was I little tough soldering the wires but not a huge deal. I'm okay with a soldering iron so I'm sure everybody here could do it as well.  I takes several hours to let the epoxy fully set but one thats done you are good to go. Plug in a new SHT75 and that it. I currently use the Davis filter which pop in using holes I drilled and it fits nicely around the 75 but I'm looking for a replacement that is a little more robust.
(http://i1283.photobucket.com/albums/a541/hwcorder/SHT75_zpsa6gp6imp.jpg) (http://s1283.photobucket.com/user/hwcorder/media/SHT75_zpsa6gp6imp.jpg.html)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Old Tele man on July 24, 2018, 11:30:03 AM
I don't *believe* that the Davis sensor is analog. I've inquired about this before because I, too, assumed it must be using one of Sensirion's analog solutions and was told by someone knowledgeable that Sensibus is a digital-only platform. johnd should know.

No, I don't know with 100% certainty I'm afraid - that level of detail would only be known within Davis Engineering.

But to the absolute best of my knowledge, Davis have only ever used (what seems to be referred to as) Sensibus or Sbus - ie digital - versions of the SHT series of T/H sensors in eg the post 2006 VP2 stations. So any thought of analogue would be - I strongly suspect - completely wide of the mark.

I've never been bothered to look into it in sufficient detail, but don't the markings on the sensor reveal its type/subtype etc? They're tiny and you probably need to take a good photo and enlarge appropriately and then cross-check with the detailed Sensirion documentation.

I’ve heard that the version of the SHT-31 that Davis uses is the “analog.” That’s why I said it but I’m probably not correct in calling it analog.

If someone on here knows how alter the Davis transmitter so that it can take a SHT-35 would be awesome! I also might add that I would be willing to buy a SHT-35 from someone who knows how to make the SHT-35 sensor that will fit into the Davis 24hr FARS & passive shields.
SHT3x-LSS datasheet

https://www.glynshop.com/erp/owweb/Daten/Datenblaetter/Sensirion/SHT3x_LSS_V0.94.pdf

Quoted from page 8:

"Heater: An on chip heating element can be addressed
by writing a command into status register. The heater
may increase the temperature of the sensor by 5 – 10°C8
beyond ambient temperature. The heater draws roughly
25 mW @ 5V supply voltage.
For example the heater can be helpful for functionality
analysis: Humidity and temperature readings before and
after applying the heater are compared. Temperature
shall increase while relative humidity decreases at the
same time. Dew point shall remain the same."


[red emphasis mine]

Anybody know if DAVIS utilize this function within the ISS?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: hwcorder on July 24, 2018, 12:00:42 PM
I don't *believe* that the Davis sensor is analog. I've inquired about this before because I, too, assumed it must be using one of Sensirion's analog solutions and was told by someone knowledgeable that Sensibus is a digital-only platform. johnd should know.

No, I don't know with 100% certainty I'm afraid - that level of detail would only be known within Davis Engineering.

But to the absolute best of my knowledge, Davis have only ever used (what seems to be referred to as) Sensibus or Sbus - ie digital - versions of the SHT series of T/H sensors in eg the post 2006 VP2 stations. So any thought of analogue would be - I strongly suspect - completely wide of the mark.

I've never been bothered to look into it in sufficient detail, but don't the markings on the sensor reveal its type/subtype etc? They're tiny and you probably need to take a good photo and enlarge appropriately and then cross-check with the detailed Sensirion documentation.

I’ve heard that the version of the SHT-31 that Davis uses is the “analog.” That’s why I said it but I’m probably not correct in calling it analog.

If someone on here knows how alter the Davis transmitter so that it can take a SHT-35 would be awesome! I also might add that I would be willing to buy a SHT-35 from someone who knows how to make the SHT-35 sensor that will fit into the Davis 24hr FARS & passive shields.
SHT3x-LSS datasheet

https://www.glynshop.com/erp/owweb/Daten/Datenblaetter/Sensirion/SHT3x_LSS_V0.94.pdf

Quoted from page 8:

"Heater: An on chip heating element can be addressed
by writing a command into status register. The heater
may increase the temperature of the sensor by 5 – 10°C8
beyond ambient temperature. The heater draws roughly
25 mW @ 5V supply voltage.
For example the heater can be helpful for functionality
analysis: Humidity and temperature readings before and
after applying the heater are compared. Temperature
shall increase while relative humidity decreases at the
same time. Dew point shall remain the same."


[red emphasis mine]

Anybody know if DAVIS utilize this function within the ISS?

No they don't. This feature is useful in high humidity situations where condensation can begin for form on the sensor.  If you can raise the temperature of the sensor far enough above the dew point you can prevent that.  However to use this functionality fully you would need a separate temperature sensor to continue to measure ambient air while the heating process in taking place.  The heated sensor of course would give an erroneous temperature reading.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: SLOweather on July 24, 2018, 12:03:36 PM
As noted in my posts in that thread, it's certainly doable. However, as also noted, using Davis wireless stations on a VP2 console limits you to whole degrees, which I found to be too constraining.

For instance...

Dry Wet  RH at 1013 mb
70   60   55.5
71   60   52.1
70   61   59.5 
69   60   58.9

Find a way to run matched sensors with at least 0.1° resolution, and it would work.

Here's an equation for it that leaves out BP:

(http://www.1728.org/relhum.png)
 
In another thread here I proposed building a chilled mirror dew point detector from an Arduino, Peltier device and a stainless steel camping mirror, but have yet to attempt it.

I was considering making some kind of "electronic wet/dry bulb" setup, and I found that SLOWeather already did it.

In California, dealing with the water source for the wet bulb sock would be better for more of the year than here in northern Michigan. I would have to take it down for winter. (or at least empty it)

Has anyone tried this setup?

It is an old thread but is still a valid and timely idea WRT this SHT-31 subject.

https://www.wxforum.net/index.php?topic=4844.0 (https://www.wxforum.net/index.php?topic=4844.0)

Greg H.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: rdsman on July 24, 2018, 12:18:58 PM
Quote

The SHT35 will be out in a pin form later this year. It'd be nice if someone could find a simple way to adapt it to a Davis ISS.


Follow this post:

http://www.wxforum.net/index.php?topic=33157.msg335987#msg335987

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: rdsman on July 24, 2018, 12:32:57 PM
Code: [Select]
Quoted from page 8:

"Heater: An on chip heating element can be addressed
by writing a command into status register. The heater
may increase the temperature of the sensor by 5 – 10°C8
beyond ambient temperature. The heater draws roughly
25 mW @ 5V supply voltage.
For example the heater can be helpful for functionality
analysis: Humidity and temperature readings before and
after applying the heater are compared. Temperature
shall increase while relative humidity decreases at the
same time. Dew point shall remain the same."

[red emphasis mine]

Anybody know if DAVIS utilize this function within the ISS?

If the VP2 is like the Vue, this wouldn't work without supplying an external voltage to the heated sensor.  In the Vue, VCC is only on during a sensor read.

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: mcrossley on July 24, 2018, 12:42:38 PM
Do we know if the ISS applies the two correction values for humidity non-linearity and temperature compensation? It would require intercepting the output from the chip and comparing the raw values with those transmitted by the ISS - way beyond my capabilities!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: miraculon on July 24, 2018, 12:59:38 PM
As noted in my posts in that thread, it's certainly doable. However, as also noted, using Davis wireless stations on a VP2 console limits you to whole degrees, which I found to be too constraining.

For instance...

Dry Wet  RH at 1013 mb
70   60   55.5
71   60   52.1
70   61   59.5 
69   60   58.9

Find a way to run matched sensors with at least 0.1° resolution, and it would work.

Here's an equation for it that leaves out BP:

(http://www.1728.org/relhum.png)
 
In another thread here I proposed building a chilled mirror dew point detector from an Arduino, Peltier device and a stainless steel camping mirror, but have yet to attempt it.

I was considering making some kind of "electronic wet/dry bulb" setup, and I found that SLOWeather already did it.

In California, dealing with the water source for the wet bulb sock would be better for more of the year than here in northern Michigan. I would have to take it down for winter. (or at least empty it)

Has anyone tried this setup?

It is an old thread but is still a valid and timely idea WRT this SHT-31 subject.

https://www.wxforum.net/index.php?topic=4844.0 (https://www.wxforum.net/index.php?topic=4844.0)

Greg H.

I just double checked and I am getting temperatures to 0.1°F increments using meteobridge and meteohub. This might be a workaround for the Davis console integer degrees issue.

Greg H.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: rdsman on July 24, 2018, 01:01:15 PM
Quote

Do we know if the ISS applies the two correction values for humidity non-linearity and temperature compensation?


It does......

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: SLOweather on July 24, 2018, 01:03:36 PM
I just double checked and I am getting temperatures to 0.1°F increments using meteobridge and meteohub. This might be a workaround for the Davis console integer degrees issue.

That'll work. Good catch. As you can see from my previous calcs, the sensors need to be closely matched, either physically, or corrected in your software, to make the calculation worthwhile.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: mcrossley on July 24, 2018, 01:48:00 PM
Quote

Do we know if the ISS applies the two correction values for humidity non-linearity and temperature compensation?


It does......
Good, in the ISS or the console?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 24, 2018, 02:12:15 PM
Well the sun came out and the wind has been1 to 5 mph a really good test for my dfars test and i am surprised so far that the dfars has been running cooler than both 24 hr fars readings by as much as .5f to 1.0f. I am shocked that so far the high volume fan is working well and if it keeps going like this then it will be really interesting tonight when i turn off the fan an use it as a regular shield and if it works out then that might solve the humidity problem at night. Stay tuned for further updates  :grin:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Old Tele man on July 24, 2018, 02:16:18 PM
Well the sun came out and the wind has been1 to 5 mph a really good test for my dfars test and i am surprised so far that the dfars has been running cooler than both 24 hr fars readings by as much as .5f to 1.0f. I am shocked that so far the high volume fan is working well and if it keeps going like this then it will be really interesting tonight when i turn off the fan an use it as a regular shield and if it works out then that might solve the humidity problem at night. Stay tuned for further updates  :grin:
Get one of those "daylight/nitelight" sensors to automatically shut off fan at dusk and then turn it back on at dawn?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: rdsman on July 24, 2018, 02:18:39 PM
Quote

Do we know if the ISS applies the two correction values for humidity non-linearity and temperature compensation?


It does......

Good, in the ISS or the console?

In the ISS.....
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on July 24, 2018, 03:00:31 PM
Nice post, CW7491! This is why the thread exists - so people can share their experiences and knowledge.

Couple questions:

1) How do you mount the 75 to the shield if you don't use glue or silicon?
2) Approximately how long did your previous 75s last before they went off the rails?

I mount it just like hwcorder showed. This is the first sensor I’ve used without a filter, so it’s only been 6 mos so far ...

As far as the wiring to the SHT75, I’ve found you’ll need to use the 6 wire cable even though the sensor only uses 4 wires, but I’m no expert, so someone else may know better.

Referencing the data sheet, the wiring is:
1. SCL - white
2. VDD - yellow
3. VSS - green
4. SDA - blue

Red - not used
Black - not used
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 24, 2018, 03:00:59 PM
I have the off/on photocell set up on my main station site but for testing l use one closer to the house, getting to old to do a lot of walking lol. So i will just up plug power tonight and see what happens if good results i will move it out to my main site and hook it up to my switchable power source.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 24, 2018, 03:02:27 PM
You can use 4 conductor wire with no problem just take the fourth wire to pin 6 leaving 4 and 5 blank.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 24, 2018, 03:06:47 PM
Quick update fars max so far 95.6   dfars 94.2 looking good.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 24, 2018, 06:36:37 PM
Final high update max fars 96.6  max dfars95.4
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WxLover16 on July 24, 2018, 07:12:28 PM
First off I wanna say how awesome it is for this topic to have ignited so much posting and enthusiasm and knowledge of everything that I love, temperature/humidity accuracy. However, so much posting of this vs. that sensor has me so confused. Some people say the 15 is better, some say the 31 is better, some say the 75 is better. I live in North GA in a generally very humid climate in summer, and my 31 has hit 100% once this summer but regularly hits the upper 90s every night, so my guess is that I have a good sensor. However, my main goal is temperature accuracy (always has been, always will be). WRT that, which is better, the 31 or 75? Also, Dendrite has possibly inspired me to remove my filter for more roller coaster readings which I love (to be able to catch all the extremes of the day, no smoothing crap!). I'm still debating it.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 24, 2018, 07:38:37 PM
Well i have four of the 31 sensors and they all show the same temp within .1degrees, really great on temperature accuracy. The 15 and  75 are pretty good in the mid ranges but i have read not so good in the really cold climates. Here in south Texas it doesn't get cold enough for me to have tried that out.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 24, 2018, 11:48:24 PM
Both the 15 and 75 are close to +/- (2°F)  @ -4°F when the 31 is only about +/- (.4°F) @ -4°.  I average around 22 days below 0°F so neither is great for cold climates.

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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on July 24, 2018, 11:56:05 PM
My 75 was pretty much in line with other stations during the winter I used it.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on July 25, 2018, 05:00:09 AM
No question the SHT31/35 are updated, improved and have better specs. If I could figure a way to use those sensors straight from the manufacturer, or without the wet bias observed from the Davis version, I would prefer it. It is purely anecdotal of course, but in terms of cold temps, I’ve tested the SHT75 as low as -8F side by side with an ASOS in New England and it was spot on.

We all have to make choices and compromises based on our unique circumstances and climate. My goal here is not to sell SHT75s (which are quickly running out of production), but to help change how Davis mounts/engineers these sensors to ultimately achieve their maximum accuracy. I invite all to try on their own, but my experience is humidity performance of the older generation SHT75, which has not been compromised by Davis mounting performed better than the SHT31 sensor with Davis mounting. I’m certain the SHT31 straight from the manufacturer would perform even better, but unfortunately that’s not an option at this point.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 25, 2018, 08:49:30 AM
Overnight report, at 6am all readings were the same with dense fog present. All three had a temp of 74.1 and humidity of 97%. When i plugged the fan in the dfars went to 98% where it is now. Just a note the fars with the 11 in it is one i made so is not contaminated like the Davis, i bought 10 sht11 sensors in factory sealed tape reel containers and mounted them on smd boards. The 11 tracks right with the new 15. If the dfars performs the same way today as it did yesterday and last night it looks like i can run the dfar and eliminate the night time fan which is great. More to come when the sun is up and the fog burned off.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 25, 2018, 09:28:24 AM
A little more info, i wish Davis would report humidity to the tenth would make it easier to compare readings. I took my modified hand held humidity meter out to the shields to see how close the readings are and the main fars was reading 96.97%, the sht11 was 97.03% and the dfars 15 was 97.98% .
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 25, 2018, 09:51:20 AM
Well some good news about reconditioned sensors.
I reconditioned 3 more sensors the other day 2-SHT31's and 1-SHT15 and just got around to testing.

I did the full rehydrate 20 hours unlike first time when disappointed not seeing any change in sensor performance only maxed at 96%. 
This time 2 of the 3 sensors reached 100% humidity in non polluting ESD bag (wet paper towel inside)  in about 8 minutes time very fast response.  I also went to 125c on 10 hour bake after reading how Spark FUN did all their sensors.

BAGS: https://www.amazon.com/gp/product/B07CX62R99/ref=oh_aui_detailpage_o04_s00?ie=UTF8&psc=1

I've been storing each sensor in these bags and WOW I've never had a sensor hit 100% until now without using software Cumulus bias. 

SHT31-100%
SHT31-98%
SHT15-100%

I'll get the 31 installed as primary sensor and see if the reconditioning helped with with high heat/hum bias.

This is the raw reading from one of the sensors s using envoy8x.  I left Davis filter on during test.
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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 25, 2018, 10:08:00 AM
That's good news hope it proves out in the long term. =D&gt;
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 25, 2018, 10:10:04 AM
That's good news hope it proves out in the long term. =D&gt;

Yes! I'm about to install the 31 that hit 100%...Excited  \:D/
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 25, 2018, 10:44:01 AM
I do wonder about over saturation.  I  couldn't remember which 31 hit 100% so redid test this time it took about 15 minutes, double the first time to reach 98 and 99%. The 100% wasn't reached on second test even with new wet paper towel.  Maybe if I waited longer..
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: DoctorKnow on July 25, 2018, 04:50:13 PM
I've been thinking and have an idea, (which I know is going against proper placement of sensors), but how about raising your ISS higher away from the ground? As I recall, when I had my 5n1, (which uses sensiron) higher at like 25 feet, it read lower on humidity. Would raising the station to say 15 feet help possibly? Just a thought... ? Or try placing the sensor near pavement? (crazy I know for temp)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 25, 2018, 05:08:25 PM
I've been thinking and have an idea, (which I know is going against proper placement of sensors), but how about raising your ISS higher away from the ground? As I recall, when I had my 5n1, (which uses sensiron) higher at like 25 feet, it read lower on humidity. Would raising the station to say 15 feet help possibly? Just a thought... ? Or try placing the sensor near pavement? (crazy I know for temp)

Think you are right especially those over lawns. I've had so much rain still haven't watered except early spring 1 time after weed and feed.  I do believe some moisture is always on grass because you see dew until about 1 pm many days.  Only mow late afternoons or grass gets stuck under carriage and plugs becoming pia to clean out.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WxLover16 on July 25, 2018, 05:25:37 PM
Dendrite, please check your PMs. Thanks.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 25, 2018, 05:57:06 PM
I've been thinking and have an idea, (which I know is going against proper placement of sensors), but how about raising your ISS higher away from the ground? As I recall, when I had my 5n1, (which uses sensiron) higher at like 25 feet, it read lower on humidity. Would raising the station to say 15 feet help possibly? Just a thought... ? Or try placing the sensor near pavement? (crazy I know for temp)

Think you are right especially those over lawns. I've had so much rain still haven't watered except early spring 1 time after weed and feed.  I do believe some moisture is always on grass because you see dew until about 1 pm many days.  Only mow late afternoons or grass gets stuck under carriage and plugs becoming pia to clean out.

Raising the ISS will only make it more difficult to achieve 100% humidity on nights with radiational cooling and dew.

The issue isn't environmental. That's proven by Ron's tests in a controlled environment. Also, up my way we've gone for long stretches without rain this summer. Yet I haven't noticed any reduction in the wet bias of my defective SHT31 sensor during the dry spells.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 25, 2018, 06:34:19 PM
Yeah wasn't suggesting that was the cure just you may be able to lower dp a little getting away from wet ground. I'm not moving sensors for sure but have tested high up before with unsatisfactory results with both abnormal warm lows and highs lower than normal. 5'-6' is correct elevation.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 25, 2018, 07:04:42 PM
I want to preface this by saying, I have only read the first half of this thread (~14 pages or so). So if my account is no longer germane, then sorry.  :-P

I have a Davis VP2. Iv'e been getting errors on Dew point from MADIS since I switched to a fan aspirated SHT31 sensor. This particular sensor is only a week old. This and my last SHT31 sensor has shown RH of +8 and Dewpoint of +3-4 degrees according to MADIS and it concerned me.
I live in Louisiana and RH is HIGH as most know. Also, Local airports are showing the same or higher dew points & RH than me almost daily. I've read many people here have little regard for the MADIS ratings and I'll address that too.
I found this thread and decided ..enough is enough!

So today I grabbed 2 Bacharach sling psychs from work. One is a Mercury and about 10 years old (maybe used twice) the other is red spirit and is brand new. Both wet & dry bulb match exactly when dry on both psychrometers.

At test, console shows 37%RH and 65 deg dew point. Temp 95 degrees. It's rather dry here today (Rare) due to a deep trough in the Gulf. Winds have been out of the NNE since yesterday (again, Very rare for July) so measuring mid-range humidity was just lucky timing.

First sling (Mercury) Psych showed 95 Dry and 76 wet worked out to 68 Deg DP , 41.5% RH.
Second (Spirit) showed 93 Dry and 74 Wet worked out to 65 Deg DP 40% RH.

This was done several times as the Temp dropped, and the readings were consistent.

Somebody Is Lying to me  :evil: And there initials are M.A.D.I.S.

If anything, I'm reading a bit dry. So, so far, I trust my station's data. I live in a heavily forested rural area with a Lake 2 miles west of me. I'm satisfied that my reading are correct. Oh and MADIS ?...I'm not re-calibrating anything so you can shove it.  \:D/

The vast amount of information here on this humidity bias and temp/humidity sensors in general is stunning. Most here have forgotten more than I'll ever learn about all this. Iv'e learned a lot and hope to learn more. Keep up the great discussions everyone.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 25, 2018, 07:27:26 PM
Somebody Is Lying to me  :evil: And there initials are M.A.D.I.S.
Oh and MADIS ?...I'm not re-calibrating anything so you can shove it.  \:D/
All CQ's are done by comparing yourself to neighboring stations, be they good or be they bad. All you can do is what you've done, use common sense and understand your data.
No one knows your PWS better than you.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 25, 2018, 07:41:42 PM
Somebody Is Lying to me  :evil: And there initials are M.A.D.I.S.
Oh and MADIS ?...I'm not re-calibrating anything so you can shove it.  \:D/
All CQ's are done by comparing yourself to neighboring stations, be they good or be they bad. All you can do is what you've done, use common sense and understand your data.
No one knows your PWS better than you.

You're right, and I know that, and I was fine knowing my sensors were correct and they were just 'grading on a curve'...but I guess what got lost in my post (my fault) was that this thread was exacerbating my doubts that 'maybe my sensors are wrong'.  Sorry MADIS, didn't mean to be so rough on ya'
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 25, 2018, 07:50:05 PM
I want to preface this by saying, I have only read the first half of this thread (~14 pages or so). So if my account is no longer germane, then sorry.  :-P

I have a Davis VP2. Iv'e been getting errors on Dew point from MADIS since I switched to a fan aspirated SHT31 sensor. This particular sensor is only a week old. This and my last SHT31 sensor has shown RH of +8 and Dewpoint of +3-4 degrees according to MADIS and it concerned me.
I live in Louisiana and RH is HIGH as most know. Also, Local airports are showing the same or higher dew points & RH than me almost daily. I've read many people here have little regard for the MADIS ratings and I'll address that too.
I found this thread and decided ..enough is enough!

So today I grabbed 2 Bacharach sling psychs from work. One is a Mercury and about 10 years old (maybe used twice) the other is red spirit and is brand new. Both wet & dry bulb match exactly when dry on both psychrometers.

At test, console shows 39%RH and 66 deg dew point. Temp 95 degrees. It's rather dry here today (Rare) due to a deep trough in the Gulf. Winds have been out of the NNE since yesterday (again, Very rare for July) so measuring mid-range humidity was just lucky timing.

First sling (Mercury) Psych showed 95 Dry and 76 wet worked out to 68 Deg DP , 41.5% RH.
Second (Spirit) showed 93 Dry and 74 Wet worked out to 65 Deg DP 40% RH.

This was done several times as the Temp dropped, and the readings were consistent.

Somebody Is Lying to me  :evil: And there initials are M.A.D.I.S.

If anything, I'm reading a bit dry. So, so far, I trust my station's data. I live in a heavily forested rural area with a Lake 2 miles west of me. I'm satisfied that my reading are correct. Oh and MADIS ?...I'm not re-calibrating anything so you can shove it.  \:D/

The vast amount of information here on this humidity bias and temp/humidity sensors in general is stunning. Most here have forgotten more than I'll ever learn about all this. Iv'e learned a lot and hope to learn more. Keep up the great discussions everyone.

All I can say this is abnormal. I would like to look into it further what CWOP station and airport do you compare to?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 25, 2018, 07:54:43 PM
Somebody Is Lying to me  :evil: And there initials are M.A.D.I.S.
Oh and MADIS ?...I'm not re-calibrating anything so you can shove it.  \:D/
All CQ's are done by comparing yourself to neighboring stations, be they good or be they bad. All you can do is what you've done, use common sense and understand your data.
No one knows your PWS better than you.
I guess what got lost in my post (my fault) was that this thread was exacerbating my doubts that 'maybe my sensors are wrong'
Ha! Get in an unfortunate line....
This will eventually get figured out, hopefully sooner than later.  :roll:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 25, 2018, 08:13:27 PM


All I can say this is abnormal. I would like to look into it further what CWOP station and airport do you compare to?

I agree. But all I can go by is the list of nearby stations at the bottom of the Data quality page.

EW6758   14.7 miles   North Georgetown, LA, US    -0.6mb   -1.6°F   -1.1°F
KAEX        17.6 miles South ALEXANDRIA INTERNATIONAL AIRPORT, LA, United States    -0.1mb   -0.6°F   1.7°F
EW6201   22.7 miles   South       Alexandria, LA, US    0.5mb   -1.1°F   0.7°F
UP886   22.7 miles   Northeast       Urania, LA,    -0.2°F   
KESF          22.9 miles   Southeast       ALEXANDRIA ESLER RGNL AIRPORT , LA,  -0.5mb   0.0°F   0.5°F
UR391   29.1 miles   South  Wilgln, LA, United States Of America                                     2.2°F   
KIER          31.1 miles   West       Natchitoches, Natchitoches Regional Airport, LA,  -0.5mb   -1.8°F   1.7°F
UP368   34.3 miles   South       Meeker, LA, United States Of America                        -0.3°F   
KAQV   42.2 miles   West       Ft. Polk, Peason Ridge, LA, United States                          -0.4mb   -0.7°F   2.7°F
KBKB   43.1 miles   Southwest       Ft. Polk, Fullerton Landing Strip, LA, United States    -0.1mb   -1.5°F   0.6°F


And of these that I looked at recently, many of their Quality checks are also flagged for high humidity/dew point.

Oh FYI I'm DW5212 (D5212 MADIS)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 25, 2018, 08:45:34 PM
Thanks for the location...Really not interested in MADAS but looking at your nearby airport Dewpoint is running about 4° higher. Which is the norm with these sensors.

Can't say anything about the sling readings you got, but they are notorious for giving different readings for different individuals. #1 reason for erratic results was finding appropriate shade. Back in the day ERT training we broke several units with 25 firefighters slinging up next to pumper truck where the only shade was.  :lol: 


https://mesowest.utah.edu/cgi-bin/droman/meso_base_dyn.cgi?stn=KAEX
https://mesowest.utah.edu/cgi-bin/droman/meso_base_dyn.cgi?stn=D5212&unit=0&timetype=LOCAL

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 25, 2018, 09:11:00 PM
Thanks for the location...Really not interested in MADAS but looking at your nearby airport Dewpoint is running about 4° higher. Which is the norm with these sensors.

Can't say anything about the sling readings you got, but they are notorious for giving different readings for different individuals. #1 reason for erratic results was finding appropriate shade. Back in the day ERT training we broke several units with 25 firefighters slinging up next to pumper truck where the only shade was.  :lol: 


https://mesowest.utah.edu/cgi-bin/droman/meso_base_dyn.cgi?stn=KAEX
https://mesowest.utah.edu/cgi-bin/droman/meso_base_dyn.cgi?stn=D5212&unit=0&timetype=LOCAL

So you're saying my Temp/Hum sensor is showing too wet?
Does local terrain/vegetation account for this? KAEX is 18 miles south of me and surrounded by miles of wide open dry cracked earth , and I'm in a deep hardwood forest near a swamp/lake.  Shouldn't I be reading a Higher dewpoint?
 And as far as shade to sling the psych. I got nothing but shade in my yard. You gotta search for sunny spot in the late afternoon  :grin:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 25, 2018, 09:38:52 PM
I can't say anything other than it does appear you are wetter.
Could be you have a good sensor and local geography makes it wetter.

I've just never seen it out of 8 different 31's they all act the same with exception some hit 100% others only 96%. 
I'm on my 3rd and final batch of baking 31's for reconditioning but out of the sensors I've tested the wet bias is still present.
 
I purchased all my sensors after the fact (not with station) could very well be there's a bad batch.

It's good you posted results, gives hope just maybe not all are bad and those complaining just have a bad batch.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 25, 2018, 09:46:29 PM
I can't say anything other than it does appear you are wetter.
Could be you have a good sensor and local geography makes it wetter.

I've just never seen it out of 8 different 31's they all act the same with exception some hit 100% others only 96%. 
I'm on my 3rd and final batch of baking 31's for reconditioning but out of the sensors I've tested the wet bias is still present.
 
I purchased all my sensors after the fact (not with station) could very well be there's a bad batch.

It's good you posted results, gives hope just maybe not all are bad and those complaining just have a bad batch.

Understood. The first 31 I had, about 2 years now, read 100% on only 2 or 3 occasions. The first time It happened, It was the first foggy night and I was watching it like a hawk. It took about 6 or 8 hours to go from 99% to 100% but it finally did hit it. This one is only a week old and conditions haven't been humid enough yet.

Edit to add: Now that I think about it, it was a passive shield setup at that time too. Nothing inferred with that just full disclosure. And also, on those 2 or 3 occasions it was early in its life. Haven't seen that in a while.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 25, 2018, 09:59:34 PM
Bobvelle where did you purchase the new 31?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 25, 2018, 10:11:25 PM
Bobvelle where did you purchase the new 31?

The latest and the older one both came from Ryan at Scaled Instruments.  The newest is the 25ft cabled version though. Which I doubt makes any difference.

Hey thanks for the MesoWest summery link. I never noticed it showed a wetbulb temp. Interesting.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 25, 2018, 10:15:50 PM
Bobvelle where did you purchase the new 31?

The latest and the older one both came from Ryan at Scaled Instruments.  The newest is the 25ft cabled version though. Which I doubt makes any difference.

Thanks
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 26, 2018, 01:51:27 PM
I can't say anything other than it does appear you are wetter.
Could be you have a good sensor and local geography makes it wetter.

I've just never seen it out of 8 different 31's they all act the same with exception some hit 100% others only 96%. 
I'm on my 3rd and final batch of baking 31's for reconditioning but out of the sensors I've tested the wet bias is still present.
 
I purchased all my sensors after the fact (not with station) could very well be there's a bad batch.

It's good you posted results, gives hope just maybe not all are bad and those complaining just have a bad batch.

Understood. The first 31 I had, about 2 years now, read 100% on only 2 or 3 occasions. The first time It happened, It was the first foggy night and I was watching it like a hawk. It took about 6 or 8 hours to go from 99% to 100% but it finally did hit it. This one is only a week old and conditions haven't been humid enough yet.

Edit to add: Now that I think about it, it was a passive shield setup at that time too. Nothing inferred with that just full disclosure. And also, on those 2 or 3 occasions it was early in its life. Haven't seen that in a while.

A “good” SHT 31 (Davis version) does well when humidities hits in the 90s up to saturation. But that’s about it. It will still have a wet bias at certain humidity and temperature  ranges. IMO.

As of now, I’m planning on getting the SHT-75 for both of my stations and using it until Davis resolves the issues with their version of the SHT-31.

I think it would be a good idea for Davis to redo the transmitter, in so that, it can take pure I2C and use the SHT-35 (the one that’s replacing the 75). Also use the SF2 filter cap. I think it would resolve their problems.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 26, 2018, 04:10:22 PM
I have followed this forum for years, but never registered or even really considered posting. I have learned a great deal from following all of your comments and experiences over the years. So let me start by saying thank you. This particular topic, however, has brought me out of the shadows …

I have owned Davis equipment since 2005. I bought a VP2. In 2006, I moved to south Texas and upgraded to a 24hr FARS and this is where my frustration with the dew point readings of the VP2 began. My new sensor would never hit 100% humidity and the rest of the scale had a wet bias, but my VP2 from 2005 never had these issues. My 2006 VP2 with FARS had an SHT11 and my 2005 had the old "analog" sensor.

I am no expert obviously, just another guy with a borderline unhealthy obsession with this stuff, but bottom line: my opinion and experience strongly supports that despite the inherent limitation of the Sensirion sensors, there is something about Davis' manufacturing process that causes an offset toward a wet bias.

I have been obsessed with trying to fix and find the source of this bias as well as trying to find a sensor that consistently hits 100% RH. The SHT11 had these issues and disappointingly so did the SHT31. I have moved around a bunch, so I have tested these things from anywhere from Texas to Maine to DC and to the Pacific Northwest. The result has always been the same. I have spent hours over the years with my sensors within 10 feet from the HO-1088 and DTS1 of an ASOS. I have purchased a psychro-dyne for quality checking. I have had the sensors in the -20sF in Maine and Vermont and over 100F in DC and Texas. Here are the main points of what I have learned:

-Outgassing of silicon and glue offsets the Sensirion sensor. The offset seems irreversible. Silicon offsets to the high side. Some glues to the low side. (I discovered this trying to use Sensirion's own filters instead of Davis' filter.)
-The Davis filter may increase response time, but is not responsible for the wet bias.
-The Sensirion sensors do seem to have a slight wet bias as has been documented in some studies, but this wet bias is some how exacerbated by how Davis mounts the sensor.
-The wet bias for Sensirion sensors in prolonged periods near saturation as mentioned in their datasheet is a separate issue from the wet bias created by the Davis mounting.
-The SHT75 straight from the manufacturer without any filter is superior to the Davis mounted SHT31 in terms of dew point and humidity.
-The SHT75 will hit up to 99%, with random very short lived jumps to 100%.
-I miss the old analog sensor

I currently run my station with an SHT75 with no filter at all. I've been doing it for about 6mos. The first 5 of those months were in a stock Davis 24hr FARS. Now it is in the Davis 7714, which is a surprisingly good passive shield (much better than the VP2 stock passive shield) (sometimes a degree or two cooler than the FARS, sometimes a degree or two warmer, at least in the north latitudes of the Seattle area...) I am concerned with contamination and dust, but after trying to put a filter on it with silicon and other glues, I have ruined a fair number of SHT75s with seemingly irreversible offsets. So far I have not seen any evidence of poor performance for lack of a filter.

I run a calibration in Cumulus for the SHT75 (1.041x-3) and field check it with the psychro-dyne. It generally is very close. The psychro-dyne is surprisingly accurate compared side by side to ASOS. I set Cumulus to report 98% as 100% and the calibration I set ensures 99% is reported as 100% too.

This is the best I have been able to do given the limitations of these sensors. I really hope the efforts here will force Davis to look at their process as I'd be happy to have an accurate Davis stock sensor that is engineered to last.

Hope this helps...

I can say I've always been told the Davis 7714 was a good passive shield and yes it blows the VP2 stock passive shield away. I just installed one today mid July at 43° near South Dakota border and running a full degree below both FARS shields and this is with very light winds.
This is over cut grass and 75-80F range currently with very high sun angle (straight up) July. Whether this changes once the sun is  low will wait and see this winter.

I'll never say a passive shield shouldn't be used again after testing this unit.  I did paint the inside bottom where sensor mounts and top inside flat black because I can never keep anything fully stock.  It's just an issue I have.    :oops:

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Old Tele man on July 26, 2018, 04:40:50 PM
I have followed this forum for years, but never registered or even really considered posting. I have learned a great deal from following all of your comments and experiences over the years. So let me start by saying thank you. This particular topic, however, has brought me out of the shadows …

I have owned Davis equipment since 2005. I bought a VP2. In 2006, I moved to south Texas and upgraded to a 24hr FARS and this is where my frustration with the dew point readings of the VP2 began. My new sensor would never hit 100% humidity and the rest of the scale had a wet bias, but my VP2 from 2005 never had these issues. My 2006 VP2 with FARS had an SHT11 and my 2005 had the old "analog" sensor.

I am no expert obviously, just another guy with a borderline unhealthy obsession with this stuff, but bottom line: my opinion and experience strongly supports that despite the inherent limitation of the Sensirion sensors, there is something about Davis' manufacturing process that causes an offset toward a wet bias.

I have been obsessed with trying to fix and find the source of this bias as well as trying to find a sensor that consistently hits 100% RH. The SHT11 had these issues and disappointingly so did the SHT31. I have moved around a bunch, so I have tested these things from anywhere from Texas to Maine to DC and to the Pacific Northwest. The result has always been the same. I have spent hours over the years with my sensors within 10 feet from the HO-1088 and DTS1 of an ASOS. I have purchased a psychro-dyne for quality checking. I have had the sensors in the -20sF in Maine and Vermont and over 100F in DC and Texas. Here are the main points of what I have learned:

-Outgassing of silicon and glue offsets the Sensirion sensor. The offset seems irreversible. Silicon offsets to the high side. Some glues to the low side. (I discovered this trying to use Sensirion's own filters instead of Davis' filter.)
-The Davis filter may increase response time, but is not responsible for the wet bias.
-The Sensirion sensors do seem to have a slight wet bias as has been documented in some studies, but this wet bias is some how exacerbated by how Davis mounts the sensor.
-The wet bias for Sensirion sensors in prolonged periods near saturation as mentioned in their datasheet is a separate issue from the wet bias created by the Davis mounting.
-The SHT75 straight from the manufacturer without any filter is superior to the Davis mounted SHT31 in terms of dew point and humidity.
-The SHT75 will hit up to 99%, with random very short lived jumps to 100%.
-I miss the old analog sensor

I currently run my station with an SHT75 with no filter at all. I've been doing it for about 6mos. The first 5 of those months were in a stock Davis 24hr FARS. Now it is in the Davis 7714, which is a surprisingly good passive shield (much better than the VP2 stock passive shield) (sometimes a degree or two cooler than the FARS, sometimes a degree or two warmer, at least in the north latitudes of the Seattle area...) I am concerned with contamination and dust, but after trying to put a filter on it with silicon and other glues, I have ruined a fair number of SHT75s with seemingly irreversible offsets. So far I have not seen any evidence of poor performance for lack of a filter.

I run a calibration in Cumulus for the SHT75 (1.041x-3) and field check it with the psychro-dyne. It generally is very close. The psychro-dyne is surprisingly accurate compared side by side to ASOS. I set Cumulus to report 98% as 100% and the calibration I set ensures 99% is reported as 100% too.

This is the best I have been able to do given the limitations of these sensors. I really hope the efforts here will force Davis to look at their process as I'd be happy to have an accurate Davis stock sensor that is engineered to last.

Hope this helps...

I can say I've always been told the Davis 7714 was a good passive shield and yes it blows the VP2 stock passive shield away. I just installed one today mid July at 43° near South Dakota border and running a full degree below both FARS shields and this is with very light winds.
This is over cut grass and 75-80F range currently with very high sun angle (straight up) July. Whether this changes once the sun is  low will wait and see this winter.

I'll never say a passive shield shouldn't be used again after testing this unit.  I did paint the inside bottom where sensor mounts and top inside flat black because I can never keep anything fully stock.  It's just an issue I have.    :oops:

All the solar radiation 'reflection' plates in our YPG tank-testing ambient air thermal-couple sensors had WHITE tops and flat-BLACK bottoms.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 26, 2018, 04:50:25 PM

All the solar radiation 'reflection' plates in our YPG tank-testing ambient air thermal-couple sensors had WHITE tops and flat-BLACK bottoms.

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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 26, 2018, 05:13:54 PM
I can say I've always been told the Davis 7714 was a good passive shield and yes it blows the VP2 stock passive shield away. I just installed one today mid July at 43° near South Dakota border and running a full degree below both FARS shields and this is with very light winds.
This is over cut grass and 75-80F range currently with very high sun angle (straight up) July. Whether this changes once the sun is  low will wait and see this winter.

I'll never say a passive shield shouldn't be used again after testing this unit.  I did paint the inside bottom where sensor mounts and top inside flat black because I can never keep anything fully stock.  It's just an issue I have.    :oops:

Questions & observations.

First, you couldn't get a more basic "Gill" type shield. Ambient Weather has their own version (SRS100LX).

What did Davis do so wrong with their stock passive shield that it can't beat something that looks like it was designed and constructed in 8th grade science class?

Did you mount an SHTxx sensor in there? If so, how?

You recently modified the FARS shield to make it passive. You said it had amazing performance. How's it holding up?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Old Tele man on July 26, 2018, 05:25:11 PM
I can say I've always been told the Davis 7714 was a good passive shield and yes it blows the VP2 stock passive shield away. I just installed one today mid July at 43° near South Dakota border and running a full degree below both FARS shields and this is with very light winds.
This is over cut grass and 75-80F range currently with very high sun angle (straight up) July. Whether this changes once the sun is  low will wait and see this winter.

I'll never say a passive shield shouldn't be used again after testing this unit.  I did paint the inside bottom where sensor mounts and top inside flat black because I can never keep anything fully stock.  It's just an issue I have.    :oops:

Questions & observations.

First, you couldn't get a more basic "Gill" type shield. Ambient Weather has their own version (SRS100LX).

What did Davis do so wrong with their stock passive shield that it can't beat something that looks like it was designed and constructed in 8th grade science class?

Did you mount an SHTxx sensor in there? If so, how?

You recently modified the FARS shield to make it passive. You said it had amazing performance. How's it holding up?
The SRS100LX has 9 plates spaced over 10.75-inches. That's a little over 1-inch spacing between each plate and the plate "lips" are deeper than each plate-to-plate spacing, so there's LOTS of air flow capability. Years ago I used one with my LaCrosse WS-8610U system once I figured out that their crappy unit was totally worthless.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 26, 2018, 05:34:30 PM
The SRS100LX has 9 plates spaced over 10.75-inches. That's a little over 1-inch spacing between each plate and the plate "lips" are deeper than each plate-to-plate spacing, so there's LOTS of air flow capability. Years ago I used one with my LaCrosse WS-8610U system once I figured out that their crappy unit was totally worthless.

The problem out this way with more spacing is it makes it easier for wind-driven snow to get in and sit on the sensor.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Old Tele man on July 26, 2018, 05:39:37 PM
The SRS100LX has 9 plates spaced over 10.75-inches. That's a little over 1-inch spacing between each plate and the plate "lips" are deeper than each plate-to-plate spacing, so there's LOTS of air flow capability. Years ago I used one with my LaCrosse WS-8610U system once I figured out that their crappy unit was totally worthless.

The problem out this way with more spacing is it makes it easier for wind-driven snow to get in and sit on the sensor.

Seldom snows here in High Deserts of AZ -- but it does snow on rare occasions -- it's summer heat, not winter snow, that's our problem.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 26, 2018, 05:49:17 PM
I can say I've always been told the Davis 7714 was a good passive shield and yes it blows the VP2 stock passive shield away. I just installed one today mid July at 43° near South Dakota border and running a full degree below both FARS shields and this is with very light winds.
This is over cut grass and 75-80F range currently with very high sun angle (straight up) July. Whether this changes once the sun is  low will wait and see this winter.

I'll never say a passive shield shouldn't be used again after testing this unit.  I did paint the inside bottom where sensor mounts and top inside flat black because I can never keep anything fully stock.  It's just an issue I have.    :oops:

Questions & observations.

First, you couldn't get a more basic "Gill" type shield. Ambient Weather has their own version (SRS100LX).

What did Davis do so wrong with their stock passive shield that it can't beat something that looks like it was designed and constructed in 8th grade science class? I tested the VP2 stock couple years ago and was running very warm +3 at times.

Did you mount an SHTxx sensor in there? If so, how? Yes, same place designed for a metal probe in middle but just used 1 screw and not bracket for metal probe

You recently modified the FARS shield to make it passive. You said it had amazing performance. How's it holding up? It did really well until it got calm and then was running +2°. Still better than the stock VP2 I would say.

I feel passive shields may give mixed results depending on longitude, siting placement over bare ground, sand, grass & wind speed. I may hate the shield at 100° but today it's still better than the fars barely, currently no wind


Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 26, 2018, 06:06:07 PM
Thanks for the info!

What did Davis do so wrong with their stock passive shield that it can't beat something that looks like it was designed and constructed in 8th grade science class? I tested the VP2 stock couple years ago and was running very warm +3 at times.

I was just looking at the VP2 specs where Davis admits that at 1040 w/m2 insolation the stock shield can reach +4F errors. I've seen worse over fresh snow (6F+ compared to my FARS).
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 26, 2018, 06:16:15 PM
Thanks for the info!

What did Davis do so wrong with their stock passive shield that it can't beat something that looks like it was designed and constructed in 8th grade science class? I tested the VP2 stock couple years ago and was running very warm +3 at times.
I've seen worse over fresh snow (6F+ compared to my FARS).
Many folks are under the impression that a FARS only benefits hot climates. I got to the point where I quit trying. ](*,)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 26, 2018, 06:38:27 PM
The flat black (not Gloss)  is meant to minimize this reflective radiation not sure why Davis doesn't manufacturer shield like this. If you look at most modern passive shields flat black inside is pretty much how all look now.  This is just example how I mounted the sensor. This shield is going to another location with different SHT31 so screws are not tightened down. 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 26, 2018, 07:11:23 PM
I personally think the Davis stock shield does well. There is lagging but as along as you sited over grass and have a nice breeze on a consistent basis, it does good.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 26, 2018, 07:19:57 PM
I personally think the Davis stock shield does well. There is lagging but as along as you sited over grass and have a nice breeze on a consistent basis, it does good.
Problem is, you can control the siting, not the breeze, especially on a consistent basis.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 26, 2018, 07:36:44 PM
I personally think the Davis stock shield does well. There is lagging but as along as you sited over grass and have a nice breeze on a consistent basis, it does good.
Problem is, you can control the siting, not the breeze, especially on a consistent basis.

Definitely true. Where one of my stations is located, it gets a good breeze most of the time. The other location is pretty much enclosed. Only really good breeze that sensor would’ve got us from the south. That’s why I have the 24hr FARS for that site.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Old Tele man on July 26, 2018, 07:41:27 PM
It's all about LOCATION, LOCATION, LOCATION:

• LOCATION where you physically site it.

• LOCATION where weather can reach it.

• LOCATION where YOU can reach it for service/repair.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 26, 2018, 08:36:32 PM
This was what the Davis 7714 shield looked like today. The red dot was when test started. Blue #1 is the passive shield, grey FARS.
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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Old Tele man on July 26, 2018, 08:48:22 PM
I ordered a Davis #07747 DFARS "kit" and will DIY it to be my "secondary/back-up" TEMP/RH system using an "old" spare ISS I have. Plan is for easy (hopefully) sensor access/change-out; it'll be mounted on same pole as current VP2, only pointing North so as NOT to physically interfere with the VP2. Should enable me to simultaneously test/play with older SHT-15 and current SHT-31 sensors (old & new) that I have stock-piled.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 27, 2018, 12:12:22 AM
I ordered a Davis DFARS "kit" and will DIY'ing it to be my "secondary/back-up" TEMP/RH system using an "old" spare ISS I have. Plan is for easy (hopefully) sensor access/change-out; it'll be mounted on same pole as current VP2, only pointing North so as NOT to physically interfere with the VP2. Should  enable me to simultaneously test/play with older SHT-15 and current SHT-31 sensors (old & new) that I have stock-piled.

Ha, I'm actually planning to do the exact same thing. Except my second Temp/Hum will be on another pole about 50ft away. Just waiting on a new ISS cover and some other odds and ends.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 27, 2018, 04:46:54 AM
No more FARS for me if these 7714's work out. I really hope we get some hot weather before slipping into fall.  Seems like all of a sudden only 70-80 & no 90's in sight so hard to confirm what I'm seeing. For sure I'll continue side by side test even into winter. Once the grass has turned and all leaves are gone, with different sun angles results could differ but very encouraging now in my heavy foliage humid environment.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: drew1021 on July 27, 2018, 07:04:33 AM
No more FARS for me if these 7714's work out. I really hope we get some hot weather before slipping into fall.  Seems like all of a sudden only 70-80 & no 90's in sight so hard to confirm what I'm seeing. For sure I'll continue side by side test even into winter. Once the grass has turned and all leaves are gone, with different sun angles results could differ but very encouraging now in my heavy foliage humid environment.

No way I'm giving up FARS. Even though I 'm frustrated to no end over the wet bias, the temperature is spot on. I will just have to wait for Davis to come up with a solution to the humidity issues we've all been experiencing.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 27, 2018, 07:39:06 AM
No more FARS for me if these 7714's work out. I really hope we get some hot weather before slipping into fall.  Seems like all of a sudden only 70-80 & no 90's in sight so hard to confirm what I'm seeing. For sure I'll continue side by side test even into winter. Once the grass has turned and all leaves are gone, with different sun angles results could differ but very encouraging now in my heavy foliage humid environment.

No way I'm giving up FARS. Even though I 'm frustrated to no end over the wet bias, the temperature is spot on. I will just have to wait for Davis to come up with a solution to the humidity issues we've all been experiencing.

Can't blame you I felt the same way until yesterday and this confirms what I saw. 2 year study by WMO gave the 7714 its highest 5 star rating running 97-99% with the reference shield.
I started a different thread since this is a shield subject not reliabilty of SHT.
https://www.wxforum.net/index.php?topic=34796.msg354556;topicseen#msg354556
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: drew1021 on July 27, 2018, 09:11:47 AM
Thanks for the link. I didn't mean to stray from the topic. I personally don't believe the FARS is the culprit here. My personal feeling is the 31 sensor is the problem, not temperature, just humidity.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 27, 2018, 09:55:08 AM
Thanks for the link. I didn't mean to stray from the topic. I personally don't believe the FARS is the culprit here. My personal feeling is the 31 sensor is the problem, not temperature, just humidity.

We've identified 3 problems with the SHT31 humidity sensor so far in this thread:


Fan aspiration affects #1 and to a much lesser extent #3 too. In non-arid climates active ventilation bombs the sensor with moist air that leads to premature aging.  It also elevates nighttime temperatures on clear nights which leads to lower humidity readings. Although, I would argue that any good passive shield will have the same effect during radiational cooling so long as it's 2 meters off the ground.

The clearest and most frustrating aspect of the dry bias is during fog events when most SHTxx sensors won't achieve 100%. This has nothing to do with your shielding choice. It's simply a limitation of the sensor. If your sensor reads 96.5% humidity in saturated air it is considered within specification.

By far the biggest problem is the wet bias (worst between 40-70%). This thread contains reports of max humidity errors in the range of 10-20%! That leads to wildly inflated heat indices in the summer. A survey of other VP2's online compared to nearby airports reveals a widespread problem that appears to stretch back to the SHT11 sensor.

EDIT 10:52AM: As for when the wet bias develops, that's uncertain. I have one good SHT31 sensor that's 7 months old and one with the wet bias that's 16 months old. Both have been in a mixture of FARS and passive shields. Others have reported receiving new sensors recently and all of them have run wet.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: drew1021 on July 27, 2018, 10:10:31 AM
Excellent summary!  8-)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on July 27, 2018, 10:59:49 AM
I've been dealing with a stomach bug and a cold turkey, coffee quitting, caffeine withdrawal migraine so my new toys aren't getting installed until this weekend. I got a couple of new 12V FARS fans, new rad plates, aerocone, new uv/solar covers, new SHT75, and new connectors to make the 12 y/o VP2 look somewhat new again. I'll post some graphs later Sunday if I can get everything installed Saturday morning (or tonight if I'm up for it). I'll be able to show you the difference in my response times between the SHT31 (no cover) and SHT75 (no cover) with the same 40 CFM fan. We'll see how high the new 75's RH can reach too.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Old Tele man on July 27, 2018, 11:10:58 AM
In my best Cuba Gooding, Jr imitation voice, "...Show me US the money DATA..." (ha,ha)!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on July 27, 2018, 11:29:24 AM
In my best Cuba Gooding, Jr imitation voice, "...Show me US the money DATA..." (ha,ha)!
You had my data at hello.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: drew1021 on July 27, 2018, 11:46:35 AM
What I've done temporarily is set a -1% bias on my console so at 100% I will max out at 97%
I would rather be off by 3% at the high end then suffer by 4to 6% mid range. I know it's not much but every little bit helps. I sure hope Davis is really working on this issue and not blowing smoke up our a**
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 27, 2018, 12:39:12 PM
I've been dealing with a stomach bug and a cold turkey, coffee quitting, caffeine withdrawal migraine so my new toys aren't getting installed until this weekend. I got a couple of new 12V FARS fans, new rad plates, aerocone, new uv/solar covers, new SHT75, and new connectors to make the 12 y/o VP2 look somewhat new again. I'll post some graphs later Sunday if I can get everything installed Saturday morning (or tonight if I'm up for it). I'll be able to show you the difference in my response times between the SHT31 (no cover) and SHT75 (no cover) with the same 40 CFM fan. We'll see how high the new 75's RH can reach too.

Nice!  Like I said earlier, I’m planning on switching over to the 75. Shoot, I might even change out the stock passive shield from my station at my friend’s farm and install the 7714 shield.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 27, 2018, 01:41:15 PM
Would I be correct to say that the SHT-75 doesn’t have the wet bias like the SHT-31?  Because I don’t see it with the comparison with these two stations...

https://mesowest.utah.edu/cgi-bin/droman/meso_table_mesowest_m.cgi?stn=KLCI&unit=0&time=&hours=24&hour1=23&day1=26&month1=07&year1=2018&radius=25&past=0&order=1


SHT-75. https://mesowest.utah.edu/cgi-bin/droman/meso_table_mesowest_m.cgi?stn=C7324&unit=0&time=&hours=24&hour1=23&day1=26&month1=07&year1=2018&radius=25&past=0&order=1
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on July 27, 2018, 02:39:00 PM
Would I be correct to say that the SHT-75 doesn’t have the wet bias like the SHT-31?  Because I don’t see it with the comparison with these two stations...

https://mesowest.utah.edu/cgi-bin/droman/meso_table_mesowest_m.cgi?stn=KLCI&unit=0&time=&hours=24&hour1=23&day1=26&month1=07&year1=2018&radius=25&past=0&order=1


SHT-75. https://mesowest.utah.edu/cgi-bin/droman/meso_table_mesowest_m.cgi?stn=C7324&unit=0&time=&hours=24&hour1=23&day1=26&month1=07&year1=2018&radius=25&past=0&order=1
I don't have the 75 in right now...that's the uncovered 31 with the 40CFM fan. LCI is an AWOS and I'm not sure what they're using for dewpoint sensing right now. Maybe someone else here knows if it's the Vaisala PTU307 or something else. CON is about 20 miles south of me and that's an ASOS.

I found this dewpoint accuracy chart for the Vaisala PTU307. I'm not sure if that is what is used in ASOS (as part of the DTS1) or if it is just a sensor with similar technology. It's interesting for you SW desert guys trying to compare dewpoints in situtations with high temperature and large dewpoint depressions. 40C (104F) with a dewpoint of -10C (14F) and an RH of 8% gives you an error of about 2.2C. Drop the dewpoint to -20C, for a 60C depression, and the error goes off the chart.

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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 27, 2018, 03:48:45 PM
Would I be correct to say that the SHT-75 doesn’t have the wet bias like the SHT-31?  Because I don’t see it with the comparison with these two stations...

https://mesowest.utah.edu/cgi-bin/droman/meso_table_mesowest_m.cgi?stn=KLCI&unit=0&time=&hours=24&hour1=23&day1=26&month1=07&year1=2018&radius=25&past=0&order=1


SHT-75. https://mesowest.utah.edu/cgi-bin/droman/meso_table_mesowest_m.cgi?stn=C7324&unit=0&time=&hours=24&hour1=23&day1=26&month1=07&year1=2018&radius=25&past=0&order=1

Ambient temperature is so different 6°, I hesitate to compare. Even DP will fluctuate some with temperature I've noticed and C7324 is 2° higher with ambient 6° lower so I think the wet bias is still there.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on July 27, 2018, 04:29:07 PM
Would I be correct to say that the SHT-75 doesn’t have the wet bias like the SHT-31?  Because I don’t see it with the comparison with these two stations...

https://mesowest.utah.edu/cgi-bin/droman/meso_table_mesowest_m.cgi?stn=KLCI&unit=0&time=&hours=24&hour1=23&day1=26&month1=07&year1=2018&radius=25&past=0&order=1


SHT-75. https://mesowest.utah.edu/cgi-bin/droman/meso_table_mesowest_m.cgi?stn=C7324&unit=0&time=&hours=24&hour1=23&day1=26&month1=07&year1=2018&radius=25&past=0&order=1

Ambient temperature is so different 6°, I hesitate to compare. Even DP will fluctuate some with temperature I've noticed and C7324 is 2° higher with ambient 6° lower so I think the wet bias is still there.
I wouldn’t compare my station to LCI. They’re on the water and I’m on a hill. There’s a lot of terrain effects out here so I gave up comparing to other stations.

Also, the wet bias is real here. My northern slope has trouble evaporating off surface moisture. So when the winds are light the dews pool here between the tree canopies.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on July 27, 2018, 07:03:27 PM
I have the 75 almost ready to go. Here it is next to a Davis sensor for size comparison.

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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 27, 2018, 07:56:56 PM
Ran a little experiment over night and today, i put a sensor in a damp wash cloth an let it sit over night showing 98%. This am i removed it from the cloth and put it in a fars to see how it would respond, well it was running around 2% high to start and as the day went on the humidity on main station was running around 39 to 45% but the test sensor was running between 5 to 7% higher. So i removed the sensor and baked it for several hours and the error corrected itself and after awhile was checking with the other station. So what they say about prolonged exposure to high humidity sure showed up and with a fan running at night in high humidity you can image the error that shows up until it dries out. One thing it does correct itself but it takes awhile and your reported readings are going to be really screwed up for awhile. I had noticed for a long time that the humidity reading was slow to come down in the am after a humid night, sometimes taking two to three hours to get dried out.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 27, 2018, 08:00:57 PM
I have the 75 almost ready to go. Here it is next to a Davis sensor for size comparison.

 [ You are not allowed to view attachments ]

Oh wow! You didn’t have to solder right?  How are you going to protect the exposed wires?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 27, 2018, 08:01:35 PM
Ran a little experiment over night and today, i put a sensor in a damp wash cloth an let it sit over night showing 98%. This am i removed it from the cloth and put it in a fars to see how it would respond, well it was running around 2% high to start and as the day went on the humidity on main station was running around 39 to 45% but the test sensor was running between 5 to 7% higher. So i removed the sensor and baked it for several hours and the error corrected itself and after awhile was checking with the other station. So what they say about prolonged exposure to high humidity sure showed up and with a fan running at night in high humidity you can image the error that shows up until it dries out. One thing it does correct itself but it takes awhile and your reported readings are going to be really screwed up for awhile. I had noticed for a long time that the humidity reading was slow to come down in the am after a humid night, sometimes taking two to three hours to get dried out.


This is with the 15, correct?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 27, 2018, 08:17:44 PM
Ran a little experiment over night and today, i put a sensor in a damp wash cloth an let it sit over night showing 98%. This am i removed it from the cloth and put it in a fars to see how it would respond, well it was running around 2% high to start and as the day went on the humidity on main station was running around 39 to 45% but the test sensor was running between 5 to 7% higher. So i removed the sensor and baked it for several hours and the error corrected itself and after awhile was checking with the other station. So what they say about prolonged exposure to high humidity sure showed up and with a fan running at night in high humidity you can image the error that shows up until it dries out. One thing it does correct itself but it takes awhile and your reported readings are going to be really screwed up for awhile. I had noticed for a long time that the humidity reading was slow to come down in the am after a humid night, sometimes taking two to three hours to get dried out.

Interesting Jerry.
I wonder if aspiration makes it worse? You just used a washcloth so maybe not.

Even in the written documentation it says sensor will need to be reset sometimes if prolonged to wet +85% conditions.
It's just not the right sensor for wet applications is our number 1 issue IMO.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 27, 2018, 08:18:09 PM
So what they say about prolonged exposure to high humidity sure showed up and with a fan running at night in high humidity you can image the error that shows up until it dries out.
I'm still trying to wrap my head around how a fan could possibly keep a sensor "wetter" than a passive shield. Evaporation or saturation is merely expedited by the fan, how could that not be desirable??
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 27, 2018, 08:34:05 PM
I used an 11 that i have and i don't know why things work the way they do i just know in a side by side comparison the fars takes longer to dry out than the standard shield. I think the fars with the big chamber it has has a lot more area to retain condensed moisture than the standard shield with the sensor located at the bottom so is more readily exposed to the drier air with all the plates being open to the air. I took a fars apart early in the am while still high humidity and the inside of the sensor chamber was really damp with beads of moisture and it takes awhile for the fan to pull enough dry air through the chambers and tubes to dry everything out so no false readings by the sensor. Just my thoughts on the matter, i am no expert on this stuff but i believe what i see with my own eyes.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 27, 2018, 08:51:01 PM
I think the fars with the big chamber it has has a lot more area to retain condensed moisture than the standard shield with the sensor located at the bottom so is more readily exposed to the drier air with all the plates being open to the air.
I don't see it. All a fan does is take ambient air and draw it across the sensor, evaporating or saturating certainly more efficient than any passive shield.
Not saying your observations are wrong, but my physics don't see your physics. :???:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on July 27, 2018, 09:26:39 PM
I have the 75 almost ready to go. Here it is next to a Davis sensor for size comparison.

 [ You are not allowed to view attachments ]

Oh wow! You didn’t have to solder right?  How are you going to protect the exposed wires?
Which exposed wires? I have some silicone conformal coating I may put on the pin end of the 75 PCB and up to where it meets the connector. Maybe I'll put a little on the wire end of the connector too.

And no, no soldering. It's an IDC connector so slide the wires in all the way and close it up tight. I think the Davis wires are 26 AWG and the connector accepts 26-28 AWG.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 27, 2018, 10:10:18 PM
I think the fars with the big chamber it has has a lot more area to retain condensed moisture than the standard shield with the sensor located at the bottom so is more readily exposed to the drier air with all the plates being open to the air.
I don't see it. All a fan does is take ambient air and draw it across the sensor, evaporating or saturating certainly more efficient than any passive shield.
Not saying your observations are wrong, but my physics don't see your physics. :???:

Interestingly, only half of the function of the fan is pulling air across the sensor as this 24hr FARS cutaway shows. The most interesting design feature is the 2 layer thermal curtain that surrounds the sensor thoroughly insulating the inner chamber.
 [ You are not allowed to view attachments ]  

When I built mine from parts. I put the sensor low in the chamber, like this.
 [ You are not allowed to view attachments ]

 I don't know if this is how Davis does it when purchased as a complete setup, but its seems to me that being in that area actually keeps the full force of the air from blowing directly on the sensor, while at the same time still 'saturated' in the air pulled in from below.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 27, 2018, 10:18:12 PM
I used an 11 that i have and i don't know why things work the way they do i just know in a side by side comparison the fars takes longer to dry out than the standard shield. I think the fars with the big chamber it has has a lot more area to retain condensed moisture than the standard shield with the sensor located at the bottom so is more readily exposed to the drier air with all the plates being open to the air. I took a fars apart early in the am while still high humidity and the inside of the sensor chamber was really damp with beads of moisture and it takes awhile for the fan to pull enough dry air through the chambers and tubes to dry everything out so no false readings by the sensor. Just my thoughts on the matter, i am no expert on this stuff but i believe what i see with my own eyes.

I get your point about the differing masses of the shields. However, on nights/mornings when the air doesn't reach full saturation (quite common here), an aspirated shield responds quicker to drier air warmed by the morning sun. This is especially true when winds at 2m are below 5mph (most mornings here). For people like me living in cities and small suburban lots that don't get a lot of air movement and/or experience heat island effects (which artificially keeps temp above dew point sometimes), passive shields aren't normally going to read drier than a shield with a higher rate of evaporation.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 27, 2018, 10:28:53 PM
I think the fars with the big chamber it has has a lot more area to retain condensed moisture than the standard shield with the sensor located at the bottom so is more readily exposed to the drier air with all the plates being open to the air.
I don't see it. All a fan does is take ambient air and draw it across the sensor, evaporating or saturating certainly more efficient than any passive shield.
Not saying your observations are wrong, but my physics don't see your physics. :???:
but its seems to me that being in that area actually keeps the full force of the air from blowing directly on the sensor, while at the same time still 'saturated' in the air pulled in from below.
I would certainly think the small area in the chamber would negate sensor placement. The filter also keeps the "full force" of the air off the sensor.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 27, 2018, 10:36:34 PM
I have the 75 almost ready to go. Here it is next to a Davis sensor for size comparison.

 [ You are not allowed to view attachments ]

Oh wow! You didn’t have to solder right?  How are you going to protect the exposed wires?
Which exposed wires? I have some silicone conformal coating I may put on the pin end of the 75 PCB and up to where it meets the connector. Maybe I'll put a little on the wire end of the connector too.


And no, no soldering. It's an IDC connector so slide the wires in all the way and close it up tight. I think the Davis wires are 26 AWG and the connector accepts 26-28 AWG.

Good deal. Will probably end up doing that.




Between where you spliced the wires and the connector.

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 27, 2018, 10:47:39 PM
I think the fars with the big chamber it has has a lot more area to retain condensed moisture than the standard shield with the sensor located at the bottom so is more readily exposed to the drier air with all the plates being open to the air.
I don't see it. All a fan does is take ambient air and draw it across the sensor, evaporating or saturating certainly more efficient than any passive shield.
Not saying your observations are wrong, but my physics don't see your physics. :???:
but its seems to me that being in that area actually keeps the full force of the air from blowing directly on the sensor, while at the same time still 'saturated' in the air pulled in from below.
I would certainly think the small area in the chamber would negate sensor placement. The filter also keeps the "full force" of the air off the sensor.

I disagree. The baffle at the bottom of the chamber produces a small area semi-stagnate air. The sensor can be placed near this area. Above that small area is a turbulence zone ...and near the top by the fan, faster and straighter moving air. Plenty of room for all that to take place.
Also, there would be no sense in adding the SF2 Sensirion filter cap if the air is not directly hitting the sensor. According to you, the cap will block this air.
By the way, my choice of the term 'full force of air' was hyperbole. We all know there is no 'force' when air is being drawn through the chamber with the typical fan.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 27, 2018, 11:05:16 PM
I think the fars with the big chamber it has has a lot more area to retain condensed moisture than the standard shield with the sensor located at the bottom so is more readily exposed to the drier air with all the plates being open to the air.
I don't see it. All a fan does is take ambient air and draw it across the sensor, evaporating or saturating certainly more efficient than any passive shield.
Not saying your observations are wrong, but my physics don't see your physics. :???:
but its seems to me that being in that area actually keeps the full force of the air from blowing directly on the sensor, while at the same time still 'saturated' in the air pulled in from below.
I would certainly think the small area in the chamber would negate sensor placement. The filter also keeps the "full force" of the air off the sensor.

I disagree. The baffle at the bottom of the chamber produces a small area semi-stagnate air. The sensor can be placed near this area. Above that small area is a turbulence zone ...and near the top by the fan, faster and straighter moving air. Plenty of room for all that to take place.
Also, there would be no sense in adding the SF2 Sensirion filter cap if the air is not directly hitting the sensor. According to you, the cap will block this air.
By the way, my choice of the term 'full force of air' was hyperbole. We all know there is no 'force' when air is being drawn through the chamber with the typical fan.
You put the sensor where you believe is the most beneficial. That being said, I think you're waaay over thinking this. Whether the sensor receives more or less of a laminar flow is irrelevant.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on July 27, 2018, 11:18:38 PM
I have the 75 almost ready to go. Here it is next to a Davis sensor for size comparison.

 [ You are not allowed to view attachments ]

Oh wow! You didn’t have to solder right?  How are you going to protect the exposed wires?
Which exposed wires? I have some silicone conformal coating I may put on the pin end of the 75 PCB and up to where it meets the connector. Maybe I'll put a little on the wire end of the connector too.


And no, no soldering. It's an IDC connector so slide the wires in all the way and close it up tight. I think the Davis wires are 26 AWG and the connector accepts 26-28 AWG.

Good deal. Will probably end up doing that.




Between where you spliced the wires and the connector.
Yeah. I may add the silicone conformal coating around those holes where the wires enter to make sure no moisture finds its way in. With this connector I didn't even have to strip the wires...just a clean straight cut across. When you close the connector it slices into the wire to make the insulated connection, but better to be safe than sorry when it comes to moisture.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 27, 2018, 11:21:14 PM
Irrelevant in regards to the discussion of wet bias sensors? Agreed.   :-P
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: kcidwx on July 28, 2018, 01:27:51 AM
After a couple of PM's on this, let me address the issue of "best practice". So where does the best practice come from about not aspirating humidity sensors. It comes from us field techs. It's frustrating when I tell people in a PM what we are seeing in the field and they come back with, "but the sensor manufacturer says this" or "the manufacturers document says that". I like to use the car analogy here. Who would you trust more in getting information about minor problems being experienced with a particular type of GM car? GM or the local independent mechanic that works on their cars everyday? Us meteorological field techs are the independent mechanics.

Just like the wet bias I discovered over a year ago with the SHT-31. Did Davis know about it? Nope! A year later someone on this forum finally told them about it. My test proved that the field techs are aware of many sensor issues that the manufacturers aren't. Not everything gets back to the manufacturers and even if it does they may not think it's a big enough problem to do anything about it. From our experiences, we develop our own best practices when it comes to sensors.

So now back to the humidity sensor best practice and where it comes from. We recycle humidity sensors on average every 18 months. We lab test the sensors when they are new right before they are put in service. They are also spot checked right after being put in service and tested in more detail every 90 days thereafter. That test data gets logged by sensor serial number. That sensor is spot checked again at the 18 month recycle time before it is pulled out of service. We lab test it again right after it is pulled out of service. What we are seeing is that we have a much higher number of sensors that we pull out of FARS installations that are right on the edge of the spec or are already slightly out of spec versus those we pull out of passive shields. Unfortunately none of us field techs has taken the time to try and fully understand why this is happening. Those nearly out of spec or out of spec sensors all show a wet bias. They often fail other sensor metrics we test as well. Getting to travel all over the country to work on different installations, it's a much bigger problem in humid regions.

Again this is not happening to ALL humidity sensors in a FARS. We pull many humidity sensors that show no ill effects after 18 months of being in a FARS. However, based on what we are seeing across the board, we recommend client configurations of temperature sensor in FARS and humidity sensor in an open passive shield (non-gill plate) to maintain optimum performance and calibration. FARS is great for humidity response time and a few other positives but what good is that if we check the sensor during the 18 month in service period and it's out of spec or fails one of the other metrics we test.

Just an FYI, I don't do any work with Davis sensors. My SHT-31 tests I did were done on the side and for my own curiosity.

So then I get asked, "What should I do since I have a combined temperature and humidity sensor?" I say put it in a FARS for temperature accuracy and hope your humidity sensor holds up. If not, replace the sensor. If you don't have the means to properly test the humidity side of the sensor and you are concerned about its accuracy long term, just get on a schedule to replace it every 12 to 18 months.

Again, for the record, I'm not saying the FARS is causing the problem with the Davis sensors in any way shape or form. It's just one possibility that needs to be investigated.

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 28, 2018, 01:51:15 AM
^^^ Thank you for your expertise on this subject. This is without a doubt the most informative thread I've ever been part of on this forum.

For me, things are pretty much status quo, for others, obviously not so much.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 28, 2018, 05:53:22 AM
  However, based on what we are seeing across the board, we recommend client configurations of temperature sensor in FARS and humidity sensor in an open passive shield (non-gill plate) to maintain optimum performance and calibration.

What is "an open passive shield (non-gill plate)"? What would that look like? Just a Bell or Cone shaped shelter?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 28, 2018, 06:23:15 AM
  However, based on what we are seeing across the board, we recommend client configurations of temperature sensor in FARS and humidity sensor in an open passive shield (non-gill plate) to maintain optimum performance and calibration.

What is "an open passive shield (non-gill plate)"? What would that look like? Just a Bell or Cone shaped shelter?

Image near top of thread Kcidwx posted of the ASOS sensor was open shield. Basically has a umbrella and fully exposed underneath.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 28, 2018, 07:35:28 AM
  However, based on what we are seeing across the board, we recommend client configurations of temperature sensor in FARS and humidity sensor in an open passive shield (non-gill plate) to maintain optimum performance and calibration.

What is "an open passive shield (non-gill plate)"? What would that look like? Just a Bell or Cone shaped shelter?

Image near top of thread Kcidwx posted of the ASOS sensor was open shield. Basically has a umbrella and fully exposed underneath.

Gotcha. Thanks
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 28, 2018, 08:49:40 AM
So then I get asked, "What should I do since I have a combined temperature and humidity sensor?" I say put it in a FARS for temperature accuracy and hope your humidity sensor holds up. If not, replace the sensor. If you don't have the means to properly test the humidity side of the sensor and you are concerned about its accuracy long term, just get on a schedule to replace it every 12 to 18 months.

OK fair enough, Ron.

But when the official parts suppliers only carry sensors with wet biases "preinstalled", what then? Davis had BETTER be working on a solution.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on July 28, 2018, 09:12:35 AM
My last Acurite check early this morning max humidity at 97%, Davis 96% aspirated so both looking normal.

At 12:40 CT I did a passive shade check against new Davis sensor with 5mph breeze running 7-9% lower humidity so really nothing changed from yesterday.

Not keeping acurite saturated in high humidity outside 24/7 so can't really determine how much drift if any is taking place toward wet side.  Max exposure time has been around 2 hours in early morning allowing it to record highest humidity.

Im guessing you’re like me. DPs are 3-4 degrees off being wet in bias

+4° dp when I did the afternoon check.  Thread has gone about as far as it can go unless anyone has something we didn't cover. Let Davis do their thing and see what they come up with.

This quote is from post 475 and we thought we were done until Davis replied! Has Davis done its thing yet? I certainly haven’t heard anything and I doubt anyone else has including the resellers of the world, like Johnd and Ryan Wilhour.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 28, 2018, 09:43:10 AM
My last Acurite check early this morning max humidity at 97%, Davis 96% aspirated so both looking normal.

At 12:40 CT I did a passive shade check against new Davis sensor with 5mph breeze running 7-9% lower humidity so really nothing changed from yesterday.

Not keeping acurite saturated in high humidity outside 24/7 so can't really determine how much drift if any is taking place toward wet side.  Max exposure time has been around 2 hours in early morning allowing it to record highest humidity.

Im guessing you’re like me. DPs are 3-4 degrees off being wet in bias

+4° dp when I did the afternoon check.  Thread has gone about as far as it can go unless anyone has something we didn't cover. Let Davis do their thing and see what they come up with.

This quote is from post 475 and we thought we were done until Davis replied! Has Davis done its thing yet? I certainly haven’t heard anything and I doubt anyone else has including the resellers of the world, like Johnd and Ryan Wilhour.

Haven't heard anything from Davis but the thread lives on.  :grin:     
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: johnd on July 28, 2018, 10:23:03 AM
This quote is from post 475 and we thought we were done until Davis replied! Has Davis done its thing yet? I certainly haven’t heard anything and I doubt anyone else has including the resellers of the world, like Johnd and Ryan Wilhour.

No, for the record, not heard anything here. But then frankly I'd be surprised to have heard anything. If anyone is interested in my personal opinion:

This reported issue doesn't sound like something that can be validated, fixed and tested quickly, even assuming that any fix is possible. Add into the mix that Davis are, I suspect, very busy with other priorities right now and this issue might have to take its place in the queue (at least beyond that some bland statement that Davis are looking into the report and maybe some updated maintenance advice).

Please don't misunderstand: I'm all for the VP2 stations being as good and consistent in specification as they can be, as I'm sure that Davis are too. But there has to be some realism about what is possible at the price point. It would be good to think that a fix can be quickly identified and implemented (though how much time and effort would be involved in trying to fix a problem of excessive long-term drift - that sounds like a 2-3 year study to me?) but there's also the real possibility that the VP2 is what it is.

The reality could - but only could, I emphasise - be that users needing real accuracy and long-term stability in RH readings should consider buying eg a Vaisala station (and a suitable maintenance contract).
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on July 28, 2018, 11:32:35 AM
Great stuff guys.

I spent the morning refurbing my VP2...new rad plates, uv/solar covers, aerocone, etc. it almost looks like I just bought it.

Anyway, I put my old SHT75 back in the FARS for now. I put silicone conformal coating on the new one so that’ll take a little time to cure at regular ambient air temps. Maybe tomorrow morning I’ll swap it out with the new one.

C7324 is my station for those who want to follow along. I wouldn’t overanalyze the readings too much until I put the new sensor in. Feel free to critique away then.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: kcidwx on July 28, 2018, 11:35:51 AM
So then I get asked, "What should I do since I have a combined temperature and humidity sensor?" I say put it in a FARS for temperature accuracy and hope your humidity sensor holds up. If not, replace the sensor. If you don't have the means to properly test the humidity side of the sensor and you are concerned about its accuracy long term, just get on a schedule to replace it every 12 to 18 months.

OK fair enough, Ron.

But when the official parts suppliers only carry sensors with wet biases "preinstalled", what then? Davis had BETTER be working on a solution.

If that's the case then you are at the mercy of the vendor. However, I would not expect a quick response from them if you even get a response at all. They will need to do a lot of troubleshooting and testing to determine the cause. That can take months, even years depending on the nature and complexity of the problem. Even then the most probable cost effective solution (if this is a widespread problem) would be for Davis to publish their own set of specs for their PCB mounted SHT-31. Not uncommon for vendors to do that. The new Davis specs might look something like this.

Temperature (-40°F to +194°F) (0 - 90 %RH)      2.0 %RH
                                           (90 - 100 %RH)   3.5 %RH
Temperature (+80°F to +120°F) (40 - 70 %RH)    4.0 %RH

Those specs are just an example for a sensor that has a wet bias in the mid humidity range during warmer temperatures.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 28, 2018, 12:21:37 PM
After a couple of PM's on this, let me address the issue of "best practice". So where does the best practice come from about not aspirating humidity sensors. It comes from us field techs. It's frustrating when I tell people in a PM what we are seeing in the field and they come back with, "but the sensor manufacturer says this" or "the manufacturers document says that". I like to use the car analogy here. Who would you trust more in getting information about minor problems being experienced with a particular type of GM car? GM or the local independent mechanic that works on their cars everyday? Us meteorological field techs are the independent mechanics.

Just like the wet bias I discovered over a year ago with the SHT-31. Did Davis know about it? Nope! A year later someone on this forum finally told them about it. My test proved that the field techs are aware of many sensor issues that the manufacturers aren't. Not everything gets back to the manufacturers and even if it does they may not think it's a big enough problem to do anything about it. From our experiences, we develop our own best practices when it comes to sensors.

So now back to the humidity sensor best practice and where it comes from. We recycle humidity sensors on average every 18 months. We lab test the sensors when they are new right before they are put in service. They are also spot checked right after being put in service and tested in more detail every 90 days thereafter. That test data gets logged by sensor serial number. That sensor is spot checked again at the 18 month recycle time before it is pulled out of service. We lab test it again right after it is pulled out of service. What we are seeing is that we have a much higher number of sensors that we pull out of FARS installations that are right on the edge of the spec or are already slightly out of spec versus those we pull out of passive shields. Unfortunately none of us field techs has taken the time to try and fully understand why this is happening. Those nearly out of spec or out of spec sensors all show a wet bias. They often fail other sensor metrics we test as well. Getting to travel all over the country to work on different installations, it's a much bigger problem in humid regions.

Again this is not happening to ALL humidity sensors in a FARS. We pull many humidity sensors that show no ill effects after 18 months of being in a FARS. However, based on what we are seeing across the board, we recommend client configurations of temperature sensor in FARS and humidity sensor in an open passive shield (non-gill plate) to maintain optimum performance and calibration. FARS is great for humidity response time and a few other positives but what good is that if we check the sensor during the 18 month in service period and it's out of spec or fails one of the other metrics we test.

Just an FYI, I don't do any work with Davis sensors. My SHT-31 tests I did were done on the side and for my own curiosity.

So then I get asked, "What should I do since I have a combined temperature and humidity sensor?" I say put it in a FARS for temperature accuracy and hope your humidity sensor holds up. If not, replace the sensor. If you don't have the means to properly test the humidity side of the sensor and you are concerned about its accuracy long term, just get on a schedule to replace it every 12 to 18 months.

Again, for the record, I'm not saying the FARS is causing the problem with the Davis sensors in any way shape or form. It's just one possibility that needs to be investigated.

Thank you for your input in this discussion. It’s been very helpful and educational for me.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 28, 2018, 12:58:22 PM
This reported issue doesn't sound like something that can be validated, fixed and tested quickly, even assuming that any fix is possible. Add into the mix that Davis are, I suspect, very busy with other priorities right now and this issue might have to take its place in the queue (at least beyond that some bland statement that Davis are looking into the report and maybe some updated maintenance advice).

Well, as long as we're discussing Davis' priorities, I'd hope they'd appreciate how inaccurate humidity readings affect their largest client base in the U.S., namely agriculture. Why do farmers buy these stations if not mainly to measure evapotranspiration? You can't measure ET accurately when humidity is well off the mark (sometimes by 10% or more) during the peak of summer when irrigation needs are most acute.


The reality could - but only could, I emphasise - be that users needing real accuracy and long-term stability in RH readings should consider buying eg a Vaisala station (and a suitable maintenance contract).

So you propose, in the absence of a meaningful solution from Davis, that VP2 owners pay 5-10x more so we can have something approaching the specifications we were given when we bought the Davis station?

Here are some other requirements you failed to mention that come with owning Vaisala or equivalent stations:
This is not a realistic solution for the vast majority of members on this forum.

No, unfortunately, the "realistic" solution is only available to those comfortable with DIY electronics projects. We're knee deep in posts describing that process now and we're waiting for results. For the average person unwilling or unable to tear things apart and solve other people's problems themselves, they are very likely screwed unless Davis can miraculously come through. That's the plain truth. And Davis had better hope against hope that one of their big buyers doesn't figure out the lay of the land.


Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 28, 2018, 01:00:38 PM

If that's the case then you are at the mercy of the vendor. However, I would not expect a quick response from them if you even get a response at all. They will need to do a lot of troubleshooting and testing to determine the cause. That can take months, even years depending on the nature and complexity of the problem. Even then the most probable cost effective solution (if this is a widespread problem) would be for Davis to publish their own set of specs for their PCB mounted SHT-31. Not uncommon for vendors to do that. The new Davis specs might look something like this.

Temperature (-40°F to +194°F) (0 - 90 %RH)      2.0 %RH
                                           (90 - 100 %RH)   3.5 %RH
Temperature (+80°F to +120°F) (40 - 70 %RH)    4.0 %RH

Try 10%, not 4%. We've had reports of worse. I've been able to replicate 9% consistently, but my max testing temperature was only ~85F. As the temp goes up, it gets worse.

EDIT: I'd also add that even as low as 60F, I'm seeing 5-6% errors on both my 15 and 31 sensors.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Old Tele man on July 28, 2018, 01:08:37 PM
It's ALL about compromise: we compromise on ACCURACY vs. PRICE, they compromise on PRICE vs. ACCURACY.

We could buy more expensive systems but balk at their prices. They could build more accurate systems but balk at their cost-to-do-so.

It's the middle, common-ground, of these compromises that the consumers tend to settle, but either ENDS of this "bath tub" analogy are still there (good vs cheap) to chose from.

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 28, 2018, 01:43:23 PM
I think a great point was made about farmers relying on this data for accuracy when humidity and evaporation rates are very important.
I'm sure a large portion of their business is agricultural with the expansion in recent years of this type of station while the weather enthusiast is probably still the largest it's important for agriculture side to trust the data.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 28, 2018, 01:44:44 PM
Great stuff guys.

I spent the morning refurbing my VP2...new rad plates, uv/solar covers, aerocone, etc. it almost looks like I just bought it.

Anyway, I put my old SHT75 back in the FARS for now. I put silicone conformal coating on the new one so that’ll take a little time to cure at regular ambient air temps. Maybe tomorrow morning I’ll swap it out with the new one.

C7324 is my station for those who want to follow along. I wouldn’t overanalyze the readings too much until I put the new sensor in. Feel free to critique away then.

Considering the geographical features you have and that being an old sensor, the 75 seems like it’s comparing fairly well to KLCI & KCON
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: hwcorder on July 28, 2018, 04:41:07 PM
So here are my stats from Gladstone. As you can see my 2 year old sht75 is running very close to local ASOS for dew point and I have increased amounts of vegetation around my property.


Site   Distance         Location   Barometer   Temperature   Dewpoint


KCLT   7.7 kilometres   East       CHARLOTTE/DOUGLAS INTERNATIONAL , NC, United States (lat 35.2225°, long -80.9543°, altitude 228 metres)   -0.7mb   -0.5°C   -0.2°C

KAKH   12.6 kilometres   Southwest       GASTONIA MUNICIPAL AIRPORT, NC, United States (lat 35.1969°, long -81.1555°, altitude 243 metres)   -0.5mb   0.5°C   0.1°C
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: galfert on July 28, 2018, 05:07:17 PM
So I thought I wanted a VP2 but now I'm not so sure. Maybe I'll wait. I'm just a hobbyists with a cheap Ambient Weather WS-2902A. I think Davis needs to fix this humidity problem and soon not months or years later. Sure things take time to fix but you all have done tremendous work in less than a month. I expect Davis to put this at the top of their priority list. Well maybe just second to releasing a decent successor, the VP3, or whatever they want to call it. The VP3 is way overdue. I implore Davis to design the VP3 with separate temperature and heated humidity sensors. That solution does not break the value / price point mark. And they need to revise their manufacturing/shipping methods to ensure better quality control. But they can't forget about their loyal VP2 customers and offer them a solution too.

I predict that if Davis doesn't fix this problem soon and also if they don't release a VP3 within a year that there will be another manufacturer that starts to take away their sales. Already the Acurite Atlas has gotten a lot of excitement and the Atlas Elite sounds like it might just be a VP2 killer.

I propose  we reach out to organizations that use VP2s and let them know what you all have discovered. For example reach out to WeatherSTEM and let them know the VP2 is not all it is cracked up to be. Maybe Davis needs to see pressure from not just this forum. We need to get Davis' big customers to be informed. Anyone know any other big Davis customers?

But not just customers need to know but also resellers. Good that Scaled Instruments knows. How about letting other vendors know? Has anyone let Ed at Ambient Weather know? Or how about Scientific Sales, do they know?

If you all want this problem fixed then let's light a fire and get Davis to respond. I'm talking about getting a real response. Not some canned "we are looking at it" BS statement. They need to quickly acknowledge the issue and promise a solution. Then they need to deliver a few months later but within 6 months.

I'm not all talk. I'll be glad to reach out to whomever. Let's make a list. And let's get our messaging straight. But maybe some of you already are a better inside contact for these organizations.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Old Tele man on July 28, 2018, 05:20:35 PM
So I thought I wanted a VP2 but now I'm not so sure. Maybe I'll wait. I'm just a hobbyists with a cheap Ambient Weather WS-2902A. I think Davis needs to fix this humidity problem and soon not months or years later. Sure things take time to fix but you all have done tremendous work in less than a month. I expect Davis to put this at the top of their priority list. Well maybe just second to releasing a decent successor, the VP3, or whatever they want to call it. The VP3 is way overdue. I implore Davis to design the VP3 with separate temperature and heated humidity sensors. That solution does not break the value / price point mark. And they need to revise their manufacturing/shipping methods to ensure better quality control. But they can't forget about their loyal VP2 customers and offer them a solution too.

I predict that if Davis doesn't fix this problem soon and also if they don't release a VP3 within a year that there will be another manufacturer that starts to take away their sales. Already the Acurite Atlas has gotten alot of excitement and the Atlas Elite sounds like it might just be a VP2 killer.

I propose  we reach out to organizations that use VP2s and let them know what you all have discovered. For example reach out to WeatherSTEM and let them know the VP2 is not all it is cracked up to be. Maybe Davis needs to see pressure from not just this forum. We need to get Davis' big customers to be informed. Anyone know any other big Davis customers?

But not just customers need to know but also resellers. Good that Scaled Instruments knows. How about letting other vendors know? Has anyone let Ed at Ambient Weather know? Or how about Scientific Sales, do they know?

If you all want this problem fixed then let's light a fire and get Davis to respond. I'm talking about getting a real response. Not some canned "we are looking at it" BS statement. They need to quickly acknowledge the issue and promise a solution. Then they need to deliver a few months later but within 6 months.

I'm not all talk. I'll be glad to reach out to whomever. Let's make a list. And let's get our messaging straight. But maybe some of you already are a better inside contact for these organizations.
Davis has been badgered by us (CWOP's) for years and seldom (but not never) responds, rather they listen and then eventually (maybe) responds. Unfortunately "silent" patience is NOT our virtue.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 28, 2018, 05:38:33 PM
So I thought I wanted a VP2 but now I'm not so sure. Maybe I'll wait. I'm just a hobbyists with a cheap Ambient Weather WS-2902A. I think Davis needs to fix this humidity problem and soon not months or years later. Sure things take time to fix but you all have done tremendous work in less than a month. I expect Davis to put this at the top of their priority list. Well maybe just second to releasing a decent successor, the VP3, or whatever they want to call it. The VP3 is way overdue. I implore Davis to design the VP3 with separate temperature and heated humidity sensors. That solution does not break the value / price point mark. And they need to revise their manufacturing/shipping methods to ensure better quality control. But they can't forget about their loyal VP2 customers and offer them a solution too.

I predict that if Davis doesn't fix this problem soon and also if they don't release a VP3 within a year that there will be another manufacturer that starts to take away their sales. Already the Acurite Atlas has gotten alot of excitement and the Atlas Elite sounds like it might just be a VP2 killer.

I propose  we reach out to organizations that use VP2s and let them know what you all have discovered. For example reach out to WeatherSTEM and let them know the VP2 is not all it is cracked up to be. Maybe Davis needs to see pressure from not just this forum. We need to get Davis' big customers to be informed. Anyone know any other big Davis customers?

But not just customers need to know but also resellers. Good that Scaled Instruments knows. How about letting other vendors know? Has anyone let Ed at Ambient Weather know? Or how about Scientific Sales, do they know?

If you all want this problem fixed then let's light a fire and get Davis to respond. I'm talking about getting a real response. Not some canned "we are looking at it" BS statement. They need to quickly acknowledge the issue and promise a solution. Then they need to deliver a few months later but within 6 months.

I'm not all talk. I'll be glad to reach out to whomever. Let's make a list. And let's get our messaging straight. But maybe some of you already are a better inside contact for these organizations.

This is exactly what Davis hates to hear.  If you think about it this thread will live on forever and it's not great for Davis advertising. You aren't the only person I'm sure with same thoughts about holding off purchase.
Best thing Davis can do is address and fix problem quickly or thread will be 100 pages long eventually and come up on every Google search for years.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on July 28, 2018, 05:44:53 PM
I certainly understand the frustration with the wet bias, but the calls for reasonability at the price point are also warranted. My primary frustration is it is quite observable that a Sensirion sensor performs better than a Davis engineered Sensirion sensor even if I have no idea why. And my experience with the Sensirion sensor is that it is quite accurate (and this is for an older generation SHT75). The VP2 is extremely well designed and reliable if maybe a bit utilitarian. My main and really only gripe is for Davis to look into and fix whatever it is about their manufacturing process that seems to offset an otherwise quite accurate sensor. If I can get a Sensirion sensor to perform better than Davis can, something is definitely wrong ...
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on July 28, 2018, 06:17:45 PM
Randy of ValentineWeather and I (probably others) have contacted Brett Lane (supervisor of Davis weather support), who at least acknowledged this issue and the discussion of it in this Forum. Short of having a march to Haywood, California not sure what else can be done. As Johnd, our reseller friend from the UK, has noted the demand for PWS is not huge so we don’t have a lot of pent-up demand for a better product so unless the Atlas device takes off, is reliable and well built, we don’t have a lot of choices. Frustrating yes, but....
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 28, 2018, 06:33:17 PM
What if Davis offered for those unhappy an option to purchase i2c protocol transmitter that worked with the VP2 so we could pick and choose the sensor we want.
Wasn't the reason they went with LLC Legacy was the older Davis stations like VP1's so they could communicate?

Just a thought  :idea: I'm sure it would make many happy. These 31's as accurate as the temperature is the humidity wasn't meant to be used in saturated atmospheres for extended periods. It's the wrong sensor for the application.


Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 28, 2018, 06:55:27 PM
So I thought I wanted a VP2 but now I'm not so sure. Maybe I'll wait. I'm just a hobbyists with a cheap Ambient Weather WS-2902A. I think Davis needs to fix this humidity problem and soon not months or years later. Sure things take time to fix but you all have done tremendous work in less than a month. I expect Davis to put this at the top of their priority list. Well maybe just second to releasing a decent successor, the VP3, or whatever they want to call it. The VP3 is way overdue. I implore Davis to design the VP3 with separate temperature and heated humidity sensors. That solution does not break the value / price point mark. And they need to revise their manufacturing/shipping methods to ensure better quality control. But they can't forget about their loyal VP2 customers and offer them a solution too.

I predict that if Davis doesn't fix this problem soon and also if they don't release a VP3 within a year that there will be another manufacturer that starts to take away their sales. Already the Acurite Atlas has gotten alot of excitement and the Atlas Elite sounds like it might just be a VP2 killer.

I propose  we reach out to organizations that use VP2s and let them know what you all have discovered. For example reach out to WeatherSTEM and let them know the VP2 is not all it is cracked up to be. Maybe Davis needs to see pressure from not just this forum. We need to get Davis' big customers to be informed. Anyone know any other big Davis customers?

But not just customers need to know but also resellers. Good that Scaled Instruments knows. How about letting other vendors know? Has anyone let Ed at Ambient Weather know? Or how about Scientific Sales, do they know?

If you all want this problem fixed then let's light a fire and get Davis to respond. I'm talking about getting a real response. Not some canned "we are looking at it" BS statement. They need to quickly acknowledge the issue and promise a solution. Then they need to deliver a few months later but within 6 months.

I'm not all talk. I'll be glad to reach out to whomever. Let's make a list. And let's get our messaging straight. But maybe some of you already are a better inside contact for these organizations.

I have already made it known to some from Auburn University. I’m about to talk to a guy from Earth Networks about it.

I personally think the quick solution is to use the Sensirion mounted 75. Be on the process of redoing the transmitter board; so that it can take the new 35 (that’s replacing the 75) next year. Also, use Sensirion filter cap and not the one they use now.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 28, 2018, 07:21:33 PM

I propose  we reach out to organizations that use VP2s and let them know what you all have discovered. For example reach out to WeatherSTEM and let them know the VP2 is not all it is cracked up to be. Maybe Davis needs to see pressure from not just this forum. We need to get Davis' big customers to be informed. Anyone know any other big Davis customers?

But not just customers need to know but also resellers. Good that Scaled Instruments knows. How about letting other vendors know? Has anyone let Ed at Ambient Weather know? Or how about Scientific Sales, do they know?

If you all want this problem fixed then let's light a fire and get Davis to respond. I'm talking about getting a real response. Not some canned "we are looking at it" BS statement. They need to quickly acknowledge the issue and promise a solution. Then they need to deliver a few months later but within 6 months.

I'm not all talk. I'll be glad to reach out to whomever. Let's make a list. And let's get our messaging straight. But maybe some of you already are a better inside contact for these organizations.

Davis don't want these guys getting upset at bad data.  :lol:
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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 28, 2018, 07:26:36 PM

I personally think the quick solution is to use the Sensirion mounted 75. Be on the process of redoing the transmitter board; so that it can take the new 35 (that’s replacing the 75) next year. Also, use Sensirion filter cap and not the one they use now.

Has it been determined the 75 doesn't have the wet bias?
I could live with it during summer and change to the 31 for winter.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 28, 2018, 07:45:01 PM

I personally think the quick solution is to use the Sensirion mounted 75. Be on the process of redoing the transmitter board; so that it can take the new 35 (that’s replacing the 75) next year. Also, use Sensirion filter cap and not the one they use now.

Has it been determined the 75 doesn't have the wet bias?
I could live with it during summer and change to the 31 for winter.

From what I’ve read on here that’s it’s better. Haven’t yet heard from the two yet that are going to install the 75 soon. Think one on here is doing it tomorrow so I’m waiting on him to do it. But even though the 31 spec sheet looks good in cold temps and the 75 not as good; some have said that the 75 does perform well with temps as low as 8° (if I remember correctly).
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on July 28, 2018, 08:06:08 PM
From the side by sides I’ve done, the unmodified 75 runs consistently drier than the Davis-mounted 31 in the midrange as low as humidity in the 20s. (And I stress it is unmodified). That said, it’s not perfect and is still a Sensirion. It struggles to hit 100%, but gets there on occasion. It hits 98 and 99 regularly. It will still exhibit a wet bias if in high humidity for a prolonged period and I’d say it may have a slight wet bias generally, but it compares really closely to my psycho-dyne. I’d love to find a way to get a 31 or 35 that Davis hasn’t touched to work with my VP2, but until then, my experience is having the 75 is markedly superior to the Davis 31. Temp specs are not as good obviously and not exactly a rigorous study, but I’ve tested side by side with an ASOS at -8F and it was right on. Not trying to convince anyone, but just my experience ...
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 28, 2018, 08:14:00 PM
From the side by sides I’ve done, the unmodified 75 runs consistently drier than the Davis-mounted 31 in the midrange as low as humidity in the 20s. (And I stress it is unmodified). That said, it’s not perfect and is still a Sensirion. It struggles to hit 100%, but gets there on occasion. It hits 98 and 99 regularly. It will still exhibit a wet bias if in high humidity for a prolonged period and I’d say it may have a slight wet bias generally, but it compares really closely to my psycho-dyne. I’d love to find a way to get a 31 or 35 that Davis hasn’t touched to work with my VP2, but until then, my experience is having the 75 is markedly superior to the Davis 31. Temp specs are not as good obviously and not exactly a rigorous study, but I’ve tested side by side with an ASOS at -8F and it was right on. Not trying to convince anyone, but just my experience ...

How much of the difference is there between the 75 & psych-dyne?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on July 28, 2018, 08:24:29 PM
While I want better and more reliable humidity readings, accurate temperature is more important to me, so whatever “bandaid” or work around is developed, it can’t reduce temperature accuracy. Right now the SHT31 is excellent when monitoring temperature. If need be, I will just live with my SHT31 to get the temperature and accept that its humidity readings are just plain flaky. And maybe this perspective is why Davis hasn’t made humidity measurement correction a high priority.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Old Tele man on July 28, 2018, 08:54:25 PM
While I want better and more reliable humidity readings, accurate temperature is more important to me, so whatever “bandaid” or work around is developed, it can’t reduce temperature accuracy. Right now the SHT31 is excellent when monitoring temperature. If need be, I will just live with my SHT31 to get the temperature and accept that its humidity readings are just plain flaky. And maybe this perspective is why Davis hasn’t made humidity correction a high priority.

The temp sensing 'device' in the SHt31 is an electronic PN-junction on silicon(?) wafer and is typically VERY accurate, because PN-junction voltage has a very specific volt-to-temperature ratio.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on July 28, 2018, 09:51:58 PM
From the side by sides I’ve done, the unmodified 75 runs consistently drier than the Davis-mounted 31 in the midrange as low as humidity in the 20s. (And I stress it is unmodified). That said, it’s not perfect and is still a Sensirion. It struggles to hit 100%, but gets there on occasion. It hits 98 and 99 regularly. It will still exhibit a wet bias if in high humidity for a prolonged period and I’d say it may have a slight wet bias generally, but it compares really closely to my psycho-dyne. I’d love to find a way to get a 31 or 35 that Davis hasn’t touched to work with my VP2, but until then, my experience is having the 75 is markedly superior to the Davis 31. Temp specs are not as good obviously and not exactly a rigorous study, but I’ve tested side by side with an ASOS at -8F and it was right on. Not trying to convince anyone, but just my experience ...

How much of the difference is there between the 75 & psych-dyne?

Usually pretty spot on. Within 1F in dew point. Sometimes 2F. The nice thing about the 75 is you can put it right in the intake of the psychro dyne to verify temp and dew point. I know some others are using the 75 so they may be able to provide more input in terms of temp/hum accuracy of the 75 across the extreme ranges
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 28, 2018, 09:55:36 PM
While I want better and more reliable humidity readings, accurate temperature is more important to me, so whatever “bandaid” or work around is developed, it can’t reduce temperature accuracy. Right now the SHT31 is excellent when monitoring temperature. If need be, I will just live with my SHT31 to get the temperature and accept that its humidity readings are just plain flaky. And maybe this perspective is why Davis hasn’t made humidity correction a high priority.

The temp sensing 'device' in the SHt31 is an electronic PN-junction on silicon(?) wafer and is typically VERY accurate, because PN-junction electron-flow has a very specific rate-to-temperature ratio.



Makes sense, all the units I've tested are within .1F maybe .2F at most of each other, old units new units doesn't matter. So either they are all off or all on the same amount with temperature.
Humidity is another story.

CW7491 has my interest up on the psycho dyne. 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 28, 2018, 10:24:31 PM
From the side by sides I’ve done, the unmodified 75 runs consistently drier than the Davis-mounted 31 in the midrange as low as humidity in the 20s. (And I stress it is unmodified). That said, it’s not perfect and is still a Sensirion. It struggles to hit 100%, but gets there on occasion. It hits 98 and 99 regularly. It will still exhibit a wet bias if in high humidity for a prolonged period and I’d say it may have a slight wet bias generally, but it compares really closely to my psycho-dyne. I’d love to find a way to get a 31 or 35 that Davis hasn’t touched to work with my VP2, but until then, my experience is having the 75 is markedly superior to the Davis 31. Temp specs are not as good obviously and not exactly a rigorous study, but I’ve tested side by side with an ASOS at -8F and it was right on. Not trying to convince anyone, but just my experience ...

How much of the difference is there between the 75 & psych-dyne?

Usually pretty spot on. Within 1F in dew point. Sometimes 2F. The nice thing about the 75 is you can put it right in the intake of the psychro dyne to verify temp and dew point. I know some others are using the 75 so they may be able to provide more input in terms of temp/hum accuracy of the 75 across the extreme ranges

That’s good!  Have you ever done a comparison between the 75 & 31 in regards to air temp?

How did your 31 compared to the psychro-dyne?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: galfert on July 28, 2018, 11:02:28 PM
Hey everyone,
It is this guy's fault... Richard Anderson design engineer at Davis. Anyone know how to reach him? Well in all seriousness it probably isn't truly his fault alone. But we have a name at Davis. Maybe he has some answers.

https://www.davisinstruments.com/blog/meet-davis-design-engineer-richard-anderson/

From the link above it says,
Quote
     he has applied over 40,000 hours toward perfecting our weather products and components. We believe that it is one of the many reasons our weather stations have stood the test of time   

Well I think we all have something to say about that.

Also in the article it says under the picture where he is using a micrometer. (what is he measuring anyway?)
Quote
Precision matters, says Richard. We design our products to the tightest tolerances for consistent manufacturing and assembly, which results in dependable, quality products.   

Really now? Is that so?

Someone should copy that blog page. Davis will probably be taking it down soon. Sorry Richard. We love you. We just want some answers.

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 28, 2018, 11:24:10 PM
Hey everyone,
It is this guy's fault... Richard Anderson design engineer at Davis. Anyone know how to reach him? Well in all seriousness it probably isn't truly his fault alone. But we have a name at Davis. Maybe he has some answers.

https://www.davisinstruments.com/blog/meet-davis-design-engineer-richard-anderson/

From the link above it says,
Quote
     he has applied over 40,000 hours toward perfecting our weather products and components. We believe that it is one of the many reasons our weather stations have stood the test of time   

Well I think we all have something to say about that.

I know that his name is on the 1999 patent application for the Davis FARS design. See: https://patents.google.com/patent/US6247360B1/en (https://patents.google.com/patent/US6247360B1/en).

However, he is just a foot soldier. Contacting him isn't going to move the needle as he will have to get in line behind his superiors. 

But if I were him, I wouldn't want my name anywhere near this mess.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: galfert on July 28, 2018, 11:43:31 PM
However, he is just a foot soldier. Contacting him isn't going to move the needle as he will have to get in line behind his superiors. 

But if I were him, I wouldn't want my name anywhere near this mess.

I know. I agree. He is just a foot soldier as you said. He is probably as upset as we are. He is most undoubtedly a very smart person, and probably corporate beurocracy has held him back from giving us the system we all really want.

But it was kinda cool to put a face to the person whom probably does have some intimate knowledge of what we are dealing with.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: galfert on July 28, 2018, 11:59:47 PM

I know that his name is on the 1999 patent application for the Davis FARS design. See: https://patents.google.com/patent/US6247360B1/en (https://patents.google.com/patent/US6247360B1/en).
.

Maybe I should file for my own FARS design patent. You know one where the temperature sensor is separate from the humidity sensor and only the temperature sensor receives air flow because they are in two separate compartments. And add a heater to the humidity sensor for periodic reconditioning. And add some insulation and distance between compartments so that the heater doesn't affect the temperature sensor. What do you think of that Richard Anderson? Hey don't steal my idea.

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: galfert on July 29, 2018, 12:35:38 AM
Okay I just came up with neat upgrade idea.

What if someone like Belfryboy or someone that knows how to work with microcontrollers builds a custom sensor replacement.

Key design aspects:
- Uses two SHT31 connected to a microcontroller.

- Purpose of the microcontroller is to switch sensors every 24 hours. The microcontroller intercepts temperature and humidity from one sensor at a time. The microcontroller will then pass along this data down the Davis bus.

- Both sensors are placed in the radiation shield using a new mounting board that creates some separation and insulation from the other. Or maybe you add a second radiation shield and keep them separate that way.

- Both sensors get heaters. But only the inactive sensor bakes for 10 hours and then gets adapted back to ambient humidity for reconditioning completion before becoming active again.

- Sensors alternate between reconditioning and being active every 24 hours.

- Possible extras are to correct for wet bias slope at different temperatures with different degrees.

- Another possible extra is that the heating and humidity reconditioning does not occur unless recent humidity was high.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: SnowHiker on July 29, 2018, 12:46:49 AM
As Johnd, our reseller friend from the UK, has noted the demand for PWS is not huge so we don’t have a lot of pent-up demand for a better product so unless the Atlas device takes off, is reliable and well built, we don’t have a lot of choices. Frustrating yes, but....
Could be a double-edged sword for Davis.  If the market is small, Davis may have a hard time affording the resources necessary for quick research, testing, development, production, and rectifying the problems with the units already sold and produced.

On the other hand, if the market is that small, Davis can't afford to lose customers, and their reputation, by not correcting the problem.

But considering how doomsday predictions usually go, I'm not going to dump my Davis stock just yet.  (As if I had any.  :lol:)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on July 29, 2018, 01:08:26 AM
From the side by sides I’ve done, the unmodified 75 runs consistently drier than the Davis-mounted 31 in the midrange as low as humidity in the 20s. (And I stress it is unmodified). That said, it’s not perfect and is still a Sensirion. It struggles to hit 100%, but gets there on occasion. It hits 98 and 99 regularly. It will still exhibit a wet bias if in high humidity for a prolonged period and I’d say it may have a slight wet bias generally, but it compares really closely to my psycho-dyne. I’d love to find a way to get a 31 or 35 that Davis hasn’t touched to work with my VP2, but until then, my experience is having the 75 is markedly superior to the Davis 31. Temp specs are not as good obviously and not exactly a rigorous study, but I’ve tested side by side with an ASOS at -8F and it was right on. Not trying to convince anyone, but just my experience ...

How much of the difference is there between the 75 & psych-dyne?

Usually pretty spot on. Within 1F in dew point. Sometimes 2F. The nice thing about the 75 is you can put it right in the intake of the psychro dyne to verify temp and dew point. I know some others are using the 75 so they may be able to provide more input in terms of temp/hum accuracy of the 75 across the extreme ranges

That’s good!  Have you ever done a comparison between the 75 & 31 in regards to air temp?

How did your 31 compared to the psychro-dyne?

The psychro dyne is dead on next to ASOS for temp and humidity. I’ve never found a difference in temp between the 75 and 31 when I’ve run them side by side. I could maybe do some more tests to get you some numeric answers. The Davis 31 is dead on for temp with the psychrodyne, but high on dewpoint usually by up to 4F, which is consistent with the comparison to ASOS. I have had the 75 next to an ASOS from -8F up to about 90F and it’s always been exactly on. I know the specs aren’t as good, but practically it seems to be a really good performer...

Thanks to some of these guys for going to Davis so they at least know the issue with their sensor. . I purchased the Acurite with the 31 after reading about it in this thread. It is really very good. I wish Davis’ 31 performed this way on the humidity side

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 29, 2018, 05:43:37 AM
Psychro dyne is made by Belfort one of the most trusted names in meteorological instruments.
 
The very best is Assmann psychrometer which is used as reference instrument tool. A little out of my price range however.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on July 29, 2018, 07:43:59 AM
The very best is Assmann psychrometer which is used as reference instrument tool. A little out of my price range however.
I'm more of a leg guy anyway.

Had a decent storm go through yesterday while I wasn't home. You can pick up some pretty cool microscale effects in temp/dew with the FARS, 75, and 1-min intervals. For those unfamiliar with the Davis text files, it's Temp-MaxTemp-MinTemp-RH-Dew

 [ You are not allowed to view attachments ]

RH on the old 75 only maxed out at 96% (31 had only been hitting 94%) last night. I put another coat of conformal coating on the new one this morning so it may not be until later tomorrow that I'll get to install it.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 29, 2018, 07:48:51 AM
Psychro dyne is made by Belfort one of the most trusted names in meteorological instruments.
 
The very best is Assmann psychrometer which is used as reference instrument tool. A little out of my price range however.

If I’m not mistaken, I’m the NWS ASOS training manual, they say to use the psychro-dyne to verify the ASOS’ DP readings.

“There is a procedure used by the ASOS Technician to check the validity of the ASOS ambient/dew point temperatures by comparing the values with the Psychron Model 566-2 psychrometer.  A measurement is considered valid if the temperature difference is within ±5 F.  dew point validity is determined by values within tolerances based on the temperature/dew point depression and if the temperature falls above or below 32 F.”

https://training.weather.gov/nwstc/DATAACQ/d.ASOShuman/ASOSTemp.htm
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 29, 2018, 08:15:34 AM
Psychro dyne is made by Belfort one of the most trusted names in meteorological instruments.
 
The very best is Assmann psychrometer which is used as reference instrument tool. A little out of my price range however.

If I’m not mistaken, I’m the NWS ASOS training manual, they say to use the psychro-dyne to verify the ASOS’ DP readings.

https://training.weather.gov/nwstc/DATAACQ/d.ASOShuman/ASOSTemp.htm

Could be...WMO according to one of the humidity studies uses the Assmann as standard...They come with a NIST certificate of 3 point calibration and corrections for thermometers. Accuracy 1% humidity .1C temperature.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on July 29, 2018, 08:27:16 AM
Not sure if this has been sent before, but here is the ASOS tech manual I found online. Chapter 5 discusses verification procedures for temperature and dewpoint (5.5.2.4)...

https://faaco.faa.gov/index.cfm/attachment/dload/?filename=/Appendix_C_-_ASOS_Tech_Manual.pdf
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 29, 2018, 09:02:11 AM
Had a decent storm go through yesterday while I wasn't home. You can pick up some pretty cool microscale effects in temp/dew with the FARS, 75, and 1-min intervals. For those unfamiliar with the Davis text files, it's Temp-MaxTemp-MinTemp-RH-Dew

Hmmm... a 12% swing down in humidity in 2 minutes (3:46-3:48) while it's raining? And the temp only changed 0.3F? That would drive me nuts.  I already had to write a routine to compute the 5 minute rolling average for humidity & dew point read by the 31 in the FARS so that I wasn't getting wild fluctuations in data sent to the website and CWOP (NWS prefers smoothed data).

To the extent it's possible, I don't want to be reporting microscale effects. People come to my site or check one of my data feeds to see what the weather is like in the city center, not to learn when a butterfly has flapped its wings.

As always, your mileage may vary, but maybe the 75 isn't for me.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on July 29, 2018, 09:33:55 AM
Had a decent storm go through yesterday while I wasn't home. You can pick up some pretty cool microscale effects in temp/dew with the FARS, 75, and 1-min intervals. For those unfamiliar with the Davis text files, it's Temp-MaxTemp-MinTemp-RH-Dew

Hmmm... a 12% swing down in humidity in 2 minutes (3:46-3:48) while it's raining? And the temp only changed 0.3F? That would drive me nuts.  I already had to write a routine to compute the 5 minute rolling average for humidity & dew point read by the 31 in the FARS so that I wasn't getting wild fluctuations in data sent to the website and CWOP (NWS prefers smoothed data).

To the extent it's possible, I don't want to be reporting microscale effects. People come to my site or check one of my data feeds to see what the weather is like in the city center, not to learn when a butterfly has flapped its wings.

As always, your mileage may vary, but maybe the 75 isn't for me.
You can see it was more stable before and after the storm passed. When that gust front first hits it becomes a pretty dynamic situation. I come from the meteorological research side of things so I find all of these brief fluctuations fascinating. I don't think someone coming to my site is worried about a few % RH during a storm when they see a rain rate of 7"/hr and 1.25" through the tipper in a half hour, but that's just me.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 29, 2018, 11:16:52 AM
Had a decent storm go through yesterday while I wasn't home. You can pick up some pretty cool microscale effects in temp/dew with the FARS, 75, and 1-min intervals. For those unfamiliar with the Davis text files, it's Temp-MaxTemp-MinTemp-RH-Dew

Hmmm... a 12% swing down in humidity in 2 minutes (3:46-3:48) while it's raining? And the temp only changed 0.3F? That would drive me nuts.  I already had to write a routine to compute the 5 minute rolling average for humidity & dew point read by the 31 in the FARS so that I wasn't getting wild fluctuations in data sent to the website and CWOP (NWS prefers smoothed data).

To the extent it's possible, I don't want to be reporting microscale effects. People come to my site or check one of my data feeds to see what the weather is like in the city center, not to learn when a butterfly has flapped its wings.

As always, your mileage may vary, but maybe the 75 isn't for me.
You can see it was more stable before and after the storm passed. When that gust front first hits it becomes a pretty dynamic situation. I come from the meteorological research side of things so I find all of these brief fluctuations fascinating. I don't think someone coming to my site is worried about a few % RH during a storm when they see a rain rate of 7"/hr and 1.25" through the tipper in a half hour, but that's just me.

Ah, gotcha. I didn't know how to interpret all the columns. Missed the gust front.  :oops: Yes, before and after the dp seems pretty stable.

Where I really get fluctuations is in the summer when there's a synoptic, upper level pattern bringing warm/moist air in from the S or SW but not enough boundary layer wind flow to prevent a competing lake breeze from Lake Superior. Tug of war ensues where you get cool/drier air from the Lake vs maritime tropical air from the Gulf. If you walk around near the ISS, you can feel the cool pockets and warm pockets as they drift about. It's wild! The local NWS seems to really hate those days because it's damn near impossible to nail the timing or even the development of the lake breeze. So either they undershoot or overshoot the high (usually undershoot because the breeze loves to show up late). Oh and I forgot to throw in downsloping wind effects as air descends from about 1200 to 600 feet in just a few miles before hitting the lake. Trying to interpret or predict temps, dew points and winds in the U.P. can be a sadomasochistic practice sometimes.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 29, 2018, 11:56:01 AM
I can't remember if we confirmed the SHT15 also had high bias. May have I remember thinking it was high and using cumulus to remove some %, so just to make sure I've started testing the reconditioned sht15. Been in saturated high humidities since 9pm yesterday 13 hours now 90% or greater.

Problem I'm getting into here in Nebraska is climate changes so high humidity and high heat bias may almost be gone going into August. Seems it does dry out some we've had over 17" rain last 3 months with 23 events over .10" so it doesn't all come at once like Houston where much of family lives they get 10" rain events and then long spells of nothing.
 [ You are not allowed to view attachments ]

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on July 29, 2018, 11:57:50 AM
“Davis Instruments is an established company with an outstanding product line and an excellent team.  Davis weather stations are sold around the world and have a leading position in the markets we serve, with foreign sales accounting for more than a third of our revenue.  Our ever-growing weather product line includes products that serve direct consumers, government, agriculture, construction, and education markets.  Our 100 employees design, engineer, manufacture, and distribute our products from our Hayward, California headquarters.”

Davis is looking for a sales manager per its website. A copy of the introduction to that position is quoted above. Folks that are looking for a position reading this Forum thread may wish to throw his or her hat in the ring to help Davis address its SHT31 issues. Maybe this new sales manager can motivate the design team to address this issue as well as pushing the company to launch a superior product to the VP2!

However, it also shows the reality of the problem. Davis has only 100 employees, so improvements to its product line are really limited.

Fellow posters--please don't make my posting a topic for discussion--I only posted it to show the reality of the situation.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 29, 2018, 12:22:23 PM
Found some info on RAWS stations with regard to temp/humidity.

First, and most helpful, is this page: https://raws.nifc.gov/standards-guidelines (https://raws.nifc.gov/standards-guidelines). If you click on the "Noncompliance Report" you'll get an active list of stations that are overdue for their yearly maintenance. Be sure to check your local RAWS on that if you use it for calibration purposes. For example, I found one that is about 35 miles from me hasn't had a visit in over 3 years!

Also, the current sensor is made by FTS and it's the THS-03 (https://ftsinc.com/meteorology/products/sensors/temperature-and-humidity-sensor/ (https://ftsinc.com/meteorology/products/sensors/temperature-and-humidity-sensor/)). Unfortunately, there's a lack of info on the specs for humidity. In a 2015 product brochure, they list the specs as +/-2% 0-60C (https://www.awra.org/memberservices/brochures/FTS_ProductCatalog_2015.pdf (https://www.awra.org/memberservices/brochures/FTS_ProductCatalog_2015.pdf). No further info available.

Also, according to this page - https://raws.nifc.gov/stationassets/relative-humidityair-temperature/relative-humidityair-temperature-wbracket (https://raws.nifc.gov/stationassets/relative-humidityair-temperature/relative-humidityair-temperature-wbracket) - there's mention of Vaisala, but those sensors seem to be no longer supported.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: galfert on July 29, 2018, 12:46:18 PM

This is exactly what Davis hates to hear.  If you think about it this thread will live on forever and it's not great for Davis advertising. You aren't the only person I'm sure with same thoughts about holding off purchase.
Best thing Davis can do is address and fix problem quickly or thread will be 100 pages long eventually and come up on every Google search for years.

I suppose you are right. I just noticed this thread is the second longest ever in number of replies on wxforum.net.

767 replies and counting.

I'm sure it will soon surpass the 1448 replies of the number one thread, "One Thing About New Mexico."
https://www.wxforum.net/index.php?topic=8240.0

This thread will forever be infamous.

Davis,
The ball is in your court. You have the opportunity to write the last chapter.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 29, 2018, 12:47:33 PM
I can't remember if we confirmed the SHT15 also had high bias.

If you're referring to the Davis labelled 15, yes. Based on my tests and others' reports in this thread, it seems to develop a wet bias with time, at least, although that could be said of most capacitive humidity sensors. I don't know if any 15s start with a wet bias. I didn't test mine when it was new.

I would note that my 15's bias isn't as extreme as my defective 31 and seems to fluctuate more. Occasionally, it runs close to specs while at other times it's off the mark (5+%). In fairness, since I've been testing it, the temp hasn't risen above 80 so I don't have a full picture, but at lower temps it seems more accurate, generally, when measuring humidity. It's also about twice as old as my bad 31 so, by rights, it should be worse, not better.

Is it possible that Davis changed the way they package or attach the sensor in the last few years making the 31 especially susceptible? Maybe the 31 is more sensitive to chemical contamination for some reason?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WxLover16 on July 29, 2018, 01:02:53 PM
The very best is Assmann psychrometer which is used as reference instrument tool. A little out of my price range however.
I'm more of a leg guy anyway.

Had a decent storm go through yesterday while I wasn't home. You can pick up some pretty cool microscale effects in temp/dew with the FARS, 75, and 1-min intervals. For those unfamiliar with the Davis text files, it's Temp-MaxTemp-MinTemp-RH-Dew

 [ You are not allowed to view attachments ]

RH on the old 75 only maxed out at 96% (31 had only been hitting 94%) last night. I put another coat of conformal coating on the new one this morning so it may not be until later tomorrow that I'll get to install it.

Those are some wild swings. I actually went in and took the filter off my 31. I love the faster response time, tho my only main concern is the fan blowing all that air straight to the humidity sensor part. I wonder how long before it starts to dry out or just go bonkers, anyone know? I was very reluctant to for that reason, but what's done is done. I will enjoy my fast response time  \:D/
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WxLover16 on July 29, 2018, 01:03:30 PM
The very best is Assmann psychrometer which is used as reference instrument tool. A little out of my price range however.
I'm more of a leg guy anyway.

Had a decent storm go through yesterday while I wasn't home. You can pick up some pretty cool microscale effects in temp/dew with the FARS, 75, and 1-min intervals. For those unfamiliar with the Davis text files, it's Temp-MaxTemp-MinTemp-RH-Dew

 [ You are not allowed to view attachments ]

RH on the old 75 only maxed out at 96% (31 had only been hitting 94%) last night. I put another coat of conformal coating on the new one this morning so it may not be until later tomorrow that I'll get to install it.

Those are some wild swings. I actually went in and took the filter off my 31 yesterday evening. I love the faster response time, tho my only main concern is the fan blowing all that air straight to the humidity sensor part. I wonder how long before it starts to dry out or just go bonkers, anyone know? I was very reluctant to for that reason, but what's done is done. I will enjoy my fast response time  \:D/
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on July 29, 2018, 02:26:58 PM
The very best is Assmann psychrometer which is used as reference instrument tool. A little out of my price range however.
I'm more of a leg guy anyway.

Had a decent storm go through yesterday while I wasn't home. You can pick up some pretty cool microscale effects in temp/dew with the FARS, 75, and 1-min intervals. For those unfamiliar with the Davis text files, it's Temp-MaxTemp-MinTemp-RH-Dew

 [ You are not allowed to view attachments ]

RH on the old 75 only maxed out at 96% (31 had only been hitting 94%) last night. I put another coat of conformal coating on the new one this morning so it may not be until later tomorrow that I'll get to install it.

Those are some wild swings. I actually went in and took the filter off my 31. I love the faster response time, tho my only main concern is the fan blowing all that air straight to the humidity sensor part. I wonder how long before it starts to dry out or just go bonkers, anyone know? I was very reluctant to for that reason, but what's done is done. I will enjoy my fast response time  \:D/
My temp is still spot on. People with filters are already seeing humidity drift anyway so I don't really worry about it. We all do this for ourselves so whatever gives you the most enjoyment I say go for it. We all want the best possible accuracy, but there's so many sensors with different specs and different ways to come up with an "official" 2m ambient temp that there's always going to be some discrepancies...different shielding, different specs, different response, different smoothing/averaging algorithms, etc.

I may gobble up a few 75s on Newark or Mouser before they become too hard to find. For ~$30 each, if they croak or drift too much in a year I could just swap another one in. By then we'll have a pin 35 that maybe we can adapt to a VP2.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: SnowHiker on July 29, 2018, 03:06:16 PM
We all want the best possible accuracy, but there's so many sensors with different specs and different ways to come up with an "official" 2m ambient temp that there's always going to be some discrepancies...different shielding, different specs, different response, different smoothing/averaging algorithms, etc.
Yep, and as we strive for more accuracy we give up a little in consistency.  If I can get accuracy that at least approaches that of historical records taken with manual equipment in Stevenson screens without spending hundreds extra to get a fraction of a degree accuracy, I'm happy.  I just want to know what it feels like outside and keep records of highs and lows and such.  Of course my "official" highs and lows may be slightly different than a neighbor with high-end equipment, but at least I'm comparing apples to apples, blissfully ignorant of any sensor drift in my uncalibrated instruments anyway.  As long as it's in the ballpark with other stations in the region I don't worry.  I also have a spare ISS and other equipment I can compare it to.

But then, I'm kind of old fashioned and tend to like simple, cheap, reliable things.  The fancier things are, the more goes wrong with them, causing more inconsistencies, in my view.  Or maybe I just use that as an excuse to be cheap.  :-P
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 29, 2018, 03:21:15 PM
The very best is Assmann psychrometer which is used as reference instrument tool. A little out of my price range however.
I'm more of a leg guy anyway.

Had a decent storm go through yesterday while I wasn't home. You can pick up some pretty cool microscale effects in temp/dew with the FARS, 75, and 1-min intervals. For those unfamiliar with the Davis text files, it's Temp-MaxTemp-MinTemp-RH-Dew

 [ You are not allowed to view attachments ]

RH on the old 75 only maxed out at 96% (31 had only been hitting 94%) last night. I put another coat of conformal coating on the new one this morning so it may not be until later tomorrow that I'll get to install it.

Those are some wild swings. I actually went in and took the filter off my 31. I love the faster response time, tho my only main concern is the fan blowing all that air straight to the humidity sensor part. I wonder how long before it starts to dry out or just go bonkers, anyone know? I was very reluctant to for that reason, but what's done is done. I will enjoy my fast response time  \:D/
My temp is still spot on. People with filters are already seeing humidity drift anyway so I don't really worry about it. We all do this for ourselves so whatever gives you the most enjoyment I say go for it. We all want the best possible accuracy, but there's so many sensors with different specs and different ways to come up with an "official" 2m ambient temp that there's always going to be some discrepancies...different shielding, different specs, different response, different smoothing/averaging algorithms, etc.

I may gobble up a few 75s on Newark or Mouser before they become too hard to find. For ~$30 each, if they croak or drift too much in a year I could just swap another one in. By then we'll have a pin 35 that maybe we can adapt to a VP2.

You have your new 75 up and running?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 29, 2018, 04:46:25 PM
From the side by sides I’ve done, the unmodified 75 runs consistently drier than the Davis-mounted 31 in the midrange as low as humidity in the 20s. (And I stress it is unmodified). That said, it’s not perfect and is still a Sensirion. It struggles to hit 100%, but gets there on occasion. It hits 98 and 99 regularly. It will still exhibit a wet bias if in high humidity for a prolonged period and I’d say it may have a slight wet bias generally, but it compares really closely to my psycho-dyne. I’d love to find a way to get a 31 or 35 that Davis hasn’t touched to work with my VP2, but until then, my experience is having the 75 is markedly superior to the Davis 31. Temp specs are not as good obviously and not exactly a rigorous study, but I’ve tested side by side with an ASOS at -8F and it was right on. Not trying to convince anyone, but just my experience ...

How much of the difference is there between the 75 & psych-dyne?

Usually pretty spot on. Within 1F in dew point. Sometimes 2F. The nice thing about the 75 is you can put it right in the intake of the psychro dyne to verify temp and dew point. I know some others are using the 75 so they may be able to provide more input in terms of temp/hum accuracy of the 75 across the extreme ranges

That’s good!  Have you ever done a comparison between the 75 & 31 in regards to air temp?

How did your 31 compared to the psychro-dyne?

The psychro dyne is dead on next to ASOS for temp and humidity. I’ve never found a difference in temp between the 75 and 31 when I’ve run them side by side. I could maybe do some more tests to get you some numeric answers. The Davis 31 is dead on for temp with the psychrodyne, but high on dewpoint usually by up to 4F, which is consistent with the comparison to ASOS. I have had the 75 next to an ASOS from -8F up to about 90F and it’s always been exactly on. I know the specs aren’t as good, but practically it seems to be a really good performer...

Thanks to some of these guys for going to Davis so they at least know the issue with their sensor. . I purchased the Acurite with the 31 after reading about it in this thread. It is really very good. I wish Davis’ 31 performed this way on the humidity side

How much of a variance in terms of RH did the psycho-dyne showed in comparison to the SHT-31?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on July 29, 2018, 04:57:19 PM
You have your new 75 up and running?
Not yet. I decided to give it another coat of conformal coating. I'll install it tomorrow evening once the shade hits the ISS in the evening.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 29, 2018, 05:00:21 PM
You have your new 75 up and running?
Not yet. I decided to give it another coat of conformal coating. I'll install it tomorrow evening once the shade hits the ISS in the evening.

Aight. Sounds good.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WxLover16 on July 29, 2018, 05:49:46 PM
Oddly enough, after taking the filter off yesterday evening, I'm really not seeing that much better response time than with it on, even with a bit drier of air in place (48%). I remember accidentally leaving it off last summer and sometimes seeing .3-4 change in an update, but now it's almost as if I still have the filter on, and this with a 24 cfm fan so it's definitely bringing in some serious ambient air with it. Hmmm...

I should add that the humidity is changing alot more than usual, just not as much as the temp.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on July 29, 2018, 06:24:42 PM
How much of a variance in terms of RH did the psycho-dyne showed in comparison to the SHT-31?
[/quote]

I only compare the dewpoint as I found variances in the temperature can make comparing RH near impossible
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 29, 2018, 06:31:26 PM
How much of a variance in terms of RH did the psycho-dyne showed in comparison to the SHT-31?

I only compare the dewpoint as I found variances in the temperature can make comparing RH near impossible
[/quote]

Gotcha.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 29, 2018, 06:40:35 PM
Oddly enough, after taking the filter off yesterday evening, I'm really not seeing that much better response time than with it on, even with a bit drier of air in place (48%). I remember accidentally leaving it off last summer and sometimes seeing .3-4 change in an update, but now it's almost as if I still have the filter on, and this with a 24 cfm fan so it's definitely bringing in some serious ambient air with it. Hmmm...

I should add that the humidity is changing alot more than usual, just not as much as the temp.

These are the sensor update rates filter or not.
ISS Weather Variable Update Intervals
(Transmitter ID Dependent)
Wind speed: . . . . . . . . . . . . . . . . . . . . . . 2.5 to 3 seconds
Wind direction: . . . . . . . . . . . . . . . . . . . . 2.5 to 3 seconds
Accumulated rainfall: . . . . . . . . . . . . . . . 20 to 24 seconds
Rain rate: . . . . . . . . . . . . . . . . . . . . . . . . 20 to 24 seconds
Outside temperature: . . . . . . . . . . . . . . . 10 to 12 seconds
Outside humidity: . . . . . . . . . . . . . . . . . . 50 seconds to 1 minute

Ultraviolet radiation: . . . . . . . . . . . . . . . . 50 seconds to 1 minute
Solar radiation: . . . . . . . . . . . . . . . . . . . 50 seconds to 1 minute

https://www.davisinstruments.com/product_documents/weather/manuals/07395-249_IM_06152.pdf
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WxLover16 on July 29, 2018, 08:06:34 PM
Oddly enough, after taking the filter off yesterday evening, I'm really not seeing that much better response time than with it on, even with a bit drier of air in place (48%). I remember accidentally leaving it off last summer and sometimes seeing .3-4 change in an update, but now it's almost as if I still have the filter on, and this with a 24 cfm fan so it's definitely bringing in some serious ambient air with it. Hmmm...

I should add that the humidity is changing alot more than usual, just not as much as the temp.

These are the sensor update rates filter or not.
ISS Weather Variable Update Intervals
(Transmitter ID Dependent)
Wind speed: . . . . . . . . . . . . . . . . . . . . . . 2.5 to 3 seconds
Wind direction: . . . . . . . . . . . . . . . . . . . . 2.5 to 3 seconds
Accumulated rainfall: . . . . . . . . . . . . . . . 20 to 24 seconds
Rain rate: . . . . . . . . . . . . . . . . . . . . . . . . 20 to 24 seconds
Outside temperature: . . . . . . . . . . . . . . . 10 to 12 seconds
Outside humidity: . . . . . . . . . . . . . . . . . . 50 seconds to 1 minute

Ultraviolet radiation: . . . . . . . . . . . . . . . . 50 seconds to 1 minute
Solar radiation: . . . . . . . . . . . . . . . . . . . 50 seconds to 1 minute

https://www.davisinstruments.com/product_documents/weather/manuals/07395-249_IM_06152.pdf

I know the update rates. All I'm saying is I'm seeing less change per update without the filter than when I saw the last time I had no filter. As opposed to .2-.5° or so change per update, I'm either seeing no change or mainly .1°, occasionally .2°. I suppose I just find it odd and perhaps not worth taking the filter off.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 29, 2018, 08:18:32 PM
Oddly enough, after taking the filter off yesterday evening, I'm really not seeing that much better response time than with it on, even with a bit drier of air in place (48%). I remember accidentally leaving it off last summer and sometimes seeing .3-4 change in an update, but now it's almost as if I still have the filter on, and this with a 24 cfm fan so it's definitely bringing in some serious ambient air with it. Hmmm...

I should add that the humidity is changing alot more than usual, just not as much as the temp.

These are the sensor update rates filter or not.
ISS Weather Variable Update Intervals
(Transmitter ID Dependent)
Wind speed: . . . . . . . . . . . . . . . . . . . . . . 2.5 to 3 seconds
Wind direction: . . . . . . . . . . . . . . . . . . . . 2.5 to 3 seconds
Accumulated rainfall: . . . . . . . . . . . . . . . 20 to 24 seconds
Rain rate: . . . . . . . . . . . . . . . . . . . . . . . . 20 to 24 seconds
Outside temperature: . . . . . . . . . . . . . . . 10 to 12 seconds
Outside humidity: . . . . . . . . . . . . . . . . . . 50 seconds to 1 minute

Ultraviolet radiation: . . . . . . . . . . . . . . . . 50 seconds to 1 minute
Solar radiation: . . . . . . . . . . . . . . . . . . . 50 seconds to 1 minute

https://www.davisinstruments.com/product_documents/weather/manuals/07395-249_IM_06152.pdf
perhaps not worth taking the filter off.
We spoke of this quite a while back, leaving the filter off has no real benefit, but then, maybe leaving it on doesn't either.  #-o :grin:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WxLover16 on July 29, 2018, 08:23:06 PM
Oddly enough, after taking the filter off yesterday evening, I'm really not seeing that much better response time than with it on, even with a bit drier of air in place (48%). I remember accidentally leaving it off last summer and sometimes seeing .3-4 change in an update, but now it's almost as if I still have the filter on, and this with a 24 cfm fan so it's definitely bringing in some serious ambient air with it. Hmmm...

I should add that the humidity is changing alot more than usual, just not as much as the temp.

These are the sensor update rates filter or not.
ISS Weather Variable Update Intervals
(Transmitter ID Dependent)
Wind speed: . . . . . . . . . . . . . . . . . . . . . . 2.5 to 3 seconds
Wind direction: . . . . . . . . . . . . . . . . . . . . 2.5 to 3 seconds
Accumulated rainfall: . . . . . . . . . . . . . . . 20 to 24 seconds
Rain rate: . . . . . . . . . . . . . . . . . . . . . . . . 20 to 24 seconds
Outside temperature: . . . . . . . . . . . . . . . 10 to 12 seconds
Outside humidity: . . . . . . . . . . . . . . . . . . 50 seconds to 1 minute

Ultraviolet radiation: . . . . . . . . . . . . . . . . 50 seconds to 1 minute
Solar radiation: . . . . . . . . . . . . . . . . . . . 50 seconds to 1 minute

https://www.davisinstruments.com/product_documents/weather/manuals/07395-249_IM_06152.pdf
perhaps not worth taking the filter off.
We spoke of this quite a while back, leaving the filter off has no real benefit, but then, maybe leaving it on doesn't either.  #-o :grin:

There is sooo much discussion in here (but perhaps the MOST informative and exciting read I've ever seen here!) of filters and good and bad humidity errors in high humidity with a FARS and the pros and cons to everything that I am confused and my head is spinning.  :lol:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 29, 2018, 09:37:30 PM
Oddly enough, after taking the filter off yesterday evening, I'm really not seeing that much better response time than with it on, even with a bit drier of air in place (48%). I remember accidentally leaving it off last summer and sometimes seeing .3-4 change in an update, but now it's almost as if I still have the filter on, and this with a 24 cfm fan so it's definitely bringing in some serious ambient air with it. Hmmm...

I should add that the humidity is changing alot more than usual, just not as much as the temp.

These are the sensor update rates filter or not.
ISS Weather Variable Update Intervals
(Transmitter ID Dependent)
Wind speed: . . . . . . . . . . . . . . . . . . . . . . 2.5 to 3 seconds
Wind direction: . . . . . . . . . . . . . . . . . . . . 2.5 to 3 seconds
Accumulated rainfall: . . . . . . . . . . . . . . . 20 to 24 seconds
Rain rate: . . . . . . . . . . . . . . . . . . . . . . . . 20 to 24 seconds
Outside temperature: . . . . . . . . . . . . . . . 10 to 12 seconds
Outside humidity: . . . . . . . . . . . . . . . . . . 50 seconds to 1 minute

Ultraviolet radiation: . . . . . . . . . . . . . . . . 50 seconds to 1 minute
Solar radiation: . . . . . . . . . . . . . . . . . . . 50 seconds to 1 minute

https://www.davisinstruments.com/product_documents/weather/manuals/07395-249_IM_06152.pdf

I know the update rates. All I'm saying is I'm seeing less change per update without the filter than when I saw the last time I had no filter. As opposed to .2-.5° or so change per update, I'm either seeing no change or mainly .1°, occasionally .2°. I suppose I just find it odd and perhaps not worth taking the filter off.

Were you using the Davis filter or the Sensirion filter?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WxLover16 on July 29, 2018, 10:14:29 PM
Oddly enough, after taking the filter off yesterday evening, I'm really not seeing that much better response time than with it on, even with a bit drier of air in place (48%). I remember accidentally leaving it off last summer and sometimes seeing .3-4 change in an update, but now it's almost as if I still have the filter on, and this with a 24 cfm fan so it's definitely bringing in some serious ambient air with it. Hmmm...

I should add that the humidity is changing alot more than usual, just not as much as the temp.

These are the sensor update rates filter or not.
ISS Weather Variable Update Intervals
(Transmitter ID Dependent)
Wind speed: . . . . . . . . . . . . . . . . . . . . . . 2.5 to 3 seconds
Wind direction: . . . . . . . . . . . . . . . . . . . . 2.5 to 3 seconds
Accumulated rainfall: . . . . . . . . . . . . . . . 20 to 24 seconds
Rain rate: . . . . . . . . . . . . . . . . . . . . . . . . 20 to 24 seconds
Outside temperature: . . . . . . . . . . . . . . . 10 to 12 seconds
Outside humidity: . . . . . . . . . . . . . . . . . . 50 seconds to 1 minute

Ultraviolet radiation: . . . . . . . . . . . . . . . . 50 seconds to 1 minute
Solar radiation: . . . . . . . . . . . . . . . . . . . 50 seconds to 1 minute

https://www.davisinstruments.com/product_documents/weather/manuals/07395-249_IM_06152.pdf

I know the update rates. All I'm saying is I'm seeing less change per update without the filter than when I saw the last time I had no filter. As opposed to .2-.5° or so change per update, I'm either seeing no change or mainly .1°, occasionally .2°. I suppose I just find it odd and perhaps not worth taking the filter off.

Were you using the Davis filter or the Sensirion filter?

Davis, never have used sensirion.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 29, 2018, 10:55:56 PM
Interesting that Campbell Scientific sells their version of the SHT-75...

https://www.campbellsci.com/cs215-l
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Old Tele man on July 29, 2018, 11:09:54 PM
Interesting that Campbell Scientific sells their version of the SHT-75...

https://www.campbellsci.com/cs215-l

Quoted from the data sheet:

"The CS215 uses the Sensirion SHT75, a combined relative humidity and temperature element, to provide accurate, stable measurements. The Sensirion SHT75 element is field-replaceable, eliminating the downtime typically required for the recalibration process. The CS215 outputs an SDI-12 signal that's measurable by many Campbell Scientific dataloggers."

What's the SDI-12 signal? Compatible with Sensibus?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 29, 2018, 11:24:00 PM
Interesting that Campbell Scientific sells their version of the SHT-75...

https://www.campbellsci.com/cs215-l

Quoted from the data sheet:

"The CS215 uses the Sensirion SHT75, a combined relative humidity and temperature element, to provide accurate, stable measurements. The Sensirion SHT75 element is field-replaceable, eliminating the downtime typically required for the recalibration process. The CS215 outputs an SDI-12 signal that's measurable by many Campbell Scientific dataloggers."

What's the SDI-12 signal? Compatible with Sensibus?

That I do not know...I’m sure someone who is smarter than I can answer that question for you.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: johnd on July 30, 2018, 02:50:21 AM

What's the SDI-12 signal? Compatible with Sensibus?

No - something fairly different. SDI-12 is another serial protocol that's quite widely used for instrumentation. It's one of the options on EM. There's a load more information online as ever, but one starting point would be:

en.wikipedia.org/wiki/SDI-12 (http://en.wikipedia.org/wiki/SDI-12)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: miraculon on July 30, 2018, 08:41:15 AM
For what it's worth, I just took my Belfort 566 psychrometer outside. We are showing 97% RH on my Davis ISS and 95% on the Davis Temp/Hum station. I believe that these are both SHT-11 (or -15) and definitely not SHT-31.

Using distilled water I got 65°F dry bulb and 64°F wet bulb.

The slide rule shows about 94% RH a web calculator gives 94.88% RH. http://linricsoftw.web701.discountasp.net/webpsycalc.aspx (http://linricsoftw.web701.discountasp.net/webpsycalc.aspx)

I ordered one of those Sensirion SHT-31 eval boards from Mouser.

Greg H.

Hmm. Interesting. Let us know the results using the 31

This morning the humidity was higher and I had the chance to check the Sensirion eval board. I have it placed in my NovaLynx thermometer housing and the shelter is in the shade this time of day.

Here is what I saw:

ISS Humidity: 92%
T/H Station: 88%
SHT-31 eval: 88.86% (Temp was 65.78°F)
Belfort Psychrometer Wet/Dry: 66°F Dry, 63½°F Wet giving 87.58%. Another reading was 67°F and 64°F for 85.38%

Conditions were changing, especially on the Sensirion board and Belfort. I tried to stay close to the therm shelter and the Sensirion while using the Belfort.

Dew Point per the Belfort was 62°F to 63°F and 63°F on the Davis ISS.

I have a feeling that this is one of those "measure it with a micrometer, mark it with chalk and cut it with an axe" kind of things...

Greg H.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 30, 2018, 08:58:25 AM
 :lol:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 30, 2018, 09:05:40 AM
Just checked my history tables on WU and noticed, to my surprise, that I was getting 100% RH readings as recently as Sept 2017 and in June and July 2017. I had no Humidity compensation added and no software formulas converting or interpreting data, just console to Weatherlink IP to WU.  This is on a SHT-31 that was at least 2 years old (or more) at that time and in a 24hr FARS.
The rest of the year was had RH highs of 98% & 99%. I guess it's just a crap shoot. This new one hasn't had the conditions right for RH that high yet... but I'm showing 98% this morning.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 30, 2018, 10:21:56 AM
This morning the humidity was higher and I had the chance to check the Sensirion eval board. I have it placed in my NovaLynx thermometer housing and the shelter is in the shade this time of day.

Here is what I saw:

ISS Humidity: 92%
T/H Station: 88%
SHT-31 eval: 88.86% (Temp was 65.78°F)
Belfort Psychrometer Wet/Dry: 66°F Dry, 63½°F Wet giving 87.58%. Another reading was 67°F and 64°F for 85.38%

Conditions were changing, especially on the Sensirion board and Belfort. I tried to stay close to the therm shelter and the Sensirion while using the Belfort.

Dew Point per the Belfort was 62°F to 63°F and 63°F on the Davis ISS.

I have a feeling that this is one of those "measure it with a micrometer, mark it with chalk and cut it with an axe" kind of things...

Greg H.


Even so, that's a 6% average humidity difference (ISS vs psychrometer) in a temp/humidity range where the Davis sensor is more accurate (above 70% in relatively cool air). However, let us know if you can test when the temp rises above 77 (25C) with a dp above 60 (15C) as that's when even larger discrepancies begin to appear.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Old Tele man on July 30, 2018, 02:18:16 PM
Devil's Advocate Humor: Sorta makes you wonder about the accuracy of all the PAST weather data, don't it?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 30, 2018, 05:46:49 PM
Sent email to metoshield owner about the issue with the SHT31 humidity and asked him to consider production of the SHT75 which is a meteorological grade sensor unlike the SHT31.

They tried to develop the SHT31 just for their shields and failed with the protocol issue but the 75 is already working in homemade units would be great addition.  Something for sure many with the wet bias would jump on.
Say a combo deal shield and sensor providing the shield works as advertised.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 30, 2018, 06:59:31 PM
So I put on the Sensirion SF2 filter on my SHT31. I decided to use Rocksett, an engeneering ceramic adhesive. Its clear, no oder, no fumes, non toxic and most importantly easy to remove. Its a mild bond on plastics. Working good so far.
http://www.flexbar.com/shop/pc/ROCKSETT-2-OZ-p4019.htm (http://www.flexbar.com/shop/pc/ROCKSETT-2-OZ-p4019.htm)
 [ You are not allowed to view attachments ]

Takes about 15 min to hold and 12hr to full cure.

I did not re-install the big Davis filter. So it sits in the FARS with just the SF2 filter. I was also able to get my 2nd Temp/Hum station installed today. It is a Passive shield SHT11 that I baked an re-hydrated yesterday. It's been going a few hours now and yes, the SHT31 is avg 4% higher RH but the temp difference is hovering around .5 deg F.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 30, 2018, 07:14:07 PM
Glad you posted that if good stuff I need something. I ordered some of those filters also primarily for the metoshield I have coming in couple weeks. Less restriction inside temperature chamber the better I would think.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 30, 2018, 07:22:51 PM
I found this also. http://ecobondit.com/ecobond-products/

http://ecobondit.com/what-we-stick-too/

and this https://www.amazon.com/Supertite-Universal-Industrial-Strength-0-63oz/dp/B077KGT3K7/ref=sr_1_8?ie=UTF8&qid=1532993103&sr=8-8&keywords=Eco-Bond+Adhesives
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 30, 2018, 07:30:46 PM
Glad you posted that if good stuff I need something. I ordered some of those filters also primarily for the metoshield I have coming in couple weeks. Less restriction inside temperature chamber the better I would think.

No long term results yet, unfortunately. But FYI. I cut the 4 prongs off and shaved the remaining nubs off flush with a razor, then scuffed the surface lightly on some 220 grit paper and blew off with canned air. Also, the rubbery (silicon ??) layer surrounding the sensor chip needed to be scuffed too. I did this with the edge of a sharp knife. applied Rocksett with a tiny screwdriver putting a VERY small amount on all 4 sides and CAREFULLY placed it over the sensor. Let it set ~15 min and re-installed into the FARS. The filter/cover is so tiny and light I just don't think it requires a super strong adhesive to stay put.
But I'll keep a lookout for it on the intake screen  :lol:   
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 30, 2018, 07:38:42 PM
I found this also. http://ecobondit.com/ecobond-products/

http://ecobondit.com/what-we-stick-too/


and this https://www.amazon.com/Supertite-Universal-Industrial-Strength-0-63oz/dp/B077KGT3K7/ref=sr_1_8?ie=UTF8&qid=1532993103&sr=8-8&keywords=Eco-Bond+Adhesives

Ecobondit curiously left out polyethylene, polypropylene plastics.... Hmmm

But Supertite looks like a winner  \:D/

Rocksett is by no means "made for" this application, but what I had on hand. Its actually for High temp aplicatons. but its the only thing I had that was non-toxic, oderless and non-volitile.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Old Tele man on July 30, 2018, 07:49:22 PM
MY THOUGHT - Use a piece of cotton string, because cotton doesn't out-gas, for a mechanical attachment.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 30, 2018, 08:00:58 PM
MY THOUGHT - Use a piece of cotton string, cause cotton don't out-gas, for a mechanical attachment.

After just installing this filter, the problem you would likely encounter is that you would have to wrap the string around the entire PCB board. The filter sits about an 1/8th inch high off the surface. The angle would be so shallow that I don't think it would apply enough mechanical pressure to hold it down...not to mention when it gets damp and elongates. This is me trying to tie that string while keeping the filter directly over the sensor  ](*,)

 :lol:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 30, 2018, 08:07:10 PM
What about what we used as kids, flour and water? How much "gas" can that have??  :-k
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 30, 2018, 08:13:58 PM
Or, secure a big paper clip....maybe one with a plastic coating on it...
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 30, 2018, 08:23:34 PM
How about using one of your nylon panty hose, the one with a run it. :lol: ..Just wrap the whole sensor. I've heard they make a excellent fine dust filter.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 30, 2018, 08:24:05 PM
Something like a rubber band would be the shiz nit. But the the filter cap is still so tiny, even a thin rubber band would likely block the hole. There is almost no extra surface area for any kind of mechanical fasteners i can think of.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 30, 2018, 08:28:28 PM
Something like a rubber band would be the shiz nit. But the the filter cap is still so tiny, even a thin rubber band would likely block the hole. There is almost no extra surface area for any kind of mechanical fasteners i can think of.

Can't use rubber. Not sure about nylon.

Maybe can't use nylon either.

Nylon is a plastic, made from petroleum. However, it is one of the least toxic plastics and outgasses little, if at all
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 30, 2018, 08:31:53 PM
Something like a rubber band would be the shiz nit. But the the filter cap is still so tiny, even a thin rubber band would likely block the hole. There is almost no extra surface area for any kind of mechanical fasteners i can think of.

Can't use rubber. Not sure about nylon.

Really? No rubber... Why not?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 30, 2018, 08:32:16 PM
How about using one of your nylon panty hose, the one with a run it. :lol: ..Just wrap the whole sensor. I've heard they make a excellent fine dust filter.
Firstly...you swore you wouldn't tell.... Secondly, this might actually work if secured well enough. Hmmmm..
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 30, 2018, 08:33:20 PM
How about using one of your nylon panty hose, the one with a run it. :lol: ..Just wrap the whole sensor. I've heard they make a excellent fine dust filter.
Firstly...you swore you wouldn't tell.... Secondly, this might actually work if secured well enough. Hmmmm..

It might just some cotton twine should work.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 30, 2018, 08:37:15 PM
Something like a rubber band would be the shiz nit. But the the filter cap is still so tiny, even a thin rubber band would likely block the hole. There is almost no extra surface area for any kind of mechanical fasteners i can think of.

Can't use rubber. Not sure about nylon.

Really? No rubber... Why not?

All rubber I've been around has a strong odor. The odor is the gaseous toxins being released. Unless it is the type of toxin that wouldn't be harmful.  Not sure :?:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 30, 2018, 08:40:44 PM
Something like a rubber band would be the shiz nit. But the the filter cap is still so tiny, even a thin rubber band would likely block the hole. There is almost no extra surface area for any kind of mechanical fasteners i can think of.

Can't use rubber. Not sure about nylon.

 

Really? No rubber... Why not?

All rubber I've been around has a strong odor. The odor is the gaseous toxins being released. Unless it isn't the type of toxin that wouldn't be harmful.  Not sure :?:

Ahh .. I got ya.

Well if its pantyhose, better make it Silk stockings instead of nylon...just in case its an issue too. besides, the silk is a lot  more durable and finer weave ..So Iv'e heard  :-)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 30, 2018, 08:51:20 PM
Regardless, I still think that Supertite Eco Glue is the best option. It is solvent free, non-toxic and water based. How much irreparable harm could it possibly do?

 Like flour and water. :-)

WOW! Right after i posted this I saw this picture on the Amazon add.

 [ You are not allowed to view attachments ]

 #-o
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 30, 2018, 08:56:08 PM
Something like a rubber band would be the shiz nit. But the the filter cap is still so tiny, even a thin rubber band would likely block the hole. There is almost no extra surface area for any kind of mechanical fasteners i can think of.

Can't use rubber. Not sure about nylon.

 

Really? No rubber... Why not?

All rubber I've been around has a strong odor. The odor is the gaseous toxins being released. Unless it isn't the type of toxin that wouldn't be harmful.  Not sure :?:

Ahh .. I got ya.

Well if its pantyhose, better make it Silk stockings instead of nylon...just in case its an issue too. besides, the silk is a lot  more durable and finer weave ..So Iv'e heard  :-)


Real silk is expensive. https://www.stockingirl.com/product/SGSS2.html

Back to the Supertite Eco Glue
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 30, 2018, 09:03:33 PM
Ya know, I think I'll try a small diameter spring (like in your ball point pen, but longer) that's wraps around the sensor board. If necessary I'll cut the sensor board to accommodate it, but pretty sure I'll find an appropriate spring. At the very least I'll put notches at both ends of the board to keep the spring stationary.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 30, 2018, 09:07:51 PM
Something like a rubber band would be the shiz nit. But the the filter cap is still so tiny, even a thin rubber band would likely block the hole. There is almost no extra surface area for any kind of mechanical fasteners i can think of.

Can't use rubber. Not sure about nylon.

 

Really? No rubber... Why not?

All rubber I've been around has a strong odor. The odor is the gaseous toxins being released. Unless it isn't the type of toxin that wouldn't be harmful.  Not sure :?:

Ahh .. I got ya.

Well if its pantyhose, better make it Silk stockings instead of nylon...just in case its an issue too. besides, the silk is a lot  more durable and finer weave ..So Iv'e heard  :-)


Real silk is expensive. https://www.stockingirl.com/product/SGSS2.html

Back to the Supertite Eco Glue
Actually, you can go on eBay and by a pillow case from some Chinese sellers for a lot cheaper. Get a low momme weight.

Ohh and Thanks for the link.   \:D/
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 30, 2018, 09:09:47 PM
Ya know, I think I'll try a small diameter spring (like in your ball point pen, but longer) that's wraps around the sensor board. If necessary I'll cut the sensor board to accommodate it, but pretty sure I'll find an appropriate spring. At the very least I'll put notches at both ends of the board to keep the spring stationary.
That's a pretty good Idea. Spring might want to roll, but if its fairly stiff it should work.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 30, 2018, 09:25:01 PM
Something like a rubber band would be the shiz nit. But the the filter cap is still so tiny, even a thin rubber band would likely block the hole. There is almost no extra surface area for any kind of mechanical fasteners i can think of.

Can't use rubber. Not sure about nylon.

 

Really? No rubber... Why not?

All rubber I've been around has a strong odor. The odor is the gaseous toxins being released. Unless it isn't the type of toxin that wouldn't be harmful.  Not sure :?:

Ahh .. I got ya.

Well if its pantyhose, better make it Silk stockings instead of nylon...just in case its an issue too. besides, the silk is a lot  more durable and finer weave ..So Iv'e heard  :-)


Real silk is expensive. https://www.stockingirl.com/product/SGSS2.html

Back to the Supertite Eco Glue
Actually, you can go on eBay and by a pillow case from some Chinese sellers for a lot cheaper. Get a low momme weight.

Ohh and Thanks for the link.   \:D/

Iv'e actually bought from her a couple times. Very good quality and great to deal with. $19 19 momme  weight un-dyed 100% mulberry silk.
https://www.ebay.com/itm/100-19mm-Mulberry-Silk-Charmeuse-Pillowcase-UndyedWhite-QUEEN-single-or-a-pair/232833685515?var=532351166292&hash=item3635f8180b%3Am%3AmQuQ1z2bT1FpM_jxNsmQvRg&_sacat=0&_nkw=100%25+19mm+Mulberry+Silk+Charmeuse+Pillowcase+UndyedWhite+QUEEN+single+or+a+pair&_from=R40&rt=nc&_trksid=m570.l1313 (https://www.ebay.com/itm/100-19mm-Mulberry-Silk-Charmeuse-Pillowcase-UndyedWhite-QUEEN-single-or-a-pair/232833685515?var=532351166292&hash=item3635f8180b%3Am%3AmQuQ1z2bT1FpM_jxNsmQvRg&_sacat=0&_nkw=100%25+19mm+Mulberry+Silk+Charmeuse+Pillowcase+UndyedWhite+QUEEN+single+or+a+pair&_from=R40&rt=nc&_trksid=m570.l1313)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 30, 2018, 09:26:35 PM
Ya know, I think I'll try a small diameter spring (like in your ball point pen, but longer) that's wraps around the sensor board. If necessary I'll cut the sensor board to accommodate it, but pretty sure I'll find an appropriate spring. At the very least I'll put notches at both ends of the board to keep the spring stationary.
That's a pretty good Idea. Spring might want to roll, but if its fairly stiff it should work.
That's what the notches are for, just make sure they're in line with the sensor.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 30, 2018, 09:33:09 PM
Ya know, I think I'll try a small diameter spring (like in your ball point pen, but longer) that's wraps around the sensor board. If necessary I'll cut the sensor board to accommodate it, but pretty sure I'll find an appropriate spring. At the very least I'll put notches at both ends of the board to keep the spring stationary.
That's a pretty good Idea. Spring might want to roll, but if its fairly stiff it should work.
That's what the notches are for, just make sure there in line with the sensor.
actually,  I meant even with notches, if the spring was too 'flimsy' it may still want to roll off the cap. but maybe not. if you went the short distance across.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 30, 2018, 10:02:50 PM
I'm spring shopping tomorrow. With an adequate one, which shouldn't be hard to find, I think it'll work fine.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 30, 2018, 10:05:45 PM
I'm spring shopping tomorrow. With an adequate one, which shouldn't be hard to find, I think it'll work fine.

I agree.
If mine ends up falling off. I may go this route too.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 30, 2018, 10:19:55 PM
Just found a Cole-Parmer 22014 Psychrometer for $45 &
 free shipping on Amazon. Includes case and extra wicks, slide rule etc.
 [ You are not allowed to view attachments ]

 :-| Not sure why. There was only one left in stock when I saw it. As soon as I ordered it, the new listing showed that "I purchased this today" and now the price is $182 + $10 Shipping.

Worse case I'll return it.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Old Tele man on July 30, 2018, 11:51:46 PM
MY THOUGHT, Part Deux:  How about a small nylon tie-wrap?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: drew1021 on July 31, 2018, 06:01:59 AM
Tie wrap might actually work. They make some very thin ones.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 31, 2018, 06:25:45 AM
Can't be just any silk has to be breathable. Pillow cases aren't the same as pantyhose.   :lol:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 31, 2018, 06:55:20 AM
Can't be just any silk has to be breathable. Pillow cases aren't the same as pantyhose.   :lol:
Yeah, I guess so. I figured the thin, light weight or mm weight would be plenty breathable.

Anyway...I just looked at a 31 that I had cooking in the 'Ole Crock pot, and I realized the mounting holes for the Davis cap are not being used. Therefore... A double wrap of thin stainless wire through the old mount holes, each lying on either edge of the filter cap, twist wires together on the back and Done.
I don't know why I didn't just do that now. I have the wire in my shop. Got it from Lowe's ... or Amazon.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 31, 2018, 07:47:22 AM
MY THOUGHT, Part Deux:  How about a small nylon tie-wrap?
I was just about to add " or monofilament line" when the stainless wire hit me. Also after realizing that the old cap mounting holes are not used, your idea about the cotton string is now looking like a possibility too. Just no way to garrantee it maintain pressure against the cap.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on July 31, 2018, 09:58:21 AM
Installed my new SHT75 yesterday evening with no filter and 9V running on the fan. I'll give it some time before analyzing the data too much. It hit 96% RH overnight without prime radiational cooling conditions.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 31, 2018, 10:19:37 AM
Dendrite, would you be able to break down the steps involved in hooking up an SHT75 to the SIM?

As I understand it, you can snip the wire off an old SHT sensor, but which wires go to which leads?

You also mentioned "conformal coating". Could you describe that process? Looks like just slathering on some silicone-like substance over the leads to insulate them.

There appears to be some kind of "jack" (for lack of a better term) that the sensor leads slip into which then attaches to the cable going to the SIM. I assume you can also procure that part from mouser.com (along with the sensor), but what is it called?

If you are busy or unavailable, no sweat.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 31, 2018, 11:40:51 AM
Interesting. SHT31 (turquoise) vs SHT11 (black) Plot shows a RH crossover point at ~86%. Between 60%-86% the SHT31 shows 3-5% higher RH. Above 86% SHT11 starts showing higher RH and continues to till 10:30 AM this morning as it's till above 95% RH here.
Disregard prior to 5pm. SHT11 Sensor just came on-line.

Note: Both Temps were tracking within .5 deg and the Temp at crossover was 78 deg F.
SHT11 is in a Passive Rad Shield, 6+ year old and just baked and re-hydrated Sunday.
SHT31 is in a FARS & is about a week old.
 [ You are not allowed to view attachments ]


Just initial observations.. doesn't mean much..But I'll see if this continues over the next few days.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 31, 2018, 12:52:19 PM
Interesting. SHT31 (turquoise) vs SHT11 (black) Plot shows a RH crossover point at ~86%. Between 60%-86% the SHT31 shows 3-5% higher RH. Above 86% SHT11 starts showing higher RH and continues to till 10:30 AM this morning as it's till above 95% RH here.
Disregard prior to 5pm. SHT11 Sensor just came on-line.

Note: Both Temps were tracking within .5 deg and the Temp at crossover was 78 deg F.
SHT11 is in a Passive Rad Shield, 6+ year old and just baked and re-hydrated Sunday.
SHT31 is in a FARS & is about a week old.

Looks like the lines cross not long after sunset.

You might, at some point, think to reverse the shield arrangement - placing the SHT11 in the FARS and the 31 in the passive shield. Reason I say that is because a FARS is going to produce a higher temperature on many nights (except overcast and windy) because it better protects against radiational cooling. The dew point should always be the same in both shields, theoretically, assuming the sensors have no bias or an identical bias. So, all things being equal, the FARS will generally have the lower humidity reading.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 31, 2018, 01:16:32 PM
Interesting. SHT31 (turquoise) vs SHT11 (black) Plot shows a RH crossover point at ~86%. Between 60%-86% the SHT31 shows 3-5% higher RH. Above 86% SHT11 starts showing higher RH and continues to till 10:30 AM this morning as it's till above 95% RH here.
Disregard prior to 5pm. SHT11 Sensor just came on-line.

Note: Both Temps were tracking within .5 deg and the Temp at crossover was 78 deg F.
SHT11 is in a Passive Rad Shield, 6+ year old and just baked and re-hydrated Sunday.
SHT31 is in a FARS & is about a week old.

Looks like the lines cross not long after sunset.

You might, at some point, think to reverse the shield arrangement - placing the SHT11 in the FARS and the 31 in the passive shield. Reason I say that is because a FARS is going to produce a higher temperature on many nights (except overcast and windy) because it better protects against radiational cooling. The dew point should always be the same in both shields, theoretically, assuming the sensors have no bias or an identical bias. So, all things being equal, the FARS will generally have the lower humidity reading.
Good point. I didn't consider that. However, on this particular evening/afternoon there was no sun as a storm blew in around noon and it was dark overcast the rest of the day. Not sure how that affects the results either.

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on July 31, 2018, 01:33:10 PM
Dendrite, would you be able to break down the steps involved in hooking up an SHT75 to the SIM?

As I understand it, you can snip the wire off an old SHT sensor, but which wires go to which leads?

You also mentioned "conformal coating". Could you describe that process? Looks like just slathering on some silicone-like substance over the leads to insulate them.

There appears to be some kind of "jack" (for lack of a better term) that the sensor leads slip into which then attaches to the cable going to the SIM. I assume you can also procure that part from mouser.com (along with the sensor), but what is it called?

If you are busy or unavailable, no sweat.
I used this for reference.

http://www.wxforum.net/index.php?topic=14175.msg189484#msg189484

White (clock) - Yellow (vdd) - Green (gnd) - Blue (data) using the Davis cord. Red and black are unused.

And yeah...I'm just using a silicone based product.

https://www.amazon.com/MG-Chemicals-Silicone-Modified-Conformal/dp/B07BSK5S7B

It says "modified" now...not sure what that means. I got mine a few years ago and it was just regular silicone conformal coating.

Sensirion does not recommend soldering directly to the pins, but rather to use a connector (1.27mm pitch). I used this to eliminate soldering altogether...

http://www.newark.com/harting/14310410301000/receptacle-4way-1row-1-27mm-har/dp/53X2803

This is direct from the datasheet...

Quote
1.2 Sockets and Soldering
For maintain high accuracy specifications the sensor shall
not be soldered. Sockets may be used such as “Preci-dip /
Mill-Max R851-83-004-20-001” or similar.
Standard wave soldering ovens may be used at maximum
235°C for 20 seconds. For manual soldering contact time
must be limited to 5 seconds at up to 350°C7.
After wave soldering the devices should be stored at
>75%RH for at least 12h to allow the polymer to rehydrate.
Alternatively the re-hydration process may be
performed at ambient conditions (>40%RH) during more
than 5 days.
In no case, neither after manual nor wave soldering, a
board wash shall be applied. In case of application with
exposure of the sensor to corrosive gases the soldering
pads of pins and PCB shall be sealed to prevent loose
contacts or short cuts.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 31, 2018, 01:40:38 PM
Thanks, dendrite! I think that will help a lot of readers, myself included.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 31, 2018, 01:51:18 PM
FYI not sure about the SHT11 but I'm finding the SHT15 runs about .5° cooler constantly. I remember some mentioned this when 31's first came out. I checked last night without sun both in FARS shields and the 31's are right on with NIST certified, temperature was upper 50's at time.  Even today the 15 is running cooler at 80°.

Wind is calm currently and look how well the passive at top #1 is doing.  :-)

 [ You are not allowed to view attachments ]
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 31, 2018, 02:03:50 PM
I'm glad you brought that up. I honestly can't tell what the exact temp diff is between my SHT31 and 11 because I can't get "2nd Temp" to report in tenths of deg. In other words, it only reports as whole numbers.
This is, I'm sure, because I am using stock software and I'm in no way setup to do precise measurements of different sensors. Not yet anyway  :-).

However, The graphs appear to tracking each other by less that 1 deg since yesterday. But again it could be .5 difference or .7 or...  :?:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 31, 2018, 03:44:59 PM
Can't be just any silk has to be breathable. Pillow cases aren't the same as pantyhose.   :lol:
I realized the mounting holes for the Davis cap are not being used. Therefore... A double wrap of thin stainless wire through the old mount holes, each lying on either edge of the filter cap, twist wires together on the back and Done.
You saved me a trip to the hardware store. I found some braided wire for hanging pictures in the house and tried it on my spare 31 and it works just fine, only one or two strands is plenty. I would say you don't really need to do the side thing, over top will be fine, especially for older eyes.
Thanks!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 31, 2018, 03:59:03 PM
Can't be just any silk has to be breathable. Pillow cases aren't the same as pantyhose.   :lol:
I realized the mounting holes for the Davis cap are not being used. Therefore... A double wrap of thin stainless wire through the old mount holes, each lying on either edge of the filter cap, twist wires together on the back and Done.
You saved me a trip to the hardware store. I found some braided wire for hanging pictures in the house and tried it on my spare 31 and it works just fine, only one or two strands is plenty. I would say you don't really need to do the side thing, over top will be fine, especially for older eyes.
Thanks!

Outstanding! \:D/
Good to know. I'll likely do this to my other 31 as a backup.

Speaking of (sort of) You think this SF2 filter would have any advantages or disadvantages in passive shield setup?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 31, 2018, 04:13:03 PM
Can't be just any silk has to be breathable. Pillow cases aren't the same as pantyhose.   :lol:
I realized the mounting holes for the Davis cap are not being used. Therefore... A double wrap of thin stainless wire through the old mount holes, each lying on either edge of the filter cap, twist wires together on the back and Done.
You saved me a trip to the hardware store. I found some braided wire for hanging pictures in the house and tried it on my spare 31 and it works just fine, only one or two strands is plenty. I would say you don't really need to do the side thing, over top will be fine, especially for older eyes.
Thanks!
Speaking of (sort of) You think this SF2 filter would have any advantages or disadvantages in passive shield setup?
What's good for the goose, is good for the gander. If I understand this filter correctly, it allows air to pass through, but also makes it waterproof when sealed correctly. I believe it is suppose to help the sensor from becoming saturated for you folks in the humid parts of the country. I don't think I really need this filter, as I not really experiencing problems like the others, but what the hell, can't hurt.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 31, 2018, 04:30:23 PM
Can't be just any silk has to be breathable. Pillow cases aren't the same as pantyhose.   :lol:
I realized the mounting holes for the Davis cap are not being used. Therefore... A double wrap of thin stainless wire through the old mount holes, each lying on either edge of the filter cap, twist wires together on the back and Done.
You saved me a trip to the hardware store. I found some braided wire for hanging pictures in the house and tried it on my spare 31 and it works just fine, only one or two strands is plenty. I would say you don't really need to do the side thing, over top will be fine, especially for older eyes.
Thanks!
Speaking of (sort of) You think this SF2 filter would have any advantages or disadvantages in passive shield setup?
... If I understand this filter correctly, it allows air to pass through, but also makes it waterproof when sealed correctly. I believe it is suppose to help the sensor from becoming saturated for you folks in the humid parts of the country....

WHAT!  Waterproof :shock:
I totally missed that. I thought it was just less "obstructive" and allowed moving air closer contact to the sensor than the Big Davis Filter. Also the other reason it was looked at was to add a dust filter in addition to the Davis filter to lessen the amount of contaminates causing premature aging of the Humidity sensor.

Well either way, I am noticing a big lag time between my SF2 filtered ,aspirated 31 and my passive shielded 11. But only on the humidity side and its the FARS sensor that's lagging. Humidity changes on my passive is way faster.  But there are way to many variables involved here to make any judgment about that, just not what I was expecting.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on July 31, 2018, 04:55:03 PM
Thanks, dendrite! I think that will help a lot of readers, myself included.
Like we were talking about before though...

If you don't like almost instantaneous response (like raw, non-averaged ASOS data) this may not be for you. Especially if you are using a custom FARS with much more CFM passing through it than the Davis fans. If that isn't a detriment to someone, then I think that person will love the 75. Of course if you find a way to put a filter over it or use the Davis stock fan, that will knock the response down. I would think a passive shield would knock it down even further, but given the recent passive vs FARS tests, who knows?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 31, 2018, 05:37:11 PM
Thanks, dendrite! I think that will help a lot of readers, myself included.
Like we were talking about before though...

If you don't like almost instantaneous response (like raw, non-averaged ASOS data) this may not be for you. Especially if you are using a custom FARS with much more CFM passing through it than the Davis fans. If that isn't a detriment to someone, then I think that person will love the 75. Of course if you find a way to put a filter over it or use the Davis stock fan, that will knock the response down. I would think a passive shield would knock it down even further, but given the recent passive vs FARS tests, who knows?

For now, I will install it as a backup in a passive shield. If my 31 starts acting up, then, yes, I will put it in my primary FARS that has an AC fan (20-25 CFM). I've got averaging routines I can apply to the readings if necessary. That's the good thing about having a completely custom site and CWOP script. At least this way, I've got something accurate if I need it.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 31, 2018, 05:42:47 PM
WHAT!  Waterproof :shock:
I totally missed that. I thought it was just less "obstructive" and allowed moving air closer contact to the sensor than the Big Davis Filter. Also the other reason it was looked at was to add a dust filter in addition to the Davis filter to lessen the amount of contaminates causing premature aging of the Humidity sensor.

It's waterproof but not water VAPOR proof. It just keeps large water drops from sitting against the sensor. That's assuming it's installed at the factory. If it's at all loose, water may find a way in underneath and, if it does, stay there longer than if you had no filter.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 31, 2018, 05:43:48 PM
Can't be just any silk has to be breathable. Pillow cases aren't the same as pantyhose.   :lol:
I realized the mounting holes for the Davis cap are not being used. Therefore... A double wrap of thin stainless wire through the old mount holes, each lying on either edge of the filter cap, twist wires together on the back and Done.
You saved me a trip to the hardware store. I found some braided wire for hanging pictures in the house and tried it on my spare 31 and it works just fine, only one or two strands is plenty. I would say you don't really need to do the side thing, over top will be fine, especially for older eyes.
Thanks!
Speaking of (sort of) You think this SF2 filter would have any advantages or disadvantages in passive shield setup?
... If I understand this filter correctly, it allows air to pass through, but also makes it waterproof when sealed correctly. I believe it is suppose to help the sensor from becoming saturated for you folks in the humid parts of the country....

WHAT!  Waterproof :shock:

You'd have to find another way, but doable I'd think.
https://www.sensirion.com/fileadmin/user_upload/customers/sensirion/Dokumente/0_Datasheets/Humidity/Sensirion_Humidity_Sensors_SF2_Filter_Cap_Datasheet.pdf
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 31, 2018, 05:48:12 PM
I was noticing when putting on the filter that the rubbery substance around the sensor allows for what I think would be a good seal if the filter is smoothed off and the wire torqued down enough. At least you would avoid a sealant if it works.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on July 31, 2018, 05:55:01 PM
My plan with the 75 is to use an old 11, cut the cable off, remove the cap and drill 2 small holes in the board.  I will then thread some wire around the plastic of the wire-to-board connector and through the holes in the sensor PCB, twisting and securing underneath. This should hold it tightly against the circuit board which I can then install in any Davis shield.

I'm uncertain how to secure a cap over this as it's too long for the Davis cap (which doesn't do much anyway) and the SF2 caps I have are not the right shape. I've read people suggesting SF1 caps, but, again, the issue is how would you secure them properly in this application? The cap isn't a deal breaker for me as I can stockpile and replace annually if this works out.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 31, 2018, 06:56:18 PM
I was noticing when putting on the filter that the rubbery substance around the sensor allows for what I think would be a good seal if the filter is smoothed off and the wire torqued down enough. At least you would avoid a sealant if it works.
Agreed.
 But I noticed 2 things while reading that link you provided.
1. touching the membrane can contaminate with oils and reduce the amount of humidity that can transfer to the sensor.  #-o Yeah.. I touched it... like  a lot. ](*,) I thought it was just a screen.

2. is ref to another link:
http://datasheets.closedcube.uk/Sensirion/Sensirion_Humidity_Sensors_SF2_Handling_Assembly_V1.pdf (http://datasheets.closedcube.uk/Sensirion/Sensirion_Humidity_Sensors_SF2_Handling_Assembly_V1.pdf)
 Where I Read the following:
 The following adhesives were tested for use with SF2 filter
caps and SHT2x: Electrolube SMA 10SL, DELO
MK096, DELO AD066, DELO 6093,EPO-TEK H70E/S,
EPO-TEK T6067, Lord MD-130
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 31, 2018, 07:05:51 PM
Oh and 1oz. of Epo-Tek® H20E  Adhesive is on sale at Amazon for $277 + shipping... But only if you hurry  :lol:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 31, 2018, 07:28:40 PM
I was noticing when putting on the filter that the rubbery substance around the sensor allows for what I think would be a good seal if the filter is smoothed off and the wire torqued down enough. At least you would avoid a sealant if it works.
Agreed.
 But I noticed 2 things while reading that link you provided.
1. touching the membrane can contaminate with oils and reduce the amount of humidity that can transfer to the sensor.  #-o Yeah.. I touched it... like  a lot. ](*,) I thought it was just a screen.
Yeah, I manhandled it too, just to see how it would go. Now that I have, I'll be using latex gloves, then washing my hands with them on before handling one of the other filters I bought (5). 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WxLover16 on July 31, 2018, 08:01:31 PM
I was noticing when putting on the filter that the rubbery substance around the sensor allows for what I think would be a good seal if the filter is smoothed off and the wire torqued down enough. At least you would avoid a sealant if it works.
Agreed.
 But I noticed 2 things while reading that link you provided.
1. touching the membrane can contaminate with oils and reduce the amount of humidity that can transfer to the sensor.  #-o Yeah.. I touched it... like  a lot. ](*,) I thought it was just a screen.
Yeah, I manhandled it too, just to see how it would go. Now that I have, I'll be using latex gloves, then washing my hands with them on before handling one of the other filters I bought (5).

So, I take it this sensirion SF2 filter is now all the craze? I might just have to get me one, or two.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 31, 2018, 08:13:48 PM
I was noticing when putting on the filter that the rubbery substance around the sensor allows for what I think would be a good seal if the filter is smoothed off and the wire torqued down enough. At least you would avoid a sealant if it works.
Agreed.
 But I noticed 2 things while reading that link you provided.
1. touching the membrane can contaminate with oils and reduce the amount of humidity that can transfer to the sensor.  #-o Yeah.. I touched it... like  a lot. ](*,) I thought it was just a screen.
Yeah, I manhandled it too, just to see how it would go. Now that I have, I'll be using latex gloves, then washing my hands with them on before handling one of the other filters I bought (5).

So, I take it this sensirion SF2 filter is now all the craze? I might just have to get me one, or two.
Well, it's certainly more protective than the stock filter, but certainly more difficult to mount properly. 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 31, 2018, 08:21:49 PM
I was noticing when putting on the filter that the rubbery substance around the sensor allows for what I think would be a good seal if the filter is smoothed off and the wire torqued down enough. At least you would avoid a sealant if it works.
Agreed.
 But I noticed 2 things while reading that link you provided.
1. touching the membrane can contaminate with oils and reduce the amount of humidity that can transfer to the sensor.  #-o Yeah.. I touched it... like  a lot. ](*,) I thought it was just a screen.
Yeah, I manhandled it too, just to see how it would go. Now that I have, I'll be using latex gloves, then washing my hands with them on before handling one of the other filters I bought (5).

So, I take it this sensirion SF2 filter is now all the craze? I might just have to get me one, or two.
Well, it's certainly more protective than the stock filter, but certainly more difficult to mount properly.

Ain't that the truth.
I'm wondering if i need to re-mount a new one now.... and then I wonder if I should just put the stock cover back on it and be done with it, because it's not worth trouble.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on July 31, 2018, 08:26:15 PM
I was noticing when putting on the filter that the rubbery substance around the sensor allows for what I think would be a good seal if the filter is smoothed off and the wire torqued down enough. At least you would avoid a sealant if it works.
Agreed.
 But I noticed 2 things while reading that link you provided.
1. touching the membrane can contaminate with oils and reduce the amount of humidity that can transfer to the sensor.  #-o Yeah.. I touched it... like  a lot. ](*,) I thought it was just a screen.
Yeah, I manhandled it too, just to see how it would go. Now that I have, I'll be using latex gloves, then washing my hands with them on before handling one of the other filters I bought (5).

So, I take it this sensirion SF2 filter is now all the craze? I might just have to get me one, or two.

All the cool kids are doing it now.  :-P
C'mon man, Put one on, then share your results er frustrations.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on July 31, 2018, 08:27:08 PM
Well for those of you who are interested in the 75 performance i have had it running as my main sensor for a few days not i i am well pleased with the overly readings i have gotten from it. Humidity wise it out does my 11 or 15 in response to rapid changes in temp and humidity with the humidity readings below 50% pretty much on where the 11 and 15 always ran 2 to 3% high. On the high end pretty even on readings but haven't had any heavy fog to see how high it will go but this am it was bouncing up to 98 and the 15 was a solid 97. The main thing is when the humidity went down the 75 dropped faster than the 15. The 15 does catch up to the 75 after some time. Now compared to the new 31 same results as the 15 on the top end and the 31 was about the same on the lower end. Had a good test of the sensors this pm when a thunderstorm came through and dumped some much needed rain. The temp was at 96 and humidity at 49 and the temp dropped to 72 and the humidity went up to 94% in about a half hour. The 75 humidity was running about 7% higher than the 15 and 31 with the temp running about 2 degrees higher. It took about 10 extra minutes for the 15 and 31 to get up to the 75 readings. I really like the 75 and it will be my main sensor for now.




Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: BCJKiwi on July 31, 2018, 11:56:24 PM
Mounting an SF1 or SF2

An Option would be to cut the legs off and sit them on top of the ~2mm thick protective coating on the Davis board.
Then place a coil spring on top of the SF1/SF2 and then fit the standard davis filter on top.

The Sensirion Filters have a circular protrusion on top and the underside of the top of the inner Davis filter carrier has rounded corners with a flat sentral portion so a coil spring should be reasonably centrally located.

The SF1 protrusion is 6.4mm dia and the SF2 is 5.05mm dia (according to the spec sheets).
So different inside diameters would be required depending on the filter in use. Suggest around 0.5mmm dia wire and a soft spring (a lot of turns) about 25 to 30mm long with ID to suit the filter. Note that on the SF2 there is very little flat area around the circular prootrusion do the Spring ID will need to be close to the 5.05 dia and preferably ground flat on the end. Have not tried this yet as am waiting on parts (SF2) and SHT15 - see below.

I also note there is very little difference in specification between the SHT15 and SHT7x. My VP2 came with an SHT11 so I have ordered a couple of SHT15s and will swap out the SHT11.
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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 01, 2018, 08:04:04 AM
Hit 97% on the 75 this morning despite BKN/OVC skies and no fog (just some light dew). I see jerryg is up to 99% RH this morning...maybe he snuck a 100% in there between Mesowest obs.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on August 01, 2018, 09:16:16 AM
Hit 97% on the 75 this morning despite BKN/OVC skies and no fog (just some light dew). I see jerryg is up to 99% RH this morning...maybe he snuck a 100% in there between Mesowest obs.

According to his website, it says the peak RH is 99%. The RW station (has SHT-15) hit 100%. Now I will say that give the 75 a few more high humidity events, it will probably trigger 100% more often.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on August 01, 2018, 10:13:00 AM
Well things got cleared up a little better this am, finally got some decent humidity lol. Now all three sensors were at 99%, the new 31 went first then the 75 went about 50 minutes later and the 2 yr old 31 that i put in place of the 15 went to 99 about an hour after the 75. The new 31 bounced to 100 a couple times. Now i still got the same results when the humidity started to drop, the 75 went first and the two 31's were about a half hour later before they dropped. It's been 50 minutes now and the 75 is still dropping at about 3% lower than the 31's.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on August 01, 2018, 10:27:33 AM
Update, one hour since 75 dropped and now the new 31 is 2% higher but the old 31 is 4% higher, wet bias showing up?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on August 01, 2018, 11:22:01 AM
All three are now in the same range of humidity, almost two hours to get there.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on August 01, 2018, 11:37:23 AM
I also note there is very little difference in specification between the SHT15 and SHT7x. My VP2 came with an SHT11 so I have ordered a couple of SHT15s and will swap out the SHT11.


And I just realized 2 days ago that I've never owned a SHT15. I went fro the 11 to 31. I'd at least like to try the 15 at some point.

As far attaching the SFx filter cap.... Hot Glue! Albeit, a very small amount on each side. Because there is relatively a "country mile" of room around the perimeter of the sensor, you wouldn't even need to put any under the cap. Just a drop on each side should lock it down.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 01, 2018, 12:19:19 PM
All three are now in the same range of humidity, almost two hours to get there.

For me this is a red flag because my 31's always run 5%+ above actual humidity (moist warm environment I assume you are in on the Gulf coast at 87°) even my new units do. And if they all are the same tells me the 75 isn't much better and running high. 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 01, 2018, 12:31:24 PM
All three are now in the same range of humidity, almost two hours to get there.

For me this is a red flag because my 31's always run 5%+ above actual humidity (moist warm environment I assume you are in on the Gulf coast at 87°) even my new units do. And if they all are the same tells me the 75 isn't much better and running high.
Didn't one of those 31s hit 100% this morning? Maybe they haven't experienced much drift yet?

Maybe he can run the data through a spreadsheet and find the average afternoon dewpoint (or RH) for each sensor. One observational point in time doesn't necessarily tell the story. A psychrometer reading would help too.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on August 01, 2018, 12:34:55 PM
Well need to keep in mind that the 75 and 31 are new and the two year old 31 has been cooked and hydrated so first time i have tried it out to see how it works. This is the first time since i have had the new sensors that we had a long period of high humidity and i don't know if just one time is enough to really get a feel for things. Right now i just like the faster response time of the 75 compared to the 31's. Heck maybe i lucked out and got a good 31 lol. :grin:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on August 01, 2018, 12:41:22 PM
All three are now in the same range of humidity, almost two hours to get there.

For me this is a red flag because my 31's always run 5%+ above actual humidity (moist warm environment I assume you are in on the Gulf coast at 87°) even my new units do. And if they all are the same tells me the 75 isn't much better and running high.

Jerryg DP is matching up fairly well with Port Lavaca Airport. Don’t know how far he is from it though. But his max DP is one degree higher than Port Lavaca.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 01, 2018, 12:41:58 PM
Yes, lets see if it stays near the 31 after saturation all day that would be my main concern and tells me it's just a senserion issue with all their sensors, some worse than others.   
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on August 01, 2018, 12:45:53 PM
Yes, lets see if it stays near the 31 after saturation all day that would be my main concern and tells me it's just a senserion issue with all their sensors, some worse than others.

What’s amazes me is how the RW stays very close to the airport (KVCT).
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on August 01, 2018, 01:10:17 PM
Just to inform you i am 10 miles se of kvct and 12 miles nw of port lavaca metar. Port lavaca station is located a few miles from lavaca bay and my readings very seldom agree because i am inland from the bay and on the other side i generally run wetter than kvct being in the country over grass and not over gravel. The rw station being on the roof of a building matches the runway and gravel of kvct lol. So i am on an island by myself with the closest pws about 7 miles away, i have to run my own comparisons lol. Another thing kvct is at 115 feet and port lavaca is at 27 feet and i am at 59 feet. I have always found it strange that the metar near the bay always has lower humidity readings than most, strange being near that much water. Of course the temps are higher due to the fact they get the warm air currents off of the water and keeps their temps up at night and cooler in the daytime. I don't get any cooling from the bay until the sea breeze kicks in and then the temp drops in the after noon and the humidity goes up, makes it real hard to get a handle on things. Sometimes the sea breeze doesn't make it to kvct and i have a big temp and humidity difference.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on August 01, 2018, 01:38:57 PM
Just to inform you i am 10 miles se of kvct and 12 miles nw of port lavaca metar. Port lavaca station is located a few miles from lavaca bay and my readings very seldom agree because i am inland from the bay and on the other side i generally run wetter than kvct being in the country over grass and not over gravel. The rw station being on the roof of a building matches the runway and gravel of kvct lol. So i am on an island by myself with the closest pws about 7 miles away, i have to run my own comparisons lol. Another thing kvct is at 115 feet and port lavaca is at 27 feet and i am at 59 feet. I have always found it strange that the metar near the bay always has lower humidity readings than most, strange being near that much water. Of course the temps are higher due to the fact they get the warm air currents off of the water and keeps their temps up at night and cooler in the daytime. I don't get any cooling from the bay until the sea breeze kicks in and then the temp drops in the after noon and the humidity goes up, makes it real hard to get a handle on things. Sometimes the sea breeze doesn't make it to kvct and i have a big temp and humidity difference.

That makes good sense. Elevation can also play a role too when it comes to DP.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on August 01, 2018, 01:47:55 PM
If I were jerryg and trying to compare to a good station, I might pick the RAWS right next to KVCT. RAWS stations have their temp/humidity sensor replaced annually. I checked the overdue maintenance report for RAWS stations and it wasn't on it. So it should be accurate (+/- 2% humidity), assuming good siting.

On a sunny hot day with not much wind, the dew point is going to bounce around. That's especially true in a FARS and even truer if there's a sensor with a small thermal mass like the SHT75 placed within that FARS. So it's best to take an average of several samples and compare that to the reference weather station from the same time. Averaging is less important when its cloudy and windy.

I looked around 11AM (CDT) and took a 5 minute average of Jerry's DPs. They ranged from 73 to 77. Avg was 74.8. Then I looked at the 11:04AM report from VCRT2 (https://www.wrh.noaa.gov/mesowest/timeseries.php?sid=VCRT2&num=48&banner=gmap&raw=0&w=325 (https://www.wrh.noaa.gov/mesowest/timeseries.php?sid=VCRT2&num=48&banner=gmap&raw=0&w=325)) and it was reporting 72. That's about a 3 degree difference. The KVCT airport was also reporting a DP of 72 as of 10:51AM.

Those readings are a bit concerning, even if they are about 9 miles away. Something to keep an eye on.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on August 01, 2018, 02:06:16 PM
Just to inform you i am 10 miles se of kvct and 12 miles nw of port lavaca metar. Port lavaca station is located a few miles from lavaca bay and my readings very seldom agree because i am inland from the bay and on the other side i generally run wetter than kvct being in the country over grass and not over gravel. The rw station being on the roof of a building matches the runway and gravel of kvct lol. So i am on an island by myself with the closest pws about 7 miles away, i have to run my own comparisons lol. Another thing kvct is at 115 feet and port lavaca is at 27 feet and i am at 59 feet. I have always found it strange that the metar near the bay always has lower humidity readings than most, strange being near that much water. Of course the temps are higher due to the fact they get the warm air currents off of the water and keeps their temps up at night and cooler in the daytime. I don't get any cooling from the bay until the sea breeze kicks in and then the temp drops in the after noon and the humidity goes up, makes it real hard to get a handle on things. Sometimes the sea breeze doesn't make it to kvct and i have a big temp and humidity difference.

That makes good sense. Elevation can also play a role too when it comes to DP.

Dew point changes with elevation at a rate of about 1F per 1000ft, generally.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on August 01, 2018, 02:51:24 PM
here is a sample of reports from victoria and port lavaca metars and mine at 1 pm victoria temp 93 dp 71 h 48  port lavaca temp 92 dp 65 h 41 and mine temp 91 dp 72 h 54 That is a lot of variations to try to compare to my dp was pretty close to victoria but way off from port lavaca just too many variations to try and compare being i am half way in between the two. I just have to trust that my 29 dollar sensor is doing as good as it can without some type of calibrated instruments to compare to on site. Heck for my site i am right and they are wrong  :lol:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 01, 2018, 03:41:17 PM
here is a sample of reports from victoria and port lavaca metars and mine at 1 pm victoria temp 93 dp 71 h 48  port lavaca temp 92 dp 65 h 41 and mine temp 91 dp 72 h 54 That is a lot of variations to try to compare to my dp was pretty close to victoria but way off from port lavaca just too many variations to try and compare being i am half way in between the two. I just have to trust that my 29 dollar sensor is doing as good as it can without some type of calibrated instruments to compare to on site. Heck for my site i am right and they are wrong  :lol:
The 65 at PKV was very brief.

https://mesowest.utah.edu/cgi-bin/droman/meso_base_dyn.cgi?stn=KPKV&unit=0&timetype=LOCAL

If you are running 1-3F higher on dewpoint you basically just have to decide if you are happy with it. Either you think the higher values physically make sense (foliage vs a rocky/dirt ASOS) or you feel there’s an instrumentational error there that you can live with. I think the psychrometer is really the only way to tell. I would measure it at your place, take a drive to one of the ASOS stations at the top of an hour and measure there, and then head home and do another measurement. I run a little higher than ASOS too, but I’d say the majority of the difference meteorologically makes sense given my lighter winds and moist ground. Some of it probably is simply sensor error and I’m OK with that as long as my temperature readings are on par. My old SHT31 was only peaking at 93% this summer. The new SHT75 has already hit 97% in 2 nights. So the old 31 definitely saw some drift.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 01, 2018, 03:49:03 PM
Current dewpoints

Jerry 1944z 67.8°
PKV 1940z 66.9°
VCT 1940z 68°
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on August 01, 2018, 04:50:42 PM
Jerry’s 75 has a lower DP than the RW station.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on August 01, 2018, 05:09:02 PM
Current dewpoints

Jerry 1944z 67.8°
PKV 1940z 66.9°
VCT 1940z 68°

You can't just take one instantaneous reading from CWOP and compare it to airports several minutes ago. When I checked earlier, it was stagnant & sunny and his DPs were bouncing around by several degrees even within a matter of seconds. He has an SHT75 loaded into a FARS. It's like a caged monkey! You have to average several of his readings. That means sitting on his website for several minutes, recording at least a couple minutes of DP readings and then deriving an average.

Of course, comparing to reference stations is only a possible indication of a problem, not confirmation. That would require a calibrated device.





Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 01, 2018, 05:22:32 PM
Current dewpoints

Jerry 1944z 67.8°
PKV 1940z 66.9°
VCT 1940z 68°

You can't just take one instantaneous reading from CWOP and compare it to airports several minutes ago. When I checked earlier, it was stagnant & sunny and his DPs were bouncing around by several degrees even within a matter of seconds. He has an SHT75 loaded into a FARS. It's like a caged monkey! You have to average several of his readings. That means sitting on his website for several minutes, recording at least a couple minutes of DP readings and then deriving an average.

Of course, comparing to reference stations is only a possible indication of a problem, not confirmation. That would require a calibrated device.
I know. I made that same point earlier...

https://www.wxforum.net/index.php?topic=34658.msg355192#msg355192

What I was getting at is that they can all be different one hour and then lined up the next. Sorry, for not clarifying what I was getting at by posting the dewpoints.  :-)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on August 01, 2018, 05:36:31 PM
Current dewpoints

Jerry 1944z 67.8°
PKV 1940z 66.9°
VCT 1940z 68°

You can't just take one instantaneous reading from CWOP and compare it to airports several minutes ago. When I checked earlier, it was stagnant & sunny and his DPs were bouncing around by several degrees even within a matter of seconds. He has an SHT75 loaded into a FARS. It's like a caged monkey! You have to average several of his readings. That means sitting on his website for several minutes, recording at least a couple minutes of DP readings and then deriving an average.

Of course, comparing to reference stations is only a possible indication of a problem, not confirmation. That would require a calibrated device.
I know. I made that same point earlier...

https://www.wxforum.net/index.php?topic=34658.msg355192#msg355192

What I was getting at is that they can all be different one hour and then lined up the next. Sorry, for not clarifying what I was getting at by posting the dewpoints.  :-)

Ah, somehow I missed that post. Yes, that's a great idea. Get an average of dewpoints for an afternoon from one or more reference station(s) and then an average of DPs of the station in question. I could see that, due to sensor or micro-scale weather differences, one hour's readings could align and another's be significantly different. The afternoon average better tells the story.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on August 01, 2018, 05:49:18 PM
As of 4:47p Jerry is reporting 96/59-60 and the RW station is reporting 96/65
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 01, 2018, 06:03:36 PM
You can do a station to station compare.

https://weather.gladstonefamily.net/qchart/AT358?date=20180801&addnl=KVCT&Add+to+charts=Add+to+charts&.cgifields=addnl


At the bottom is nearby sites select what you want and add to charts.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on August 01, 2018, 08:17:18 PM


He has an SHT75 loaded into a FARS. It's like a caged monkey!
:lol:
And I have a SHT11 loaded in passive shield in the shade... It's like a Three-toed Sloth on Xanax. :grin:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on August 01, 2018, 08:43:24 PM
Well as of an hour ago. I have a new set-up. Yeah I got rid of the SHT11 after a couple days of data.

St.#1 --SHT31  //   24 hr FARS  //  mostly in sun  // latest ver. ISS running raw (So no -0.9 deg offset needed)

St.#2 --SHT31 // Passive shield // mostly shaded // older ver. ISS running with the suggested -0.9 deg offset.

 Also St.# 2 is an older SHT31 that I baked and re-hydrated Monday.

I will record and report what I see. Several different variables going here, but it should still be interesting data.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on August 01, 2018, 11:26:05 PM
here is a sample of reports from victoria and port lavaca metars and mine at 1 pm victoria temp 93 dp 71 h 48  port lavaca temp 92 dp 65 h 41 and mine temp 91 dp 72 h 54 That is a lot of variations to try to compare to my dp was pretty close to victoria but way off from port lavaca just too many variations to try and compare being i am half way in between the two. I just have to trust that my 29 dollar sensor is doing as good as it can without some type of calibrated instruments to compare to on site. Heck for my site i am right and they are wrong  :lol:

How did the 31 and 15 compared against the 75?   Your 75 was reporting DPs lower than the metars and the RW station at times during this afternoon
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: galfert on August 02, 2018, 06:17:54 AM
In reference to the SF2 mounting issues...

Why not use Closedcube Breakout SHT31 solution that includes SHT31 in small PCB with mounting holes for the SF2 filter, and includes the SF2 filter. Then all you need to do is solder the 4 leads.

Closedcube makes a very small SHT31 breakout and they have a bit larger one they call PRO. I'm not sure what the difference is. Also there is a D (digital) version and an A (Analog) version. Has anyone determined if Davis uses the Analog or the Digital?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on August 02, 2018, 07:09:28 AM
In reference to the SF2 mounting issues...

Why not use Closedcube Breakout SHT31 solution that includes SHT31 in small PCB with mounting holes for the SF2 filter, and includes the SF2 filter. Then all you need to do is solder the 4 leads.

Closedcube makes a very small SHT31 breakout and they have a bit larger one they call PRO. I'm not sure what the difference is. Also their is a D (digital) version and an A (Analog) version. Has anyone determined if Davis uses the Analog or the Digital?
Then you have the issue of finding a way to mount it to the FARS mounting plate or the VP2 Passive shield mount. So either way there is some customizing going on .. not to mention, now you gotta pull out the soldering iron...and then encapsulation/epoxy/silicone.
However, this may be the perfect solution for the Meteoshield, and for that matter, the 7714 since there is no specific mount for the Davis sensor in these to begin with.
I am too Interested in what version (digital or Analog) works with the VP2 protocol.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on August 02, 2018, 07:33:39 AM
You need to keep in mind that the break out is i2c formatted and will not work with Davis iss.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: galfert on August 02, 2018, 07:40:14 AM
You need to keep in mind that the break out is i2c formatted and will not work with Davis iss.

Maybe the Digital version is the i2c and the Analog is the one for Davis.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 02, 2018, 08:29:03 AM
You need to keep in mind that the break out is i2c formatted and will not work with Davis iss.

Maybe the Digital version is the i2c and the Analog is the one for Davis.
The Davis uses the SHT31-LSS
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jas340 on August 02, 2018, 11:22:17 PM
FWIW my Davis VP2 is just 2 months old. It has been running about 5 degrees wet since day 1. My Vue also ran wet. Until this thread I though I was the only one. I almost got rid of my Vaisala WXT because it did not match the humidity of the VP2!!!
It's good to know I'm not alone however I feel kind of taken. For years everyone has raved about Davis quality.

The rain gauge is no where near accurate if the wind is blowing. The anemometer not reliable under 2mph. Both are these are just nature of the beast due to design limitations and are really not a problem.

There is no excuse for the SHT-31 issue. It is a $3.50 sensor. How about spending $7.00 wholesale on a much better sensor?????

 :oops: :oops: :oops: :oops: :oops: :oops: :oops: :oops: :oops: :oops: #-o #-o #-o #-o #-o #-o #-o #-o #-o #-o

Edit: I made a total noob mistake and disparaged the Davis brand even though my stations humidity is spot on with 2 reference stations. I am using my old Vue console with the new VP2 and decided to input a -4 degree calibration offset for humidity. Low and behold I had a +4 offset already in there. Zeroed it out and now I am spot on. I don't remember ever putting an offset in and I've had the console 7 years or so. In conclusion, my bad. I do live in a semi-arid climate etc. but am glad the sensor is accurate when new (in my case).
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on August 03, 2018, 02:21:44 PM
I got my Barani pro passive shield in a while ago and i got it up and running for test and so far it is working great in a short test time. The wind has been calm to light so far and with it calm the pro has been running .5f lower than the fars and when the wind has been up around 4 mph or more it has been running about 1 degree lower. I sure hope this proves out in the long term i will use it and get rid of the fan problem and maybe help the sensor with the high humidity problem.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on August 03, 2018, 02:44:15 PM
I got my Barani pro passive shield in a while ago and i got it up and running for test and so far it is working great in a short test time. The wind has been calm to light so far and with it calm the pro has been running .5f lower than the fars and when the wind has been up around 4 mph or more it has been running about 1 degree lower. I sure hope this proves out in the long term i will use it and get rid of the fan problem and maybe help the sensor with the high humidity problem.

Please keep us up to date.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: galfert on August 03, 2018, 02:59:05 PM
You need to keep in mind that the break out is i2c formatted and will not work with Davis iss.

Maybe the Digital version is the i2c and the Analog is the one for Davis.
The Davis uses the SHT31-LSS

So does that mean LSS version is not the Analog version? I read that LSS is a legacy version. So are three 3 versions of the SH31 or just 2?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on August 03, 2018, 03:00:38 PM
I just moved the pro over to the main station, did not want to hook it up until i could see that it was going to work as advertised. Now is am going to keep an eye on it and my other fars that runs even in readings with it. When i reversed the sensor cable the results where still the same so it has nothing to do with one fars verses the other. As long as the pro shield runs no worse in temp than the fars i will be happy. Just the thought of not having to mess with a powered shield makes me giddy lol.Right now i am keeping an eye on the humidity readings too. Tonight will really be interesting when the humidity starts to go up and how the fars and the pro compare.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on August 03, 2018, 03:02:46 PM
There are 3 version of the sensor one is analog the other is digital with i2c format and the third is what Davis uses the LSS chip which is also digital just a different format.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on August 04, 2018, 05:53:59 PM
Well, as much as I hate to say this, looks like I'll be jumping on the "bash the 31" wet bias bandwagon. After weeks of scrutinizing even more than usual, I've come to the conclusion that I'm suffering the same symptoms, maybe just not quite as bad as everyone else. As a generality, I'm running 3-4F in dew higher than my "official buddies". The thing is, neighboring stations tend to run wet also, so makes me dubious, but yes, most are Davis's. My 11 and 15 behaved the same way, which also makes me wonder. I guess I'll dial back the humidity 2% and see how that looks for now. Crap...
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 04, 2018, 06:39:42 PM
Well, as much as I hate to say this, looks like I'll be jumping on the "bash the 31" wet bias bandwagon. After weeks of scrutinizing even more than usual, I've come to the conclusion that I'm suffering the same symptoms, maybe just not quite as bad as everyone else. As a generality, I'm running 3-4F in dew higher than my "official buddies". The thing is, neighboring stations tend to run wet also, so makes me dubious, but yes, most are Davis's. My 11 and 15 behaved the same way, which also makes me wonder. I guess I'll dial back the humidity 2% and see how that looks for now. Crap...

Welcome aboard and about time.
I can confirm Davis has made shipping changes but I'm skeptical that's the issue. Haven't figured out yet because the Acurite SHT31 version doesn't display the same high bias.  I'm also testing on my NEW just built 2 days ago outdoor shaded porch and it's running close to 4-5% lower humidity all the time except in the morning dew when they all come together around 97% today.  It's not the LCC version however and isn't embedded into plastic like the Davis either.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on August 04, 2018, 06:59:06 PM
Well, as much as I hate to say this, looks like I'll be jumping on the "bash the 31" wet bias bandwagon. After weeks of scrutinizing even more than usual, I've come to the conclusion that I'm suffering the same symptoms, maybe just not quite as bad as everyone else. As a generality, I'm running 3-4F in dew higher than my "official buddies". The thing is, neighboring stations tend to run wet also, so makes me dubious, but yes, most are Davis's. My 11 and 15 behaved the same way, which also makes me wonder. I guess I'll dial back the humidity 2% and see how that looks for now. Crap...
Welcome aboard and about time.
Didn't want to proclaim the sky was falling until I truly gave my particular situation a thorough evaluation. Perhaps being in a drier climate does help mitigate the bias, but from my perspective, it's there. Taking out 2% has me perfectly in-line.... for now..... ](*,)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 04, 2018, 09:00:08 PM
69.1°F with 98% RH already and clear skies at 9pm EDT. This should be a good night to see if the new SHT75 can hit 99% or 100%.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on August 04, 2018, 09:58:13 PM
69.1°F with 98% RH already and clear skies at 9pm EDT. This should be a good night to see if the new SHT75 can hit 99% or 100%.

I do know that Jerry has hit 99% with his 75

Noticed that one of the Airport stations has already reached 100%. I’m sure that’s due to the calculation made from the dewpoint sensor.

But I’ll be watching your station tonight
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on August 04, 2018, 09:59:39 PM
69.1°F with 98% RH already and clear skies at 9pm EDT. This should be a good night to see if the new SHT75 can hit 99% or 100%.

Sounds like some really FROGGY weather...you down in the Louisiana bayous?

He is from New Hamphshire. Here is his station...

https://mesowest.utah.edu/cgi-bin/droman/meso_table_mesowest_m.cgi?stn=C7324&unit=0&time=&hours=24&hour1=01&day1=05&month1=08&year1=2018&radius=25&past=0&order=1
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 04, 2018, 10:24:01 PM
With the humidity we've had this summer it has certainly felt more like LA than NH. ;)

We've had about 7" of rain the past 2 weeks. The water table is high (despite D1 drought status) and we have mushrooms growing everywhere. When the wind is under 3mph, the moisture builds up and pools near the sfc. I see quick temp drops and dewpoint rises with the 75 and FARS when this occurs. When the wind picks up (5-10mph+) the slightly drier air mixes down and the T/Td respond in the other direction. We usually have dew forming on the lawn 2-3hrs before sunset when we get into these wet, stagnant airmasses.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on August 05, 2018, 12:39:34 AM
Well, as much as I hate to say this, looks like I'll be jumping on the "bash the 31" wet bias bandwagon. After weeks of scrutinizing even more than usual, I've come to the conclusion that I'm suffering the same symptoms, maybe just not quite as bad as everyone else. As a generality, I'm running 3-4F in dew higher than my "official buddies". The thing is, neighboring stations tend to run wet also, so makes me dubious, but yes, most are Davis's. My 11 and 15 behaved the same way, which also makes me wonder. I guess I'll dial back the humidity 2% and see how that looks for now. Crap...

Welcome aboard and about time.
I can confirm Davis has made shipping changes but I'm skeptical that's the issue. Haven't figured out yet because the Acurite SHT31 version doesn't display the same high bias.  I'm also testing on my NEW just built 2 days ago outdoor shaded porch and it's running close to 4-5% lower humidity all the time except in the morning dew when they all come together around 97% today.  It's not the LCC version however and isn't embedded into plastic like the Davis either.

I’m seeing the same thing with the Acurite SHT31. It’s really very accurate from what I’m seeing.  The humidity doesn’t seem to have any bias. 4-5% lower than the Davis 31 outside the high humidity range is about what I’m seeing and that’s right on with the psychro dyne. I’m not sure what Acurite has figured out that Davis hasn’t. It’s frustrating and promising all at the same time.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on August 05, 2018, 01:13:46 AM
Well, as much as I hate to say this, looks like I'll be jumping on the "bash the 31" wet bias bandwagon. After weeks of scrutinizing even more than usual, I've come to the conclusion that I'm suffering the same symptoms, maybe just not quite as bad as everyone else. As a generality, I'm running 3-4F in dew higher than my "official buddies". The thing is, neighboring stations tend to run wet also, so makes me dubious, but yes, most are Davis's. My 11 and 15 behaved the same way, which also makes me wonder. I guess I'll dial back the humidity 2% and see how that looks for now. Crap...

Welcome aboard and about time.
I can confirm Davis has made shipping changes but I'm skeptical that's the issue. Haven't figured out yet because the Acurite SHT31 version doesn't display the same high bias.  I'm also testing on my NEW just built 2 days ago outdoor shaded porch and it's running close to 4-5% lower humidity all the time except in the morning dew when they all come together around 97% today.  It's not the LCC version however and isn't embedded into plastic like the Davis either.
It’s frustrating and promising all at the same time.
Yes.
The frustrating part is how long will the frustrating part last?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 05, 2018, 07:29:43 AM
Well we did it. We had a bit of fog this morning, but nothing too thick. You could see a hint of blue sky through it at sunrise. The 75 showed 99% RH beginning around midnight and about a half hour ago it started bouncing around between 99% and the elusive 100%. It seems to have conveniently missed the 5 min Mesowest obs though so you won’t see it there. It’s fluctuating between 98% and 99% now.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 05, 2018, 07:59:16 AM
Here's my 75 before I sealed it with the silicone conformal coating.

 [ You are not allowed to view attachments ]
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 05, 2018, 08:13:27 AM
Here's my 75 before I sealed it with the silicone conformal coating.

 [ You are not allowed to view attachments ]

So are there any how to make these 75's with clear precise instructions?. I've tried to follow but usually give up.  :-(
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 05, 2018, 08:33:13 AM
Here's my 75 before I sealed it with the silicone conformal coating.

 [ You are not allowed to view attachments ]

So are there any how to make these 75's with clear precise instructions?. I've tried to follow but usually give up.  :-(
I just cut the sensor off of an old Davis temp/hum. You can see how the wiring goes in my pic. The connector I used required no soldering although you could get a connector with pins that you could solder the wires to. I just make a clean snip of the wires and insert them into the connector and then gently close it to make the connection. The 75 already comes with the 10K pull up resistor between pins 1 and 4 so you don’t need to worry about that. I sealed it with coating from that little bridge to the sensor down to the connector just to be on the safe side. The Davis cord is 6 wires, but the red and black don’t get used so you can just cut them away.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bobvelle on August 05, 2018, 08:33:37 AM
OK, I like this the best.

Hot glue on three sides. Yeah, three sides is probably overkill, 2 sides is plenty, but I had the gun warmed-up and was on a role  :-)

The melted area on the SF2 top edge was not from the hot glue. It was a failed attempt to melt a stainless wire to it (NOT while on the sensor board) Obviously it did not work. The material is not a typical plastic.
Anyway , just thought I'd share.

 [ You are not allowed to view attachments ]
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 05, 2018, 08:39:27 AM
Here's my 75 before I sealed it with the silicone conformal coating.

 [ You are not allowed to view attachments ]

So are there any how to make these 75's with clear precise instructions?. I've tried to follow but usually give up.  :-(
I just cut the sensor off of an old Davis temp/hum. You can see how the wiring goes in my pic. The connector I used required no soldering although you could get a connector with pins that you could solder the wires to. I just make a clean snip of the wires and insert them into the connector and then gently close it to make the connection. The 75 already comes with the 10K pull up resistor between pins 1 and 4 so you don’t need to worry about that. I sealed it with coating from that little bridge to the sensor down to the connector just to be on the safe side. The Davis cord is 6 wires, but the red and black don’t get used so you can just cut them away.

Okay that seems simple enough.
Do you have a link to connector, I'll use one of my old sensor cables.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 05, 2018, 09:06:30 AM
http://www.newark.com/webapp/wcs/stores/servlet/ProductDisplay?catalogId=10001&langId=-1&urlRequestType=Base&partNumber=53X2803&storeId=10194

I'd get a few. As openvista and I realized, they can be easy to break when you're first getting used to using them.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 05, 2018, 09:44:49 AM
http://www.newark.com/webapp/wcs/stores/servlet/ProductDisplay?catalogId=10001&langId=-1&urlRequestType=Base&partNumber=53X2803&storeId=10194

I'd get a few. As openvista and I realized, they can be easy to break when you're first getting used to using them.

Thanks I'll make up a couple and have ready for next summer for sure. I plan on continuing with the 31's this winter but may change mind if I find the 75's accurate enough as temperatures drop into -20 range. The 15's aren't great once temps go sub zero and specs show about the same is what worries me.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on August 05, 2018, 11:51:16 AM
http://www.newark.com/webapp/wcs/stores/servlet/ProductDisplay?catalogId=10001&langId=-1&urlRequestType=Base&partNumber=53X2803&storeId=10194

I'd get a few. As openvista and I realized, they can be easy to break when you're first getting used to using them.

Thanks I'll make up a couple and have ready for next summer for sure. I plan on continuing with the 31's this winter but may change mind if I find the 75's accurate enough as temperatures drop into -20 range. The 15's aren't great once temps go sub zero and specs show about the same is what worries me.

Do a test of the 75 for this winter and see how it does and let us know.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 05, 2018, 12:38:21 PM
I never noticed an issue with my old 75 in near 0F temps during the winter I used it. Unfortunately we never had temps frigid enough (-15F or colder) to really test it out.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 05, 2018, 01:04:00 PM
I never noticed an issue with my old 75 in near 0F temps during the winter I used it. Unfortunately we never had temps frigid enough (-15F or colder) to really test it out.

What were you comparing too? Only way to know is have a side by side comparison. You can't trust others around you, I can't even trust the ASOS which started running +2° warm March 2017 Madis even picked the +2° jump.
Our city council members have taken notice finally after my persistence and some including me are pushing to have KVTN ASOS removed off of city property.  It's actually dangerous we had multiple icing instances last winter and ASOS never reported ice formation because of warm bias.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 05, 2018, 01:29:11 PM
I never noticed an issue with my old 75 in near 0F temps during the winter I used it. Unfortunately we never had temps frigid enough (-15F or colder) to really test it out.

What were you comparing too? Only way to know is have a side by side comparison. You can't trust others around you, I can't even trust the ASOS which started running +2° warm March 2017 Madis even picked the +2° jump.
Our city council members have taken notice finally after my persistence and some including me are pushing to have KVTN ASOS removed off of city property.  It's actually dangerous we had multiple icing instances last winter and ASOS never reported ice formation because of warm bias.
I had a 6470 probe from my soil station in my old 7714 passive shield from my WMII days. It only read whole numbers, but it was usually +/- 1F on those winter mornings. And yeah, the normal biases that you get used to with local stations on a day-to-day basis were still on par on those cold mornings. I have nothing scientific to give you though...just empirical. CW7491 had it testing accurately at -8F IIRC.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 05, 2018, 01:49:51 PM
I'll definitely test it for everyone this winter in one of the shields. -26° was coldest last January, -24° at airport with +bias. 
Year before we were down to -32°F in town and station just east town now annexed the SHT31 recorded -36°.
I worry the sht75 may not be accurate at those temperatures. 
 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on August 05, 2018, 04:14:49 PM
I never noticed an issue with my old 75 in near 0F temps during the winter I used it. Unfortunately we never had temps frigid enough (-15F or colder) to really test it out.
Our city council members have taken notice finally after my persistence and some including me are pushing to have KVTN ASOS removed off of city property.  It's actually dangerous we had multiple icing instances last winter and ASOS never reported ice formation because of warm bias.
The primary purpose of any ASOS is obviously to serve those who fly in and out of the associated airport. I assume by moving it off city property will take it well off the airport proper, no?
Not saying there isn't a problem, but the bottom line is my old employer will have the final say of where and even if it gets moved. And if it does, it certainly won't be off the airport proper.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 05, 2018, 05:07:01 PM
Privately owned airport. Not sure who has the right to do what but if it was my property it would be gone.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on August 05, 2018, 05:59:19 PM
Privately owned airport. Not sure who has the right to do what but if it was my property it would be gone.
Well, I guess pilots could just rely on a windsock... ;)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 05, 2018, 06:13:29 PM
Doesn't the ASOS freezing rain sensor work independently from the other sensors? If it's sensing icing, it should report it whether the temp is 32F or 34F. There's probably some algorithm that overrides it once you get above 35F though.

NDOT has a whole arsenal of roadside weather stations. I would think they have road surface temperature and condition sensors (wet, icing, dry, etc) along with ptype.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 05, 2018, 06:26:50 PM
Privately owned airport. Not sure who has the right to do what but if it was my property it would be gone.
Well, I guess pilots could just rely on a windsock... ;)

Why not? This airport ASOS currently has instruments giving false data, I consider more dangerous than a simple windsock.  A council member who also runs the weather coop is fed up too. We've been discussing this for a year now.
If it was up to me it would be gone already especially after last winter with multiple icing conditions missed by ASOS.
 
They know about it, both of us have reported the issue multiple times. It's ongoing they ignore it so next step is getting city involved.

Not sure about the freezing rain sensor if it does it failed too.

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on August 05, 2018, 06:39:51 PM
Privately owned airport. Not sure who has the right to do what but if it was my property it would be gone.
Well, I guess pilots could just rely on a windsock... ;)
It's ongoing they ignore it so next step is getting city involved.
Who's they? I know the city owns the airport, now the question is who maintains the ASOS? If it's the city, then you're obviously on the right track.
If I were you and you're still not getting satisfaction, I'd give these guys a call and tell them you're concerns. If they can't help, they should be able to tell you who can. GL!
https://www.faa.gov/about/office_org/field_offices/fsdo/?state=NE
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 05, 2018, 06:43:44 PM
Privately owned airport. Not sure who has the right to do what but if it was my property it would be gone.
Well, I guess pilots could just rely on a windsock... ;)
It's ongoing they ignore it so next step is getting city involved.
Who's they? I know the city owns the airport, now the question is who maintains the ASOS? If it's the city, then you're obviously on the right track.
If I were you and you're still not getting satisfaction, I'd give these guys a call and tell them you're concerns. If they can't help, they should be able to tell you who can. GL!
https://www.faa.gov/about/office_org/field_offices/fsdo/?state=NE
Isn't VTN a first order station? Wouldn't that fall under the NWS?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 05, 2018, 06:45:42 PM
I may be wrong, but I think all ASOS stations up here are handled by NWS GYX. AWOS is maintained by FAA.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 05, 2018, 06:48:01 PM
Probably we just want it fixed but keeps hitting on deaf ears. Ice doesn't lie or get out of calibration so we know which instruments are off. (THE ASOS)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on August 05, 2018, 06:49:25 PM
Privately owned airport. Not sure who has the right to do what but if it was my property it would be gone.
Well, I guess pilots could just rely on a windsock... ;)
It's ongoing they ignore it so next step is getting city involved.
Who's they? I know the city owns the airport, now the question is who maintains the ASOS? If it's the city, then you're obviously on the right track.
If I were you and you're still not getting satisfaction, I'd give these guys a call and tell them you're concerns. If they can't help, they should be able to tell you who can. GL!
https://www.faa.gov/about/office_org/field_offices/fsdo/?state=NE
Isn't VTN a first order station?
I have no idea what a "first order station" is.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on August 05, 2018, 06:52:44 PM
I may be wrong, but I think all ASOS stations up here are handled by NWS GYX. AWOS is maintained by FAA.
The way I understand, ASOS's are NWS/FAA territory and AWOS's are airport management maintained. But as I said a while back on this matter, I'm sure one size doesn't fit all.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 05, 2018, 06:53:32 PM
Privately owned airport. Not sure who has the right to do what but if it was my property it would be gone.
Well, I guess pilots could just rely on a windsock... ;)
It's ongoing they ignore it so next step is getting city involved.
Who's they? I know the city owns the airport, now the question is who maintains the ASOS? If it's the city, then you're obviously on the right track.
If I were you and you're still not getting satisfaction, I'd give these guys a call and tell them you're concerns. If they can't help, they should be able to tell you who can. GL!
https://www.faa.gov/about/office_org/field_offices/fsdo/?state=NE
Isn't VTN a first order station?
I have no idea what a "first order station" is.
http://www.weather2000.com/1st_order_wbans.txt

Basically your long term climatological sites.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on August 05, 2018, 06:55:39 PM
Huh, news to me. Thanks!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 05, 2018, 07:01:42 PM


Basically your long term climatological sites.

And all of a sudden its +2° high. How surprising  :evil:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on August 05, 2018, 07:16:46 PM
I guess I'll dial back the humidity 2% and see how that looks for now. Crap...
Granted, the air mass has changed little, but taking out 2% has put my 31 in it's wheelhouse because the readings are tracking to the point of astonishment. :shock: Shows how good this sensor should be. Too bad this can't be over the entire 0-100% range.  ](*,)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: drew1021 on August 05, 2018, 07:20:09 PM
Our local ASOS went haywire many years ago and stayed that way for years resulting in a +2-3 degree error. Stood out like a sore thumb. Frustrated me to no end. Finally one day out of the blue it was fixed. I inquired about it to the forecast office for our region and was told that a construction project several years before had cut the sensor bundle and never was repaired properly. They had to run a new line. Wala!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 05, 2018, 07:21:06 PM
Who was they? They was forecast office primarily and emails to ASOS aomc@noaa.gov<aomc@noaa.gov>; all ignored.  Went back and looked at some old blogs:

03/04/2018 6:30am update: Fog (freezing) this morning with just below freezing temperatures both East and West side Valentine. ICE has formed on chain link fencing west Valentine with temperature between 31.1 to 31.5°. Airport visitors with ASOS thermometer still running 2 degrees warmer (33.8°) could be in for a surprise. Make sure you deice because the chain link doesn't lie.
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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 05, 2018, 07:31:19 PM
Our local ASOS went haywire many years ago and stayed that way for years resulting in a +2-3 degree error. Stood out like a sore thumb. Frustrated me to no end. Finally one day out of the blue it was fixed. I inquired about it to the forecast office for our region and was told that a construction project several years before had cut the sensor bundle and never was repaired properly. They had to run a new line. Wala!

Thanks for sharing.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on August 05, 2018, 07:54:05 PM
Wow how can you guys live in that kind of cold? Down here everyone panics and drips pipes and cover plants when the temp gets down to 32 lol. Normally we may get into the twenties a couple of times in a normal winter. I am amazed that the sensor perform at all at those temps. Well let me know this winter when it is getting that cold and i will send you a jar of hot air fresh from the outside.  :lol:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: galfert on August 05, 2018, 08:40:37 PM
Privately owned airport. Not sure who has the right to do what but if it was my property it would be gone.
Well, I guess pilots could just rely on a windsock... ;)
It's ongoing they ignore it so next step is getting city involved.
Who's they? I know the city owns the airport, now the question is who maintains the ASOS? If it's the city, then you're obviously on the right track.
If I were you and you're still not getting satisfaction, I'd give these guys a call and tell them you're concerns. If they can't help, they should be able to tell you who can. GL!
https://www.faa.gov/about/office_org/field_offices/fsdo/?state=NE

Thank you for this contact information. I'm going to contact the Orlando office of the FAA. I've been dealing with a bad ASOS at KORL Orlando Executive Airport reporting bad SLP. You can read what I've been dealing with here.
https://www.wxforum.net/index.php?topic=34821.0
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on August 05, 2018, 08:48:06 PM
I have always wondered what type of measuring equipment is used when they measure -75 to -100 are colder temps. Most home use temps don't go that low or perform well in the minus ranges. The record cold temp here was 8 degrees and the high was 111 degress, very rare to get into the 100's in the summer and near 20 in the winter so nearly all measuring devices work just fine.
the sht31 temp appears to really be the strong point of the sensor and the humidity the weak point, too bad, could really be a great sensor otherwise.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on August 05, 2018, 08:57:18 PM

...

the sht31 temp appears to really be the strong point of the sensor and the humidity the weak point, too bad, could really be a great sensor otherwise.

Yes the temperature sensor of the SHT31 is right on. Unfortunately, the humidity if off. Most people, myself included, desire accurate temperature vs. humidity accuracy. Too bad this sensor doesn’t capture both readings correctly and consistently.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on August 05, 2018, 10:23:29 PM
Privately owned airport. Not sure who has the right to do what but if it was my property it would be gone.
Well, I guess pilots could just rely on a windsock... ;)
It's ongoing they ignore it so next step is getting city involved.
Who's they? I know the city owns the airport, now the question is who maintains the ASOS? If it's the city, then you're obviously on the right track.
If I were you and you're still not getting satisfaction, I'd give these guys a call and tell them you're concerns. If they can't help, they should be able to tell you who can. GL!
https://www.faa.gov/about/office_org/field_offices/fsdo/?state=NE

Thank you for this contact information. I'm going to contact the Orlando office of the FAA. I've been dealing with a bad ASOS at KORL Orlando Executive Airport reporting bad SLP. You can read what I've been dealing with here.
https://www.wxforum.net/index.php?topic=34821.0
If I may, lemme give you a little piece of advice; you go in "guns a blazing", they'll cut you off like a gangrenous appendage and there ain't squat you can do about it other than call your congressional rep in DC, then GL with that. You go in with "hey, I may have some safety related stuff for you guys", you're more likely to get an audience that will hear you beef. Whether they act upon it or not will be determined by how credible you're deemed, so have your ducks in a row.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on August 05, 2018, 10:41:16 PM

...

the sht31 temp appears to really be the strong point of the sensor and the humidity the weak point, too bad, could really be a great sensor otherwise.

Yes the temperature sensor of the SHT31 is right on. Unfortunately, the humidity if off. Most people, myself included, desire accurate temperature vs. humidity accuracy. Too bad this sensor doesn’t capture both readings correctly and consistently.

I know I keep belaboring the point, but based on my experience with the home-brew SHT75 and especially the $17 Acurite SHT31, I’m not convinced this is a Sensirion issue as much as it’s a Davis issue.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on August 05, 2018, 11:01:43 PM

...

the sht31 temp appears to really be the strong point of the sensor and the humidity the weak point, too bad, could really be a great sensor otherwise.

Yes the temperature sensor of the SHT31 is right on. Unfortunately, the humidity if off. Most people, myself included, desire accurate temperature vs. humidity accuracy. Too bad this sensor doesn’t capture both readings correctly and consistently.

I know I keep belaboring the point, but based on my experience with the home-brew SHT75 and especially the $17 Acurite SHT31, I’m not convinced this is a Sensirion issue as much as it’s a Davis issue.

You may be right. At a minimum, Davis contributed to the problem with poor shipping procedures, etc.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 06, 2018, 04:34:27 AM
Have you guys been watching Jerry's new SHT75 dewpoint vs the airport?
Looks like to me the 75dp is running higher too. I would say on average 2° dp temperature. 
The ASOS is only 8 miles away and recorded high temperature +2° over the SHT75 in passive shield for high yesterday which also seems excessive considering wind was well mixed with average wind speed 14 mph during high temperature period. 
Correction Jerry posted another thread his high temp was 91. Uploads to cwop are spaced so missed the actual high.

Jerry   https://mesowest.utah.edu/cgi-bin/droman/meso_base_dyn.cgi?stn=AT358&unit=0&timetype=LOCAL
ASOS   https://mesowest.utah.edu/cgi-bin/droman/meso_base_dyn.cgi?stn=KVCT

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 06, 2018, 04:57:39 AM

...

the sht31 temp appears to really be the strong point of the sensor and the humidity the weak point, too bad, could really be a great sensor otherwise.

Yes the temperature sensor of the SHT31 is right on. Unfortunately, the humidity if off. Most people, myself included, desire accurate temperature vs. humidity accuracy. Too bad this sensor doesn’t capture both readings correctly and consistently.

I know I keep belaboring the point, but based on my experience with the home-brew SHT75 and especially the $17 Acurite SHT31, I’m not convinced this is a Sensirion issue as much as it’s a Davis issue.

So the 75 is running lower DP for sure?
Good to know I have a couple coming. Was wondering watching Jerry's.
The Acurite runs 5% lower humidity during the daytime and still manages to hit 98% in mornings like today when my almost new Davis SHT31 seems to be stuck at 97% now.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: drew1021 on August 06, 2018, 06:28:48 AM
Just a thought, I wonder if Davis is purchasing 31's that don't make the cut at a reduced price.
No proof, just wondering.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on August 06, 2018, 08:43:26 AM

...

the sht31 temp appears to really be the strong point of the sensor and the humidity the weak point, too bad, could really be a great sensor otherwise.

Yes the temperature sensor of the SHT31 is right on. Unfortunately, the humidity if off. Most people, myself included, desire accurate temperature vs. humidity accuracy. Too bad this sensor doesn’t capture both readings correctly and consistently.

I know I keep belaboring the point, but based on my experience with the home-brew SHT75 and especially the $17 Acurite SHT31, I’m not convinced this is a Sensirion issue as much as it’s a Davis issue.

So the 75 is running lower DP for sure?
Good to know I have a couple coming. Was wondering watching Jerry's.
The Acurite runs 5% lower humidity during the daytime and still manages to hit 98% in mornings like today when my almost new Davis SHT31 seems to be stuck at 97% now.

Yes, in my experience the 75 runs lower than the Davis 31. By about 1-2F in DP on average. That’s not to say there is no wet bias at all particularly after periods of prolonged high humidity as outlined in the data sheet. But it tends to “dry out” more quickly. The Acurite 31 seems to be superior to the 75 in the little bit of side by side I’ve done with the Davis 31. Consistently 3-5% lower RH as I think you’ve said you’ve seen. Temp is right on. Of course it hasn’t been exposed to prolonged periods of high humidity either.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 06, 2018, 10:16:16 AM
Have you guys been watching Jerry's new SHT75 dewpoint vs the airport?
Looks like to me the 75dp is running higher too. I would say on average 2° dp temperature. 
The ASOS is only 8 miles away and recorded high temperature +2° over the SHT75 in passive shield for high yesterday which also seems excessive considering wind was well mixed with average wind speed 14 mph during high temperature period. 
Correction Jerry posted another thread his high temp was 91. Uploads to cwop are spaced so missed the actual high.

Jerry   https://mesowest.utah.edu/cgi-bin/droman/meso_base_dyn.cgi?stn=AT358&unit=0&timetype=LOCAL
ASOS   https://mesowest.utah.edu/cgi-bin/droman/meso_base_dyn.cgi?stn=KVCT
I've been following him on surface maps for a few days and he seems to be pretty close to what you'd expect being between VCT and the more humid PKV. You can see his dewpoint fluctuate depending on the component of onshore flow when the moisture boundary is between those 2 METAR sites.

Man, you tell me not to compare to other stations and then you do it. (just kidding around) ;)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on August 06, 2018, 11:27:23 AM
Well i got a chance to test out a theory this morning, the sun made it out for a short period and the winds were light. I suspected for some time that in high humidity areas like mine the fan running at night gets the interior of the shield wet and causing a lag in temp and humidity readings in the am. When looking at the cwop info it showed the temp lagging rising in the am and the humidity slow to go down. I put the fars fan on the photocell set up i have so the fan was off at night and on at daylight. This morning the fars temp and humidity followed the two shields right off the bat, no delay at all. The temp was slightly higher and the humidity was two percent lower in the fars with the fan off all night but when the fan came on the temp and humidity read the same as the other two. Now if i get this result everytime it will show that the sensors are in a very damp chamber during the night and causing errors in the am until it dries out. This is a test, only a test lol.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 06, 2018, 01:22:02 PM
I notice with my 75 that with the high temperature response the dewpoint has a lot of variation as well...in fact more than I think it should be. I think the problem has to do with the update interval. The temp is updating every 10-12s while the RH is 50-60s. If I see a 1-2F swing in temp in a minute, the RH doesn't have time to "catch up" and sync with the temp.

It would be ideal if the RH updated exactly when the temperature does. Therefore, for any update in time you know that the RH measurement corresponds to the temp.

Many of you tech guys would know better than me...how reasonable would it be to have a firmware update that sent a new RH ob with every temp measurement? I know we've complained about the integer resolution with 2nd temp sensors before and how it's a byte issue.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 06, 2018, 02:35:21 PM
Cumulus has a setting to calculate dewpoint so any changes with temperature it should adjust. Can't confirm how often it adjust but it's there. 
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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 06, 2018, 03:14:25 PM
I can confirm with cumulus both DP and humidity adjust with temperature sometimes every couple seconds and doesn't wait for humidity change.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 06, 2018, 03:26:32 PM
I can confirm with cumulus both DP and humidity adjust with temperature sometimes every couple seconds and doesn't wait for humidity change.
Not to go OT, but I’ll have to look into Cumulus more. You confused me a bit there...with Cumulus, DP and RH change every few seconds, but doesn’t wait for an RH change? Does Cumulus have faster update intervals than the VP2 console? I think I once saw someone say here that you can get temperature in tenths resolution with temp/hum stations using it?

Now the slightly OT part..will it report wind in 0.1 mph resolution using a 3rd party anemometer that has the ability? The one I have coming in has 0.01m/s resolution capability so I’d love to get the max potential out of it.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 06, 2018, 04:17:30 PM
I may of said that wrong the DP changes with temperature every update couple seconds sometimes, because both are in tenths, Rh is changing less often. 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 06, 2018, 04:46:02 PM
I may of said that wrong the DP changes with temperature every update couple seconds sometimes, because both are in tenths, Rh is changing less often.
Ah okay. I get that with weatherlink. But therein lies the problem. If the RH holds at say 74% for me in a 1-min update, my temp can fluctuate 1-2F during the course of that minute and the dewpoint follows it given the "frozen" RH value.

What I would like to do is keep my 75 in the FARS for temp, but use a Davis 31 in a 2nd temp/hum station with my old 7714 shield. Then I would take the temp from my FARS/75 and dew from the passive/31 to calculate RH and post to my site/CWOP. I think I saw that idea mentioned here before and that Cumulus would report 2nd temp readings in tenths. Basically it would be like an ASOS setup...FARS for temp and then a separate passive shield for the dewpoint.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 06, 2018, 04:56:46 PM
I may of said that wrong the DP changes with temperature every update couple seconds sometimes, because both are in tenths, Rh is changing less often.
Ah okay. I get that with weatherlink. But therein lies the problem. If the RH holds at say 74% for me in a 1-min update, my temp can fluctuate 1-2F during the course of that minute and the dewpoint follows it given the "frozen" RH value.

What I would like to do is keep my 75 in the FARS for temp, but use a Davis 31 in a 2nd temp/hum station with my old 7714 shield. Then I would take the temp from my FARS/75 and dew from the passive/31 to calculate RH and post to my site/CWOP. I think I saw that idea mentioned here before and that Cumulus would report 2nd temp readings in tenths. Basically it would be like an ASOS setup...FARS for temp and then a separate passive shield for the dewpoint.

Interesting. I didn't realize Cumulus could do that.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 06, 2018, 05:28:41 PM
I may of said that wrong the DP changes with temperature every update couple seconds sometimes, because both are in tenths, Rh is changing less often.
Ah okay. I get that with weatherlink. But therein lies the problem. If the RH holds at say 74% for me in a 1-min update, my temp can fluctuate 1-2F during the course of that minute and the dewpoint follows it given the "frozen" RH value.

What I would like to do is keep my 75 in the FARS for temp, but use a Davis 31 in a 2nd temp/hum station with my old 7714 shield. Then I would take the temp from my FARS/75 and dew from the passive/31 to calculate RH and post to my site/CWOP. I think I saw that idea mentioned here before and that Cumulus would report 2nd temp readings in tenths. Basically it would be like an ASOS setup...FARS for temp and then a separate passive shield for the dewpoint.

Interesting. I didn't realize Cumulus could do that.
Scratch that. It's meteobridge/meteohub.

https://www.wxforum.net/index.php?topic=34658.msg354339#msg354339
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on August 07, 2018, 03:43:20 PM
Well eating crow today, in fact several of us because we were convinced the ASOS at KVTN was off. Council member/ Coop manager here in Valentine got permission to check ASOS ourselves and the unexpected happened. It was running it's usual +2°F above my station and coop so as drove out there was thinking "we have them now proof and will take pictures for evidence".

Took the certified thermometer out up and stuck probe into vaisala shield intake and and to my  #-o was dead on.  Spent 20 minutes waiting for any discrepancy and it was matching 5 minute average updates every time meanwhile in town it was 2° cooler still.
No conspiracy theory it's just warmer at airport. +2°
Icing last winter probably just didn't occur at airport. 

I feel better knowing.  I mentioned we may have a reverse of heat island going on because of intown foliage and vegetation.
 


In short, life is good again!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 07, 2018, 03:58:31 PM

 



In short, life is good again!

Much better   :-), the intake screen was 1/2 plugged also I'm heading back out to clean off.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on August 07, 2018, 05:16:49 PM
You didn't compare any dews while you were there???????????
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 07, 2018, 05:28:45 PM
You didn't compare any dews while you were there???????????

Negative on that.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: drew1021 on August 07, 2018, 07:10:47 PM
I would be suprised if it was a continuous 2 degree discrepancy. Even with well stirred air?
Also looks like a lot of metal around the sensor shield, could this effect the readings? Not trying to be negative, just my curiosity and observation.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 07, 2018, 07:21:00 PM
I would be suprised if it was a continuous 2 degree discrepancy. Even with well stirred air?
Also looks like a lot of metal around the sensor shield, could this effect the readings? Not trying to be negative, just my curiosity and observation.

Yes lots of metal right there. Ended up being +4° over in town. The coop guy thinks something is still wrong he's been running it 30+ years and claims airport was never been +4-5 degrees warmer and he is less than mile away.
All I can say I went out there twice and it was running +2° both times. If I left thermometer and see what it records vs ASOS I guess maybe next.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: drew1021 on August 07, 2018, 07:36:41 PM
It would be nice if you could place a remote setup next to the the ASOS for a few weeks just to see if any of discrepancies occur. Good luck with that idea.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 08, 2018, 09:38:52 AM
It would be nice if you could place a remote setup next to the the ASOS for a few weeks just to see if any of discrepancies occur. Good luck with that idea.

We may be doing a full day long test soon, just record high and low with portable certified thermometer. At least can see if the temperature spikes are real. Spot checking you may miss a issue because temps took off late 5:30-6:30 pm. One thing I noticed the air intake for blower seems rather low to ground, estimate about 4.3'.  Lower than my FARS intakes at home 5.0'.  I always thought between 5-6' was standard.
Next time I go out there I'll tape measure the intake.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 08, 2018, 10:21:24 AM
It would be nice if you could place a remote setup next to the the ASOS for a few weeks just to see if any of discrepancies occur. Good luck with that idea.

We may be doing a full day long test soon, just record high and low with portable certified thermometer. At least can see if the temperature spikes are real. Spot checking you may miss a issue because temps took off late 5:30-6:30 pm. One thing I noticed the air intake for blower seems rather low to ground, estimate about 4.3'.  Lower than my FARS intakes at home 5.0'.  I always thought between 5-6' was standard.
Next time I go out there I'll tape measure the intake.
Yeah...I think they generally run 1.5-2m so that would be a little low.

It wouldn't really explain 1-2F too high during low solar situations though. Something to keep in mind about the rime icing via FZFG though...the ground sfc and railings will be colder than the 2m temp on those radiational cooling nights. So, just like with a ground frost situation with a 2m temp of say 35F, you could have icing on the fence railings with the ASOS 2m temp showing 33F. It wouldn't explain why you, and other stations, were running 31-31.5F though.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: drew1021 on August 08, 2018, 11:29:17 AM
It would be nice if you could place a remote setup next to the the ASOS for a few weeks just to see if any of discrepancies occur. Good luck with that idea.

We may be doing a full day long test soon, just record high and low with portable certified thermometer. At least can see if the temperature spikes are real. Spot checking you may miss a issue because temps took off late 5:30-6:30 pm. One thing I noticed the air intake for blower seems rather low to ground, estimate about 4.3'.  Lower than my FARS intakes at home 5.0'.  I always thought between 5-6' was standard.
Next time I go out there I'll tape measure the intake.

Yeah I noticed that the intake looked a bit low. With that said if anything the nighttime temperatures would be cooler, especially on nights with maximum radiational cooling.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on August 08, 2018, 06:23:27 PM
Finally mounted my SF2 filter, left the Davis filter off. Response time and data looks fine, didn't really expect different.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 08, 2018, 07:06:23 PM
Results of ASOS test are in.   Sunny light winds.  Started test early to catch the low temperature. Uncanny how high temperature captured were exact. No explanation on low temperature difference.
ASOS High- 93.2°        Test NIST certified thermometer -93.23°
ASOS Low  -55.0°        Test    "              "       "            -53.51°



In town heavy foliage, Passive 7714 SHT31    High-91.7°        Low-53.3°
                                 Davis FARS   SHT31    High-91.3          Low-53.4°
 
Measured the blower inlet from ground exactly 5'0"
Kept thermometer in worm bait box.

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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on August 08, 2018, 07:17:08 PM
No explanation on low temperature difference.
If I missed this from earlier, I apologize, but is your probe inside the aspirated chamber while testing?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 08, 2018, 07:49:33 PM
I'll need a drawing exactly where sensor is located I measured at inlet screen. Not exactly sure where the sensor chamber is located but it has to be close.

Found this. Red arrow was where measurements taken on outside of screen.
This sucker is really pulling some air.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on August 08, 2018, 07:56:58 PM
No explanation on low temperature difference.
If I missed this from earlier, I apologize, but is your probe inside the aspirated chamber while testing?
This sucker is really pulling some air.
Was pretty sure they did. My old 67CFM doesn't look so bad now, does it? :-P
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on August 08, 2018, 09:01:11 PM
Talking about sucking in air the rm young shield i have has a squirrel cage type fan and it sucks up anywhere from 5 to 11 m/s and use a 1 amp 12 volt module to power it compared to the davis which is a puny .86 m/s. I can hear it all over the yard and it will ruin a sensor in no time in high humidity. I plan on using it for temp probe only for comparison with the other shields, it is just too air flow.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Evening thunder on August 10, 2018, 08:27:18 AM
I thought I’d share my experience here, this topic confirms my long-held suspicions of wet bias with Davis sensors (not just the SHT31). I've not read the entire thing though.

I'm in the UK, in a valley, where on many nights the humidity rises above 95% with radiation mist/fog quite common. My set up is a 24h FARS and an older passive ISS, running two SHT31 sensors (18 and 24 months old)

These two sensors both drifted at the top end within 6 months (and in opposite directions for a while), The first went from reaching 98-99% maximum to only 95-96% (thick fog). I thought it may be a dodgy sensor so bought the 2nd sensor, but that also drifted down while by summer 2017 the first sensor was back to reaching 98-99% lol.
Currently, they reach about 97-98%, with the 2nd sensor 1-2% wetter through the mid-range.

Concerning the mid-range, and yep.. they seem to have a wet bias. I upload to a website that allows an easy comparison between two stations. This is me compared to the nearby official Met Office station (5 miles away) yesterday (a cooler, fresher day than many this summer with a well-mixed environment until about 9pm).

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My dew point is typically 2-3C higher (x1.8 for difference in F) Of course these are only hourly observation points so brief fluctuations could skew the graph, however this is a typical story.

Humidity for August so far

 [ You are not allowed to view attachments ]

As you can see I'm usually approaching 10% more humid. We are at virtually the same elevation with similar surroundings and I can’t see any genuine reasons for this when the wind is in the direction that it was yesterday. It does seem with me at least the problem is not just with warm humid airmasses.

Not happy with that but what can I do.. I don't really have a controlled environment to recondition sensors (and if they have to be removed from the Davis board? that's a no-go for me anyway).
Nearby Davis stations do read similar to me but probably have the same issue.


Also, a brief history of the previous sensors I've used in case it's of interest:

My 1st SHT11 failed in August 2009, I think it had less of a wet bias, but only reached 95-96%.
The 2nd SHT11 had a significant wet bias in the mid-range, and reached 98-99%.
I had a SHT75 'Davis ready' sensor from a seller on Ebay. That possibly had little bias but it only reached about 95% so think I only used it in the old/backup ISS.
I also had a SHT15 (3rd party), and it reached 97-98%. It still had some degree of wet-bias.

So as you can see wet biased mid-range and no 100% readings seems to have been a general theme for me.

I still have the 15 and 75 lying around so am going to plug them into my backup ISS and see what they say compared to the SHT31.

I still like the SHT31 for temperature and prefer its humidity response to the 11's I used. If a stable 'corrected' sensor came out I'd buy that, although my records won't be consistent but they aren't anyway with those sensor changes and drift lol.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on August 10, 2018, 09:27:33 AM
I thought I’d share my experience here, this topic confirms my long-held suspicions of wet bias with Davis sensors (not just the SHT31). I've not read the entire thing though.

I'm in the UK, in a valley, where on many nights the humidity rises above 95% with radiation mist/fog quite common. My set up is a 24h FARS and an older passive ISS, running two SHT31 sensors (18 and 24 months old)

These two sensors both drifted at the top end within 6 months (and in opposite directions for a while), The first went from reaching 98-99% maximum to only 95-96% (thick fog). I thought it may be a dodgy sensor so bought the 2nd sensor, but that also drifted down while by summer 2017 the first sensor was back to reaching 98-99% lol.
Currently, they reach about 97-98%, with the 2nd sensor 1-2% wetter through the mid-range.

Concerning the mid-range, and yep.. they seem to have a wet bias. I upload to a website that allows an easy comparison between two stations. This is me compared to the nearby official Met Office station (5 miles away) yesterday (a cooler, fresher day than many this summer with a well-mixed environment until about 9pm).

 [ You are not allowed to view attachments ]

My dew point is typically 2-3C higher (x1.8 for difference in F) Of course these are only hourly observation points so brief fluctuations could skew the graph, however this is a typical story.

Humidity for August so far

 [ You are not allowed to view attachments ]

As you can see I'm usually approaching 10% more humid. We are at virtually the same elevation with similar surroundings and I can’t see any genuine reasons for this when the wind is in the direction that it was yesterday. It does seem with me at least the problem is not just with warm humid airmasses.

Not happy with that but what can I do.. I don't really have a controlled environment to recondition sensors (and if they have to be removed from the Davis board? that's a no-go for me anyway).
Nearby Davis stations do read similar to me but probably have the same issue.


Also, a brief history of the previous sensors I've used in case it's of interest:

My 1st SHT11 failed in August 2009, I think it had less of a wet bias, but only reached 95-96%.
The 2nd SHT11 had a significant wet bias in the mid-range, and reached 98-99%.
I had a SHT75 'Davis ready' sensor from a seller on Ebay. That possibly had little bias but it only reached about 95% so think I only used it in the old/backup ISS.
I also had a SHT15 (3rd party), and it reached 97-98%. It still had some degree of wet-bias.

So as you can see wet biased mid-range and no 100% readings seems to have been a general theme for me.

I still have the 15 and 75 lying around so am going to plug them into my backup ISS and see what they say compared to the SHT31.

I still like the SHT31 for temperature and prefer its humidity response to the 11's I used. If a stable 'corrected' sensor came out I'd buy that, although my records won't be consistent but they aren't anyway with those sensor changes and drift lol.

If I were you, I would get a brand new 75 sensor and do the necessary work to fit it in with the transmitter. hwcorder has the 75 and it matches up well with the Airport stations. In fact, it reached up to 100% one morning. Jerryg also uses the 75.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 10, 2018, 09:51:11 AM
I think the 75 may drift somewhat too. My old one was only maxing out at 96% when I plugged it back in a few weeks ago, but the new one is regularly hitting 98-100%. The caveat is that the old 75 was collecting dust for a couple of years so maybe there's another issue there. My 31 was only peaking at 93% so at least the old 75 was better than that.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 10, 2018, 09:55:11 AM
We also have our first consistent dry breeze in awhile today and my dewpoint is running right on par with the LCI AWOS and a little under the CON ASOS. With the 31, I was rarely lower than them during the day. With my 40CFM fan it can get a little jumpy with wet ground and light winds as we fluctuate between drier, mixed air and more moist, surface dewpoint pooling.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 10, 2018, 11:14:58 AM
Are you using the 75 without a filter?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 10, 2018, 11:30:24 AM
Are you using the 75 without a filter?
Yup. Used the 31 without a filter too.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on August 10, 2018, 01:31:25 PM
Just a warning for those planning to mount a 75 using the wire-to-board connector. Those things are flimsy crap. I got it working on my 4th try (after burning through 3 other connectors). I then moved the sensor, and it wouldn't read correctly after that.

For those wondering, yes, the wires were in there properly (you could pull on them and they wouldn't come out). The wire end of the connector was as seated as you could make it, but it always kinda moves, no matter what.

Even if I could seal it when it was connected properly, I have no confidence that connector would last long term.

I know dendrite got it working and that's great. I guess I'm saying I'm not sure how easily that success can be replicated.

Caveat emptor. YMMV.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 10, 2018, 02:09:57 PM
Just a warning for those planning to mount a 75 using the wire-to-board connector. Those things are flimsy crap. I got it working on my 4th try (after burning through 3 other connectors). I then moved the sensor, and it wouldn't read correctly after that.

For those wondering, yes, the wires were in there properly (you could pull on them and they wouldn't come out). The wire end of the connector was as seated as you could make it, but it always kinda moves, no matter what.

Even if I could seal it when it was connected properly, I have no confidence that connector would last long term.

I know dendrite got it working and that's great. I guess I'm saying I'm not sure how easily that success can be replicated.

Caveat emptor. YMMV.

This sounds like fun. Not!  I think I ordered 6 for 2 sensors. Is the issue with the wire (plug) side or sensor? If on wire side I may be able to solder a better connection.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on August 10, 2018, 02:55:55 PM
Wire side.

Yup, I'm going the solder route too.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on August 10, 2018, 03:31:58 PM
If it is the same ones i got they really suck, one fell apart and the other made connection about half the time even though it was seated right. El Stinko.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on August 10, 2018, 04:18:51 PM
I have experience soldering... just not in such tight spaces. So I spread the leads apart vertically to give myself extra space.

Well... after shorting the leads with excess solder, I attempted to "fix" my mess and the sensor became too hot to touch. It's pretty fubar'd now.

And with that I'm out of the 75 project.

To those who plan to proceed, you will need good soldering skills/tools, a well-lit workspace, and some way to safely immobilize the sensor.

For the average VP2 owner who just wants accurate humidity readings, the wait continues for a viable solution.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on August 10, 2018, 05:29:54 PM
Don't know if anyone else has tried compensating by dialing in what humidity you want, but six days ago I dropped mine 2%. Since then I've been between 11 and 91%, 109 and 70°F, and have been tickled with the results. Of course I'm completely aware that this is certainly no fix and haven't "challenged" the upper and lower limit to boot, but in between, I'm very pleased. At least it's better than a sharp stick in the eye til there's a fix.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 10, 2018, 05:46:31 PM
Ugh. Sorry they didn't work well for you guys. I hate suggesting something and having them fail for everyone. Mine is working great, but they're definitely tough to work with.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 10, 2018, 05:59:30 PM
Don't know if anyone else has tried compensating by dialing in what humidity you want, but six days ago I dropped mine 2%. Since then I've been between 11 and 91%, 109 and 70°F, and have been tickled with the results. Of course I'm completely aware that this is certainly no fix and haven't "challenged" the upper and lower limit to boot, but in between, I'm very pleased. At least it's better than a sharp stick in the eye til there's a fix.

Not really an option for most. Many have sensors that struggle to reach 98% with half stopping at 96% even in soupy fog so dialing back what most need around 6-8% (not 2%) would really look worse on foggy mornings and you are reporting 88-90% humidity. 

I would rather be reporting 48% during afternoon when it's really 41% and less noticeable unless it's in the high heat index region when that extra 6-8% starts sticking out like a sore thumb.   
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on August 10, 2018, 06:01:43 PM
When I switched from the 11 to the 15 a few years back, I took the sensor board to a dude down the street that works on old stereo stuff. Not only did he nail the job, he did it with his naked eye :eek:, threw him a Jackson and was gone. Maybe some of you guys can find someone similar.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 10, 2018, 06:05:33 PM
I have experience soldering... just not in such tight spaces. So I spread the leads apart vertically to give myself extra space.

Well... after shorting the leads with excess solder, I attempted to "fix" my mess and the sensor became too hot to touch. It's pretty fubar'd now.

And with that I'm out of the 75 project.

To those who plan to proceed, you will need good soldering skills/tools, a well-lit workspace, and some way to safely immobilize the sensor.

For the average VP2 owner who just wants accurate humidity readings, the wait continues for a viable solution.
Sensirion doesn't recommend soldering to the 75 leads. These are the ones they recommend on the data sheet.

https://www.mouser.com/ProductDetail/Mill-Max/851-43-004-20-001000?qs=iJx90KzHWBrT6JXc8SwU%252bg%3D%3D

I used these and they work fine, but again...tight spaces.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on August 10, 2018, 06:09:49 PM
Don't know if anyone else has tried compensating by dialing in what humidity you want, but six days ago I dropped mine 2%. Since then I've been between 11 and 91%, 109 and 70°F, and have been tickled with the results. Of course I'm completely aware that this is certainly no fix and haven't "challenged" the upper and lower limit to boot, but in between, I'm very pleased. At least it's better than a sharp stick in the eye til there's a fix.

Not really an option for most. Many have sensors that struggle to reach 98% with half stopping at 96% even in soupy fog so dialing back what most need around 6-8% (not 2%) would really look worse on foggy mornings and you are reporting 88-90% humidity. 
 
Yeah, figured as much since "only" 2% was needed for me to get in line.
I imagine I'm likely see less than 11% before there's a fix ](*,) and will obviously see how it does.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 10, 2018, 06:13:47 PM
Just a warning for those planning to mount a 75 using the wire-to-board connector. Those things are flimsy crap. I got it working on my 4th try (after burning through 3 other connectors). I then moved the sensor, and it wouldn't read correctly after that.

For those wondering, yes, the wires were in there properly (you could pull on them and they wouldn't come out). The wire end of the connector was as seated as you could make it, but it always kinda moves, no matter what.

Even if I could seal it when it was connected properly, I have no confidence that connector would last long term.

I know dendrite got it working and that's great. I guess I'm saying I'm not sure how easily that success can be replicated.

Caveat emptor. YMMV.

This sounds like fun. Not!  I think I ordered 6 for 2 sensors. Is the issue with the wire (plug) side or sensor? If on wire side I may be able to solder a better connection.
If you give it a shot...

Notice how the connector opens and closes. Use a little flat head precision screwdriver to gently open it all the way. There will be a subtle "click" into the fully open position. Cut the wires straight across, line them up, and insert them. They will freely and loosely go in to a certain point. Then you need to give them some extra force to push them through all the way to make the connection.

The next part is where they tend to break. When closing the connector over the wires, do it gently with even pressure. I don't recommend trying to close it all the way as there are multiple locations where the plastic can break apart. I found that out twice. Heck, just closing it with your thumb and index finger may be enough.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 10, 2018, 06:51:44 PM
Just a warning for those planning to mount a 75 using the wire-to-board connector. Those things are flimsy crap. I got it working on my 4th try (after burning through 3 other connectors). I then moved the sensor, and it wouldn't read correctly after that.

For those wondering, yes, the wires were in there properly (you could pull on them and they wouldn't come out). The wire end of the connector was as seated as you could make it, but it always kinda moves, no matter what.

Even if I could seal it when it was connected properly, I have no confidence that connector would last long term.

I know dendrite got it working and that's great. I guess I'm saying I'm not sure how easily that success can be replicated.

Caveat emptor. YMMV.

This sounds like fun. Not!  I think I ordered 6 for 2 sensors. Is the issue with the wire (plug) side or sensor? If on wire side I may be able to solder a better connection.
If you give it a shot...

Notice how the connector opens and closes. Use a little flat head precision screwdriver to gently open it all the way. There will be a subtle "click" into the fully open position. Cut the wires straight across, line them up, and insert them. They will freely and loosely go in to a certain point. Then you need to give them some extra force to push them through all the way to make the connection.

The next part is where they tend to break. When closing the connector over the wires, do it gently with even pressure. I don't recommend trying to close it all the way as there are multiple locations where the plastic can break apart. I found that out twice. Heck, just closing it with your thumb and index finger may be enough.

Thanks for the tip.
I'll copy and keep handy.
Made the mistake ordering through Newark because sensor was cheaper vs Mouser. WOW! They sit on order for 3 days, I finally called and was told by someone with really bad english because I was first time customer they hold order. WHAT? Never heard of such BS...Anyway it will be next Wednesday before it gets here. I get stuff from Mouser shipped same day sometimes, incredibly different companies.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on August 10, 2018, 07:09:00 PM
I'd have cancel it and told them to pound sand, then happily told them you'd do business elsewhere.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: hwcorder on August 10, 2018, 07:43:31 PM
I have experience soldering... just not in such tight spaces. So I spread the leads apart vertically to give myself extra space.

Well... after shorting the leads with excess solder, I attempted to "fix" my mess and the sensor became too hot to touch. It's pretty fubar'd now.

And with that I'm out of the 75 project.

To those who plan to proceed, you will need good soldering skills/tools, a well-lit workspace, and some way to safely immobilize the sensor.

For the average VP2 owner who just wants accurate humidity readings, the wait continues for a viable solution.

There are female pin connectors that you can solder instead of the actual sensor. You can find them at mouser or newark 14.

Also I made a bonehead mistake of not mentioning I  do use the humidity offset feature on weather display then add back in the offset on the high-end to reach 100%.
The raw readings on my 75 reach 98 tops.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on August 10, 2018, 07:51:30 PM
I have experience soldering... just not in such tight spaces. So I spread the leads apart vertically to give myself extra space.

Well... after shorting the leads with excess solder, I attempted to "fix" my mess and the sensor became too hot to touch. It's pretty fubar'd now.

And with that I'm out of the 75 project.

To those who plan to proceed, you will need good soldering skills/tools, a well-lit workspace, and some way to safely immobilize the sensor.

For the average VP2 owner who just wants accurate humidity readings, the wait continues for a viable solution.

There are female pin connectors that you can solder instead of the actual sensor. You can find them at mouser or newark 14.

Also I made a bonehead mistake of not mentioning I  do use the humidity offset feature on weather display then add back in the offset on the high-end to reach 100%.
The raw readings on my 75 reach 98 tops.

So that’s why your DPs is matching up well with the nearby airport stations?

BTW: I sent you a PM
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 10, 2018, 08:21:20 PM
I have experience soldering... just not in such tight spaces. So I spread the leads apart vertically to give myself extra space.

Well... after shorting the leads with excess solder, I attempted to "fix" my mess and the sensor became too hot to touch. It's pretty fubar'd now.

And with that I'm out of the 75 project.

To those who plan to proceed, you will need good soldering skills/tools, a well-lit workspace, and some way to safely immobilize the sensor.

For the average VP2 owner who just wants accurate humidity readings, the wait continues for a viable solution.

There are female pin connectors that you can solder instead of the actual sensor. You can find them at mouser or newark 14.

Also I made a bonehead mistake of not mentioning I  do use the humidity offset feature on weather display then add back in the offset on the high-end to reach 100%.
The raw readings on my 75 reach 98 tops.

Makes a big difference. It would be nice if Davis offered this as a fix.  HINT
I make my numbers look better with Cumulus also but still need sensor to be capably of reaching 98% on its own. 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 10, 2018, 08:35:14 PM
Mine has regularly been reaching 99-100%. We’ll see how long that lasts.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: hwcorder on August 11, 2018, 09:28:49 AM
I have experience soldering... just not in such tight spaces. So I spread the leads apart vertically to give myself extra space.

Well... after shorting the leads with excess solder, I attempted to "fix" my mess and the sensor became too hot to touch. It's pretty fubar'd now.

And with that I'm out of the 75 project.

To those who plan to proceed, you will need good soldering skills/tools, a well-lit workspace, and some way to safely immobilize the sensor.

For the average VP2 owner who just wants accurate humidity readings, the wait continues for a viable solution.

There are female pin connectors that you can solder instead of the actual sensor. You can find them at mouser or newark 14.

Also I made a bonehead mistake of not mentioning I  do use the humidity offset feature on weather display then add back in the offset on the high-end to reach 100%.
The raw readings on my 75 reach 98 tops.

So that’s why your DPs is matching up well with the nearby airport stations?

BTW: I sent you a PM

On Weather Display I use a -1 slope factor which would lower my value by 2% through the mid range but then use a +2 offset.  So in effect though the mid ranges my final reading and raw reading match. When I get to the higher end of the scale my final reading is slightly higher than the raw output.  Once my 75 reaches 98% the adjusted reading reaches 100%.


Quote
It would be nice if Davis offered this as a fix.  HINT

Yes and I would think it would not be too hard to add some kind of adjustment slope in a firmware update.  Of coarse if they were to do that they would be admitting that there is a problem.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: galfert on August 11, 2018, 09:53:27 AM

Yes and I would think it would not be too hard to add some kind of adjustment slope in a firmware update.  Of coarse if they were to do that they would be admitting that there is a problem.

Well Davis certainly has a problem, whether they want to admit it or not. If you look at the custom report that I generated the number of VP2s in use is rapidly decreasing. Davis better release their VP3 soon as I've said before or someone is going to take their place. And they should also address this issue with their existing loyal VP2 customers.

https://www.wxforum.net/index.php?topic=34900.0


Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: hwcorder on August 12, 2018, 08:59:48 AM
Well the wiring in my 75 seems to have developed a short somewhere so i switched it out with a new 31 I have had lying around in a stock Davis filter.  Switched it out yesterday and was surprised to see it reach 100% last night with ease.  We did have a heavy shower come through yesterday around sunset that put down about 1/3 of an inch followed by partly cloudy skies last night with some light fog this morning.  I guess I will leave it in there for a few days to see how it tracks along with surrounding ASOS stations and to see if a substantial wet bias develops.  Still using the MeteoShield Pro for those of you wondering.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on August 12, 2018, 09:28:52 AM
Well the wiring in my 75 seems to have developed a short somewhere so i switched it out with a new 31 I have had lying around in a stock Davis filter.  Switched it out yesterday and was surprised to see it reach 100% last night with ease.  We did have a heavy shower come through yesterday around sunset that put down about 1/3 of an inch followed by partly cloudy skies last night with some light fog this morning.  I guess I will leave it in there for a few days to see how it tracks along with surrounding ASOS stations and to see if a substantial wet bias develops.  Still using the MeteoShield Pro for those of you wondering.

Nice. The 31 I have in one of my field stations reaches 100% fairly easily. It does good with humidities in the 90 percentile. It’s just when it goes below 75% is when the wet bias really begins to show.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 12, 2018, 10:43:34 AM
Well the wiring in my 75 seems to have developed a short somewhere so i switched it out with a new 31 I have had lying around in a stock Davis filter.  Switched it out yesterday and was surprised to see it reach 100% last night with ease.  We did have a heavy shower come through yesterday around sunset that put down about 1/3 of an inch followed by partly cloudy skies last night with some light fog this morning.  I guess I will leave it in there for a few days to see how it tracks along with surrounding ASOS stations and to see if a substantial wet bias develops.  Still using the MeteoShield Pro for those of you wondering.
I had problems with moisture on the PCB with my first SHT75 until I put the silicone conformal coating on it. Now only the sensor and tip of the PCB that the sensor sits on is exposed.

I've been sitting at 97-98% with 1.60" of rain in 24hrs and mine is still running fine.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on August 12, 2018, 10:50:35 AM
Looks like Onset might have figured out how to protect the Sensirion sensors from excessive moisture intake.

Check out this doc: http://www.onsetcomp.com/files/manual_pdfs/11427-N%20MAN-S-THB.pdf (http://www.onsetcomp.com/files/manual_pdfs/11427-N%20MAN-S-THB.pdf)

It talks about the housing material over the sensor. It's quite the "cap" they devised.

I'm not sure which Sensirion model they use, but they do mention the manufacturer in at least one of their other docs detailing testing procedures (http://www.onsetcomp.com/files/15932-B-Testing-Sensirion-RH-Sensors-White-Paper.pdf (http://www.onsetcomp.com/files/15932-B-Testing-Sensirion-RH-Sensors-White-Paper.pdf)).

Any Onset HOBO owners reading this thread who have tested their humidity/dew point? Is there a pronounced bias in the middle (40-70%)?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: hwcorder on August 12, 2018, 11:12:26 AM
Well the wiring in my 75 seems to have developed a short somewhere so i switched it out with a new 31 I have had lying around in a stock Davis filter.  Switched it out yesterday and was surprised to see it reach 100% last night with ease.  We did have a heavy shower come through yesterday around sunset that put down about 1/3 of an inch followed by partly cloudy skies last night with some light fog this morning.  I guess I will leave it in there for a few days to see how it tracks along with surrounding ASOS stations and to see if a substantial wet bias develops.  Still using the MeteoShield Pro for those of you wondering.
I had problems with moisture on the PCB with my first SHT75 until I put the silicone conformal coating on it. Now only the sensor and tip of the PCB that the sensor sits on is exposed.

I've been sitting at 97-98% with 1.60" of rain in 24hrs and mine is still running fine.

Found the short in the white wire. Looks like it got pulled away from the potting epoxy I used somehow. Probably since I have been messing with it pulling it in and out of different shields.  Had to order some more 4 pin connectors from mouser so I can resolder.  Like others I had little luck with the crimp style connectors. Kept getting an inconsistent connection. 

Speaking of soldering, I found these pin connectors on Mouser made by Mill Max.  They come with solder cups where it looks as though you can insert the wire then solder it in.  If it is as it looks this would be much easier for some of you who have bad luck trying to solder the 1.27mm tabs. I have left the link below.
https://www.mouser.com/ProductDetail/Mill-Max/851-13-004-10-003000?qs=sGAEpiMZZMs%252bGHln7q6pm8Vn94ktop%2fJsOvP78LpBImemlioZUsStg%3d%3d
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 12, 2018, 11:18:33 AM
Did you see what Jerry and I ordered?  I have 2 on the way may not be here until fall however. https://www.aliexpress.com/item/2pcs-Waterproof-Temperature-and-humidity-sensor-shell-SHT10-SHT21-SHT15-shT11-sht20-SHT75-sensor-protective-sleeve/32725766858.html?spm=a2g0s.9042311.0.0.69af4c4dlQrpxK

Others have some like https://www.emesystems.com/sht75pg/documents/SHT75PG.pdf

dendrite are there no worries about contamination using silicone conformal coating?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 12, 2018, 11:21:35 AM


Speaking of soldering, I found these pin connectors on Mouser made by Mill Max.  They come with solder cups where it looks as though you can insert the wire then solder it in.  If it is as it looks this would be much easier for some of you who have bad luck trying to solder the 1.27mm tabs. I have left the link below.
https://www.mouser.com/ProductDetail/Mill-Max/851-13-004-10-003000?qs=sGAEpiMZZMs%252bGHln7q6pm8Vn94ktop%2fJsOvP78LpBImemlioZUsStg%3d%3d

I'll try these. So stick wires in solder then the SHT75 plus in, is that the idea?

Found this on how to solder with them.

https://www.eptac.com/ask/what-is-the-correct-way-to-fill-solder-cups/
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: hwcorder on August 12, 2018, 11:30:26 AM


Speaking of soldering, I found these pin connectors on Mouser made by Mill Max.  They come with solder cups where it looks as though you can insert the wire then solder it in.  If it is as it looks this would be much easier for some of you who have bad luck trying to solder the 1.27mm tabs. I have left the link below.
https://www.mouser.com/ProductDetail/Mill-Max/851-13-004-10-003000?qs=sGAEpiMZZMs%252bGHln7q6pm8Vn94ktop%2fJsOvP78LpBImemlioZUsStg%3d%3d

I'll try these. So stick wires in solder then the SHT75 plus in, is that the idea?

Seems so. I would think these will be a lot easier to keep the spacing of the wires while being able to get a good solder on them.  You can do whatever weatherproofing you desire, give it time to cure then insert the 75 so there is virtually no risk of chemical contamination.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on August 12, 2018, 11:34:13 AM
Maybe Onset hasn't figured it out? Because a quick check of one of their stations in Bryn Mawr (https://www.wunderground.com/personal-weather-station/dashboard?ID=KPAHAVER17#history (https://www.wunderground.com/personal-weather-station/dashboard?ID=KPAHAVER17#history)) shows a dew point of 72/73 when all the surrounding airports are reporting 70.

The more I observe other Sensirion sensors in other branded weather stations, the more I realize the problem isn't limited to Davis. I also see the problem in Atlas Elites and RainWise stations.

It probably goes back to what Ron (kcidwx) said about Sensirion sensors being designed for indoor use and HVAC primarily and not as meteorological instruments.

Doesn't seem like the cap matters, at least not for 11/15/31 sensors. Seems it's more about how often they've been in saturated air.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 12, 2018, 11:47:22 AM
How are these sht75's working if interface is I2c? Thought that wouldn't work.

https://www.mouser.com/ProductDetail/Sensirion/SHT75?qs=%2fha2pyFaduhV3NYEe1MP5y%2fYDjvd38ndllSCUE15w5k%3d
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: hwcorder on August 12, 2018, 12:11:07 PM
How are these sht75's working if interface is I2c? Thought that wouldn't work.

https://www.mouser.com/ProductDetail/Sensirion/SHT75?qs=%2fha2pyFaduhV3NYEe1MP5y%2fYDjvd38ndllSCUE15w5k%3d

Unless there is an I2C version not advertised by Senserion of the 75 then this must be a typo.  I don't order my 75s from Mouser but from Newark 14. Main reason being they have a distribution warehouse in South Carolina so I get them within a day or two even with UPS gound.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on August 12, 2018, 12:17:57 PM
How are these sht75's working if interface is I2c? Thought that wouldn't work.

https://www.mouser.com/ProductDetail/Sensirion/SHT75?qs=%2fha2pyFaduhV3NYEe1MP5y%2fYDjvd38ndllSCUE15w5k%3d

It's a typo. I ordered my 75 from them and had it working briefly before I broke it.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on August 12, 2018, 12:53:14 PM
OK, after looking at a bunch of Rainwise stations, I should back up and say I only occasionally see the wet bias problem.

Whereas with Davis VP2s, well, it's uncommon to find one that's accurate in the middle range of humidity (40-70%).

Also, to be fair, the Acurite Atlases listed on Wunderground may very well be other stations at this point since the "beta" test period ended, I believe, and the product has yet to be released.


Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on August 12, 2018, 01:11:57 PM
OK, after looking at a bunch of Rainwise stations, I should back up and say I only occasionally see the wet bias problem.

Whereas with Davis VP2s, well, it's uncommon to find one that's accurate in the middle range of humidity (40-70%).

Also, to be fair, the Acurite Atlases listed on Wunderground may very well be other stations at this point since the "beta" test period ended, I believe, and the product has yet to be released.

Do we know for sure that Rainwise uses Sensirion? Maybe just for humidity? I noticed their specs for temp go down to -67F and I have tried to research online what they use for temp and humidity, but came up without much info.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on August 12, 2018, 01:16:49 PM
The Rainwise III uses the SHT15. I don't think they use separate temp & humidity sensors, although someone correct me if I'm wrong.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on August 12, 2018, 01:22:46 PM
The Rainwise III uses the SHT15. I don't think they use separate temp & humidity sensors, although someone correct me if I'm wrong.

Good to know and thanks for the info. I've been trying to figure that one out for a while.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 12, 2018, 01:56:16 PM
OK, after looking at a bunch of Rainwise stations, I should back up and say I only occasionally see the wet bias problem.

Whereas with Davis VP2s, well, it's uncommon to find one that's accurate in the middle range of humidity (40-70%).

Also, to be fair, the Acurite Atlases listed on Wunderground may very well be other stations at this point since the "beta" test period ended, I believe, and the product has yet to be released.

Do we know for sure that Rainwise uses Sensirion? Maybe just for humidity? I noticed their specs for temp go down to -67F and I have tried to research online what they use for temp and humidity, but came up without much info.

It looks like they may have changed sensor at sometime. The RWIII now says -40 to 60c range and use to be different like -68F. There was a forum member who lived in Alaska that would come here and brag about how his station would still work at those cold temperatures.   https://www.rainwise.com/products/attachments/6801/20150710121434.pdf
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Old Tele man on August 12, 2018, 02:04:34 PM
Did you see what Jerry and I ordered?  I have 2 on the way may not be here until fall however. https://www.aliexpress.com/item/2pcs-Waterproof-Temperature-and-humidity-sensor-shell-SHT10-SHT21-SHT15-shT11-sht20-SHT75-sensor-protective-sleeve/32725766858.html?spm=a2g0s.9042311.0.0.69af4c4dlQrpxK

Others have some like https://www.emesystems.com/sht75pg/documents/SHT75PG.pdf

dendrite are there no worries about contamination using silicone conformal coating?

Anybody else notice that the 'dewpoint accuracy' graph at the bottom of page 4 [ https://www.emesystems.com/sht75pg/documents/SHT75PG.pdf ] shows a VERY interesting anomaly: the dp 'error' suddenly kinks UPWARD at and above 90%RH.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 12, 2018, 02:13:33 PM
Yeah that's the correct 75 spec graph. Why everyone was so high on the 31's being flatline 2% originally (now it changed some with updated specs) and we've found out Davis version is nowhere near specs  +/- 8% is more inline and why others are going back to the SHT75 that hasn't been destroyed with the waterproofing process Davis uses melting chip into board. My Acurite 31 still looks good so not blaming sensirion at all for this mess. 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dport on August 12, 2018, 02:35:57 PM
Maybe Onset hasn't figured it out? Because a quick check of one of their stations in Bryn Mawr (https://www.wunderground.com/personal-weather-station/dashboard?ID=KPAHAVER17#history (https://www.wunderground.com/personal-weather-station/dashboard?ID=KPAHAVER17#history)) shows a dew point of 72/73 when all the surrounding airports are reporting 70.

The more I observe other Sensirion sensors in other branded weather stations, the more I realize the problem isn't limited to Davis. I also see the problem in Atlas Elites and RainWise stations.

It probably goes back to what Ron (kcidwx) said about Sensirion sensors being designed for indoor use and HVAC primarily and not as meteorological instruments.

Doesn't seem like the cap matters, at least not for 11/15/31 sensors. Seems it's more about how often they've been in saturated air.

My station is just a couple miles from the Onset station you mentioned.  I’m running a 24 hr FARS VP2 that was just installed in June.  We had heavy rains yesterday and my station hit 99% humidity on many occasions last night (this is stock with no calibration adjustments).  The Onset hit a max of 97%. Currently, my dew is 72 while the Onset is 73. The nearby airports (mainly philly international) have a weird dry bias that doesn’t make sense. 

Here is my station if you want to compare to the Onset:
https://www.wunderground.com/personal-weather-station/dashboard?ID=KPABRYNM2#history
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on August 12, 2018, 02:50:01 PM
OK, after looking at a bunch of Rainwise stations, I should back up and say I only occasionally see the wet bias problem.

Whereas with Davis VP2s, well, it's uncommon to find one that's accurate in the middle range of humidity (40-70%).

Also, to be fair, the Acurite Atlases listed on Wunderground may very well be other stations at this point since the "beta" test period ended, I believe, and the product has yet to be released.

Do we know for sure that Rainwise uses Sensirion? Maybe just for humidity? I noticed their specs for temp go down to -67F and I have tried to research online what they use for temp and humidity, but came up without much info.

It looks like they may have changed sensor at sometime. The RWIII now says -40 to 60c range and use to be different like -68F. There was a forum member who lived in Alaska that would come here and brag about how his station would still work at those cold temperatures.   https://www.rainwise.com/products/attachments/6801/20150710121434.pdf

The email I received from them, which was around a month ago, stated that they use the SHT-15 for MK III stations
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on August 12, 2018, 03:26:55 PM
Did you see what Jerry and I ordered?  I have 2 on the way may not be here until fall however. https://www.aliexpress.com/item/2pcs-Waterproof-Temperature-and-humidity-sensor-shell-SHT10-SHT21-SHT15-shT11-sht20-SHT75-sensor-protective-sleeve/32725766858.html?spm=a2g0s.9042311.0.0.69af4c4dlQrpxK

Others have some like https://www.emesystems.com/sht75pg/documents/SHT75PG.pdf

So basically the SF1 filter caps are not able to protect the sensor during extreme humid conditions?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on August 12, 2018, 04:05:29 PM
The nearby airports (mainly philly international) have a weird dry bias that doesn’t make sense. 

When I checked (around 11:30AM EDT), every nearby airport was reporting 70 -- in all cardinal directions. It's unusual to see agreement like that. Again, not saying definitively that the station in question runs wet (verification would require a trustworthy instrument next to it).

Pulling up a map of the area now (3:45PM EDT) shows light winds from the N & NE and 10 mi visibility. There are 9 airports averaging about 70 again, a couple RAWS stations about the same, and state department of transportation stations (in northern Delaware) avg around 70/71. Meanwhile dew points at area PWS's are averaging about 73.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 12, 2018, 04:09:56 PM
Did you see what Jerry and I ordered?  I have 2 on the way may not be here until fall however. https://www.aliexpress.com/item/2pcs-Waterproof-Temperature-and-humidity-sensor-shell-SHT10-SHT21-SHT15-shT11-sht20-SHT75-sensor-protective-sleeve/32725766858.html?spm=a2g0s.9042311.0.0.69af4c4dlQrpxK

Others have some like https://www.emesystems.com/sht75pg/documents/SHT75PG.pdf
So basically the SF1 filter caps are not able to protect the sensor during extreme humid conditions?

My understanding the whole sensor fits inside waterproof case was idea. Davis encapsulates connections in plastic/rubber material on board. 
Something needs to protect the connections or they won't last exposed to elements. May get by for awhile but eventually failure.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 12, 2018, 04:11:56 PM
Anybody else notice that the 'dewpoint accuracy' graph at the bottom of page 4 [ https://www.emesystems.com/sht75pg/documents/SHT75PG.pdf ] shows a VERY interesting anomaly: the dp 'error' suddenly kinks UPWARD at and above 90%RH.
Most of them do it. They have the 31 at a steady 2% through the full range, but my hunch is it's a little higher than that in that 80/90-100% range. I mean, why would the 30 and 35 have a decrease in accuracy in high RH and not the 31?

https://www.sensirion.com/en/environmental-sensors/humidity-sensors/digital-humidity-sensors-for-various-applications/
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on August 12, 2018, 04:14:58 PM
Did you see what Jerry and I ordered?  I have 2 on the way may not be here until fall however. https://www.aliexpress.com/item/2pcs-Waterproof-Temperature-and-humidity-sensor-shell-SHT10-SHT21-SHT15-shT11-sht20-SHT75-sensor-protective-sleeve/32725766858.html?spm=a2g0s.9042311.0.0.69af4c4dlQrpxK

Others have some like https://www.emesystems.com/sht75pg/documents/SHT75PG.pdf
So basically the SF1 filter caps are not able to protect the sensor during extreme humid conditions?

My understanding the whole sensor fits inside waterproof case was idea. Davis encapsulates connections in plastic/rubber material on board. 
Something needs to protect the connections or they won't last exposed to elements. May get by for awhile but eventually failure.

Gotcha. Wonder if this casing would cause any lagging in responsiveness?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 12, 2018, 04:30:21 PM
Did you see what Jerry and I ordered?  I have 2 on the way may not be here until fall however. https://www.aliexpress.com/item/2pcs-Waterproof-Temperature-and-humidity-sensor-shell-SHT10-SHT21-SHT15-shT11-sht20-SHT75-sensor-protective-sleeve/32725766858.html?spm=a2g0s.9042311.0.0.69af4c4dlQrpxK

Others have some like https://www.emesystems.com/sht75pg/documents/SHT75PG.pdf
So basically the SF1 filter caps are not able to protect the sensor during extreme humid conditions?

My understanding the whole sensor fits inside waterproof case was idea. Davis encapsulates connections in plastic/rubber material on board. 
Something needs to protect the connections or they won't last exposed to elements. May get by for awhile but eventually failure.

Gotcha. Wonder if this casing would cause any lagging in responsiveness?

Won't know for awhile, but if Davis wants to get away from how they encapsulate on board (maybe damaging sensor) they should look into a waterproof design like this. This is how the probe sensors are designed in elongated tube fashion with waterproof enclosure.   So we are just replicating that design with these. 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on August 12, 2018, 05:18:09 PM
Did you see what Jerry and I ordered?  I have 2 on the way may not be here until fall however. https://www.aliexpress.com/item/2pcs-Waterproof-Temperature-and-humidity-sensor-shell-SHT10-SHT21-SHT15-shT11-sht20-SHT75-sensor-protective-sleeve/32725766858.html?spm=a2g0s.9042311.0.0.69af4c4dlQrpxK

Others have some like https://www.emesystems.com/sht75pg/documents/SHT75PG.pdf
So basically the SF1 filter caps are not able to protect the sensor during extreme humid conditions?

My understanding the whole sensor fits inside waterproof case was idea. Davis encapsulates connections in plastic/rubber material on board. 
Something needs to protect the connections or they won't last exposed to elements. May get by for awhile but eventually failure.

Gotcha. Wonder if this casing would cause any lagging in responsiveness?

Won't know for awhile, but if Davis wants to get away from how they encapsulate on board (maybe damaging sensor) they should look into a waterproof design like this. This is how the probe sensors are designed in elongated tube fashion with waterproof enclosure.   So we are just replicating that design with these.

Gotcha. Probably order one myself soon.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on August 12, 2018, 05:20:00 PM
Did you see what Jerry and I ordered?  I have 2 on the way may not be here until fall however. https://www.aliexpress.com/item/2pcs-Waterproof-Temperature-and-humidity-sensor-shell-SHT10-SHT21-SHT15-shT11-sht20-SHT75-sensor-protective-sleeve/32725766858.html?spm=a2g0s.9042311.0.0.69af4c4dlQrpxK

Others have some like https://www.emesystems.com/sht75pg/documents/SHT75PG.pdf
So basically the SF1 filter caps are not able to protect the sensor during extreme humid conditions?

My understanding the whole sensor fits inside waterproof case was idea. Davis encapsulates connections in plastic/rubber material on board. 
Something needs to protect the connections or they won't last exposed to elements. May get by for awhile but eventually failure.

Gotcha. Wonder if this casing would cause any lagging in responsiveness?
I put the SF2 on my 31 about a week ago and have seen no difference. I would think the SF1 is similar.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on August 12, 2018, 05:45:30 PM
I'm honestly not sure if this was shown here before, but I'm going to see if I can buy this directly from Sensirion (I got my 15 directly from them) without having to buy 10,000, and see if my stereo buddy down the street can work his magic again installing it.
https://www.sensirion.com/fileadmin/user_upload/customers/sensirion/Dokumente/0_Datasheets/Humidity/Sensirion_Humidity_Sensors_SHT3x_Datasheet_Filter_Membran.pdf
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: graculus on August 12, 2018, 07:01:10 PM
I think you'll find it's an option rather than a separate part, digi-key shows SHt31's with it installed https://www.digikey.com/products/en/sensors-transducers/humidity-moisture-sensors/529?k=sht31
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on August 12, 2018, 07:27:20 PM
I think you'll find it's an option rather than a separate part, digi-key shows SHt31's with it installed https://www.digikey.com/products/en/sensors-transducers/humidity-moisture-sensors/529?k=sht31
Thanks for the link. Yes, that's my point, you get the sensor and the cover and Davis has nothing to do with it. I can only hope that digi-key's handling process is as Sensirion dictates.
Now, can my guru get it mounted to my existing board??
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 12, 2018, 07:34:06 PM
Are we sure these work with Davis ISS? I was under impression Davis had special protocol it's digital LLC.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on August 12, 2018, 07:39:35 PM
Are we sure these work with Davis ISS? I was under impression Davis had special protocol it's digital LLC.
This is waaaay over my pay grade, but all I see is SHT31 and i2c thinking this is compatible. Merely an assumption.
Time for those in the know....
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 12, 2018, 07:41:09 PM
Are we sure these work with Davis ISS? I was under impression Davis had special protocol it's digital LLC.
This is waaaay over my pay grade, but all I see is SHT31 and i2c thinking this is compatible. Merely an assumption.
Time for those in the know....

I2c is not for sure.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on August 12, 2018, 07:48:15 PM
Are we sure these work with Davis ISS? I was under impression Davis had special protocol it's digital LLC.
This is waaaay over my pay grade, but all I see is SHT31 and i2c thinking this is compatible. Merely an assumption.
Time for those in the know....

I2c is not for sure.
Like I said... If no one else can give an absolute whether this will work or not, I'll be calling digi key tomorrow for their input, they may be clueless too.
Sure would be nice if it was, at least it would be another option to try.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on August 12, 2018, 08:06:54 PM
It will not work it has to be the sht31LS  which is a sensibus format digital chip
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on August 12, 2018, 08:20:17 PM
It will not work it has to be the sht31LS  which is a sensibus format digital chip
Of course it is...  ](*,)
Thanks.  :evil:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on August 12, 2018, 09:19:08 PM
Valentine Weather, several hundred posts ago, thought we had exhausted the points in this thread and should wait for a response from Davis. Well, I think this thread continues with great thought, but has anybody heard from Davis on the SHT31 issue?

As an aside, despite our problems with Davis, I am amazed at the VP2 product line. Today, I was installing a separate transmitter for my anemometer (part no. 6332) as I only get one decent wind reading from my 3 VP2s, so I consolidated to use only the one decent one. So I have used 4 of the available stations! Maybe a neighbor will take the remaining 4 transmitting stations? Considering the VP2 product line is now about 15 years old, I am amazed how it has stood the test of time with a few glitches along the way, including the SHT31.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 12, 2018, 09:25:41 PM
Valentine Weather, several hundred posts ago, thought we had exhausted the points in this thread and should wait for a response from Davis. Well, I think this thread continues with great thought, but has anybody heard from Davis on the SHT31 issue?

As an aside, despite our problems with Davis, I am amazed at the VP2 product line. Today, I was installing a separate transmitter for my anemometer (part no. 6332) as I only get one decent wind reading from my 3 VP2s, so I consolidated to use only the one decent one. So I have used 4 of the available stations! Maybe a neighbor will take the remaining 4 transmitting stations? Considering the VP2 product line is now about 15 years old, I am amazed how it has stood the test of time with a few glitches along the way, including the SHT31.
If Apple bought out Davis we'd have a new VP every  year. ;)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: hwcorder on August 12, 2018, 09:28:52 PM
Valentine Weather, several hundred posts ago, thought we had exhausted the points in this thread and should wait for a response from Davis. Well, I think this thread continues with great thought, but has anybody heard from Davis on the SHT31 issue?

As an aside, despite our problems with Davis, I am amazed at the VP2 product line. Today, I was installing a separate transmitter for my anemometer (part no. 6332) as I only get one decent wind reading from my 3 VP2s, so I consolidated to use only the one decent one. So I have used 4 of the available stations! Maybe a neighbor will take the remaining 4 transmitting stations? Considering the VP2 product line is now about 15 years old, I am amazed how it has stood the test of time with a few glitches along the way, including the SHT31.
If Apple bought out Davis we'd have a new VP every  year. ;)

Yeah and the price would double!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: galfert on August 12, 2018, 09:42:45 PM
Valentine Weather, several hundred posts ago, thought we had exhausted the points in this thread and should wait for a response from Davis. Well, I think this thread continues with great thought, but has anybody heard from Davis on the SHT31 issue?

As an aside, despite our problems with Davis, I am amazed at the VP2 product line. Today, I was installing a separate transmitter for my anemometer (part no. 6332) as I only get one decent wind reading from my 3 VP2s, so I consolidated to use only the one decent one. So I have used 4 of the available stations! Maybe a neighbor will take the remaining 4 transmitting stations? Considering the VP2 product line is now about 15 years old, I am amazed how it has stood the test of time with a few glitches along the way, including the SHT31.
If Apple bought out Davis we'd have a new VP every  year. ;)

Yeah and the price would double!

Yeah and it would look cool just like a Netatmo, as it would have been designed for looks first and function second...."You are putting it outside in the sun wrong!"
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on August 12, 2018, 10:12:50 PM
Valentine Weather, several hundred posts ago, thought we had exhausted the points in this thread and should wait for a response from Davis. Well, I think this thread continues with great thought, but has anybody heard from Davis on the SHT31 issue?
IMHO, there's no way that Davis will respond to this issue within the VP2's production lifetime. We (this forum) are a TINY portion of their sales and the other 99% are oblivious, just as we were until recent. Too much time, R&D is needed to address this issue properly to the satisfaction of us and Davis (read money). Honestly, I only see a resolution when a new model is released, and that's only IF Davis thinks it's a problem in the first place, which I'm dubious.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on August 12, 2018, 10:32:53 PM
Valentine Weather, several hundred posts ago, thought we had exhausted the points in this thread and should wait for a response from Davis. Well, I think this thread continues with great thought, but has anybody heard from Davis on the SHT31 issue?
IMHO, there's no way that Davis will respond to this issue within the VP2's production lifetime. We (this forum) are a TINY portion of their sales and the other 99% are oblivious, just as we were until recent. Too much time, R&D is needed to address this issue properly to the satisfaction of us and Davis (read money). Honestly, I only see a resolution when a new model is released, and that's only IF Davis thinks it's a problem in the first place, which I'm dubious.

Unfortunately, CW2274, I think you are right. But if this SHT31 issue becomes the impetus for the long awaited VP3 or whatever Davis calls it, so be it. I will just have to update my equipment—unlike my cellphone—I still use an iPhone 5S! How embarrassing!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: galfert on August 12, 2018, 10:59:51 PM
Valentine Weather, several hundred posts ago, thought we had exhausted the points in this thread and should wait for a response from Davis. Well, I think this thread continues with great thought, but has anybody heard from Davis on the SHT31 issue?
IMHO, there's no way that Davis will respond to this issue within the VP2's production lifetime. We (this forum) are a TINY portion of their sales and the other 99% are oblivious, just as we were until recent. Too much time, R&D is needed to address this issue properly to the satisfaction of us and Davis (read money). Honestly, I only see a resolution when a new model is released, and that's only IF Davis thinks it's a problem in the first place, which I'm dubious.

Okay here goes nothing.....[deep sigh...anticipating backlash]... I was wondering if anyone of you dealing with this VP2 humidity sensor issue, and with the financial means, to spend some money for more testing hardware for the sake of knowledge. My proposal is to use the the newest Ambient Weather WS-2902A weather station as a comparison.

Surely I'm not suggesting replacing a VP2 for a WS-2902A. But being an owner of a WS-2902A I have been thoroughly impressed. I've wanted a VP2 for quite some time. And I'm affected by this issue because I still think I will eventually get a VP2 or whatever comes next. So finding a resolve for this issue with Davis is important to me. But you know what?...My neighbor replaced his 2006 VP2 for a WS-2902A just last month. I've been meaning to introduce myself and ask him why.

I know on paper the VP2 should have better sensors that are more precise with less deviation percentage specifications. And there are many many things far superior on the VP2 like separate anemometer/wind vane, FARS option, faster console refresh, PC software connectivity, and add-on sensors (soil, leaf...etc..etc). But I'm just curious how good the WS-2902A humidity sensor is.

I suppose another option is to compare an existing VP2 to a nearby WS-2902. But I haven't found a good example yet. Most WS-2902's are poorly sited, and you would need to have intimate knowledge of the VP2 being compared if it had a SHT31 or SHT15 or if it was modified to SHT75 and how old that sensor is. So this means it isn't something I can do myself.

But someone with an VP2 could compare to a nearby WS-2902 or they could buy their own WS-2902A to do some testing.

Why am I proposing this test?
- Because I own a WS-2902A and it seems to be a popular choice with much sales growth compared to all else, at least in the PWS market and those that connect to WU. I know this shouldn't be the only indicator as I'm sure there are tons of VP2s that don't connect to WU.
- No one knows what sensor is in the WS-2902A and it might be an indication of something new that is really good and maybe better than anything we have seen before. I haven't taken mine apart but I'm tempted. But don't want to void my warranty.

Why do I believe the WS-2902A humidity sensor is so good.
- I've seen it report consistently humidity levels of 95, 96, 97, 98, and 99. (it never hits 100).
- Seems to be accurate in the mid range compared to airport here in Florida humid Summer weather.
- Most WS-2902 devices I find on WU seem to all have really nice data. Consistent graphs. Lots of data points and no gaps and quick response.
- Most WS-2902 that are nearby all match up perfectly.

So what could we gain from this and how does it help with the VP2 issue?
- Determine if the WS-2902A is any good in dealing with humidity. You all have compared your sensors to Vaisala, or Rainwise....why not look at another cheap option that may be really good and seemingly ignored solely because it seems cheap.
- Find out what sensor is in the WS-2902A and if it is any good.
- If this is good stuff we could hope that Davis employs the use of the WS-2902A sensor as a future upgrade.
- If this is good stuff and made available for Davis, it will be cheap as the WS-2902A is cheap.

So what is bad about the WS-2902A
- The radiation shield is little and I've notice a degree maybe two sometimes spike on intense sunlight days.
- As I mentioned it is an all in one
- Software options limited

Interesting Facts about the WS-2902A
- You can connect it to a Meteobridge if you also get an ObserverIP device. In fact it may be cheaper to get a separate ObserverIP and just the outdoor array module and forget the display console. It isn't sold this way but you can get just those two parts and have a fully functioning station.
- The WS-2902 is the prior version. They change the name to WS-2902A when a new firmware was introduced to change the WiFi setup method. But a WS-2902 can be converted into a WS-2902A by just upgrading firmware. I may seem use the two terms interchangeably but I'm not. I say WS-2902 when talking generic and regarding established base or WS-2902A when referring to a new purchase or a known specific device running newest firmware.
- WU just shows both WS-2902A and WS-2902 as just WS-2902.
- WS-2902A hit #1 in sales group on Amazon a few weeks back and is currently at #3 or #4.

[ducks and covers]
 :roll:

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 12, 2018, 11:13:20 PM
Valentine Weather, several hundred posts ago, thought we had exhausted the points in this thread and should wait for a response from Davis. Well, I think this thread continues with great thought, but has anybody heard from Davis on the SHT31 issue?
IMHO, there's no way that Davis will respond to this issue within the VP2's production lifetime. We (this forum) are a TINY portion of their sales and the other 99% are oblivious, just as we were until recent. Too much time, R&D is needed to address this issue properly to the satisfaction of us and Davis (read money). Honestly, I only see a resolution when a new model is released, and that's only IF Davis thinks it's a problem in the first place, which I'm dubious.

Unfortunately, CW2274, I think you are right. But if this SHT31 issue becomes the impetus for the long awaited VP3 or whatever Davis calls it, so be it. I will just have to update my equipment—unlike my cellphone—I still use an iPhone 5S! How embarrassing!

Hope you guys are wrong...Myself I think we could see a design change in sensor with a encapsulated probe, similar to higher end sensors instead of molding sensor into board for water resistance. 
I've said several times the SHT31 in the acurite's work fine. Still reach 98% but doesn't show the high bias so it's either the LS version Davis uses or the process of embedding with plastic onto board.
 
Hopefully they are currently testing this theory out among others they may have. The email said give the engineers time to get this figured out.
Before any major change they need to make sure and this takes time.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on August 12, 2018, 11:30:07 PM
Valentine Weather, several hundred posts ago, thought we had exhausted the points in this thread and should wait for a response from Davis. Well, I think this thread continues with great thought, but has anybody heard from Davis on the SHT31 issue?
IMHO, there's no way that Davis will respond to this issue within the VP2's production lifetime. We (this forum) are a TINY portion of their sales and the other 99% are oblivious, just as we were until recent. Too much time, R&D is needed to address this issue properly to the satisfaction of us and Davis (read money). Honestly, I only see a resolution when a new model is released, and that's only IF Davis thinks it's a problem in the first place, which I'm dubious.

- WS-2902A hit #1 in sales group on Amazon a few weeks back and is currently at #3 or #4.

[ducks and covers]
 :roll:
Just because it's popular sure as hell doesn't make it good. For me personally, I would never go downhill on my PWS, bad humidity 31 or not. I wonder how many WS's I would have needed to replace in the 11 years I've had my lone VP2 in this Arizona sun. Phftt, forget everything else in the VP2's favor, that's gravy.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 13, 2018, 03:58:40 AM
Valentine Weather, several hundred posts ago, thought we had exhausted the points in this thread and should wait for a response from Davis. Well, I think this thread continues with great thought, but has anybody heard from Davis on the SHT31 issue?
IMHO, there's no way that Davis will respond to this issue within the VP2's production lifetime. We (this forum) are a TINY portion of their sales and the other 99% are oblivious, just as we were until recent. Too much time, R&D is needed to address this issue properly to the satisfaction of us and Davis (read money). Honestly, I only see a resolution when a new model is released, and that's only IF Davis thinks it's a problem in the first place, which I'm dubious.

- WS-2902A hit #1 in sales group on Amazon a few weeks back and is currently at #3 or #4.

[ducks and covers]
 :roll:
Just because it's popular sure as hell doesn't make it good. For me personally, I would never go downhill on my PWS, bad humidity 31 or not. I wonder how many WS's I would have needed to replace in the 11 years I've had my lone VP2 in this Arizona sun. Phftt, forget everything else in the VP2's favor, that's gravy.

Exactly why I have no comment on what stations are most popular on WU. Really don't care about popularity usually just the opposite. I would own one of those throwaway Iphones with the masses and need to replace every other year if it survives without being fully armored. Meanwhile my 5 year old Note2 doesn't have a scratch on the glass lives in my pocket with keys, change with no protection (zero) been through multiple drops and survived, saves me money every year because it may be the last phone I'll ever need.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on August 13, 2018, 06:41:53 AM
Just had to comment, bad things about vp2 is probably super cap, which usually is years down the road and probably happens long after most cheap stations have been replaced. The sensor issue is new and not as big a deal as replacing a cheap station. Why i have and will have vp2, nearly a year ago Harvey blew in here as a cat 4 storm and i had 8 inch tree limbs as well as whole trees blown down, carport blew over the garage roof, radio tower down power poles snapped off, etc. my vp2 worked all the time on battery and the anny had a wind run of nearly 22,000 miles and is still working today and iss never missed a beat. Outdoor tough is worth a lot and i am willing to put up with a few problems that can be solved with a new sensor or capacitor. Just my thoughts.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: hwcorder on August 13, 2018, 10:19:13 AM
Obs from the Persian Gulf.  A little off topic I know but I seriously wonder if there are any Davis VP2s around here and how they compare to official obs.  I just cant imagine a dew point in the 90's with a HI of 140!  My old boss who was a Weather Officer with the Navy back in the 70's used to tell me about the insane inversions they get there and how it made tacking aircraft with radar very difficult.

(http://i1283.photobucket.com/albums/a541/hwcorder/persiangulfobs_zpsqy34pd0w.png) (http://s1283.photobucket.com/user/hwcorder/media/persiangulfobs_zpsqy34pd0w.png.html)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 13, 2018, 10:34:32 AM
Obs from the Persian Gulf.  A little off topic I know but I seriously wonder if there are any Davis VP2s around here and how they compare to official obs.  I just cant imagine a dew point in the 90's with a HI of 140!  My old boss who was a Weather Officer with the Navy back in the 70's used to tell me about the insane inversions they get there and how it made tacking aircraft with radar very difficult.

(http://i1283.photobucket.com/albums/a541/hwcorder/persiangulfobs_zpsqy34pd0w.png) (http://s1283.photobucket.com/user/hwcorder/media/persiangulfobs_zpsqy34pd0w.png.html)
This is an Ambient

https://www.wunderground.com/personal-weather-station/dashboard?ID=IABUDHAB125&cm_ven=localwx_pwsdash#history/tdata/s20180813/e20180813/mdaily

93F/90F at one point. No thank you.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dport on August 14, 2018, 12:07:20 PM
High end humidity update.  I've had my VP2 for 2.5 months now.  The SHT31 seems to be performing great.  I regularly hit 99% humidity and this morning was stuck at 99% for 5 straight hours (no calibration adjustments made at all).  https://www.wunderground.com/personal-weather-station/dashboard?ID=KPABRYNM2#history/tdata/s20180814/e20180814/mdaily

There might be a slight wet bias in the mid ranges, but not much.  Perhaps I just got lucky received a "good" temp/hum sensor.  We shall see if I can continue to hit 99% in the coming months. 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 14, 2018, 01:26:36 PM
High end humidity update.  I've had my VP2 for 2.5 months now.  The SHT31 seems to be performing great.  I regularly hit 99% humidity and this morning was stuck at 99% for 5 straight hours (no calibration adjustments made at all).  https://www.wunderground.com/personal-weather-station/dashboard?ID=KPABRYNM2#history/tdata/s20180814/e20180814/mdaily

There might be a slight wet bias in the mid ranges, but not much.  Perhaps I just got lucky received a "good" temp/hum sensor.  We shall see if I can continue to hit 99% in the coming months.
I've always noticed mine starts losing the ability to reach upper RH values after the first warm season. Come the following May, 99-100% becomes a max of 97-98%.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: hwcorder on August 14, 2018, 06:34:19 PM
So I completed and installed my DIY probe made out of a Pilot Pin.  Obviously the hardest part is getting the wires connected to socket the connector.  I can tell you that those solder cup connectors I ordered from Mouser where no easier to work with and seemed more fragile.  Ended up using the connector I was originally going to use.  After a lot of patience I protected the connections with 4 very small heat sink wraps for each of the 4 wires and then covered that one with a larger wrap that covered the shielded part of the cable to the body of the connector itself.  I then fit the connector through a slot in a piece of pcb board which of cut and shaved into a round piece that would fit into the body of the pin.  All of this was done using a Dremel Tool.  This was all sealed and weatherproofed with hot glue. 

Where the sht75  (and hopefully a future 35) plugs in, fits perfectly into an Viasala HMP60 filter housing. It does not screw in but the fit is quite snug and I doubt anything could get through.  At first I was using the gel grip in the pin to further seal the filter and pin housing but noticed that it was generating static charge so I removed it.  I think filter is very well sealed but I might wrap some electrical tape around it just to be sure.  Here are some pics.  I will say it will be very nice to be able to remove and replace the probe with ease into the MeteoShield Pro which I have decided to use at least for now.

      (http://i1283.photobucket.com/albums/a541/hwcorder/IMG_20180814_133943381_HDR_zpsvhtvt4tx.jpg) (http://s1283.photobucket.com/user/hwcorder/media/IMG_20180814_133943381_HDR_zpsvhtvt4tx.jpg.html)

(http://i1283.photobucket.com/albums/a541/hwcorder/IMG_20180814_134039566_HDR_zpsbdheq6cv.jpg) (http://s1283.photobucket.com/user/hwcorder/media/IMG_20180814_134039566_HDR_zpsbdheq6cv.jpg.html)

(http://i1283.photobucket.com/albums/a541/hwcorder/IMG_20180814_135645666_HDR_zpsy5ukkzy7.jpg) (http://s1283.photobucket.com/user/hwcorder/media/IMG_20180814_135645666_HDR_zpsy5ukkzy7.jpg.html)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 14, 2018, 07:07:48 PM
So I completed and installed my DIY probe made out of a Pilot Pin.  Obviously the hardest part is getting the wires connected to socket the connector.  I can tell you that those solder cup connectors I ordered from Mouser where no easier to work with and seemed more fragile.  Ended up using the connector I was originally going to use.  After a lot of patience I protected the connections with 4 very small heat sink wraps for each of the 4 wires and then covered that one with a larger wrap that covered the shielded part of the cable to the body of the connector itself.  I then fit the connector through a slot in a piece of pcb board which of cut and shaved into a round piece that would fit into the body of the pin.  All of this was done using a Dremel Tool.  This was all sealed and weatherproofed with hot glue. 

Where the sht75  (and hopefully a future 35) plugs in, fits perfectly into an Viasala HMP60 filter housing. It does not screw in but the fit is quite snug and I doubt anything could get through.  At first I was using the gel grip in the pin to further seal the filter and pin housing but noticed that it was generating static charge so I removed it.  I think filter is very well sealed but I might wrap some electrical tape around it just to be sure.  Here are some pics.  I will say it will be very nice to be able to remove and replace the probe with ease into the MeteoShield Pro which I have decided to use at least for now.
Nice. I like the creativity there. If you had removed the clip from the pen I would've thought it was meant for holding a temp probe.  :grin:

Is that data available online?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on August 14, 2018, 08:30:47 PM
Ended up using the connector I was originally going to use.

Which one was that?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: hwcorder on August 14, 2018, 09:07:25 PM
So I completed and installed my DIY probe made out of a Pilot Pin.  Obviously the hardest part is getting the wires connected to socket the connector.  I can tell you that those solder cup connectors I ordered from Mouser where no easier to work with and seemed more fragile.  Ended up using the connector I was originally going to use.  After a lot of patience I protected the connections with 4 very small heat sink wraps for each of the 4 wires and then covered that one with a larger wrap that covered the shielded part of the cable to the body of the connector itself.  I then fit the connector through a slot in a piece of pcb board which of cut and shaved into a round piece that would fit into the body of the pin.  All of this was done using a Dremel Tool.  This was all sealed and weatherproofed with hot glue. 

Where the sht75  (and hopefully a future 35) plugs in, fits perfectly into an Viasala HMP60 filter housing. It does not screw in but the fit is quite snug and I doubt anything could get through.  At first I was using the gel grip in the pin to further seal the filter and pin housing but noticed that it was generating static charge so I removed it.  I think filter is very well sealed but I might wrap some electrical tape around it just to be sure.  Here are some pics.  I will say it will be very nice to be able to remove and replace the probe with ease into the MeteoShield Pro which I have decided to use at least for now.
Nice. I like the creativity there. If you had removed the clip from the pen I would've thought it was meant for holding a temp probe.  :grin:

Is that data available online?

Thanks!  Yes I upload to Wunderground and CWOP.
 https://www.wunderground.com/personal-weather-station/dashboard?ID=KNCBELMO18#history
https://weather.gladstonefamily.net/site/D5686

Ended up using the connector I was originally going to use.

Which one was that?
Here they are. The post are a little more flexible that the other ones so you can spread the pins out some. Not too much though.
https://www.mouser.com/ProductDetail/Harwin/M50-3030442?qs=sGAEpiMZZMs%252bGHln7q6pm8Vn94ktop%2fJmuuS%252b8ZjnXtI6OeGFBMW0g%3d%3d
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on August 14, 2018, 10:12:33 PM
So I completed and installed my DIY probe made out of a Pilot Pin.  Obviously the hardest part is getting the wires connected to socket the connector.  I can tell you that those solder cup connectors I ordered from Mouser where no easier to work with and seemed more fragile.  Ended up using the connector I was originally going to use.  After a lot of patience I protected the connections with 4 very small heat sink wraps for each of the 4 wires and then covered that one with a larger wrap that covered the shielded part of the cable to the body of the connector itself.  I then fit the connector through a slot in a piece of pcb board which of cut and shaved into a round piece that would fit into the body of the pin.  All of this was done using a Dremel Tool.  This was all sealed and weatherproofed with hot glue. 

Where the sht75  (and hopefully a future 35) plugs in, fits perfectly into an Viasala HMP60 filter housing. It does not screw in but the fit is quite snug and I doubt anything could get through.  At first I was using the gel grip in the pin to further seal the filter and pin housing but noticed that it was generating static charge so I removed it.  I think filter is very well sealed but I might wrap some electrical tape around it just to be sure.  Here are some pics.  I will say it will be very nice to be able to remove and replace the probe with ease into the MeteoShield Pro which I have decided to use at least for now.
Nice. I like the creativity there. If you had removed the clip from the pen I would've thought it was meant for holding a temp probe.  :grin:

Is that data available online?

Thanks!  Yes I upload to Wunderground and CWOP.
 https://www.wunderground.com/personal-weather-station/dashboard?ID=KNCBELMO18#history
https://weather.gladstonefamily.net/site/D5686

Ended up using the connector I was originally going to use.

Which one was that?
Here they are. The post are a little more flexible that the other ones so you can spread the pins out some. Not too much though.
https://www.mouser.com/ProductDetail/Harwin/M50-3030442?qs=sGAEpiMZZMs%252bGHln7q6pm8Vn94ktop%2fJmuuS%252b8ZjnXtI6OeGFBMW0g%3d%3d

How does the raw readings of the 75 humidity sensor compares to the airport? Why I ask is that you have done some offset stuff on WD.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: hwcorder on August 14, 2018, 10:30:05 PM
So I completed and installed my DIY probe made out of a Pilot Pin.  Obviously the hardest part is getting the wires connected to socket the connector.  I can tell you that those solder cup connectors I ordered from Mouser where no easier to work with and seemed more fragile.  Ended up using the connector I was originally going to use.  After a lot of patience I protected the connections with 4 very small heat sink wraps for each of the 4 wires and then covered that one with a larger wrap that covered the shielded part of the cable to the body of the connector itself.  I then fit the connector through a slot in a piece of pcb board which of cut and shaved into a round piece that would fit into the body of the pin.  All of this was done using a Dremel Tool.  This was all sealed and weatherproofed with hot glue. 

Where the sht75  (and hopefully a future 35) plugs in, fits perfectly into an Viasala HMP60 filter housing. It does not screw in but the fit is quite snug and I doubt anything could get through.  At first I was using the gel grip in the pin to further seal the filter and pin housing but noticed that it was generating static charge so I removed it.  I think filter is very well sealed but I might wrap some electrical tape around it just to be sure.  Here are some pics.  I will say it will be very nice to be able to remove and replace the probe with ease into the MeteoShield Pro which I have decided to use at least for now.
Nice. I like the creativity there. If you had removed the clip from the pen I would've thought it was meant for holding a temp probe.  :grin:

Is that data available online?

Thanks!  Yes I upload to Wunderground and CWOP.
 https://www.wunderground.com/personal-weather-station/dashboard?ID=KNCBELMO18#history
https://weather.gladstonefamily.net/site/D5686

Ended up using the connector I was originally going to use.

Which one was that?
Here they are. The post are a little more flexible that the other ones so you can spread the pins out some. Not too much though.
https://www.mouser.com/ProductDetail/Harwin/M50-3030442?qs=sGAEpiMZZMs%252bGHln7q6pm8Vn94ktop%2fJmuuS%252b8ZjnXtI6OeGFBMW0g%3d%3d

How does the raw readings of the 75 humidity sensor compares to the airport? Why I ask is that you have done some offset stuff on WD.

The way I have it done on WD the adjusted readings and the raw readings from the sht75 are the same. But in the high end the raw readings only will reach 97-98% my adjusted reading will go to 100%.  I guess the short answer would be it tracks pretty darn close to local ASOS stations.

Edit: When I say the adjusted and raw readings are the same I meant through the mid ranges <75%.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on August 15, 2018, 05:23:02 PM
So I completed and installed my DIY probe made out of a Pilot Pin.  Obviously the hardest part is getting the wires connected to socket the connector.  I can tell you that those solder cup connectors I ordered from Mouser where no easier to work with and seemed more fragile.  Ended up using the connector I was originally going to use.  After a lot of patience I protected the connections with 4 very small heat sink wraps for each of the 4 wires and then covered that one with a larger wrap that covered the shielded part of the cable to the body of the connector itself.  I then fit the connector through a slot in a piece of pcb board which of cut and shaved into a round piece that would fit into the body of the pin.  All of this was done using a Dremel Tool.  This was all sealed and weatherproofed with hot glue. 

Where the sht75  (and hopefully a future 35) plugs in, fits perfectly into an Viasala HMP60 filter housing. It does not screw in but the fit is quite snug and I doubt anything could get through.  At first I was using the gel grip in the pin to further seal the filter and pin housing but noticed that it was generating static charge so I removed it.  I think filter is very well sealed but I might wrap some electrical tape around it just to be sure.  Here are some pics.  I will say it will be very nice to be able to remove and replace the probe with ease into the MeteoShield Pro which I have decided to use at least for now.
Nice. I like the creativity there. If you had removed the clip from the pen I would've thought it was meant for holding a temp probe.  :grin:

Is that data available online?

Thanks!  Yes I upload to Wunderground and CWOP.
 https://www.wunderground.com/personal-weather-station/dashboard?ID=KNCBELMO18#history
https://weather.gladstonefamily.net/site/D5686

Ended up using the connector I was originally going to use.

Which one was that?
Here they are. The post are a little more flexible that the other ones so you can spread the pins out some. Not too much though.
https://www.mouser.com/ProductDetail/Harwin/M50-3030442?qs=sGAEpiMZZMs%252bGHln7q6pm8Vn94ktop%2fJmuuS%252b8ZjnXtI6OeGFBMW0g%3d%3d

How does the raw readings of the 75 humidity sensor compares to the airport? Why I ask is that you have done some offset stuff on WD.

The way I have it done on WD the adjusted readings and the raw readings from the sht75 are the same. But in the high end the raw readings only will reach 97-98% my adjusted reading will go to 100%.  I guess the short answer would be it tracks pretty darn close to local ASOS stations.

Edit: When I say the adjusted and raw readings are the same I meant through the mid ranges <75%.

So how does it do over 75%?  Wet or dry bias?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 16, 2018, 07:53:25 AM
My acurite continues to act like a SHT31 should running 3-5% below Davis daytime and has no problem reaching 99%.
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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on August 16, 2018, 08:28:03 AM
Jealous of that temperature, i got 78 degrees this am heading up to upper 90's today.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 16, 2018, 09:08:24 AM
Jealous of that temperature, i got 78 degrees this am heading up to upper 90's today.

Summers are nice but you pay for it on the other end.. ;) If you don't like cold.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: hwcorder on August 16, 2018, 09:13:49 AM
So I completed and installed my DIY probe made out of a Pilot Pin.  Obviously the hardest part is getting the wires connected to socket the connector.  I can tell you that those solder cup connectors I ordered from Mouser where no easier to work with and seemed more fragile.  Ended up using the connector I was originally going to use.  After a lot of patience I protected the connections with 4 very small heat sink wraps for each of the 4 wires and then covered that one with a larger wrap that covered the shielded part of the cable to the body of the connector itself.  I then fit the connector through a slot in a piece of pcb board which of cut and shaved into a round piece that would fit into the body of the pin.  All of this was done using a Dremel Tool.  This was all sealed and weatherproofed with hot glue. 

Where the sht75  (and hopefully a future 35) plugs in, fits perfectly into an Viasala HMP60 filter housing. It does not screw in but the fit is quite snug and I doubt anything could get through.  At first I was using the gel grip in the pin to further seal the filter and pin housing but noticed that it was generating static charge so I removed it.  I think filter is very well sealed but I might wrap some electrical tape around it just to be sure.  Here are some pics.  I will say it will be very nice to be able to remove and replace the probe with ease into the MeteoShield Pro which I have decided to use at least for now.
Nice. I like the creativity there. If you had removed the clip from the pen I would've thought it was meant for holding a temp probe.  :grin:

Is that data available online?

Thanks!  Yes I upload to Wunderground and CWOP.
 https://www.wunderground.com/personal-weather-station/dashboard?ID=KNCBELMO18#history
https://weather.gladstonefamily.net/site/D5686

Ended up using the connector I was originally going to use.

Which one was that?
Here they are. The post are a little more flexible that the other ones so you can spread the pins out some. Not too much though.
https://www.mouser.com/ProductDetail/Harwin/M50-3030442?qs=sGAEpiMZZMs%252bGHln7q6pm8Vn94ktop%2fJmuuS%252b8ZjnXtI6OeGFBMW0g%3d%3d

How does the raw readings of the 75 humidity sensor compares to the airport? Why I ask is that you have done some offset stuff on WD.

The way I have it done on WD the adjusted readings and the raw readings from the sht75 are the same. But in the high end the raw readings only will reach 97-98% my adjusted reading will go to 100%.  I guess the short answer would be it tracks pretty darn close to local ASOS stations.

Edit: When I say the adjusted and raw readings are the same I meant through the mid ranges <75%.

So how does it do over 75%?  Wet or dry bias?

I would say it continues to do very well all the way up until the mid 90% range before the dry bias shows up. Sensor seems to peek at 98%.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 16, 2018, 09:30:35 AM
I would say it continues to do very well all the way up until the mid 90% range before the dry bias shows up. Sensor seems to peek at 98%.
Yeah, I think Davis and Sensirion may be a little at fault here. My new 75 has no problem with 99%-100%, but my 5 y/o one maxes out at 96%. I assume the age of yours is somewhere in between and you're maxing at 98%.

Like you, I find both do well during the day with lower RH.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: hwcorder on August 16, 2018, 09:43:57 AM
I would say it continues to do very well all the way up until the mid 90% range before the dry bias shows up. Sensor seems to peek at 98%.
Yeah, I think Davis and Sensirion may be a little at fault here. My new 75 has no problem with 99%-100%, but my 5 y/o one maxes out at 96%. I assume the age of yours is somewhere in between and you're maxing at 98%.

Like you, I find both do well during the day with lower RH.

I also like the fact that it is quicker to come down from a night of high humidity than the 31.  Something think jerryg also noticed in his tests.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on August 19, 2018, 03:32:32 PM
Did you see what Jerry and I ordered?  I have 2 on the way may not be here until fall however. https://www.aliexpress.com/item/2pcs-Waterproof-Temperature-and-humidity-sensor-shell-SHT10-SHT21-SHT15-shT11-sht20-SHT75-sensor-protective-sleeve/32725766858.html?spm=a2g0s.9042311.0.0.69af4c4dlQrpxK

Others have some like https://www.emesystems.com/sht75pg/documents/SHT75PG.pdf

dendrite are there no worries about contamination using silicone conformal coating?

Just been looking at the pics of that thing on aliexpress, I hope it isn’t as big as it looks in the picture. Would think it would cause the sensor to lag if it’s that big. Hopefully I’m wrong but I’m looking forward to try it out when it gets here.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on August 19, 2018, 04:47:57 PM
Well good news and bad news, i got my order in yesterday and they are not that big but they are not very good. I put one out and it ran 1 degree warmer than it should so ipulled it out and ran some checks with my temp. standard and with just the filter part and not the body it stayed .4 degrees warmer than it should . I held it between my fingers for 5 seconds and it took the temp increase almost 5 minutes to recover from a 1.5 degree increase. Well it is from China and i guess i was hoping for the best. I squeeze the filter and it blew apart, looked like compressed formed paper. But the good news is the body makes a good mount for the sensor, just had to add the sf1 filter to it and i have it running in my main shield right now. I swear i know you can not get anything from China worth a damn, i ordered three break out board with sht15 sensors and nothing but crap, one max humidity at 79 another at 82 and the third minimum was running 7 high total loss. I think they were the rejects from the factory. If it has made in China on it it is pure crap.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Old Tele man on August 19, 2018, 05:00:14 PM
In my best imitation Forrest Gump voice, "Cheap is, as cheap is."
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 21, 2018, 08:29:38 AM
Very thick, prolonged radiation fog this morning. The SHT75 has been at 99% for awhile with a low of 54.2F.

CON has been running 1/4-1/8SM visibility and alternating between a dewpoint depression of 0F and 1F. So with rounding, my hunch is that they're actually around 99% RH in reality. They had a low of 53F.

LCI is coming out of the fog now, but had a similar deal. Mostly 0F, but some 1F dewpoint depressions in the 5 minute obs...probably about 99% RH. They were also 1/8-1/4SM and had a low of 54F.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on August 21, 2018, 08:47:01 AM
Very thick, prolonged radiation fog this morning. The SHT75 has been at 99% for awhile with a low of 54.2F.

CON has been running 1/4-1/8SM visibility and alternating between a dewpoint depression of 0F and 1F. So with rounding, my hunch is that they're actually around 99% RH in reality. They had a low of 53F.

LCI is coming out of the fog now, but had a similar deal. Mostly 0F, but some 1F dewpoint depressions in the 5 minute obs...probably about 99% RH. They were also 1/8-1/4SM and had a low of 54F.

Awesome! I opened my package that has the 75 sensor in it and I didn’t realize how small that sensor really was. Lol
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 21, 2018, 10:30:12 AM
Very thick, prolonged radiation fog this morning. The SHT75 has been at 99% for awhile with a low of 54.2F.

CON has been running 1/4-1/8SM visibility and alternating between a dewpoint depression of 0F and 1F. So with rounding, my hunch is that they're actually around 99% RH in reality. They had a low of 53F.

LCI is coming out of the fog now, but had a similar deal. Mostly 0F, but some 1F dewpoint depressions in the 5 minute obs...probably about 99% RH. They were also 1/8-1/4SM and had a low of 54F.

Awesome! I opened my package that has the 75 sensor in it and I didn’t realize how small that sensor really was. Lol
Very small and very responsive. My setup really is like the raw data from an ASOS. I may move my sensor up to a full 2m (6.5ft) to pull it away from the sfc heating as much as possible without going over the 2m mark (40 CFM fan remember). Either that or I'll put a 5 minute averaging algorithm on the data. For now I'm just accepting the raw readings for what they are. If I put my hand briefly under the air intake of the shield I see a spike in temperature in my data.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on August 21, 2018, 11:07:10 AM
Very thick, prolonged radiation fog this morning. The SHT75 has been at 99% for awhile with a low of 54.2F.

CON has been running 1/4-1/8SM visibility and alternating between a dewpoint depression of 0F and 1F. So with rounding, my hunch is that they're actually around 99% RH in reality. They had a low of 53F.

LCI is coming out of the fog now, but had a similar deal. Mostly 0F, but some 1F dewpoint depressions in the 5 minute obs...probably about 99% RH. They were also 1/8-1/4SM and had a low of 54F.

Awesome! I opened my package that has the 75 sensor in it and I didn’t realize how small that sensor really was. Lol
Very small and very responsive. My setup really is like the raw data from an ASOS. I may move my sensor up to a full 2m (6.5ft) to pull it away from the sfc heating as much as possible without going over the 2m mark (40 CFM fan remember). Either that or I'll put a 5 minute averaging algorithm on the data. For now I'm just accepting the raw readings for what they are. If I put my hand briefly under the air intake of the shield I see a spike in temperature in my data.

Nice!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on August 21, 2018, 08:07:31 PM
Well... it happened. Looks like my good 31 just went bad on humidity.

My first sign was when I noticed this evening the FARS dewpoint exceeded the "bad" 31 in my passive shield. That was my "oh shite" moment.

I just measured a 10% humidity error with my Kestrel. Today has been quite brisk with well-mixed conditions, overcast, NW winds. The airport, which is about 15 miles inland (normally difficult to compare to except on days like this), is reporting a 50 degree dewpoint (I was showing 53.3 vs 49.6 on my Kestrel).

What preceded this? Fog and mist last night. Reached 94% humidity for the first time since July 1st.

Maybe it will snap back once we get a dry spell (tomorrow?). But I'm not holding my breath.

 :evil: :evil: :evil:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: galfert on August 21, 2018, 11:21:36 PM
Well... it happened. Looks like my good 31 just went bad on humidity.


So that was what? Like 2 and a half weeks old? This is the new 31 in the 7714 right?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on August 22, 2018, 07:21:06 AM
Well... it happened. Looks like my good 31 just went bad on humidity.


So that was what? Like 2 and a half weeks old? This is the new 31 in the 7714 right?

No this was the newer (9 months old) 31 in my FARS that was still accurate on humidity as of the day before. The 31 in my passive shield (16 months old) has run wet since I first started testing it months ago.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on August 22, 2018, 10:51:30 AM
UPDATE: Looks like the FARS 31 has returned to calibration! Only took 48 hours  ](*,)

We're presently at ~50% humidity. I guess my sensor isn't hosed (yet) and it still returns to spec after it's been below 80% humidity for some time (~9 hours).
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 23, 2018, 11:07:47 PM
Has anyone ever tried artificially warming a 2nd SHT sensor to always keep the RH below 75% and then calculate the dewpoint from that?
 
Something similar to the warmed probe from Vaisala.

https://www.vaisala.com/sites/default/files/documents/CEN-TIA-Warmed-Probe-Application-Note-B211246EN-A_0.pdf
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on August 25, 2018, 07:51:21 PM
Welp, so much for that. I got another new 31 today from Ryan that was properly packaged, ie, no bubble wrap. As is, my 8 month old 31 in use has a 2% humidity subtraction for the wet bias, and does a very nice job there, for now. The new (properly packaged 31, have you), is even wetter at ~1.3F higher dew and ~2% higher humidity than the old 31 in use. I brought the ISS inside for testing to stabilize the data and found that the old, drier 31 tracts right in line (with the 2% subtraction) with SHT11 in my Vue, dew and humidity. Point being, the Vue's SHT11 looks spot on with no correction. Davis obviously assembles both, why does the console not have the wet bias?
This QC blows, and frankly is unacceptable. ](*,)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on August 25, 2018, 08:58:39 PM
Welp, so much for that. I got another new 31 today from Ryan that was properly packaged, ie, no bubble wrap. As is, my 8 month old 31 in use has a 2% humidity subtraction for the wet bias, and does a very nice job there, for now. The new (properly packaged 31, have you), is even wetter at ~1.3F higher dew and ~2% higher humidity than the old 31 in use. I brought the ISS inside for testing to stabilize the data and found that the old, drier 31 tracts right in line (with the 2% subtraction) with SHT11 in my Vue, dew and humidity. Point being, the Vue's SHT11 looks spot on with no correction. Davis obviously assembles both, why does the console not have the wet bias?
This QC blows, and frankly is unacceptable.  ](*,)

It probably all boils down to how Davis mounts the 31 chip into the PCB. They should’ve done at least what RW does and others by making a temp/hum probe. But who knows. All I can say is that, as of now, the best thing you can do is to switch to the SHT-75. I’m in the process of doing just that for my two field stations and for one of the fruit growers that I know here in Chilton County, AL.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 26, 2018, 07:46:22 AM
Welp, so much for that. I got another new 31 today from Ryan that was properly packaged, ie, no bubble wrap. As is, my 8 month old 31 in use has a 2% humidity subtraction for the wet bias, and does a very nice job there, for now. The new (properly packaged 31, have you), is even wetter at ~1.3F higher dew and ~2% higher humidity than the old 31 in use. I brought the ISS inside for testing to stabilize the data and found that the old, drier 31 tracts right in line (with the 2% subtraction) with SHT11 in my Vue, dew and humidity. Point being, the Vue's SHT11 looks spot on with no correction. Davis obviously assembles both, why does the console not have the wet bias?
This QC blows, and frankly is unacceptable. ](*,)

I got one also the other day and for a new sensor it's not good.  Struggles to reach 98% and has wet bias.
I think these are old sensors maybe couple years now open air stored at Davis prior to shipping out to retailer so don't blame SI at all.
Seems some of best sensors reported are people seeing 100% coming with new stations until they go bad.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 26, 2018, 07:50:32 AM
Welp, so much for that. I got another new 31 today from Ryan that was properly packaged, ie, no bubble wrap. As is, my 8 month old 31 in use has a 2% humidity subtraction for the wet bias, and does a very nice job there, for now. The new (properly packaged 31, have you), is even wetter at ~1.3F higher dew and ~2% higher humidity than the old 31 in use. I brought the ISS inside for testing to stabilize the data and found that the old, drier 31 tracts right in line (with the 2% subtraction) with SHT11 in my Vue, dew and humidity. Point being, the Vue's SHT11 looks spot on with no correction. Davis obviously assembles both, why does the console not have the wet bias?
This QC blows, and frankly is unacceptable.  ](*,)

It probably all boils down to how Davis mounts the 31 chip into the PCB. They should’ve done at least what RW does and others by making a temp/hum probe. But who knows. All I can say is that, as of now,

Agree 100%. My acurite sht31 doesn't have the same issues.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on August 26, 2018, 04:22:54 PM
Welp, so much for that. I got another new 31 today from Ryan that was properly packaged, ie, no bubble wrap. As is, my 8 month old 31 in use has a 2% humidity subtraction for the wet bias, and does a very nice job there, for now. The new (properly packaged 31, have you), is even wetter at ~1.3F higher dew and ~2% higher humidity than the old 31 in use. I brought the ISS inside for testing to stabilize the data and found that the old, drier 31 tracts right in line (with the 2% subtraction) with SHT11 in my Vue, dew and humidity. Point being, the Vue's SHT11 looks spot on with no correction. Davis obviously assembles both, why does the console not have the wet bias?
This QC blows, and frankly is unacceptable. ](*,)
don't blame SI at all.

Oh, absolutely not! I even confirmed with Ryan that I could return this one if it sucks, read above :roll:, and I will. He's even RMA-ing all his bubble wrapped stuff back to Davis, although it doesn't seem to matter....I'll see if he'll let me buy three or so and see if I get lucky with one and return the others.
Maybe if we swamp Davis with 31 returns they'll get the picture a little more in focus. :-x
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: JudinNorman on August 26, 2018, 04:45:04 PM
Welp, so much for that. I got another new 31 today from Ryan that was properly packaged, ie, no bubble wrap. As is, my 8 month old 31 in use has a 2% humidity subtraction for the wet bias, and does a very nice job there, for now. The new (properly packaged 31, have you), is even wetter at ~1.3F higher dew and ~2% higher humidity than the old 31 in use. I brought the ISS inside for testing to stabilize the data and found that the old, drier 31 tracts right in line (with the 2% subtraction) with SHT11 in my Vue, dew and humidity. Point being, the Vue's SHT11 looks spot on with no correction. Davis obviously assembles both, why does the console not have the wet bias?
This QC blows, and frankly is unacceptable. ](*,)
don't blame SI at all.

Oh, absolutely not! I even confirmed with Ryan that I could return this one if it sucks, read above :roll:, and I will. He's even RMA-ing all his bubble wrapped stuff back to Davis, although it doesn't seem to matter....I'll see if he'll let me buy three or so and see if I get lucky with one and return the others.
Maybe if we swamp Davis with 31 returns they'll get the picture a little more in focus. :-x


Has anyone had any correspondence with Davis on this issue?

I'm pretty bummed out my 10+ year monitor 2 dewpoints track local weather service station while my just over a year old sht31 has gone out of calibration with dewpoints a few degrees higher usually.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 26, 2018, 04:51:16 PM
I'm running a 75 right now (Thanks Jerry) and do see dp running lower around 2 degrees  vs SHT31's . This removes any wet bias. Problem is when I did the damp rag test for 2 hours max was only 97%.
I'm testing inside the PRO meteo shield along with a 31 and plan on doing this for few days and see what I get in the morning against the SHT31 in same shield. Also want to watch temperature. They run almost identical on temps so far in the 80-90° range.

Currently the 75 is very close to airport 1.3 mi away on DP 63°  https://mesowest.utah.edu/cgi-bin/droman/meso_base_dyn.cgi?stn=KVTN&unit=0&timetype=LOCAL/

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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on August 26, 2018, 04:57:08 PM
Welp, so much for that. I got another new 31 today from Ryan that was properly packaged, ie, no bubble wrap. As is, my 8 month old 31 in use has a 2% humidity subtraction for the wet bias, and does a very nice job there, for now. The new (properly packaged 31, have you), is even wetter at ~1.3F higher dew and ~2% higher humidity than the old 31 in use. I brought the ISS inside for testing to stabilize the data and found that the old, drier 31 tracts right in line (with the 2% subtraction) with SHT11 in my Vue, dew and humidity. Point being, the Vue's SHT11 looks spot on with no correction. Davis obviously assembles both, why does the console not have the wet bias?
This QC blows, and frankly is unacceptable. ](*,)
don't blame SI at all.

Oh, absolutely not! I even confirmed with Ryan that I could return this one if it sucks, read above :roll:, and I will. He's even RMA-ing all his bubble wrapped stuff back to Davis, although it doesn't seem to matter....I'll see if he'll let me buy three or so and see if I get lucky with one and return the others.
Maybe if we swamp Davis with 31 returns they'll get the picture a little more in focus. :-x
Has anyone had any correspondence with Davis on this issue?
Oh, they're aware....
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on August 26, 2018, 05:15:25 PM
...
Has anyone had any correspondence with Davis on this issue?

I'm pretty bummed out my 10+ year monitor 2 dewpoints track local weather service station while my just over a year old sht31 has gone out of calibration with dewpoints a few degrees higher usually.

If you go back to earlier postings in this thread, yes, Davis is aware of the issues associated with the SHT31, but to the best of my knowledge, no one has received any feedback since it was brought to Davis’ attention.

Along these lines, I bought an additional VP2 station from Ryan at SI about three months ago, and recently its 31 sensor has become unreliable too—even the normally reliable temperature is whacky. I now have three VP2s with the SHT31 about 30 feet apart so I can easily compare. And, this morning none of my stations reported 100% humidity, but all of my windows were dripping wet and nearby reliable stations showed 100% humidity!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 26, 2018, 08:24:42 PM
I'm running a 75 right now (Thanks Jerry) and do see dp running lower around 2 degrees  vs SHT31's . This removes any wet bias. Problem is when I did the damp rag test for 2 hours max was only 97%.
I'm testing inside the PRO meteo shield along with a 31 and plan on doing this for few days and see what I get in the morning against the SHT31 in same shield. Also want to watch temperature. They run almost identical on temps so far in the 80-90° range.

Currently the 75 is very close to airport 1.3 mi away on DP 63°  https://mesowest.utah.edu/cgi-bin/droman/meso_base_dyn.cgi?stn=KVTN&unit=0&timetype=LOCAL/
Yeah...give it a little time. I've routinely been hitting 98-99% every night with the brief touching of 100% some days. Have you also seen a big difference in the temp response compared to the 31...especially during the day with better mixing?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 26, 2018, 09:24:31 PM
Haven't noticed yet but been busy. I did see difference in temps I'll need to check out. May post graph later.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 26, 2018, 09:45:43 PM
Red box is when the 75 went in service. Both sensors in same Pro shield.
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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: galfert on August 26, 2018, 10:20:23 PM
What software is that you are using to make those nice comparisons graphs?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 26, 2018, 10:23:15 PM
Randy...do you have all of those sensors online? I know your E3958 one.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on August 26, 2018, 10:24:16 PM
I'm running a 75 right now (Thanks Jerry) and do see dp running lower around 2 degrees  vs SHT31's . This removes any wet bias. Problem is when I did the damp rag test for 2 hours max was only 97%.
I'm testing inside the PRO meteo shield along with a 31 and plan on doing this for few days and see what I get in the morning against the SHT31 in same shield. Also want to watch temperature. They run almost identical on temps so far in the 80-90° range.

Currently the 75 is very close to airport 1.3 mi away on DP 63°  https://mesowest.utah.edu/cgi-bin/droman/meso_base_dyn.cgi?stn=KVTN&unit=0&timetype=LOCAL/

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So can I view your 75 data online on your website?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 26, 2018, 11:31:24 PM
Main sensor E3958 and website click on globe for live data is now the SHT75 with no offset for humidity like I had for the 31.
The other sensors are not online. I'm using envoy8x and the software it comes with WDTU which is capable of exporting to excel for graphs.

5 min update to cwop here:
https://mesowest.utah.edu/cgi-bin/droman/meso_base_dyn.cgi?stn=E3958&unit=0&timetype=LOCAL

If you want to compare dp with airport here:

https://mesowest.utah.edu/cgi-bin/droman/meso_base_dyn.cgi?stn=KVTN&unit=0&timetype=LOCAL/
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on August 27, 2018, 12:23:32 AM
Main sensor E3958 and website click on globe for live data is now the SHT75 with no offset for humidity like I had for the 31.
The other sensors are not online. I'm using envoy8x and the software it comes with WDTU which is capable of exporting to excel for graphs.

5 min update to cwop here:
https://mesowest.utah.edu/cgi-bin/droman/meso_base_dyn.cgi?stn=E3958&unit=0&timetype=LOCAL

If you want to compare dp with airport here:

https://mesowest.utah.edu/cgi-bin/droman/meso_base_dyn.cgi?stn=KVTN&unit=0&timetype=LOCAL/


Thanks!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 27, 2018, 10:42:37 AM
Looks like you're running the usual 2F cooler than the ASOS, but you've been consistently 2F cooler on dewpoint as well. You were fairly well mixed overnight though. I'm interested to see how the daytime dewpoint track.

I noticed your MADIS data is consistently staggered 1hr from the analysis. What's up with that? It looks like MADIS is lagging 1hr behind your station. Is this some kind of CDT vs CST issue? It looks like your graphs would be fine if you set your clocks back an hour.

https://weather.gladstonefamily.net/site/E3958
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 27, 2018, 11:01:14 AM
Looks like you're running the usual 2F cooler than the ASOS, but you've been consistently 2F cooler on dewpoint as well. You were fairly well mixed overnight though. I'm interested to see how the daytime dewpoint track.

I noticed your MADIS data is consistently staggered 1hr from the analysis. What's up with that? It looks like MADIS is lagging 1hr behind your station. Is this some kind of CDT vs CST issue? It looks like your graphs would be fine if you set your clocks back an hour.

https://weather.gladstonefamily.net/site/E3958

Madis is most likely a time zone issue. We are right on the switch over to Mountain.
This might not be the best days ahead to track dp. Later this week back into 90's I think.
Everything is 60° currently. 
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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 27, 2018, 04:36:36 PM
Looks like you're running the usual 2F cooler than the ASOS, but you've been consistently 2F cooler on dewpoint as well. You were fairly well mixed overnight though. I'm interested to see how the daytime dewpoint track.


https://weather.gladstonefamily.net/site/E3958

I noticed today my Main online live Console still had the -.9 left from using the 31 and I'm pretty sure I took it out last night. So yes the live online was .9 low until about 3:10 pm today. Can't believe I forgot it.
Not sure what's up I remember specifically taking it out at console maybe it didn't save is all I can think of. Graphs it was out for sure on the envoy8x. I do have 2 different consoles and the envoy8x anytime I do something with sensors so it gets confusing at times with this offset BS.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on August 27, 2018, 09:05:12 PM
Been keeping track of your DP readings Randy. Been impressed so far. Seems like it’s neck to neck with the ASOS. Sometimes it’s lower.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 27, 2018, 09:10:06 PM
Been keeping track of your DP readings Randy. Been impressed so far. Seems like it’s neck to neck with the ASOS. Sometimes it’s lower.


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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 28, 2018, 09:36:27 AM
Did a chest freezer test on sht31 vs 75 this morning for anyone considering change over living in colder climate. Results unscientific with sensors tied together. Let freezer go through 3 cooling cycles so took over an hour. Temperature difference was either .5 or .6 with the SHT75 reading warmer.
As freezer warmed up the 75 would really leave the 31 behind  (+3)  before compressor started again. Then on cool down it rapidly made the 3 degree difference up catching the 31 within couple minutes of compressor kicking on.  The 75 does adjust rapidly to temperature change.
  Below zero
SHT75 -8.2  SHT31 -8.7
SHT75 -8.1  SHT31 -8.6
SHT75 -7.9  SHT31 -8.4
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on August 28, 2018, 10:14:52 AM
Did a chest freezer test on sht31 vs 75 this morning for anyone considering change over living in colder climate. Results unscientific with sensors tied together. Let freezer go through 3 cooling cycles so took over an hour. Temperature difference was either .5 or .6 with the SHT75 reading warmer.
As freezer warmed up the 75 would really leave the 31 behind  (+3)  before compressor started again. Then on cool down it rapidly made the 3 degree difference up catching the 31 within couple minutes of compressor kicking on.  The 75 does adjust rapidly to temperature change.
  Below zero
SHT75 -8.2  SHT31 -8.7
SHT75 -8.1  SHT31 -8.6
SHT75 -7.9  SHT31 -8.4

That’s not bad at all. Rarely if Alabama gets below zero but looks like the 75 isn’t a bad performer in that range. Wonder what your calibrated thermometer would’ve read?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 28, 2018, 10:19:58 AM
Did a chest freezer test on sht31 vs 75 this morning for anyone considering change over living in colder climate. Results unscientific with sensors tied together. Let freezer go through 3 cooling cycles so took over an hour. Temperature difference was either .5 or .6 with the SHT75 reading warmer.
As freezer warmed up the 75 would really leave the 31 behind  (+3)  before compressor started again. Then on cool down it rapidly made the 3 degree difference up catching the 31 within couple minutes of compressor kicking on.  The 75 does adjust rapidly to temperature change.
  Below zero
SHT75 -8.2  SHT31 -8.7
SHT75 -8.1  SHT31 -8.6
SHT75 -7.9  SHT31 -8.4

That’s not bad at all. Rarely if Alabama gets below zero but looks like the 75 isn’t a bad performer in that range. Wonder what your calibrated thermometer would’ve read?
The specs in that range are around +/- 1.2C for the 75 and +/- 0.25 for the 31. So yeah, only a consistent 0.5F difference isn't too bad. Heck, maybe the 31 is -0.2C and the 75 is +0.3C.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 28, 2018, 11:49:34 AM
I'll know more this winter with 31 as temperature backup. The freezer may not be that reliable but sure looked decent after 3 cycles. The 75's for sure are within specs both humidity and temperature unlike the 31's.

Update: I don't have a certified in that range or surely would of used it. I tested the certified not this range it read -9.7 so about 1° lower than the 31.. Guess its possible the SHT31 is reading on high side and why not that big a difference between the 75...If anyone has a certified below zero I would like to see results.   
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 28, 2018, 11:55:23 AM
I went back and looked at the certified and it was only -9.7. Response time was so fast I thought it was more. 

Very possible if the certified at 0c is more accurate than the 31 means the 31's aren't even accurate on the low end. It is little odd the 75 was within .5F.

Here are specs on certified:±0.05°C  within –2.00 to 2.00°C (28.4° to 35.6°F),
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on August 28, 2018, 12:00:40 PM
I may have to run that test, my calibrated meter has a calibration point of -10 degrees and is right on calibration wise. I will have to try it when i get the new sensor out of the standard shield.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 28, 2018, 12:08:35 PM
I may have to run that test, my calibrated meter has a calibration point of -10 degrees and is right on calibration wise. I will have to try it when i get the new sensor out of the standard shield.

Good deal Jerry.
I'm wondering about the 31 accuracy down low now.
Added the specs to certified thermometer I used above post. 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 28, 2018, 12:23:37 PM
I may have to run that test, my calibrated meter has a calibration point of -10 degrees and is right on calibration wise. I will have to try it when i get the new sensor out of the standard shield.

Good deal Jerry.
I'm wondering about the 31 accuracy down low now.
Added the specs to certified thermometer I used above post.
How well do they all match up at a warmer temp in the fridge?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 28, 2018, 01:07:23 PM
I may have to run that test, my calibrated meter has a calibration point of -10 degrees and is right on calibration wise. I will have to try it when i get the new sensor out of the standard shield.

Good deal Jerry.
I'm wondering about the 31 accuracy down low now.
Added the specs to certified thermometer I used above post.
How well do they all match up at a warmer temp in the fridge?

Not sure they match well in meteo shield, within a tenth in 50's.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on August 28, 2018, 02:26:41 PM
Interesting results, calibrated meter accuracy +/- .3F   meter -3.4  31-3.7 75-2.4 15-1.4 so figuring the meter between -3.1 to -3.7 it looks like the 31 is the winner which i would have expected based on specs. the 75 is pretty close to specs for it, not good on low end. The 15 is much better in the south than the north lol.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 28, 2018, 02:43:02 PM
Interesting results, calibrated meter accuracy +/- .3F   meter -3.4  31-3.7 75-2.4 15-1.4 so figuring the meter between -3.1 to -3.7 it looks like the 31 is the winner which i would have expected based on specs. the 75 is pretty close to specs for it, not good on low end. The 15 is much better in the south than the north lol.
Nice work from you two guys. Now I anxiously await the pin 35 and an easy way to implement it with a VP2.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on August 28, 2018, 03:08:26 PM
Put another 75 in the freezer and first check was meter -2.4 and 75 -2.1 will let them cool a while longer but looks like reference temp is important on the 75 at low temps. If 31 sensors are basically close then comparing to 31 should work out. I will check other 31 later to see if it is close like the first one.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 28, 2018, 03:15:35 PM


I was letting them cycle all the way to bottom and record where they turn.  Can't trust between because some sensors drop faster then others.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on August 28, 2018, 03:27:51 PM
I was letting them cycle all the way to bottom and record where they turn.  Can't trust between because some sensors drop faster then others.
Yes. Even monitoring just one sensor I always wait for it to reverse course and go from there, then you're sure.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 28, 2018, 03:38:12 PM
I'm data logging both the 75 and 31 so there is no question.  8-) The certified will just record lowest temperature.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on August 28, 2018, 03:43:49 PM
Well the meter has been at -3.7 for a while and the 75 is at -2.9 still looking like the 75 is short on temp. But the specs at this temp tells it like it is.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 28, 2018, 03:55:50 PM
It's all being data logged but here is a look at zero on the SHT31 the 75 was + (.5). I cranked freezer to colder setting so test still going. I'll let it cycle a few times to make sure it repeats.
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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 28, 2018, 03:59:25 PM
At -4 we are at (+1) on 75.
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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 28, 2018, 04:07:31 PM
8 below
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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 28, 2018, 04:09:36 PM
9 below
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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 28, 2018, 04:13:19 PM
10 below. No way I can use the 75 in winter.
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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 28, 2018, 04:17:43 PM
11 below Certified is running with the SHT31 also.
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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 28, 2018, 04:33:12 PM
-12.1  below and it's going to turn the certified recorded -12.29, Sht 31 -12.1, SHT75 -10.0 F
I've seen enough. ending my test. Once it got to -2 degrees difference seemed to stay. Not sure what it would do even colder but I can't use in winter when its been -39F here.
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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on August 28, 2018, 04:35:49 PM
11 below Certified is running with the SHT31 also.
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That’s a bummer but then again, the spec sheet did say that it should start drifting below 0°F
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 28, 2018, 04:43:41 PM
Nice obs Randy. I'll probably get a new 31 and swap it out from the 75 before we start getting well below freezing. I still like the idea of the low thermal mass pin model, but I'll have to wait for the 35. rdsman is doing good work over in that other thread with reading the data from the 35.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on August 28, 2018, 05:01:59 PM
Well it looks to me like none of the sensors like it cold  :lol:. You need to move down here where freezing temps are a rarity. The meter has been steady on -3.9 and the 31 has been holding at -2.9 no better than the 75. It looks like in cold weather areas you need to cull out the sensors to find one like the first 31. Heck you get down to that temp what difference does a degree or to make anyway, cold is cold  :lol:.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 28, 2018, 05:05:22 PM
Nice obs Randy. I'll probably get a new 31 and swap it out from the 75 before we start getting well below freezing. I still like the idea of the low thermal mass pin model, but I'll have to wait for the 35. rdsman is doing good work over in that other thread with reading the data from the 35.

Really, I need to visit but so busy over here. LOL

Yeah this was good info for those running the 75 and have an occasional below zero day. I average over 20 a year so it's a little much so will also swapp the 75 for the 31 come cold weather unless we get other options like the 35.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 28, 2018, 05:07:06 PM
Well it looks to me like none of the sensors like it cold  :lol:. You need to move down here where freezing temps are a rarity. The meter has been steady on -3.9 and the 31 has been holding at -2.9 no better than the 75. It looks like in cold weather areas you need to cull out the sensors to find one like the first 31. Heck you get down to that temp what difference does a degree or to make anyway, cold is cold  :lol:.

Most of my family is down in Houston area. But I live in weather paradise right here.. :lol:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 28, 2018, 05:33:20 PM
Final graph on low temperature test.
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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on August 28, 2018, 06:09:24 PM
Frankly, at -11F, I was quite impressed with the 31. Even the dew seemed to play along with the 75.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on August 28, 2018, 06:16:22 PM
Man Randy if you consider -30 weather heaven then i think you brain froze at some point  :lol:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on August 28, 2018, 06:24:19 PM
Man Randy if you consider -30 weather heaven then i think you brain froze at some point  :lol:
Took the words right outta my mouth, then I thought about the three or four days in a row last year I was over 116F and decided to  :-$.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on August 28, 2018, 08:44:36 PM
Update subzero  meter -5.2  three 31's read -5.4   -5.5    and the worst one -6.0
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 29, 2018, 06:11:32 AM
I've ran multiple 31's since they came out (usually 3 different stations together) and the one thing I like besides accuracy on temperature new or old given exact same conditions they will run .1f or less of each other for hours usually at night where conditions are the same. Nothing like the old 11 and 15's where you could have new sensors running 1/2 degree apart.

Testing these passive shields has changed the consistency a little. All were fars units prior and now I have 3 different passives and only 1 fars running. 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on August 29, 2018, 09:01:13 AM
I always thought that about the 31 but i have been watching my 3 75's for the last hour and they too have been sitting on 77.0 and 94% humidity with a .1 temp difference now and then. It looks like the 75 has that same ability before the sun comes up and changes things. So 75 in warmer times and 31 in colder times for temp. Like you said it should double the useful life of your sensors using them 6 months out of the year.  :grin:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 29, 2018, 11:16:35 AM
Yes IMO the 76  75 is solid performer where humidity plays a big role which happens to be where most people live because of man's attraction to water which was once needed for commerce and travel.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on August 29, 2018, 12:13:48 PM
Yes IMO the 76  75 is solid performer where humidity plays a big role which happens to be where most people live because of man's attraction to water which was once needed for commerce and travel.

It would be interesting to see how the 75 performs in the 0°-30°F range. Based from another thread, it seems like the 75 performs well when temps are in the upper 20s in comparison with the 35
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on August 29, 2018, 01:10:15 PM
I don't think that was a temp comparison or that it had anything to do with temp accuracy. It's his attempt to convert the I2C to sensibus. He's trying to understand how the 75 is read and adapt the 35 to that. Basically he has it down to 0.0-0.1F and 0.0-0.1% RH. IOW, what the 75 would report as say 85.4F, the 35 would read it as 85.3F or 85.4F. So there is probably some 0.05F error in there which he is trying to fine tune. Personally, a 0.0-0.1F conversion error on top of the 0.1-0.2F sensor accuracy is good enough for me.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on August 29, 2018, 03:16:06 PM
I don't think that was a temp comparison or that it had anything to do with temp accuracy. It's his attempt to convert the I2C to sensibus. He's trying to understand how the 75 is read and adapt the 35 to that. Basically he has it down to 0.0-0.1F and 0.0-0.1% RH. IOW, what the 75 would report as say 85.4F, the 35 would read it as 85.3F or 85.4F. So there is probably some 0.05F error in there which he is trying to fine tune. Personally, a 0.0-0.1F conversion error on top of the 0.1-0.2F sensor accuracy is good enough for me.

Aight. Gotcha.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: rdsman on August 29, 2018, 04:20:25 PM
I don't think that was a temp comparison or that it had anything to do with temp accuracy. It's his attempt to convert the I2C to sensibus. He's trying to understand how the 75 is read and adapt the 35 to that. Basically he has it down to 0.0-0.1F and 0.0-0.1% RH. IOW, what the 75 would report as say 85.4F, the 35 would read it as 85.3F or 85.4F.

You are correct sir..........
 I probably need to amend that post!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 31, 2018, 09:03:58 AM
For those interested in the sht75 and may not have soldering skills or any luck with the crimping version, I've found this option. Haven't done this myself however so nothing confirmed.
The plug/cord was made for the sht75.... called 3M-CABLES FOR EK-H4 made for evaluation kit.
 
The plug is 3 meters and rather expensive at $80. You only need 1 however as the sensors go bad just replace the SHT75.
Looks like the connector on the stock plugs is the RJ12 which has 6 gold connectors. Only 4 will be used however. You will need to figure out which 4 go where for the sensor to work and crimp plug accordingly with tool.

I referenced this video about plugs on youtube to determine what was being used. The stock sht31 plug has 6 contacts the red and black are not connected to the SHT31 sensor.

Youtube video on plugs: https://www.youtube.com/watch?v=fjNCUDrdULw

Link to sht75 plug/cord sold by digikey: https://www.digikey.com/product-detail/en/sensirion-ag/3M-CABLES-FOR-EK-H4/1649-1055-ND/5982944

Anyone with more knowledge please chime in on this.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 31, 2018, 09:28:22 AM
Yesterday the SHT75 and 31 temperature were the same all day within .3F because wind speeds were up so humidity should have also followed and been the same. They weren't. 
This is graph shows humidity difference throughout day. As you can see morning and evening are the same but during critical time of day when temperatures go up the 75 runs in normal range vs the 31 averaging about 5% higher sometimes more sometimes less.  DP averages about 2°F higher on 31 also. 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on August 31, 2018, 09:34:30 AM
For those interested in the sht75 and may not have soldering skills or any luck with the crimping version, I've found this option. Haven't done this myself however so nothing confirmed.
The plug/cord was made for the sht75.... called 3M-CABLES FOR EK-H4 made for evaluation kit.
 
The plug is 3 meters and rather expensive at $80. You only need 1 however as the sensors go bad just replace the SHT75.
Looks like the connector on the stock plugs is the RJ12 which has 6 gold connectors. Only 4 will be used however. You will need to figure out which 4 go where for the sensor to work and crimp plug accordingly with tool.

I referenced this video about plugs on youtube to determine what was being used. The stock sht31 plug has 6 contacts the red and black are not connected to the SHT31 sensor.

Youtube video on plugs: https://www.youtube.com/watch?v=fjNCUDrdULw

Link to sht75 plug/cord sold by digikey: https://www.digikey.com/product-detail/en/sensirion-ag/3M-CABLES-FOR-EK-H4/1649-1055-ND/5982944

Anyone with more knowledge please chime in on this.

According to the datasheet (https://www.sensirion.com/fileadmin/user_upload/customers/sensirion/Dokumente/0_Datasheets/Humidity/Sensirion_Humidity_Sensors_EK-H4_Datasheet.pdf (https://www.sensirion.com/fileadmin/user_upload/customers/sensirion/Dokumente/0_Datasheets/Humidity/Sensirion_Humidity_Sensors_EK-H4_Datasheet.pdf)), the cable terminates in an RJ45 jack. So you would need to chop that off and splice it into the end of a 31/15/11 sensor cable or the like.

Also might want to put some conformal coating over the sensor end of the cable too to prevent water ingress.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 31, 2018, 09:38:01 AM
For those interested in the sht75 and may not have soldering skills or any luck with the crimping version, I've found this option. Haven't done this myself however so nothing confirmed.
The plug/cord was made for the sht75.... called 3M-CABLES FOR EK-H4 made for evaluation kit.
 
The plug is 3 meters and rather expensive at $80. You only need 1 however as the sensors go bad just replace the SHT75.
Looks like the connector on the stock plugs is the RJ12 which has 6 gold connectors. Only 4 will be used however. You will need to figure out which 4 go where for the sensor to work and crimp plug accordingly with tool.

I referenced this video about plugs on youtube to determine what was being used. The stock sht31 plug has 6 contacts the red and black are not connected to the SHT31 sensor.

Youtube video on plugs: https://www.youtube.com/watch?v=fjNCUDrdULw

Link to sht75 plug/cord sold by digikey: https://www.digikey.com/product-detail/en/sensirion-ag/3M-CABLES-FOR-EK-H4/1649-1055-ND/5982944

Anyone with more knowledge please chime in on this.

According to the datasheet (https://www.sensirion.com/fileadmin/user_upload/customers/sensirion/Dokumente/0_Datasheets/Humidity/Sensirion_Humidity_Sensors_EK-H4_Datasheet.pdf (https://www.sensirion.com/fileadmin/user_upload/customers/sensirion/Dokumente/0_Datasheets/Humidity/Sensirion_Humidity_Sensors_EK-H4_Datasheet.pdf)), the cable terminates in an RJ45 jack. So you would need to chop that off and splice it into the end of a 31/15/11 sensor cable or the like.

Also might want to put some conformal coating over the sensor end of the cable too to prevent water ingress.

So should work no problem with a crimping tool $13 and RJ12 jack?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on August 31, 2018, 09:52:55 AM
For those interested in the sht75 and may not have soldering skills or any luck with the crimping version, I've found this option. Haven't done this myself however so nothing confirmed.
The plug/cord was made for the sht75.... called 3M-CABLES FOR EK-H4 made for evaluation kit.
 
The plug is 3 meters and rather expensive at $80. You only need 1 however as the sensors go bad just replace the SHT75.
Looks like the connector on the stock plugs is the RJ12 which has 6 gold connectors. Only 4 will be used however. You will need to figure out which 4 go where for the sensor to work and crimp plug accordingly with tool.

I referenced this video about plugs on youtube to determine what was being used. The stock sht31 plug has 6 contacts the red and black are not connected to the SHT31 sensor.

Youtube video on plugs: https://www.youtube.com/watch?v=fjNCUDrdULw

Link to sht75 plug/cord sold by digikey: https://www.digikey.com/product-detail/en/sensirion-ag/3M-CABLES-FOR-EK-H4/1649-1055-ND/5982944

Anyone with more knowledge please chime in on this.

According to the datasheet (https://www.sensirion.com/fileadmin/user_upload/customers/sensirion/Dokumente/0_Datasheets/Humidity/Sensirion_Humidity_Sensors_EK-H4_Datasheet.pdf (https://www.sensirion.com/fileadmin/user_upload/customers/sensirion/Dokumente/0_Datasheets/Humidity/Sensirion_Humidity_Sensors_EK-H4_Datasheet.pdf)), the cable terminates in an RJ45 jack. So you would need to chop that off and splice it into the end of a 31/15/11 sensor cable or the like.

Also might want to put some conformal coating over the sensor end of the cable too to prevent water ingress.

So should work no problem with a crimping tool $13 and RJ12 jack?

You wouldn't need to crimp if you can re-purpose an existing Davis sensor cable. But if you want to go with a fresh RJ12 plug, then, yes, you'd need to wire in the jack appropriately.

Here's the guide for the Davis cable:
White (clock)
Yellow (vdd)
Green (gnd)
Blue (data)

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 31, 2018, 10:07:49 AM
Good deal Jerry just PM'd me, I was asking him for direction on this also.  He said another option is just splice old plug directly no crimping tool necessary. 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on September 01, 2018, 06:59:29 PM
Waiting on the glue to dry. The connector is definitely flimsy and you would need the board for support. This is going to be installed in my station at my friend’s farm with the Davis stock passive shield.

I’m now curious if the Campbell Scientific version of the 75 would work with the Davis transmitter? If I could use the CS version (which is a probe) for the MeteoShield Pro shield, that would be awesome.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on September 01, 2018, 10:13:37 PM
So what are the observations of the 75 so far? I like mine (sitting at 59.1F and 97% RH @ 10pm right now) and will probably stick with it until November. Unfortunately I feel we'll be waiting awhile for a pin SHT35 although if we could get the i2c to sensibus converter we could use that breakout from Closed Cube.

https://www.tindie.com/products/closedcube/sht35-d-digital-humidity-and-temperature-sensor/

Anyway, hope all of you are seeing RH readings more in spec with the 75.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on September 01, 2018, 10:29:26 PM
Yesterday the SHT75 and 31 temperature were the same all day within .3F because wind speeds were up so humidity should have also followed and been the same. They weren't. 
This is graph shows humidity difference throughout day. As you can see morning and evening are the same but during critical time of day when temperatures go up the 75 runs in normal range vs the 31 averaging about 5% higher sometimes more sometimes less.  DP averages about 2°F higher on 31 also.
Just saw this. I noticed your dewpoint charts on Gladstone are right in line and you've been running neck and neck generally with VTN.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on September 02, 2018, 09:00:11 AM
Better vs 31 on humidity. I'll swap 75 for 31 in November for temperature. I may keep 75 in one of the shields so differences can be logged at low temperatures . At -11 we know its reading +2 high and seemed to bottom out at +2 but without a colder deep freeze won't know for sure.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on September 02, 2018, 10:18:42 AM
Well i can not be sure but i think i just had a 75 fall victim to the fars, i had a new 75 running in the fars for comparison and it hit 99 a couple times but alas it now won't go over 95 percent sometimes bumps 96. We had several nights of light fog and the humidity was high for quite a while each night and now low humidity readings. I am going to try drying and hydrating it to see what happens.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on September 02, 2018, 10:34:47 AM
Well i can not be sure but i think i just had a 75 fall victim to the fars, i had a new 75 running in the fars for comparison and it hit 99 a couple times but alas it now won't go over 95 percent sometimes bumps 96. We had several nights of light fog and the humidity was high for quite a while each night and now low humidity readings. I am going to try drying and hydrating it to see what happens.

Are 75's no better with prolonged high humidity? Seems like they should reset after awhile.
I would swap it out and put in DIY test chamber or wet rag test and see what it will do. 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on September 02, 2018, 11:06:04 AM
Maybe i am just unlucky lol, i did put it in my tester and it maxed out at 95 so i am letting it dry in oven for most of the day and will try it again. Hope it comes alive again. It may just have been a bad sensor.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: hwcorder on September 02, 2018, 11:07:46 AM
Maybe i am just unlucky lol, i did put it in my tester and it maxed out at 95 so i am letting it dry in oven for most of the day and will try it again. Hope it comes alive again. It may just have been a bad sensor.

Were you using the stock filter or something else?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on September 02, 2018, 11:15:23 AM
Using the factory filter sf1.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on September 02, 2018, 11:24:36 AM
Well i can not be sure but i think i just had a 75 fall victim to the fars, i had a new 75 running in the fars for comparison and it hit 99 a couple times but alas it now won't go over 95 percent sometimes bumps 96. We had several nights of light fog and the humidity was high for quite a while each night and now low humidity readings. I am going to try drying and hydrating it to see what happens.

Was it installed in your RM Young FARS?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on September 02, 2018, 11:28:13 AM
Maybe i am just unlucky lol, i did put it in my tester and it maxed out at 95 so i am letting it dry in oven for most of the day and will try it again. Hope it comes alive again. It may just have been a bad sensor.

Were you using the stock filter or something else?

hwcorder, whenever you get a chance, check your PM. Got a question about Campbell Scientific. Thanks!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: hwcorder on September 02, 2018, 11:28:39 AM
Using the factory filter sf1.

Hmmm, hopefully just a fluke then.  Mine recently has been topping out slightly lower about 96-97% as opposed to the 98% it used to.  Its about two years old however.  Probably time to switch it out but it does so well in the lower to mid range I've been putting it off.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on September 02, 2018, 11:33:41 AM
It was in the standard Davis fars with stock fan.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on September 02, 2018, 12:27:03 PM
It was in the standard Davis fars with stock fan.

Oh wow. Let’s just hope it was a bad sensor.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on September 02, 2018, 07:07:08 PM
For those interested in the sht75 and may not have soldering skills or any luck with the crimping version, I've found this option. Haven't done this myself however so nothing confirmed.
The plug/cord was made for the sht75.... called 3M-CABLES FOR EK-H4 made for evaluation kit.
 
The plug is 3 meters and rather expensive at $80. You only need 1 however as the sensors go bad just replace the SHT75.
Looks like the connector on the stock plugs is the RJ12 which has 6 gold connectors. Only 4 will be used however. You will need to figure out which 4 go where for the sensor to work and crimp plug accordingly with tool.

I referenced this video about plugs on youtube to determine what was being used. The stock sht31 plug has 6 contacts the red and black are not connected to the SHT31 sensor.

Youtube video on plugs: https://www.youtube.com/watch?v=fjNCUDrdULw

Link to sht75 plug/cord sold by digikey: https://www.digikey.com/product-detail/en/sensirion-ag/3M-CABLES-FOR-EK-H4/1649-1055-ND/5982944

Anyone with more knowledge please chime in on this.

Wonder if you can get this thing cheaper? So far, I’ve not been able to find it. With the help of my step father, we must’ve screwed something up. My 75 tells me it’s -40° to -70°. Lol
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on September 02, 2018, 07:24:51 PM
Was it tested and working prior to sealing?

Sounds like the numbers I was getting so after I destroyed all connectors (5) gave up.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on September 02, 2018, 07:28:25 PM
Was it tested and working prior to sealing?

Sounds like the numbers I was getting so after I destroyed all connectors (5) gave up.

Foolishly had it sealed up before testing but was able to take the sensor itself off the mounting board. It does have some glue on the back of the sensor though.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on September 02, 2018, 08:28:18 PM
Plug the sensor back in and press down on the connector i had one that went flaky on the reading and it would come and go when i pressed on the plug. Those teeth that bite into the wires are sort of flimsy.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on September 03, 2018, 07:00:39 AM
Valentine, I see where your 75 finally went up to 100%
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on September 03, 2018, 07:44:45 AM
Valentine, I see where your 75 finally went up to 100%

Actually it didn't, what you see is my SHT31 on FARS. Little disappointed in the 75's ability to go beyond 97% in passive shield.
I think it would make 98% in FARS however because there is zero wind not even whisper at ground level.
Dense fog this morning so should of been there. Truthfully these SHT75 are no better than the 31's it's a crap shoot whether sensor can reach even 98%. Where they are superior is the no high humidity bias but for cold climates inferior on temperature. Too bad we don't have a sensor that can do it all.

If sensor makes 98% Cumulus will display 100%. I have another 31 that goes to 100% every time at remote station inside a PRO shield also.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on September 03, 2018, 08:17:19 AM
Valentine, I see where your 75 finally went up to 100%

Actually it didn't, what you see is my SHT31 on FARS. Little disappointed in the 75's ability to go beyond 97% in passive shield.
I think it would make 98% in FARS however because there is zero wind not even whisper at ground level.
Dense fog this morning so should of been there. Truthfully these SHT75 are no better than the 31's it's a crap shoot whether sensor can reach even 98%. Where they are superior is the no high humidity bias but for cold climates inferior on temperature. Too bad we don't have a sensor that can do it all.

If sensor makes 98% Cumulus will display 100%. I have another 31 that goes to 100% every time at remote station inside a PRO shield also.

Oh wow.  That’s a bummer!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on September 03, 2018, 08:24:19 AM
Like I said had 75 been inside the fars it would make another 1% 98% no problem from all my observations fars does better on mornings with no wind.
I've only had 1 sensor out of maybe a dozen ever reach 100% on its own and that's the sensor at remote site now. Thinking of bringing it back home as main unit.
How long it last is  :?:.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on September 03, 2018, 09:01:25 AM
3 of 4 shields hit 98% finally. Takes longer on passive shields vs the fars. All have 31's, I will go back to the 75's next spring.  :-(
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on September 03, 2018, 09:46:42 AM
4 mornings in a row with 99/100% at my place. I guess YMMV. Anyone test a psychrometer or the acurites in those 97/98% conditions?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on September 03, 2018, 10:03:36 AM
4 mornings in a row with 99/100% at my place. I guess YMMV. Anyone test a psychrometer or the acurites in those 97/98% conditions?

Under porch the acurite hit 98%
 [ You are not allowed to view attachments ]
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: hwcorder on September 03, 2018, 11:46:04 AM
From my experience with any of these sensors whether it be the sht 15, 75, or 31 if it doesn't have a wet bias in the low to mid RH it will usually not read above 98%.  If it does reach the 99% to 100% it tends to read too wet a lower values. Very rare does it seem like I find a sensor that does both.  My preference is to have better accuracy in the mid ranges than to have a sensor that reads 100%, but that"s just me. 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on September 03, 2018, 01:05:00 PM
I agree, i don't believe in our price range we will ever get the perfect sensor. I got over the must have 100% reading a while ago and temp. is pretty straight forward in the normal range, so have leaned toward best dp reading. What is most important anyway 100% reading on humidity accurate temp. or best dp?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on September 03, 2018, 04:59:26 PM
Something different, have the SHT75 back in service but this time inside FARS. FARS sensor chamber is big enough for the skinny 75 and big 31 at same time. So should I feel a need to swap over to 31 it will be quick and easy.

Added extension on the 75 also so can reach TX1 easily without moving transmitters around.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Old Tele man on September 03, 2018, 06:34:32 PM
Something different, have the SHT75 back in service but this time inside FARS. FARS sensor chamber is big enough for the skinny 75 and big 31 at same time. So should I feel a need to swap over to 31 it will be quick and easy.

Added extension on the 75 also so can reach TX1 easily without moving transmitters around.

Has anybody tried using a short 1-2 foot RJ45 male/female extension cable between the ISS and sensor to eliminate having to open the ISS each time while changing between sensors?

If yes, where did you get it from?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on September 03, 2018, 06:54:22 PM
From my experience with any of these sensors whether it be the sht 15, 75, or 31 if it doesn't have a wet bias in the low to mid RH it will usually not read above 98%.  If it does reach the 99% to 100% it tends to read too wet a lower values. Very rare does it seem like I find a sensor that does both.  My preference is to have better accuracy in the mid ranges than to have a sensor that reads 100%, but that"s just me.

Yes, my 31 which is normally accurate in the mid ranges (when it hasn't spent extended time above 80% and gets out of calibration for awhile), will no longer read above 95% and even then only briefly. Practically speaking, 94% is the top end. Originally, I believe 97% was the top with maybe flashes of 98% in extended foggy periods. This is a sensor first installed at the end of November last year. Not even a year old!

Feeling pretty frustrated right now because we had some wet weather recently and the sensor got out of calibration. This afternoon humidity was in the 70s and the sensor wouldn't recalibrate. Last time it took almost 48 hours to return to normal. More rain is on the way tonight thru Wednesday. So... thinking to myself... do I yank the -8% offset I put in this afternoon (after testing with my Kestrel)? That's probably the only course of action I have or it will run dry when it's raining and immediately afterward.

It's getting quite ridiculous the lengths we all have to go to so we can have even somewhat accurate humidity readings.

UPDATE 11PM: Once the sun set and humidity started climbing into the 80s, the biases in the 31 converged. So I was able to safely remove the offset... at least until humidity drops below 80% again. 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on September 03, 2018, 06:57:33 PM
Something different, have the SHT75 back in service but this time inside FARS. FARS sensor chamber is big enough for the skinny 75 and big 31 at same time. So should I feel a need to swap over to 31 it will be quick and easy.

Added extension on the 75 also so can reach TX1 easily without moving transmitters around.

Has anybody tried using a short 1-2 foot RJ45 male/female extension cable between the ISS and sensor to eliminate having to open the ISS each time while changing between sensors?

If yes, where did you get it from?

Good Idea.
PIA feeding those cables inside ISS but I'm getting good at it now..  :-)
In my case I would need to protect the unused exposed connectors from elements. Same with any extension, couldn't leave exposed unless  weather proofed with something like sticky Tacky tape. Wrap in wax paper and sticky Tacky tape works great on camera connectors also making everything waterproof. Reason I wrap in paper first it's much easier to get off and doesn't adhere to connections making a mess.
 
I had the sht31 30' cable going into fars shield already which reached TX1 primary ISS.  Also needed a 30' cable for sht75 to reach so spliced in 4 wire flat cable I already had on hand. (Only takes 4 wires) for temperature sensor.  Now won't need to keep jocking stuff around every time.  \:D/
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on September 03, 2018, 07:17:18 PM
I picked up three more 31's from Ryan to see if I could get lucky.... ummmm, no. All three ran exactly 3% and 1.9 to 2.0F higher than my 8 month old 31, which is still running "only" 2% wet, not 5% like these new ones. Seems like the older 31's (I have two, both over two years old) are more accurate. They're all wrong, but the 3 new ones are more wrong, but consistently so. :???:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Old Tele man on September 03, 2018, 07:30:29 PM
Something different, have the SHT75 back in service but this time inside FARS. FARS sensor chamber is big enough for the skinny 75 and big 31 at same time. So should I feel a need to swap over to 31 it will be quick and easy.

Added extension on the 75 also so can reach TX1 easily without moving transmitters around.

Has anybody tried using a short 1-2 foot RJ45 male/female extension cable between the ISS and sensor to eliminate having to open the ISS each time while changing between sensors?

If yes, where did you get it from?

Good Idea.
PIA feeding those cables inside ISS but I'm getting good at it now..  :-)
In my case I would need to protect the unused exposed connectors from elements. Same with any extension, couldn't leave exposed unless  weather proofed with something like sticky Tacky tape. Wrap in wax paper and sticky Tacky tape works great on camera connectors also making everything waterproof. Reason I wrap in paper first it's much easier to get off and doesn't adhere to connections making a mess.
 
I had the sht31 30' cable going into fars shield already which reached TX1 primary ISS.  Also needed a 30' cable for sht75 to reach so spliced in 4 wire flat cable I already had on hand. (Only takes 4 wires) for temperature sensor.  Now won't need to keep jocking stuff around every time.  \:D/

I plan to slide a piece of "heat-shrink wrap" over the whole connector, leaving about 1" extra on each end, and then, after heat-shrinking, I'll tape both ends with Gorilla Tape to waterproof. But I wonder what others are using?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on September 03, 2018, 07:56:28 PM
This stuff is messier than hell, but boy do it work.
https://www.amazon.com/Gardner-Bender-LTB-400-Electrical-Waterproof/dp/B000FPAN2K
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on September 03, 2018, 08:12:14 PM
I have decided to ask Ryan for a new SHT31 to replace the one in my new 24 hour fan VP2, not because of being wet, rather temperature inaccuracies at dusk. I guess I can live with a wet hum-temp sensor but when the temperatures are inaccurate, even for 2-3 hours, that is unacceptable. My 2 year old VP2 with only a daytime fan has never had a temperature issue—always spot on.  I can easily compare results they are on the same fence 25 feet apart. 

Somewhere the QC on the Davis SHT has failed us.

Somewhat related to this, we had a huge downpour this afternoon. My new VP2 showed .66 and my older VP2 was .90 which matched my nearby CoCo gauge! Maybe I got a lemon but I will reserve judgment pending cleaning of the rainbase and tipper mechanism! Until this storm, my new VP2 was spot on with the CoCo gauge but it hadn’t experienced a rain like we got today.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on September 03, 2018, 08:16:11 PM
This stuff is messier than hell, but boy do it work.
https://www.amazon.com/Gardner-Bender-LTB-400-Electrical-Waterproof/dp/B000FPAN2K

Yikes I don't like it. I prefer any of the the self fusing electrical tapes. Doesn't have to be scotch 70 others work just as well.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on September 03, 2018, 08:19:07 PM
I have decided to ask Ryan for a new SHT31 to replace the one in my new 24 hour fan VP2, not because of being wet, rather temperature inaccuracies at dawn.

That would be a first for me. Never seen a 31 temperature go bad.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on September 03, 2018, 08:20:09 PM
temperature inaccuracies at dawn.
:???: How is it only inaccurate at dawn?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Old Tele man on September 03, 2018, 08:23:03 PM
This stuff is messier than hell, but boy do it work.
https://www.amazon.com/Gardner-Bender-LTB-400-Electrical-Waterproof/dp/B000FPAN2K

Yikes I don't like it. I prefer any of the the self fusing electrical tapes. Doesn't have to be scotch 70 others work just as well.

Yes, the "volatiles" are probably hell on the RH% sensor! How close to sensor is too close?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on September 03, 2018, 08:26:12 PM
This stuff is messier than hell, but boy do it work.
https://www.amazon.com/Gardner-Bender-LTB-400-Electrical-Waterproof/dp/B000FPAN2K

Yikes I don't like it. I prefer any of the the self fusing electrical tapes. Doesn't have to be scotch 70 others work just as well.

Yes, the "volatiles" are probably hell on the RH% sensor!
Had no idea this was meant to be in the "smelling range" of the sensor. Obviously not meant for that.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on September 03, 2018, 08:28:11 PM
This stuff is messier than hell, but boy do it work.
https://www.amazon.com/Gardner-Bender-LTB-400-Electrical-Waterproof/dp/B000FPAN2K

Yikes I don't like it. I prefer any of the the self fusing electrical tapes. Doesn't have to be scotch 70 others work just as well.
I actually use fusing tape before the liquid stuff. Just me I guess...
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on September 03, 2018, 08:44:08 PM
temperature inaccuracies at dawn.
:???: How is it only inaccurate at dawn?

My typo—should be dusk not dawn. The temperature after the sun sets doesn’t drop and stays higher than it should be for a couple of hours. The comparison that MADIS does on my two stations is crystal clear and it is obvious when viewing temperatures on fellow PWSs on WU shortly after sunset. It is like the sensor doesn’t want to cool off for a bit, then later in the night and all the next day, til dusk, it is fine—spot on. Never had a SHT31 do that!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on September 03, 2018, 08:45:21 PM
Running a 75 and 31 in same fars sensor chamber. The SHT75 runs .2-.4 below the 31 mid 70's ambient outside. So looks very similar to the SHT15's I've tested. Be interesting if this continues or just in the 70's range.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on September 03, 2018, 09:01:37 PM
temperature inaccuracies at dawn.
:???: How is it only inaccurate at dawn?

My typo—should be dusk not dawn.
Even still..... :?   Don't have an obstructed SF2 on the goofy one, do ya?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on September 03, 2018, 09:04:23 PM
temperature inaccuracies at dawn.
:???: How is it only inaccurate at dawn?

My typo—should be dusk not dawn.
Even still..... :?   Don't have an obstructed SF2 on the goofy one, do ya?

Nope!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on September 03, 2018, 09:17:45 PM
From my experience with any of these sensors whether it be the sht 15, 75, or 31 if it doesn't have a wet bias in the low to mid RH it will usually not read above 98%.  If it does reach the 99% to 100% it tends to read too wet a lower values. Very rare does it seem like I find a sensor that does both.  My preference is to have better accuracy in the mid ranges than to have a sensor that reads 100%, but that"s just me.

The Acurite unit I have (https://www.acurite.com/rain-gauge-station-lightning-detection-01021m.html) does not have the humidity problems like the Davis SHT sensors have. Only issue that this Acurite unit has is lagging.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on September 03, 2018, 10:14:24 PM
From my experience with any of these sensors whether it be the sht 15, 75, or 31 if it doesn't have a wet bias in the low to mid RH it will usually not read above 98%.  If it does reach the 99% to 100% it tends to read too wet a lower values. Very rare does it seem like I find a sensor that does both.  My preference is to have better accuracy in the mid ranges than to have a sensor that reads 100%, but that"s just me.

The Acurite unit I have (https://www.acurite.com/rain-gauge-station-lightning-detection-01021m.html) does not have the humidity problems like the Davis SHT sensors have.
Huh? It doesn't even have an outdoor ISS, or whatever acurite calls it. How would you know?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on September 04, 2018, 08:44:12 AM
I put a 31 back in use to do some comparing with the 75 and so far they have been tracking each other real well. Right now they are both showing 98% humidity and 75.7 temp. The temp is the same on both sensors and the humidity has been tracking real well. I am waiting for the humidity to get down into the mid ranges to see if the high bias shows up on the 31. Have had a lot of rain the last few days and the humidity yesterday stayed above 80% most of the day so sensors have been on the damp side for some time and will get a chance to see if the 31 lags the 75 when humidity goes down.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on September 04, 2018, 09:54:39 AM
Removed the bulky 31 filter and added trimmed down version, now the 31 is running slightly lower on temperature.   Did a flip flop so the SHT31 stock filter appears to be causing about +.4 F swing. I'll keep a watch on this and log throughout day.
Both sensors are in same FARS sensor chamber so should be good indication.
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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on September 04, 2018, 10:09:53 AM
Update, humidity drop was about the same for both sensors no wet bias seen.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on September 04, 2018, 10:40:37 AM
From my experience with any of these sensors whether it be the sht 15, 75, or 31 if it doesn't have a wet bias in the low to mid RH it will usually not read above 98%.  If it does reach the 99% to 100% it tends to read too wet a lower values. Very rare does it seem like I find a sensor that does both.  My preference is to have better accuracy in the mid ranges than to have a sensor that reads 100%, but that"s just me.

The Acurite unit I have (https://www.acurite.com/rain-gauge-station-lightning-detection-01021m.html) does not have the humidity problems like the Davis SHT sensors have.
Huh? It doesn't even have an outdoor ISS, or whatever acurite calls it. How would you know?

Simple. Put the sensor inside of Acurite’s radiation shield. Since their shielding isn’t great, I put it under a tree. Their monitor or “console” tells me the readings. During the days when temps are high and in humid conditions, the Acurite unit tracks well with the ASOS 15 miles north.  The Davis unit shows DPs around 2-4° higher than either two.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on September 04, 2018, 11:37:23 AM
Well didn't take long for the 75 and 31 temps to come back together after filter change and now 75 is running -.2F lower as before.

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on September 06, 2018, 09:28:18 AM
Well my 75 was showing 99% this am without any fog just humidity. It has been reading just fine from low humidity to top end, much smoother in the readings than the 31 which at times would seem to get hung up on the higher side of what it should read.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Intheswamp on September 09, 2018, 10:09:10 AM
Ok, I still haven't installed the new (bought a year ago?) temp/humidity sensor.  Looks like I need to.  I looked this morning and was surprised to see a report of 21% humidity.  This is after it being so foggy I couldn't even see the ISS this morning at 220' distant.  Uh,...no.  So, this week I hope to swap the sensors out.

(https://farm2.staticflickr.com/1889/43662204375_e76eeb62d2_o.jpg) (https://flic.kr/p/29whdca)09909_2018_HumidityChart (https://flic.kr/p/29whdca) by Intheswamp (https://www.flickr.com/photos/71532701@N07/), on Flickr
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Intheswamp on September 09, 2018, 10:14:48 AM
Hmmm, and one more chart...weather trends.  It seems the humidity sensor is nose-diving as the fog clears out.   #-o
(https://farm2.staticflickr.com/1862/43662288605_9127d55024_b.jpg) (https://flic.kr/p/29whDep)0909_2018_WxTrends (https://flic.kr/p/29whDep) by Intheswamp (https://www.flickr.com/photos/71532701@N07/), on Flickr
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on September 09, 2018, 11:44:47 AM
For those of you who might want to try the 75 here is the link to the best price that i have found so far. They are in factory packaging and shipping packaged the proper way. http://www.futureelectronics.com/en/Technologies/Product.aspx?ProductID=SHT75SENSIRIONAG9049996&IM=0&utm_source=octopart.co
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on September 09, 2018, 02:57:57 PM
Good find Jerry.  =D&gt;
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Intheswamp on September 09, 2018, 10:54:47 PM
Interesting recovery for the plummeting humidity...
 (https://farm2.staticflickr.com/1868/43866978284_bb978d1660_b.jpg) (https://flic.kr/p/29QnJkN)09092018_temphumidity (https://flic.kr/p/29QnJkN) by Intheswamp (https://www.flickr.com/photos/71532701@N07/), on Flickr
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Intheswamp on September 10, 2018, 08:38:40 AM
Well, it looks like we're getting another episode of the plummeting humidity chart this morning...another heavy fog morning.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: galfert on September 10, 2018, 09:32:11 AM
I've noticed a few Davis stations around me with plummeting humidity graphs. It always seems to drop to around that 30% range and then it recovers when the humidity drops around 75%. I've seen several graphs just like yours.

If I go back on those stations historic data I notice that they gradually lose the top end coming down from 96% down to about 88%. Then they hold that 88% top end for a while. Then it may get a bit random and unpredictable for a few weeks as it tried to keep things together. That's the big indicator that its about to blow. And then wham drops down to 30% when it should be above 75% to 100%. Never to ever recover to normal working condition unless the sensor is replaced. Seen it with VP2 and Vue. I've talked to a few of my neighbors with this issue.

Its like a drunk college kid. He is sober and working in 96% efficiency. Then he starts drinking and getting drunk loosing capacity down to 88%. Then they get unpredictable as they try and sober up. Someone gives them coffee or smacks them around. But they are a wreck and stumbling and dizzy. Then they feel like they are going to blow chunks and they try and hold it in for bit. Then wham they toss their cookies and pass out momentarily on the toilet down to barely breathing at 30% capacity. Then they feel better for a while and recover to 75%. But they are still drunk and think they are okay to drink more....only to pass out again. I don't know why I came up with this analogy. Just feeling silly this morning. Yes I'm sober.
  UU
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Intheswamp on September 10, 2018, 01:28:50 PM
Words of experience....   :-k :-D
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on September 11, 2018, 09:31:31 PM
Rocking the 100% RH tonight. I haven't seen 100% this much since my analog sensor.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on September 11, 2018, 10:06:13 PM
Rocking the 100% RH tonight. I haven't seen 100% this much since my analog sensor.
Please forgive me, but after fifty pages of this thread, I've forgotten what you're "rocking" with, 75, 31, fan, no fan....
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on September 12, 2018, 07:51:56 AM
Rocking the 100% RH tonight. I haven't seen 100% this much since my analog sensor.
Please forgive me, but after fifty pages of this thread, I've forgotten what you're "rocking" with, 75, 31, fan, no fan....
75 + 40CFM fan + no filter
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on September 12, 2018, 09:11:44 AM
Fine mist water droplets drawn onto sensor is making 100% much easier I'm sure. Sooner or later sensor will fail without any protection. This was what Jerry was experiencing in humid south Texas gulf region even using filter with FARS, killing sensors so decided to go with passive unit.

You can replace the 75 at $20 each futureelectronics link was posted above by Jerry. I ordered a few replacements myself.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on September 12, 2018, 10:47:31 AM
I have some 100 mesh silk (not the real stuff), it comes much higher mesh also. Plan is cover all sensors eventually. 200-300 mesh may have been better choice but water still beads on top of the 100 mesh when tested and no air restriction I can tell. Its very sheer stuff and completely odorless.
 
I'm just going to wrap sensors, have single layer on sensor side of board. This is a cut down SHT31 I was using in Meteoshield Pro and simple wire tie you might find on loaf of bread. Nothing fancy but should work.
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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on September 12, 2018, 01:27:23 PM
Fine mist water droplets drawn onto sensor is making 100% much easier I'm sure. Sooner or later sensor will fail without any protection. This was what Jerry was experiencing in humid south Texas gulf region even using filter with FARS, killing sensors so decided to go with passive unit.

You can replace the 75 at $20 each futureelectronics link was posted above by Jerry. I ordered a few replacements myself.
I had my old 75 installed this way for an entire Fall-Winter-Spring period and didn't notice any changes in readings to the naked eye. A lot of you guys have 31s failing or drifting and that's with filters and passive/stock FARS shields. It hasn't been easy to hit 100%. It was mostly 99% with occasional obs of 100% and this was after almost 2" of synoptic rain followed by 12hrs of drizzle. The local METAR reported 0F to 1F dewpoint depressions too so RH was definitely in that 98-100% range at times.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on September 12, 2018, 02:37:00 PM
I had my old 75 installed this way for an entire Fall-Winter-Spring period and didn't notice any changes in readings to the naked eye. A lot of you guys have 31s failing or drifting and that's with filters and passive/stock FARS shields. It hasn't been easy to hit 100%. It was mostly 99% with occasional obs of 100% and this was after almost 2" of synoptic rain followed by 12hrs of drizzle. The local METAR reported 0F to 1F dewpoint depressions too so RH was definitely in that 98-100% range at times.

The 75 may be a more robust sensor being it has been used extensively in meteorological field.  Being last year of production I noticed many sellers like futureelectronics.com have on order (1900) so may be around awhile longer.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on September 12, 2018, 07:23:28 PM

Well this was an interesting day, we have had several days of cloudy rainy high humidity days in a row and today we finally got some sunshine and my sensors were both showing humidity around 84 to 86 percent when the one i put out for testing this am was showing 79 to 81 percent as well as a bunch of stations area wide. I put a new sensor in the standard shield and it showed the lower humidity but the main station was still high so i swapped it out and it was then showing the lower reading. Well i remember reading that the sensor could read high if exposed to high humidity for 60 or more hours and that's exactly what i had going on. Now staying in high humidity doesn't happen that often but it sure gives you a 3% higher reading just like the specs said and until the humidity lowers for some time your readings will have that offset.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on September 12, 2018, 08:15:06 PM
The more i think about it that really sucks if you live in an area where long periods of humidity above 80% exist, i guess you would have to change out your sensor every couple of days to keep from having wrong readings of humidity and dew point. Here is what it says, Sensor works stable within recommended normal range –
see Figure 4. Long term exposures to conditions outside
normal range, especially at humidity >80%RH, may
temporarily offset the RH signal (+3 %RH after 60h). After
return to normal range it will slowly return towards
calibration state by itself. See Section 1.4 “Reconditioning
Procedure” to accelerate eliminating the offset. Prolonged
exposure to extreme conditions may accelerate ageing.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on September 12, 2018, 10:36:05 PM
Sure seems strange that above 80% is not normal range and can cause an error.
what kind of sensor is this? How can a manufacturer make a humidity sensor that can't take humidity over 80% for several days, sure seems like some kind of poor planning to me. Could this be what is causing the error in the 31?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on September 13, 2018, 08:15:10 AM
Sure seems strange that above 80% is not normal range and can cause an error.
what kind of sensor is this? How can a manufacturer make a humidity sensor that can't take humidity over 80% for several days, sure seems like some kind of poor planning to me. Could this be what is causing the error in the 31?

It could play a role, but I think it’s the way Davis mounts the 31 chip into the pcb. But I could be wrong.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on September 13, 2018, 08:51:54 AM


It could play a role, but I think it’s the way Davis mounts the 31 chip into the pcb. But I could be wrong.

Yes, something is different between Davis 31's and Acurite 31's. Either the calibration or manufacturing process knocks Davis units off. The Acurite 31 calibration is very similar to the SHT75 running 2° DP lower mid to upper ranges.  I wonder if Davis didn't ask for a different calibration for desert regions so it wasn't going to 0% humidity? Making the mid to upper areas run wet. Just an  :idea:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on September 13, 2018, 09:38:47 AM
It may be time for Davis to have a separate temperature and dewpoint option. I think that will be a project of mine come next spring as I'll be busy with my coop and getting the ultrasonic anemometer installed this fall. I'm thinking an SHT in a FARS for temp and an SHT in a passive with some form of light, supplemental heat for dewpoint.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on September 13, 2018, 04:35:00 PM


It could play a role, but I think it’s the way Davis mounts the 31 chip into the pcb. But I could be wrong.

I wonder if Davis didn't ask for a different calibration for desert regions so it wasn't going to 0% humidity?
I think the version of software on the console dictates whether it'll hit 0% or not.
Now that we're drying out here, the humidity/dews are really going down so I keep waiting for my -2% correction to catch up with me, but not so...yet. Myself and TUS have both hit 8% with dews of 28F and 29F respectively, so the shoe drop hasn't happened as I keep waiting to see when the -2% will start to show itself. In the mean time, the -2% has been money tween 8% and 91%.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on September 13, 2018, 06:03:54 PM


 I think the version of software on the console dictates whether it'll hit 0% or not.

I've never heard that,  the 15's had dry bias issue below 10%, remember? Has a change been made to compensate, causing the wet bias we now see. The Davis versions are different from the acurite 31 is why I was saying it may be a special calibration request. These 31's with LS protocol are specifically made for Davis and not publically available as far as I know or we would be purchasing instead of the 75's.   
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on September 13, 2018, 06:21:27 PM


 I think the version of software on the console dictates whether it'll hit 0% or not.

I've never heard that,  the 15's had dry bias issue below 10%, remember?
Hell Randy, I can't remember what was for dinner last night. Not saying the 15 didn't, I honestly don't recall.
As far as the 0%, this is for the Vue console, didn't look at the VP2. See version 2.14.

 

Version History

Firmware version 4.30

    Modified heat index table to better support high temp and high humidity environments.

Firmware version 4.18

    Supports new barometric pressure sensor. It will also support previous barometric sensors with a Mfg
    Code of MB or higher (e.g. MB, MC, etc.)

Firmware version 3.00

    Updated internal communication between console and data logger for improved performance.

Firmware version 2.14

    Solved problem of inadvertent low transmitter battery message. It was displaying as soon as a low voltage was detected, now it will not display unless a low battery has been detected for a full 2 minutes.
    Outside humidity no longer displays 0%.
    Modified heat index table to match the algorithm used by the WeatherLink software.

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on September 13, 2018, 10:34:21 PM
Does anyone knows where you can get a crossover adapter for the RJ12?  If I can get a RJ 12 adapter, then I won’t have to worry about getting rid of the 45 plug that’s on the EH-4 cable.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on September 14, 2018, 11:29:56 AM
Does anyone knows where you can get a crossover adapter for the RJ12?  If I can get a RJ 12 adapter, then I won’t have to worry about getting rid of the 45 plug that’s on the EH-4 cable.
The thing is you need the wires to line up (clk, vdd, gnd, data) according to the Davis ISS requirements.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on September 14, 2018, 11:48:59 AM
Another mostly 99% RH night with a few 100% obs thrown in. The fog started forming before sunset last night and was very thick by morning. The local ASOS at CON was reporting M1/4SM visibility at times. However, they never had an hourly METAR with 100% RH during the fog...the best they could do was a 1F dewpoint depression (DD). CON is a 1st order climate station with official obs going back to the 1860s. So there's an example of a 1st order ASOS reporting the lowest possible automated visibility in fog (M1/4SM) with an RH under 100%. So either the ASOS is wrong or we don't need to be seeing 100% RH every time we experience some fog. The funny thing is that CON reported a couple hours of 100% RH once the visibility began to rise. They even had a 100% with a visibility of 7SM (not even a BR ob). The LCI AWOS near me also reported M1/4SM, but they did have a few hourly METARs with a 0F DD. 1P1 to my north is a little different. That's an AWOS that reports to the exact 0.1C in every observation (not 1F resolution converted to degC). They had 1/4SM visibility and the RH maxed out at 97%.

So in summary...lowest vis this morning and the lowest DD during the FG (fog) period (1/2SM vis or lower)

Me 0F (100%)
CON M1/4SM 1F (97%)
LCI M1/4SM 0F (100%)
1P1 1/4SM 0.9F (97%)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on September 15, 2018, 08:32:23 AM
heh...right on cue...

It looks like my 75 is failing. It stayed at 89-90% RH overnight despite fog and drizzle. Everyone around me is in the 97-100% range. We’ll finally dry out later today so I’ll give it another night to see if it improves. I’ll probably get another 31 from Ryan and use that through winter. Maybe we’ll have a way to use the 35s by spring.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on September 15, 2018, 03:23:01 PM
Well all i have to say is these sensors really suck. I am in a high humidity area and they just plain suck. We have had humidity above 80% for the last three days, i had put out two new 75's in my two Barani shields. Well every thing was fine to start with the two reading the same most of the time but yesterday morning  when both sensors had been on 97% all night one started to go down in readings until this am when it was at 75% and the other one was at 98%, so when the rain stopped for a bit i changed it out and when i got it into the house it had returned to 98% and started dropping down to the indoor humidity just like it should. The replacement was showing 95% and holding while the other was at 98%, after a while the replacement was down to 91% and the other was still at 98%, i checked with my acurite and it showed 91%, so i changed out the 98% one for a new sensor thinking it was moisture in the shield but it too settled at 91% so it was the sensor and not the shield. These sensors are really a piece of crap above 80% but the manufacturer does tell you in the specs that it is a piece of crap in high humidity and should only be used between 20% and 80% to insure accuracy. I guess i will have to change out sensors every morning when humidity is high, wow what a great sensor Davis has. Even the new 31 has the same warning about high humidity screwing the readings up. ](*,)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on September 15, 2018, 07:39:48 PM
 :lol:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on September 15, 2018, 08:24:19 PM
Welp got a rj12 plug on using the cable from Sensirion but the transmitter does not report any data from the 75 sensor.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on September 15, 2018, 09:04:08 PM
If you are sure you got the sensor plugged in the right way then you need to ohm out the cable and make sure you got the wiring right from the jack to the rj connector and that the pins on the jack go to the correct pins on the rj plug.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on September 15, 2018, 11:37:45 PM
Here is pin assignment for the Sensirion cable this will need to be changed to match like jerry said Ohm meter.

https://www.sensirion.com/fileadmin/user_upload/customers/sensirion/Dokumente/0_Datasheets/Humidity/Sensirion_Humidity_Sensors_EK-H4_Datasheet.pdf

Here's the guide for the Davis cable:
White (clock)
Yellow (vdd)
Green (gnd)
Blue (data)

Here is cable image.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on September 16, 2018, 03:33:03 PM
Aight. Here is what my setup currently looks like. Keep in mind that the transmitter doesn’t read anything.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on September 16, 2018, 03:39:06 PM
Aight. Here is what my setup currently looks like. Keep in mind that the transmitter doesn’t read anything.

Just taking a good look at the previous picture posted, wires might not be soldered in the right slots 🤦‍♂️
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on September 16, 2018, 10:10:40 PM
The way the jack is wired now blue should go to pin 6 on rj plug yellow to 2 green to 3 and white to 1
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on September 17, 2018, 07:32:45 AM
My RH dropped out to 0% this morning. The 31 is going back in when I get home. I wonder if water got into the connector.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on September 17, 2018, 07:42:09 AM
My RH dropped out to 0% this morning. The 31 is going back in when I get home. I wonder if water got into the connector.
I would say sensor more likely or you would of lost everything if connector shorted.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on September 17, 2018, 10:08:52 AM
My RH dropped out to 0% this morning. The 31 is going back in when I get home. I wonder if water got into the connector.
I would say sensor more likely or you would of lost everything if connector shorted.
Yeah who knows. My previous 75 never had an issue and made it through an entire winter. The sensor itself should've dried out yesterday with full sun and mid 80s.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on September 17, 2018, 10:25:03 AM
Early on Davis sensor failure was more common resulting in the water resistant version we now see.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on September 18, 2018, 08:34:14 PM
The SHT31 has been in for 24hrs now. I have a new one coming in from Ryan, but I gave the old one a gentle cleaning with a wet paper towel and put it in. The temp is spot on albeit much less responsive than my 75. The RH performed well all day. Max RH this morning in the rain was 98%, but it's already 97-98% this evening with fog developing. My max RH with this sensor, when I pulled it in July, was down to 93%. So giving it a little cleaning seems to have given it new life.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on September 18, 2018, 10:08:41 PM
Comparison of 75 and 31 in same stock Davis Fars shield at temperatures mid range this is the difference between the two sensors.
 [ You are not allowed to view attachments ]
 [ You are not allowed to view attachments ]
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on September 19, 2018, 10:34:51 AM
Comparison of 75 and 31 in same stock Davis Fars shield at temperatures mid range this is the difference between the two sensors.

Looks to me like your 75 is slowly getting wetter too. It appears to only average about 1-1.5F below the 31 in dew point during the daytime.

From my informal survey of dozens of VP2s, as well as my own experience with a wet 31, the sensors tend to run at least 2.5 to 3 degrees above the actual dew point when air temps are between 60-80F (15-25C) and humidity is between 30-70%. That translates to something like 6-9% high in humidity.

As time goes on, the SHT sensors (11, 15, 31 and I would assume the 75 since it's basically the 15) also get wetter between 70-90% and drier above 90%. It's the worst of all possible worlds. So if you see a newer, supposedly good sensor converging with your known bad sensor at night, that generally isn't good news.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on September 19, 2018, 10:44:15 AM
You may be right. I'm still at 97%-98% on all sensors 10am and may not drop out of 90's for another day or so, pushing that 60 hours of saturation before rain stops.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on September 19, 2018, 11:40:02 AM
You may be right. I'm still at 97%-98% on all sensors 10am and may not drop out of 90's for another day or so, pushing that 60 hours of saturation before rain stops.

That being the case, then it's all about whether the 75 reads significantly lower in dew point once humidity drops below 80% for awhile. I've seen it take anywhere between 6 to 48 hours for a good sensor to automatically recalibrate after being in high humidity for more than 24 hours (Sensirion may say 60 hours, but it's more like 1-2 days). If the air continues flirting with the 80 percentiles then it can take a long time (days) for the recalibration to occur. If humidity falls into the 60s or lower, then it usually takes just a matter of hours to sort the good from the bad sensors.

My "good" 31 actually reads worse than my bad 31 before it recalibrates. I'm talking an average of 3.5 to 4F (10+%) high in dp when the humidity first starts dropping below 80%. In case anyone doubts this, I've run tests with an accurate device (factory tested to +/-2% and confirmed by testing it at my local airport).

The good news is that both your sensors are still getting up to 97-98%. That tells me they are relatively healthy. Though dendrite would have said the same about his 75 until it suddenly went bad. 

I should point out that max humidity isn't always an indication of whether a sensor is accurate throughout the entire range. My "good" 31 will now only max out at 95% when the air's been saturated for hours. 94% is more common. Generally, though, it's within 2% when humidity is consistently below 80%.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on September 19, 2018, 12:32:41 PM
Jerry and I are testing the 100 mesh silk for filters. Jerry has reported recovery time is much faster vs the sf1 filter on the 75, so we are hoping we found the right combo. The silk has faster response times both ways and will hold droplets of water until pressed underneath so looks like it should filter dust rather well also.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Dj1225 on September 19, 2018, 05:19:23 PM
Can someone tell me why the humidity would spike almost 20% in 5 min?  [ You are not allowed to view attachments ]
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on September 19, 2018, 05:29:09 PM
Well, because your temp went down 2.1F and your dew went up 8.4F.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Dj1225 on September 19, 2018, 06:00:12 PM
Has anyone else had spikes in temps and dew point in such a short amount of time like that?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on September 19, 2018, 06:25:46 PM
Over in the Atlas thread (http://www.wxforum.net/index.php?topic=35084.msg358927#msg358927 (http://www.wxforum.net/index.php?topic=35084.msg358927#msg358927)), it was just revealed by one of the beta testers (nincehelser) that the temp/hum sensor is, indeed, SHT31. The Atlas was just released this week.

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on September 19, 2018, 08:12:07 PM
Has anyone else had spikes in temps and dew point in such a short amount of time like that?
You definitely had some kind of boundary go through there based on the temp drop and consistent rise in dewpoint. Maybe the 78F is a little inflated, but you have a consistent 75F+ there. Was there any precip during that time even if very light? It looks like some kind of weak seabreeze given the E wind and the shift the a muggier southerly component.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on September 19, 2018, 08:25:20 PM
Has anyone else had spikes in temps and dew point in such a short amount of time like that?
You definitely had some kind of boundary go through there based on the temp drop and consistent rise in dewpoint.
See the loop at 20Z.
https://weather.cod.edu/satrad/nexrad/index.php?type=LIX-N0Q-1-200
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Dj1225 on September 19, 2018, 11:13:29 PM
Has anyone else had spikes in temps and dew point in such a short amount of time like that?
You definitely had some kind of boundary go through there based on the temp drop and consistent rise in dewpoint.
See the loop at 20Z.
https://weather.cod.edu/satrad/nexrad/index.php?type=LIX-N0Q-1-200

I have had a some type of Davis weather station for over 20 years and I've never seen the sea breeze make the humidity jump that high in such a short amount of time. I wonder if it would have shown such a spike in that short of time if I had the 31 installed instead of the 75? CW2274 you live out in the desert have you ever seen your humidity spike that much in such a short time with the monsoon or the influx of Pacific moisture?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on September 20, 2018, 12:21:50 AM
Has anyone else had spikes in temps and dew point in such a short amount of time like that?
You definitely had some kind of boundary go through there based on the temp drop and consistent rise in dewpoint.
See the loop at 20Z.
https://weather.cod.edu/satrad/nexrad/index.php?type=LIX-N0Q-1-200

 CW2274 you live out in the desert have you ever seen your humidity spike that much in such a short time with the monsoon or the influx of Pacific moisture?
Honestly, I'm not sure, but if so, I'd be infrequent. It's certainly possible your sensor is acting up, obviously study it closely, you'll be the best judge as whether it's going out the door or not. I posted the radar loop because you could see a 'boundary' or 'outflow' at the time of your spike move from south to north, but the Gulfport airport did not show the spike, but that's only one observation per hour compared to yours every 5 minutes.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on September 20, 2018, 10:22:14 AM
I saw similar jumps as that with my 75...mostly with quick moving t'storms where the sun quickly comes back out and there's some evaporative cooling. The type of situation where you see the sun blazing and the steam rising off the roof.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on September 20, 2018, 12:13:21 PM
Been above 85% now solid 30 hours most of the time near 100%.
I've actually had 2-SHT31's reach 100% on their own and this SHT31 graph is inside Davis FARS has reached 99%. I finally had 1- SHT75 out of 3 different ones reach 98% but took almost 20 hours of actual humidity near 100% saturation. I'm disappointed in the 75's ability to go high. Still have more to test but so far the 3 I've tested all duds on high end.

I find the SHT31's are better on top end near 100% but as we know have the high bias also.
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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on September 20, 2018, 12:38:02 PM
At same time here is under porch where I keep the acurite 31 dry. Here you can see it's also reached 99% overnight but as you can see has dropped down to 96% currently with rain falling and the Davis 31's are all near 99% currently. This is what I mean when I say the 31's don't all act the same. The Acurite version doesn't display the same wet bias but still reaches 99% under porch cover.
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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on September 20, 2018, 01:04:56 PM
All of mine reach 99% outside in the shields  :grin: i don't see why yours don't unless you are just cursed  :lol:.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on September 20, 2018, 01:14:20 PM
heh...my 75s have always trumped the 31s on reaching the high end too. I'm not sure I've ever had a 31 over 98%.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on September 20, 2018, 01:42:32 PM
All of mine reach 99% outside in the shields  :grin: i don't see why yours don't unless you are just cursed  :lol:.

I've had horrible luck so far.  :-(
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Dj1225 on September 20, 2018, 01:55:43 PM
All of mine reach 99% outside in the shields  :grin: i don't see why yours don't unless you are just cursed  :lol:.
Mine also has no trouble reaching 99%
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on September 20, 2018, 06:55:43 PM
Well trying another 75. Did the damp rag test for about 2.5 hours it finally hit 98% went another hour and didn't go beyond.  3.5 hours total..

So I heated some water in big pot and gave sensor some hot steam humidity just a few minutes around 138° and saw 97% once. Took out cooled down to room temperature, 40% humidity.
Did the wet rag test again and it quickly reached 98% this time followed by 99% within 10 minutes total time where its still holding now for 15 minutes.

So my question do these sensors need broke-in?

As I was getting ready to post just noticed 100%....Hee haw  \:D/

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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on September 20, 2018, 06:59:56 PM
Well trying another 75. Did the damp rag test for about 2.5 hours it finally hit 98% went another hour and didn't go beyond.  4.5 hours total..

So I heated some water in big pot and gave sensor so hot steam humidity just a few minutes around 138° and saw 97% once. Took out cooled down to room temperature, 40% humidity.
Did the wet rag test again and it quickly reached 98% this time followed by 99% within 10 minutes total time where its still holding now for 15 minutes.

So my question do these sensors need broke-in?

As I was getting ready to post just noticed 100%....Hee haw  \:D/

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I think I’ve read somewhere (maybe on this forum) that Davis does say that the sensor needs to be “broke in.”
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on September 20, 2018, 07:07:18 PM
Now I wonder if the key was the few minutes ( about 10) in the hot steam, at least to reduce the time... Next time I may try the steam first.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on September 20, 2018, 07:09:55 PM
Could we trouble you to try a 31 also? :-)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on September 20, 2018, 07:23:48 PM
As I was getting ready to post just noticed 100%....Hee haw  \:D/
Not to be Debbie Downer, but I just noticed your dew is higher than your temp. So, I guess the choice is 99%, or 100% with a 'technically' too high of a dew.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on September 20, 2018, 07:35:38 PM
As I was getting ready to post just noticed 100%....Hee haw  \:D/
Not to be Debbie Downer, but I just noticed your dew is higher than your temp. So, I guess the choice is 99%, or 100% with a 'technically' too high of a dew.
\

It came together, dewpoint is calculated so it was slower posting. I've got 8 stations so the envoy8x is behind occasionally and wasn't there why I captured the image.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on September 20, 2018, 07:37:07 PM
As I was getting ready to post just noticed 100%....Hee haw  \:D/
Not to be Debbie Downer, but I just noticed your dew is higher than your temp. So, I guess the choice is 99%, or 100% with a 'technically' too high of a dew.
\

It came together, dewpoint is calculated so it was slower posting. I've got 8 stations so the envoy8x is behind occasionally and wasn't there why I captured the image.
=D&gt;
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on September 20, 2018, 07:42:46 PM
Could we trouble you to try a 31 also? :-)

Currently fresh out of 31's but worth a try. The way these measure humidity is resistance across a little bridge so maybe a little heat and steam loosens it up. Just a guess but seemed to work that time.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on September 20, 2018, 08:03:53 PM
Could we trouble you to try a 31 also? :-)

Currently fresh out of 31's but worth a try. The way these measure humidity is resistance across a little bridge so maybe a little heat and steam loosens it up. Just a guess but seemed to work that time.
IIRC, back when I picked up my 15, I called Sensirion and asked about all the 'conditioning' in their technical literature and if that was necessary, I basically got a "no", perhaps that was bad info. At this point, who knows.  :roll:

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on September 20, 2018, 08:20:30 PM
Could we trouble you to try a 31 also? :-)

Currently fresh out of 31's but worth a try. The way these measure humidity is resistance across a little bridge so maybe a little heat and steam loosens it up. Just a guess but seemed to work that time.
IIRC, back when I picked up my 15, I called Sensirion and asked about all the 'conditioning' in their technical literature and if that was necessary, I basically got a "no", perhaps that was bad info. At this point, who knows.  :roll:

Don't know either. Jerry has built a chamber with fan to speed up saturation so all the sensors he puts in the field he knows what the humidity will do.

Guess I'll find out if it helped or not next time we have high humidity.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WxLover16 on September 20, 2018, 09:06:22 PM
heh...my 75s have always trumped the 31s on reaching the high end too. I'm not sure I've ever had a 31 over 98%.

Very odd because the 31 I just bought almost a month ago (installed 8/24) has hit 100% a whole weeks worth of data (if not more, I'd have to look), something I've never seen before. I was almost thinking the 100 was erroneous as I'd never seen it that much before. I'll just chalk it up as my best sensor yet?  \:D/

Also, even though I don't have a filter on it, I also didn't on the previous and it rarely hit 99%.

Edit: Forgot to add I have a 24 cfm fan going in it, dunno if that matters or not...
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on September 22, 2018, 06:48:11 AM
Well as temperatures are dropping into the low 40's the trend I'm seeing on the sht75 humidity is now running slightly higher than the 31's.  Not by much just 1-2%. I was wondering if the Davis sht31 wet bias was only active at higher temperatures and yes that appears to be.

The 31 for sure is the sensor of choice at colder climates come winter with it's better low temperature accuracy and no difference in humidity it's a no brainer for me which sensor to use.
 
Graph shows Dp's from yesterday with ambient temperatures 40's-50's. Both sensors in same stock Davis FARS.
It's clear the 75 is running slightly higher at these lower dp temperatures. 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on September 22, 2018, 07:29:32 AM
This is what I like about the 75 notice response to change is really apparent. Sensors are in same fars shield.
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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on September 22, 2018, 08:50:23 AM
My new transmitter (2016+), aerocone, and new SHT31 came in from SI today. My old transmitter (late 2006) was running an older 31 so I had a -0.9F offset on it. With the new transmitter it is back to no calibration. I assume I did everything correctly on that front?

Anyway, here's the readings before and after the switch. Conditions were similar throughout the day...overcast with a S wind and no precip. It looks like my dewpoint dropped a solid 1F with the new 31 and the temperature response seems to have improved (appears to be more "swings" in the readings). My dewpoint is right in line with the METARs this morning.

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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on September 22, 2018, 09:02:53 AM
Transmitter manufacture date is the key. Look at the first 2 numbers after the letters, as long as they are 16,17,18 etc. no offset is needed with 31.
Yes you can see the humidity is lower on new sensor for sure.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on September 28, 2018, 05:15:19 PM
Heads up for anyone wanting to change over to SHT75 and not great at soldering.
Digikey is about out of the Senserion cables 10' long with the 4 prong sht75 pin adapter.  These need the RJ plug changed or splice in existing Davis RJ connector.
 
Jerry just did a couple up for another forum member and reported the 2-outside wires need swapped on RJ to work.

Reason I'm posting these cables are no longer being made and soon the SHT75 is also out of production.
Only place I've found the cables is Digikey with 6 left today. 3 weeks ago 70+ in stock so someone or ones, snagging them up. 

https://www.digikey.com/product-detail/en/sensirion-ag/3M-CABLES-FOR-EK-H4/1649-1055-ND/5982944

Oh wow!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on October 02, 2018, 08:58:15 PM
For those with the 75: Does the 75 goes back to calibration after prolong periods of RH being over 80% or does the sensor began to have a permanent wet bias?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on October 02, 2018, 09:12:33 PM
I had a very wet period where the humidity was over 80 for several days and my sensor was showing about 2 higher, so i replaced it and after a day i put it back out this pm and it is tracking with the others, the 2 seems to be gone. All of the sensors have that caution warning about this quirk. At least it looks like it will clear up.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on October 02, 2018, 09:17:39 PM
I had a very wet period where the humidity was over 80 for several days and my sensor was showing about 2 higher, so i replaced it and after a day i put it back out this pm and it is tracking with the others, the 2 seems to be gone. All of the sensors have that caution warning about this quirk. At least it looks like it will clear up.

Good deal
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on October 02, 2018, 09:18:30 PM
I had a very wet period where the humidity was over 80 for several days and my sensor was showing about 2 higher, so i replaced it and after a day i put it back out this pm and it is tracking with the others, the 2 seems to be gone. All of the sensors have that caution warning about this quirk. At least it looks like it will clear up.

Are you still using the 75 for your website?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on October 02, 2018, 10:01:39 PM
Yes
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on October 04, 2018, 05:44:05 PM
Got the 75 installed in one of my field stations yesterday evening. Recorded a peak RH of 98% this morning which matches up with most Davis stations that has the 31 inside of a FARS. I’ve noticed the DP being the same to 1° lower at times compared to the stations with the 31 sensors. Other times, it’s been 2-3° lower than the 31s. The surrounding airports still around 1-3° cooler with most of the time only being 1-2° cooler. Only one time today that the DP on the 75 was cooler than the surrounding airports (1-3°).

You can view the surrounding NWS/RAWS stations here:

https://mesowest.utah.edu/cgi-bin/droman/trend_zip_mm.cgi?zipcode=35171&radius=55&noho=1&rawsflag=1&orderby=p&hour1=21&day1=04&month1=10&year1=2018&type=0&mode=

You can view my field station with the 75 here:

https://mesowest.utah.edu/cgi-bin/droman/meso_base_dyn.cgi?stn=E6358&unit=0&time=&hours=24&hour1=21&day1=04&month1=10&year1=2018&radius=25&past=0&order=1

Here is a nearby station that has the 31 inside of a FARS

https://mesowest.utah.edu/cgi-bin/droman/meso_base_dyn.cgi?stn=F0139&unit=0&time=&hours=24&hour1=21&day1=04&month1=10&year1=2018&radius=25&past=0&order=1
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on October 04, 2018, 06:17:43 PM
I noticed today that the nearby VP2 station that has the 31 inside of FARS recorded the same peak DP of 75°. The lowest DP was recorded early this morning and was only 1° difference.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on October 04, 2018, 06:55:09 PM
Do you guys think it's possible that Davis has addressed and corrected the problem and haven't said anything (I truly don't think they'll ever admit a problem exists), or happenstance that 31's seem playing well more so with others?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on October 04, 2018, 06:59:25 PM
Do you guys think it's possible that Davis has addressed and corrected the problem and haven't said anything (I truly don't think they'll ever admit a problem exists), or happenstance that 31's seem playing well more so with others?

Don’t know...All I know is that the nearby station with the 31 is three years old. Sensor hasn’t been replaced.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on October 04, 2018, 07:32:56 PM
Do you guys think it's possible that Davis has addressed and corrected the problem and haven't said anything (I truly don't think they'll ever admit a problem exists), or happenstance that 31's seem playing well more so with others?
Don’t know...All I know is that the nearby station with the 31 is three years old. Sensor hasn’t been replaced.
Not positive, but I think the 31 wasn't even available then. I installed my first one late 3/16 and I believe that was pretty much hot off the press.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on October 04, 2018, 07:48:45 PM
Do you guys think it's possible that Davis has addressed and corrected the problem and haven't said anything (I truly don't think they'll ever admit a problem exists), or happenstance that 31's seem playing well more so with others?
Don’t know...All I know is that the nearby station with the 31 is three years old. Sensor hasn’t been replaced.
Not positive, but I think the 31 wasn't even available then. I installed my first one late 3/16 and I believe that was pretty much hot off the press.

I’m mistaken then. It’s been 2 years and not three.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WxLover16 on October 04, 2018, 07:49:03 PM
Do you guys think it's possible that Davis has addressed and corrected the problem and haven't said anything (I truly don't think they'll ever admit a problem exists), or happenstance that 31's seem playing well more so with others?
Don’t know...All I know is that the nearby station with the 31 is three years old. Sensor hasn’t been replaced.
Not positive, but I think the 31 wasn't even available then. I installed my first one late 3/16 and I believe that was pretty much hot off the press.

I believe I read on here somewhere that they started being shipped on some VP2's in December 2015. I could be wrong though.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on October 04, 2018, 08:01:54 PM
Do you guys think it's possible that Davis has addressed and corrected the problem and haven't said anything (I truly don't think they'll ever admit a problem exists), or happenstance that 31's seem playing well more so with others?
Don’t know...All I know is that the nearby station with the 31 is three years old. Sensor hasn’t been replaced.
Not positive, but I think the 31 wasn't even available then. I installed my first one late 3/16 and I believe that was pretty much hot off the press.

I believe I read on here somewhere that they started being shipped on some VP2's in December 2015. I could be wrong though.
No sir. 31's weren't factory installed until last year. That doesn't mean they weren't available at that point for the retro fit on older ISS's like mine.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on October 05, 2018, 12:34:29 PM
I got a new ISS and 31 from Ryan a few weeks ago and I've been liking it so far. I had been using an old late 2006 ISS that came with an analog T/H when I bought it. After switching to digital sensors I had noticed I'd get strange spikes in RH a couple of times a day...i.e. I'd have an hour of something like 83-86% RH but I'd get 1 ob in there with 95% that would really stand out. That's gone now with the new transmitter.

The past few days I've been hitting 99% RH at night and the dewpoint has seemed on par during the day. I'm running it with the filter off and the fan at 9V.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Old Tele man on October 05, 2018, 12:42:41 PM
I got a new ISS and 31 from Ryan a few weeks ago and I've been liking it so far. I had been using an old late 2006 ISS that came with an analog T/H when I bought it. After switching to digital sensors I had noticed I'd get strange spikes in RH a couple of times a day...i.e. I'd have an hour of something like 83-86% RH but I'd get 1 ob in there with 95% that would really stand out. That's gone now with the new transmitter.

The past few days I've been hitting 99% RH at night and the dewpoint has seemed on par during the day. I'm running it with the filter off and the fan at 9V.
9VDC? Obviously, that's not a stock Davis FARS fan; what brand is it?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on October 05, 2018, 01:19:34 PM
I got a new ISS and 31 from Ryan a few weeks ago and I've been liking it so far. I had been using an old late 2006 ISS that came with an analog T/H when I bought it. After switching to digital sensors I had noticed I'd get strange spikes in RH a couple of times a day...i.e. I'd have an hour of something like 83-86% RH but I'd get 1 ob in there with 95% that would really stand out. That's gone now with the new transmitter.

The past few days I've been hitting 99% RH at night and the dewpoint has seemed on par during the day. I'm running it with the filter off and the fan at 9V.
9VDC? Obviously, that's not a stock Davis FARS fan; what brand is it?
I'll have to find a link later. It's one of those 12V circular Xerox drum fans. I connect it to a variable AC adapter and run it at 9V because  it doesn't like to startup at 12V even though it's rated for it. I got tired of forgetting it's off when the power comes back on after an outage. At 9V, no issues.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on October 05, 2018, 10:51:08 PM
So far with the 75, I like it pretty good. During the day, the DP is either the same or a few degrees lower than the nearby 31s. Sometimes the DP is the same or +/-1° than the nearby airports. Other times, it’s around a 2-3° difference with the 75 being wetter. For rest of y’all that have the 75, have y’all noticed the same thing? I will say that some weeds and other vegetation growing some underneath the station but nowhere high enough to be close the shield/sensor.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on October 08, 2018, 08:20:55 PM
This maybe should be in a new thread, but I recently found the preliminary data sheet on the new SHT85 ...

https://www.mouser.com/catalog/specsheets/Sensirion_10022018_HT_DS_SHT85_V0.9_D1.pdf
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on October 08, 2018, 08:59:44 PM
This maybe should be in a new thread, but I recently found the preliminary data sheet on the new SHT85 ...

https://www.mouser.com/catalog/specsheets/Sensirion_10022018_HT_DS_SHT85_V0.9_D1.pdf
\:D/ However.... The Big But....Is the protocol compatible with our ISS's???
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: galfert on October 08, 2018, 09:18:34 PM
This maybe should be in a new thread, but I recently found the preliminary data sheet on the new SHT85 ...

https://www.mouser.com/catalog/specsheets/Sensirion_10022018_HT_DS_SHT85_V0.9_D1.pdf
\:D/ However.... The Big But....Is the protocol compatible with our ISS's???

I searched the whole document. I only see references to I2C protocol. So doesn't look good. No mention of LSS. I suppose there could always be a separate document for the LSS version.

I liked that the SHT85 has a built-in heater! That is new I think.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on October 08, 2018, 09:31:33 PM
If you look at the data sheet on SHT75 its says IC not IC2 so this is same problem other newer sensors have so probably won't work. I'm no expert however on what will work with Davis.
From what I have pieced together the Davis version of SHT31 was special production order because they don't advertise the Davis version at all on Sensirion website.
Davis uses a protocol that is backward compatible and proprietary.
The 75 looks like the only way to avoid the wet bias.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: graculus on October 08, 2018, 10:25:32 PM
I liked that the SHT85 has a built-in heater! That is new I think.

The SHT1x and SHT3x series have all had heaters.

Most unlikely IMHO that they will offer this assembly (selected SHT3x on a stick?) in the older interface :-(
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: rdsman on October 09, 2018, 09:32:32 AM
Here is my take on this:

The SHT85 is pin for pin compatible with the SHT75 (electrically) and could be simply plugged in without any rewiring .  Although the SHT85 appears to be I2C only, a simple firmware upgrade for the ISS would be required.  As far as I know, this could only be done by Davis.

See:

https://www.mouser.com/catalog/additional/Sensirion_HT_AN_SHT85_Transitioning_SHT7x_to_SHT85_V0.9_D1.pdf



Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on October 09, 2018, 09:50:41 AM
Here is my take on this:

The SHT85 is pin for pin compatible with the SHT75 (electrically) and could be simply plugged in without any rewiring .  Although the SHT85 appears to be I2C only, a simple firmware upgrade for the ISS would be required.  As far as I know, this could only be done by Davis.

See:

https://www.mouser.com/catalog/additional/Sensirion_HT_AN_SHT85_Transitioning_SHT7x_to_SHT85_V0.9_D1.pdf

Would be nice, either Davis or others figure out how. Specs on the X85 look really good.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on October 09, 2018, 05:45:30 PM
Well what is really disappointing is it still has the same operating conditions of all the sensors, 20 to 80% above that the 3% error, etc. They just don't care about using the sensor in high humidity sites. It looks like the same old junk to me. We need Davis to change over to a sensor that is not so sensitive to high humidity.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on October 10, 2018, 07:59:35 PM
Anyone else getting the sensirion cables for the SHT75, Digikey has them back in stock. Remember price 1 order includes 4 cables per order. https://www.digikey.com/product-detail/en/sensirion-ag/3M-CABLES-FOR-EK-H4/1649-1055-ND/5982944

Existing Jack needs cut off and replaced with RJ12. I just added RJ12 jack on 8 cables I have and tested. No idea what I'll do with that many cables.

Wires from sensirion cable insert into jack pins this order. The cable has a raised knob on one side this side needs 75 JD3 label up.
Green-1
White-2
Brown-3
Yellow-6


FYI anyone working with these SHT75s the pins can bend easily so be careful use plenty of light making sure you are lined up properly before inserting.  They don't insert all the way, only so far so pins are exposed.

 [ You are not allowed to view attachments ]  
 [ You are not allowed to view attachments ]




Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on October 17, 2018, 02:02:05 PM
I recently received a new SHT31 sensor assembly. I installed it in my AC FARS on October 7th.

After performing almost 2 dozen tests with a 3 month old, factory certified Kestrel 5000 (+/- 2%) and cross referencing with the nearby airport and another SHT31 I own with a consistent bias (of about +8%), I can confidently state that the sensor arrived with an average 6% middle range wet bias. It's consistent from all the way up to the low 80 percentiles at which point it's about half that (+3%) until you reach accuracy around 90-93%.  The lowest humidity I've tested is 51%, but I have no reason to believe the error won't extend down another 20% or so into the 30 percentiles (that's the behavior of my other SHT31 sensors).

The new sensor has been maxing out at 98% but only after hours of exposure to saturated air. Effectively, 97% has been the ceiling since it was installed. I expect that maximum value to slowly decline over the next several months to around 95-96%, as has been the case with my 2 other 31s so far (along with reports of the same from many other owners and surveys of nearby VP2s).

I should point out that due to a cold snap, I've only tested in the 30-50F range. No idea how it performs in warmer weather.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on October 17, 2018, 02:23:41 PM
Just to emphasize... unlike my last 31, the new one is never accurate in the middle ranges. My previous 31 would encounter a long period of 80%+ humidity and gain a wet bias that would last an indeterminate amount of time until it dropped below 80% long enough to recalibrate. There's no way you can deal with that other than to wait. Meanwhile, my readings would be 8-10% off for hours or even days. Embarrassing.

At least when the error is consistent, I am able to custom program a couple offsets below 90% and a couple others above 95% and be done. So this sensor is an improvement in that way, but still 3x outside it's spec when BRAND NEW. And who knows if its behavior remains stable over time.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on October 17, 2018, 03:41:58 PM
The 31 I'm using I bought 3/16 and has about 8 months of service on it. I have no where near the error you're experiencing as I only use a -2% correction factor. As far as I can tell, it's very good there as I've been down to 11% and up to 96% with the -2%. Now, does that mean older ones are more reliable or that my dry climate helps or both or neither or.... #-o
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on October 17, 2018, 04:12:58 PM
My thinking about all of this maybe we need to add dewpoint temperature when we talk about Relative Humidity being its only relative to the ambient temperature and not a good indicator of actual moisture content in the air which IMO is the issue.
 
100% at 32°F (0°c) for days is still very little moisture and won't oversaturate the sensor membrane is my thinking. Now take 85% at 65°+dp the sensor membrane is absorbing lots of moisture in just hours.

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: drew1021 on October 17, 2018, 04:34:34 PM
This has been my exact observation in the 10 months my 31 has been in use. I used my Belford 566 as my reference. Hopefully Davis will find a solution to the problem. I would be willing to pay more for better accuracy.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on October 17, 2018, 04:37:58 PM
The 96% I hit was only a few days ago, without the correction, I was at 98% with the temp at 59.9F, dew, 59.3. The dew remained there for about 11 hours, then seemed to respond fine to the drying air later the next morning.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: wxmanmhd on October 17, 2018, 07:30:09 PM
I have a Vantage Pro2 with 24 hr Aspiration Fan and was wondering if with a steady air movement across the SHT 31 sensor will keep it a little lower than the shield without a fan. Has anyone compared that?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on October 17, 2018, 07:34:32 PM
Has anyone compared that?
For years and years. The fan is there to keep ambient air flowing across the sensor at all times, no matter what the wind is doing.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: wxmanmhd on October 17, 2018, 08:21:54 PM
I am sorry I forgot to mention that if it affects the humidity part of the sensor.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on October 17, 2018, 08:40:20 PM
I am sorry I forgot to mention that if it affects the humidity part of the sensor.
So it is said. To what degree I think has much to do with what climate the 31 is exposed to.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on October 18, 2018, 08:43:26 AM
My thinking about all of this maybe we need to add dewpoint temperature when we talk about Relative Humidity being its only relative to the ambient temperature and not a good indicator of actual moisture content in the air which IMO is the issue.
 
100% at 32°F (0°c) for days is still very little moisture and won't oversaturate the sensor membrane is my thinking. Now take 85% at 65°+dp the sensor membrane is absorbing lots of moisture in just hours.

When I conduct a test, I am recording dew points and then deriving the correct humidity from that using a dew point calculator (see: http://andrew.rsmas.miami.edu/bmcnoldy/Humidity.html (http://andrew.rsmas.miami.edu/bmcnoldy/Humidity.html) - the underlying formula is widely used in weather software). Otherwise, if there were a difference in temperature between the test device and the VP2, the humidity wouldn't be comparable.  Dew point should always be the same regardless of measured temp. *

Regardless, and very curiously, I nearly always arrive at a +6% difference. Same goes for my backup VP2 with an older SHT31 - it has a consistent +8% bias. Comparisons to the airport confirm the dewpoint errors and test device accuracy.


* NOTE: Just in case anyone is a stickler, normally I let the test device (Kestrel) acclimate until there's less than 1F difference between it and SHT31. This way even the reported humidities should match within about 1% and I can double check my calculations. I do not hold the device in my hand, but rather let it hang from a hook and orient the sensor into the wind on my covered, but otherwise open porch (this way I can measure in rain and snow too). I've compared the readings taken on the porch to those using a tripod next to the ISS and they are the same. The issue with using a tripod is sun exposure, which is ill-advised. I constructed an umbrella for it, but that doesn't do enough to reduce radiation errors and acts as a sail in the wind. The north-facing porch method nearly always matches the airport to within 1F when dew points are stable and well-mixed synoptic conditions prevail. 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on October 18, 2018, 10:21:37 AM
I have a Vantage Pro2 with 24 hr Aspiration Fan and was wondering if with a steady air movement across the SHT 31 sensor will keep it a little lower than the shield without a fan. Has anyone compared that?

Theoretically, fan aspiration shouldn't lower or raise accurately measured humidity. Some of the best psychrometers are fan aspirated. That said, when compared to a passively ventilated shield, on some clear, still mornings the FARS may report slightly higher temperatures (and thus lower humidities) compared to some lesser quality passive shields. The FARS temp in this scenario is actually more accurate and, therefore, the humidity should be as well. However, if you observe widespread dew and the measured humidity is less than 100% (which it will be in virtually all VP2s due to an unrelated dry bias in the sensor), it's because the air and surrounding objects are not shielded from longwave radiation which has a cooling effect driving the temp down to the dew point. This is somewhat akin to the difference of how the temperature feels when you are standing in the sun vs the same temperature in the shade.

There's a more general problem with measuring humidity inside radiation shields (something kcidwx, the NWS tech, covered much earlier in this thread). If the air has been saturated for awhile, the inside of the shield can collect moisture. You can argue that a fan will make a shield wetter when the air is loaded, particularly in low wind conditions. But the opposite is also true: a fan accelerates evaporation and should return the shield to balance with the outside air once humidity begins to drop.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on October 18, 2018, 03:23:52 PM
I just did a dry/wet bulb test and it showed the humidity to be 87.9 and my console reading was 88% this is with the sht 75, looks real good, i tested several times with the same results. I found a good chart to calculate this with, it uses dry/wet/barometer inputs to calculate with.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on October 18, 2018, 03:58:16 PM
I just did a dry/wet bulb test and it showed the humidity to be 87.9 and my console reading was 88% this is with the sht 75, looks real good, i tested several times with the same results. I found a good chart to calculate this with, it uses dry/wet/barometer inputs to calculate with.

OK, but just to be clear, I'm not questioning whether the console's (or your weather software's) dew point calculation is off. I'm saying my new SHT31 relative humidity sensor runs wet below about 90%. I only mentioned the dew point formula just in case anyone was questioning my method of calculating humidity discrepancies from dew points.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on October 18, 2018, 05:10:31 PM
When it comes to dp and wet bulb stuff i get semi lost, but the formulas on the web sure help. I also noticed with my 31 as the humidity dropped below 90% the readings compared to the 75 would slowly get larger with the 31 getting up to 3% higher around 80%. I see the same thing with the 15 but haven't seen it on the 75 so far. I just don't like the fact that the operating range is 20 to 80% everything else is up for grabs as far as accuracy goes. I am not going to hold my breath until Davis uses a sensor without this problem.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Old Tele man on October 18, 2018, 05:18:41 PM
Sensor design & technology is good, but NOT perfect. Sometimes, "close enuf" is all you're gonna get...unless you don't mind mortgaging the house.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on October 18, 2018, 06:12:17 PM
 :lol:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: wxmanmhd on October 18, 2018, 08:21:54 PM
Thanks for all the great info. I also was thinking of some evaporation with the FARS and agree sensor’s are pretty good vs the cost.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on October 18, 2018, 08:47:32 PM
Well, just to make it interesting, we finally had a day above 55F. And wouldn't you know it, the sensor is now running at +4% (rather than +6%) in the warmer air!  ](*,) That still equates to about a +2.5F dew point error.

Not looking for perfection, but when the manufacturer states that their sensor is +/- 2% *through the entire range* and that's not even close to true (not even at a given temp like 25C), I take issue. If it was consistently even +/- 4%, I could live with it. But having a varying wet bias from 35-85% (!) followed by a dry bias is unacceptable. This is my 3rd SHT31 and 2 of them have run wet at all humidities below 90% and the 3rd ran wet after extended periods above 80%. It's become near impossible to get these sensors to operate within specification and that's coming from a professional software developer.

Kestrel's custom designed humidity sensor is *damn good* (truly within +/- 2% at every temp I've tested so far) and it's in a $250 device. So, no, you don't need to 2nd mortgage your house to afford a good humidity sensor.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: drew1021 on November 04, 2018, 09:45:09 AM
I don't know about everyone else but I am getting impatient waiting for Davis to come up with a solution to the infamous wet bias of the 31. Its frustrating when all around me there are <200 dollar stations, ambient weather, showing reasonable humidity values as compared to my Belfort 566 and local ASOS.  Example, my Belfort was reading 67%, local ASOS 65%, my davis 73% ](*,)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on November 04, 2018, 10:20:39 AM
I don't think we will get anything better until Davis changes over to the i2c format and thus gets a larger variety of sensors to choose from. They have no options right now with the ls format. I fixed my problem by switching to the 75 sensor and making my own but even that is going to be a problem with the 75 being done away with. The maker of the sensor admits to the sensor being a lemon above 90% and should not be used in a wet setup. I blame Davis for picking a sensor for their stations with that type of warning from the maker.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: drew1021 on November 04, 2018, 10:58:22 AM
Agree, but is Davis going to listen to us and begin working on the fix? If enough people complain or care enough to voice their concerns to Davis then maybe. The bottom line is if sales of the VP2 suffer then we have a better chance.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on November 04, 2018, 02:40:11 PM
Agree, but is Davis going to listen to us and begin working on the fix? If enough people complain or care enough to voice their concerns to Davis then maybe. The bottom line is if sales of the VP2 suffer then we have a better chance.
I think that the VP2 owners here on this forum are a minuscule segment of their business and not worth a "drop everything that you're doing and fix it for these whiners" type of scenario. If it was a problem that was so obvious to ALL, then it might be a different story, but I think the vast majority are oblivious, or don't care enough to voice a complaint.
I very well could be and hope I'm wrong, but doubtful because I'd think there'd be a fix already.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: drew1021 on November 04, 2018, 03:00:55 PM
Agree, but is Davis going to listen to us and begin working on the fix? If enough people complain or care enough to voice their concerns to Davis then maybe. The bottom line is if sales of the VP2 suffer then we have a better chance.
I think that the VP2 owners here on this forum are a minuscule segment of their business and not worth a "drop everything that you're doing and fix it for these whiners" type of scenario. If it was a problem that was so obvious to ALL, then it might be a different story, but I think the vast majority are oblivious, or don't care enough to voice a complaint.
I very well could be and hope I'm wrong, but doubtful because I'd think there'd be a fix already.

Couldn't agree more, I guess chalk one up to wishful thinking on my behalf. Maybe I'll look into the Ambient Weather WS-2000. I've read their specs and on paper it looks worse than the VP2, however by my observation all the local ones look to be pretty accurate. I read that the sensor they use is the sht-30. I don't really care for the all in one units, however wind is not of the highest priority for me. Rain on the other hand is. Ambients specs aren't that great, +\- 10% What to do.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on November 04, 2018, 03:08:10 PM
Agree, but is Davis going to listen to us and begin working on the fix? If enough people complain or care enough to voice their concerns to Davis then maybe. The bottom line is if sales of the VP2 suffer then we have a better chance.
I think that the VP2 owners here on this forum are a minuscule segment of their business and not worth a "drop everything that you're doing and fix it for these whiners" type of scenario. If it was a problem that was so obvious to ALL, then it might be a different story, but I think the vast majority are oblivious, or don't care enough to voice a complaint.
I very well could be and hope I'm wrong, but doubtful because I'd think there'd be a fix already.
What to do.
Granted, my SHT31 is "only" set for a minus 2% correction factor, considerably less than most, but you'll not see me throwing my VP2 out the window....not even a debate.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: galfert on November 04, 2018, 07:09:17 PM
How much traction or effect do you all think starting an online petition would do for getting Davis to take notice? I think besides the petition alone there needs to be a way to reach out to other places beyond this hobbyist forum. We would need a way to secure contact information for weather hardware industries. This is something that a hardware resellers have. So I wonder if it would be possible to enlist the support of a Scaled Instruments or Scientific Sales or Ambient (they sell Davis too). In some ways I wonder how willing they would be to be a part of a notification process that VP2s are not performing their best, as it might hurt sales. There is also the possibility that it goes against some sort of reseller contract. Another way may be to be active at yearly industry conferences where Davis shows up with a booth. Someone could pass flyers at the event. The wording needs to be delicate and not disparaging. The message should be something like Davis VP2 hardware has shown by independent tests that there is a wet bias and a ceiling of around 94% to 96% humidity ceiling and that it would be instrumental to sign the petition to ensure that Davis does something about it. That's it. No harsh language calling anything crap, or that Davis isn't worth it. Nothing like that. It needs to be supportive of Davis. There should be some positive language praising the merits of owning a Davis system. I think it is always best to critique something by also adding in a positive. If you watch a bad movie and you just complain about how terrible it was and that is all you say then you are going to turn a lot of people off or cause others to jump on the defense. But if you critique and then have a balanced view and sing some praises also about what is good then you get better reception for your input, you seem more credible, and people don't have to jump on the defense. Same works with children when you are trying to raise them to make better choices. Then it becomes a showing that you aren't really there to complain, but that you want to help and be part of the solution, and that you care.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on November 04, 2018, 07:35:25 PM
Your post has merit, but make no doubt, Davis is completely aware of our 'feelings' regarding this situation. I as stated earlier today, I truly believe the 'cost to weight ratio' will leave Davis unresponsive. IMO, we'll only see a change when the next line of PWS's comes out, even if they stay in the PWS business. Who knows? I would certainly think it's lucrative enough to think they will.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dalecoy on November 05, 2018, 01:18:14 PM
I don't know about everyone else but I am getting impatient waiting for Davis to come up with a solution to the infamous wet bias of the 31.

I'm curious about a couple of things here:

1.  What form would that "solution" have?  Would it be hardware?  Software?  Or...

2.  If a hardware solution, then would this be "send your ISS in for replacement"?  Or optionally "do it yourself"?

3.  What production lots of VP2s would be involved?  That is, would it be available for all ages of VP2?  Cabled in addition to wireless?  Etc.

4.  What would be the estimated cost?

5.  How many units would you expect Davis to sell?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: SnowHiker on November 06, 2018, 12:40:02 AM

I'm curious about a couple of things here:

1.  What form would that "solution" have?  Would it be hardware?  Software?  Or...

2.  If a hardware solution, then would this be "send your ISS in for replacement"?  Or optionally "do it yourself"?

3.  What production lots of VP2s would be involved?  That is, would it be available for all ages of VP2?  Cabled in addition to wireless?  Etc.

4.  What would be the estimated cost?

5.  How many units would you expect Davis to sell?
My thoughts are:

1. If it's a hardware problem, as is my understanding, then a hardware solution would be preferable.  If it's a software problem a software solution would be preferable.  If it's a hardware fix that also requires a software update, then both.

2.  Yes, sending it in or optionally fixing it yourself if feasible.  Sounds good to me.

3.  The production lots that are affected.  At the least those whose owners complain and have units that are under warranty.  More depending on how far Davis feels it is worth it to them to please their existing customers.

4.  It is Davis that needs to come up with an acceptable solution and to determine the cost of that solution, not the consumer experiencing the problem.

5.  Also not a concern for the consumer who is looking for a solution.  How many sales might Davis lose because of the problem, whether rectified or unaddressed?

I'm actually not affected by the problem, but these questions strike me as strange as the answers should be self-evident, so I assume you're trying to prove some kind of point; like it's not worth it for Davis to address and rectify the problem.  If so, maybe you're right, but that doesn't mean you should throw design or manufacturing flaws back in the face of the affected consumers.  So why not just come out and make your point?  Especially as the one you addressed your questions to had acknowledged that those who are aware of and bothered by the problem may not be enough for Davis to address (see reply # 1325).
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on November 06, 2018, 08:20:08 AM
The problem cannot be adequately addressed through software because each sensor has varying amounts of bias, and those biases can change with time. I have 3 sensors. They all behave differently. No one solution would ensure each operates within specifications.

It's clearly a hardware problem. As has been pointed out many times already, Sensirion warns explicitly in the datasheet that the sensor isn't designed for environments with humidities above 80%. I mean how asinine is it that Davis (or anyone else) thought "screw it, we're doing it anyway?!" As Ron (kcidwx) said, this is an INDOOR sensor not intended for meteorology. He's an NWS field tech who's tested and verified the problem even with non-Davis branded SHT31s. Arguing that other manufacturers have implemented this sensor outdoors "successfully" is problematic because AFAIK, those claiming that haven't tracked non-Davis SHT31s long-term (particularly not with calibrated instrumentation). Just to make matters worse, Davis uses non-approved chemicals in mounting & packaging the sensor (material safety handling). So either way you slice it, Davis blew through vendor warnings about sensor corruption and the results are predictable.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: EA1EF on November 06, 2018, 09:07:38 AM
We thing that probably the simplest way are adopted by Belfryboy that use Sensirion original recommended proof materials based in housing with Dupont original Teflon PTFE

Probably The original Davis housing for SHT sensors aren't optimized for the sensor specs and outdoor, and users are that pay so expensive the Davis bad design because Davis caps are so far Sensitrion specs and recommendations.

Handling recommendations
https://www.sensirion.com/fileadmin/user_upload/customers/sensirion/Dokumente/2_Humidity_Sensors/Sensirion_Humidity_Sensors_Handling_Instructions.pdf

https://www.sensirion.com/en/environmental-sensors/humidity-sensors/filter-cap-sf1/

https://www.sensirion.com/en/environmental-sensors/humidity-sensors/filter-cap-sf2/




Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on November 06, 2018, 10:23:34 AM
The problem cannot be adequately addressed through software because each sensor has varying amounts of bias, and those biases can change with time. I have 3 sensors. They all behave differently. No one solution would ensure each operates within specifications.

It's clearly a hardware problem. As has been pointed out many times already, Sensirion warns explicitly in the datasheet that the sensor isn't designed for environments with humidities above 80%. I mean how asinine is it that Davis (or anyone else) thought "screw it, we're doing it anyway?!" As Ron (kcidwx) said, this is an INDOOR sensor not intended for meteorology. He's an NWS field tech who's tested and verified the problem even with non-Davis branded SHT31s. Arguing that other manufacturers have implemented this sensor outdoors "successfully" is problematic because AFAIK, those claiming that haven't tracked non-Davis SHT31s long-term (particularly not with calibrated instrumentation). Just to make matters worse, Davis uses non-approved chemicals in mounting & packaging the sensor (material safety handling). So either way you slice it, Davis blew through vendor warnings about sensor corruption and the results are predictable.
Agree. It's not a meteorological sensor.

If someone has a separate temp/hum station, the probable way to go (if you're really bothered by the wet bias) is to measure the temp with your FARS/ISS and then use a passive shield with a 2nd SHTxx with some form of supplemental heat inside of the shield to keep the RH almost always < 80%. Then apply your own calculations to determine a new RH using the ISS 2m temp and the passive 2m dewpoint. I'm not sure if you could apply a resistor to the Davis temp/hum voltage line to create a little heat without affecting the sensor performance. That would be a question better answered by one of the electrical guys here.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on November 06, 2018, 12:53:23 PM
Agree. It's not a meteorological sensor.

If someone has a separate temp/hum station, the probable way to go (if you're really bothered by the wet bias) is to measure the temp with your FARS/ISS and then use a passive shield with a 2nd SHTxx with some form of supplemental heat inside of the shield to keep the RH almost always < 80%. Then apply your own calculations to determine a new RH using the ISS 2m temp and the passive 2m dewpoint. I'm not sure if you could apply a resistor to the Davis temp/hum voltage line to create a little heat without affecting the sensor performance. That would be a question better answered by one of the electrical guys here.

Aside from the fact that what you just described, while conceptually simple, would take a variety of non-trivial hardware and software skills to accomplish, the implicit assumption is that so long as you can keep the SHT31 below 80% humidity it will report accurate readings. Otherwise, why go to all the effort? (I'm saying this for the benefit of others who may not have read the thread in its entirety.) Mine arrived with +6% bias below 80% (at temps above 55F; +4% below it). I know Randy and others have reported receiving new 31s that were wet from the start. It seems, based on past discussion, that this initially defective behavior is due to bad material handling by Davis. Reconditioning attempts have largely failed.

If the chemical contamination issues were sorted out, and the sensor were verified to be accurate before leaving Davis' warehouse, then keeping it below 80% humidity long term may be a worthwhile, if complicated, endeavor to pursue.

Speaking more generally and to no one in particular, if you think accurate humidity isn't that important, you should go ask someone who runs a decent size farm whether correct evapo-transpiration calculations matter to them. Spoiler alert: a 5+% shift in humidity during peak growing season could mean wasting thousands of gallons of water ($$$) or, worse yet, under applying irrigation to thirsty crops.  Keep in mind that agriculture is one of the primary targets for Davis products. So when the VP2 OVER reports humidity, irrigation is UNDER applied and this could cost farmers real money from lost yields (especially in hot and dry years).
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: galfert on November 06, 2018, 12:59:38 PM
Speaking more generally and to no one in particular, if you think accurate humidity isn't that important, you should go ask someone who runs a decent size farm whether correct evapo-transpiration calculations matter to them. Spoiler alert: a 5+% shift in humidity during peak growing season could mean wasting thousands of gallons of water ($$$) or, worse yet, under applying irrigation to thirsty crops.  Keep in mind that agriculture is one of the primary targets for Davis products. So when the VP2 OVER reports humidity, irrigation is UNDER applied and this could cost farmers real money from lost yields (especially in hot and dry years).

Which is why I say someone needs to show up to some farm convention and find the Davis booth and start handing out informative flyers to everyone around the area. I bet Davis would then care very much and they will address the issue. If you can't get in the venue then hand out flyers at the entrance.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on November 06, 2018, 01:12:01 PM
Speaking more generally and to no one in particular, if you think accurate humidity isn't that important, you should go ask someone who runs a decent size farm whether correct evapo-transpiration calculations matter to them. Spoiler alert: a 5+% shift in humidity during peak growing season could mean wasting thousands of gallons of water ($$$) or, worse yet, under applying irrigation to thirsty crops.  Keep in mind that agriculture is one of the primary targets for Davis products. So when the VP2 OVER reports humidity, irrigation is UNDER applied and this could cost farmers real money from lost yields (especially in hot and dry years).

Which is why I say someone needs to show up to some farm convention and find the Davis booth and start handing out informative flyers to everyone around the area. I bet Davis would then care very much and they will address the issue. If you can't get in the venue then hand out flyers at the entrance.

I'm all for action over words (which is why I crafted my own custom solution to this problem after spending countless hours assessing and documenting it), but that sounds like a pretty good way to get sued by Davis. I suppose if you have the cash or if you are a consumer litigator, that might work. As with anything, there's always the likelihood that the truth is irrelevant to the proceedings -- in the courtroom or the court of public opinion. What's the saying? It's easier to fool people than to convince them they've been fooled?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: galfert on November 06, 2018, 02:00:32 PM
Speaking more generally and to no one in particular, if you think accurate humidity isn't that important, you should go ask someone who runs a decent size farm whether correct evapo-transpiration calculations matter to them. Spoiler alert: a 5+% shift in humidity during peak growing season could mean wasting thousands of gallons of water ($$$) or, worse yet, under applying irrigation to thirsty crops.  Keep in mind that agriculture is one of the primary targets for Davis products. So when the VP2 OVER reports humidity, irrigation is UNDER applied and this could cost farmers real money from lost yields (especially in hot and dry years).

Which is why I say someone needs to show up to some farm convention and find the Davis booth and start handing out informative flyers to everyone around the area. I bet Davis would then care very much and they will address the issue. If you can't get in the venue then hand out flyers at the entrance.

I'm all for action over words (which is why I crafted my own custom solution to this problem after spending countless hours assessing and documenting it), but that sounds like a pretty good way to get sued by Davis. I suppose if you have the cash or if you are a consumer litigator, that might work. As with anything, there's always the likelihood that the truth is irrelevant to the proceedings -- in the courtroom or the court of public opinion. What's the saying? It's easier to fool people than to convince them they've been fooled?

You have a very good point. So maybe we consult a lawyer or maybe we don't need to. Just use common sense. You could present the information without even using the Davis name. You could list other brand hardware that uses SHT31 and mention how they don't have these wet bias problems then post the question in big letters, "Does your weather station have wet bias?" Then close it out saying that other leading weather station hardware has been shown to have wet bias in independent tests. You see this type of advertising and information all the time.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on November 07, 2018, 05:23:07 PM
From the testing I’ve done, I think the Sensirion sensor is actually remarkably good for the price. While the sensor has its limitations as discussed here (exposure to prolonged high humidity etc) my experience is there is simply something about Davis’ application of the sensor that accounts for most of the poor performance.

I am in NYC for work and took a run through Central Park. To validate my obsession with this stuff, I carried my $15 Acurite SHT31 and stopped at the Central Park ASOS in time for the 351p observation. This is completely anecdotal and not exactly scientific, but it’s just another data point. This Acurite sensor isn’t exposed to long periods of high humidity, although I have had it outside near saturation for hours at a time to see how it performs. And I don’t exactly baby it. I take it on runs, throw it in my bag etc. No special handling by any means. I even took pictures for the 5 minutes, on the minute, leading up to the 351p ASOS observation since ASOS uses a running 5 min average. The entire time the Acurite SHT31 was representative of the 351p picture shown below. In the other picture you can barely see the Acurite on the corner fence post for a sense of proximity. Even without a radiation shield and considering mass of the plastic moulding, the results were spot on. Rounding 61.4° and 40.9% to 61° and 41% gives a DP of 37°, which is exactly the same as ASOS at the exact same time.


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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on November 10, 2018, 11:04:16 AM
Another at 751a ...

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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on November 10, 2018, 12:27:16 PM
We already know from previous discussion up thread that the Acurite indoor units can be expected to perform quite well for a time outdoors. But no one has exposed them long term. After all, they aren't shielded from the elements whatsoever so siting them properly year round would be ill advised.

Sensirion puts the following disclaimer in the datasheet for the sensor:

Quote
The sensor shows best performance when operated within recommended normal temperature and humidity range of 5 – 60 °C and 20 – 80 %RH, respectively. Long term exposure to conditions outside normal range, especially at high humidity, may temporarily offset the RH signal (e.g. +3%RH after 60h at >80%RH). After returning into the normal temperature and humidity range the sensor will slowly come back to calibration state by itself. Prolonged exposure to extreme conditions may accelerate ageing.

"Extreme conditions" here are defined as being below 5C (41F) or above 80% humidity regularly. Notice also how they use the term "normal". Under what scenario would an environment featuring temps above 41 and humidity between 20-80% be considered "normal"? The only one I can think of would be climate controlled (home, business, lab, greenhouse, humidor, etc).
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on November 10, 2018, 12:38:54 PM
The (Acurite SHT31)  will fit inside the Davis 7714... Yes the humidity doesn't suffer same fate as Davis. Much lower in those DP's above 55 but doesn't suffer at highest end either. Basically the Acurite 31 FOLLOWS THE DATA SHEET like we would EXPECT. Why I feel problem only exists with the LS series special order Davis SHT31.   
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on November 10, 2018, 02:56:40 PM
We already know from previous discussion up thread that the Acurite indoor units can be expected to perform quite well for a time outdoors. But no one has exposed them long term. After all, they aren't shielded from the elements whatsoever so siting them properly year round would be ill advised.

Sensirion puts the following disclaimer in the datasheet for the sensor:

Quote
The sensor shows best performance when operated within recommended normal temperature and humidity range of 5 – 60 °C and 20 – 80 %RH, respectively. Long term exposure to conditions outside normal range, especially at high humidity, may temporarily offset the RH signal (e.g. +3%RH after 60h at >80%RH). After returning into the normal temperature and humidity range the sensor will slowly come back to calibration state by itself. Prolonged exposure to extreme conditions may accelerate ageing.

"Extreme conditions" here are defined as being below 5C (41F) or above 80% humidity regularly. Notice also how they use the term "normal". Under what scenario would an environment featuring temps above 41 and humidity between 20-80% be considered "normal"? The only one I can think of would be climate controlled (home, business, lab, greenhouse, humidor, etc).

Agreed and I am not trying to suggest that Sensirion sensors are without their limitations and errors. As you point out, they are quite transparent about them. And yes, I haven't exposed the Acurite sensor to "extreme conditions" for more than a few continuous days. But I have exposed it to >80% humidity in excess of 60 continuous hours. I wanted to see if it reaches 100% humidity (it does >99.4%) and to see how the Sensirion offset, when exposed to "extreme conditions," compares to a new (<1 year old) Davis SHT31 that has never been outside (I keep it for such comparisons). Even after that exposure, the Davis SHT31 shows a wet bias in excess of the Acurite mounted SHT31. As I said initially, nothing scientific here, but just providing a data point and sharing my experience.

From what I have seen, Davis' SHT31 seems to have a wet bias before it's exposed to anything. The Davis SHT31 suffers from errors inherent to the Sensirion sensor plus whatever errors they introduce in the application of the sensor. The Acurite sensor performs extremely well (which is surprising given my experience with Davis and Acurite more generally). Here is to hoping that Davis can and will make whatever changes they need to get the maximum performance out of the Sensirion sensor even given its inherent limitations because I think it would be a great improvement.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on November 11, 2018, 04:44:08 PM
I discovered something in the specs for the sht1x and sht7x series sensors that i had not read before or just overlooked. I was wondering why all my old sensors and the new 75's stopped at 99% and the Sensirion specs state (Values higher than 99%RH indicate fully saturated air and
must be processed and displayed as 100%RH1) this is why they don't go over 99% because Davis doesn't show the readings over 99% as 100% unless they are 99.5% or higher. The highest reading in dense fog on mine has been 99.4% with most at 99.1 to 99.2 percent. But according to specs they should be displayed as 100% by Davis are they should show the specs as 0 to 99% instead of 100%. So again it looks like Davis doesn't follow the Sensirion instructions. The 31's don't show that in the specs and both mine hit 100% in fog with one showing 99.9% and the other 99.7% so that looks like a change made with this sensor. ](*,)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on November 11, 2018, 05:23:01 PM
If you've stated this before, sorry, I've forgotten, but how to you see resolution to the 0.1%? I just know that whenever I hit 99% (no idea what is is to the tenth), the temp/dew spread is always 0.4F.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: openvista on November 11, 2018, 05:46:59 PM
I discovered something in the specs for the sht1x and sht7x series sensors that i had not read before or just overlooked. I was wondering why all my old sensors and the new 75's stopped at 99% and the Sensirion specs state (Values higher than 99%RH indicate fully saturated air and
must be processed and displayed as 100%RH1) this is why they don't go over 99% because Davis doesn't show the readings over 99% as 100% unless they are 99.5% or higher. The highest reading in dense fog on mine has been 99.4% with most at 99.1 to 99.2 percent. But according to specs they should be displayed as 100% by Davis are they should show the specs as 0 to 99% instead of 100%. So again it looks like Davis doesn't follow the Sensirion instructions. The 31's don't show that in the specs and both mine hit 100% in fog with one showing 99.9% and the other 99.7% so that looks like a change made with this sensor. ](*,)

Well, that wouldn't be a problem for any of my three SHT31 sensors. None of them have ever gotten higher than 98% including the one I just received a month or so ago. To reach 98% the newest sensor has to sit in saturated air for at least a few hours. Most of the time the max is 97%. But, according to Sensirion, as long as it reaches 96.5% it's considered within specification (max error 3.5%). 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on November 11, 2018, 06:52:21 PM
I have a hand held unit that used the sht11 that i removed and fixed it up so i can plug any sht sensor that uses the Davis format and it reads the humidity to a tenth. Both of my 31's read 100% in fog. Unless you are lucky and get one of the older sht's that gets to 99.5% you won't see a 100 ever. But according the the specs 99.1 should trigger the 100% reading but it looks like Davis fixed the software to not read a 100% less than 99.5%. So i use my weather display software to adjust my readings to show 100% by adding 1% to the readings which kicks in when the reading goes above 93% and removes it when i goes back down to 93. My 31's are good on the high end but read high on the lower end so have to use the adjustments to lower the readings on the low end. I use the best sensor that reads pretty good on the low end and tops out at 99 on the high end and get pretty good results that way. I have found that when i get a decent reading on one end the other is off on the other end.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on November 11, 2018, 07:13:22 PM
Unless you are lucky and get one of the older sht's that gets to 99.5% you won't see a 100 ever.

I have found that when i get a decent reading on one end the other is off on the other end.
Thanks. As I stated here before, the 31 I have now (couple years old) refused to hit 100%, sat on 99 (zero correction factor), and that's after being WOXOF for hours, but all my comps were long since at 100%.

Of course, robbing Peter to pay Paul.

 :-| I guess I'm basically rehashing old news.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on November 11, 2018, 07:24:33 PM
Yeah it's been pretty well beaten to death  :lol:. My point is Davis should have made the software to read 100 with anything over 99 and didn't. I guess i lucked out with my two 31's reading 100 but they always do when the fog is around but when we get some really dry air after a frontal passage they read high on the low end, of course down here we only get low humidity in the winter when dry air comes in from the north. I have two of the Davis sht15's that both read 99 and do pretty well on the low end too. I use the pick of the litter when it comes to the one i use on the main station but for someone who just has the one it is pretty much luck of the draw whether you get a good one or not.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Old Tele man on November 12, 2018, 07:53:05 PM
SHT31's new labelling: WINTER, SPRING, SUMMER, FALL
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on May 21, 2019, 02:25:06 PM
I talked to one of Davis’ tech folks. He forwarded my email to their design team regarding the suggestion of them switching the temp/hum sensor from the Sensirion SHT-31 to the IST HYT-221 sensor. He won’t be able to get a response from them but he did basically say that they may not switch due to compatibility issues and would have to redesign the transmitter board. But he wasn’t a 100% percent certain in which way they would go.

The Sensirion SHT-75 is the only option Davis users have if you live in an humid environment and want fairly reliable DP readings.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on May 21, 2019, 02:42:56 PM
Here are the specs:
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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: galfert on May 21, 2019, 02:48:32 PM
The required change is to go with an I2C bus and get rid of LSS. Then they can use any current and future sensor. This is inevitable and Davis will need to do this at some point.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on May 21, 2019, 03:13:02 PM
The required change is to go with an I2C bus and get rid of LSS. Then they can use any current and future sensor. This is inevitable and Davis will need to do this at some point.

Agreed. Hopefully they will do it very soon.  Unless there is a better option than the HYT-221, I hope they will use that sensor. Supposedly, it can handle very high humidity environments and is chemical resistant. According to IST, this sensor is good for agricultural use, which is one of Davis main targets for their stations.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on May 21, 2019, 03:16:25 PM
Here are the specs:
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It would be interesting if the temperature sensor performs well during the winter in your location Randy.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on May 21, 2019, 03:19:01 PM
Here are the specs:
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It would be interesting if the temperature sensor performs well during the winter in your location Randy.

Looks about the same as SHT15 and 75. Not as good as 31 which is basic flatline through those sub-zero temperatures.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on May 21, 2019, 03:51:30 PM
The required change is to go with an I2C bus and get rid of LSS. Then they can use any current and future sensor. This is inevitable and Davis will need to do this at some point.

Inevitable like a color screen on its console, and wifi built-in?

Sorry, but I couldn’t resist poking fun at Davis, again, and its horribly dated technology.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on May 21, 2019, 04:18:10 PM
The required change is to go with an I2C bus and get rid of LSS. Then they can use any current and future sensor. This is inevitable and Davis will need to do this at some point.

Inevitable like a color screen on its console, and wifi built-in?

Sorry, but I couldn’t resist poking fun at Davis, again, and its horribly dated technology.
Ahhh yes, but yet you proudly display owning three of them... :-P
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on May 21, 2019, 04:23:50 PM
The required change is to go with an I2C bus and get rid of LSS. Then they can use any current and future sensor. This is inevitable and Davis will need to do this at some point.

Inevitable like a color screen on its console, and wifi built-in?

Sorry, but I couldn’t resist poking fun at Davis, again, and its horribly dated technology.
Ahhh yes, but yet you proudly display owning three of them... :-P

Unfortunately, there isn’t a better system on the market at the Davis price point.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: galfert on May 21, 2019, 04:48:23 PM
The required change is to go with an I2C bus and get rid of LSS. Then they can use any current and future sensor. This is inevitable and Davis will need to do this at some point.

Inevitable like a color screen on its console, and wifi built-in?

Sorry, but I couldn’t resist poking fun at Davis, again, and its horribly dated technology.

With the release of the WLL the color console with WiFi solution no longer needs to come from Davis. All we need now is for someone to come up with an app that mimics a display console. A cheap tablet is all you'll need and you are all set. We'll see that soon from some 3rd party developer.

Since the WLL release, for me this I2C bus support is the last remaining advancement into having something worthy of being called a VP3. Then I can have a new ISS with I2C sensors, a WLL that is the console to receive sensor data and 3rd party software to do what you want with the data (as Meteobridge is already supporting via WLL API), and soon the tablet display app utilizing the same WLL API that Meteobridge is using and you got yourself a VP3.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on June 18, 2019, 09:40:15 PM
Here is a interesting video that briefly talks about the IST HYT sensors
https://youtu.be/X9vW5JroqE4
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on June 21, 2019, 07:08:39 PM
Well, after 3 years of service, I retired my first 31 and slapped in another. So far so good, 16-ish hrs each day now are below 10% humidity and seems to be performing nicely. Will have to wait for the monsoon to see about the other end. I also finally replaced my original 12 year old C123, it was down to 2.3v. Certainly got my money's worth outta it. =D&gt;
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on June 22, 2019, 04:01:29 PM
Well, after 3 years of service, I retired my first 31 and slapped in another. So far so good, 16-ish hrs each day now are below 10% humidity and seems to be performing nicely. Will have to wait for the monsoon to see about the other end. I also finally replaced my original 12 year old C123, it was down to 2.3v. Certainly got my money's worth outta it. =D&gt;

Nice! The Sensirion humidity sensors are better suited for your neck of the woods.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on June 22, 2019, 04:40:00 PM
Well, after 3 years of service, I retired my first 31 and slapped in another. So far so good, 16-ish hrs each day now are below 10% humidity and seems to be performing nicely. Will have to wait for the monsoon to see about the other end. I also finally replaced my original 12 year old C123, it was down to 2.3v. Certainly got my money's worth outta it. =D&gt;

Nice! The Sensirion humidity sensors are better suited for your neck of the woods.
No doubt. The old one was dropping off the table below 5% or so. This one so far has not. I'll watch it like a hawk as usual, maybe Davis has figured out how to mount it properly. Maybe not....I'll keep y'all advised.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on June 22, 2019, 09:20:12 PM
Well, after 3 years of service, I retired my first 31 and slapped in another. So far so good, 16-ish hrs each day now are below 10% humidity and seems to be performing nicely. Will have to wait for the monsoon to see about the other end. I also finally replaced my original 12 year old C123, it was down to 2.3v. Certainly got my money's worth outta it. =D&gt;

Nice! The Sensirion humidity sensors are better suited for your neck of the woods.
No doubt. The old one was dropping off the table below 5% or so. This one so far has not. I'll watch it like a hawk as usual, maybe Davis has figured out how to mount it properly. Maybe not....I'll keep y'all advised.

Please do! Not sure mounting will help much except to reduce wet bias.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on June 22, 2019, 09:58:55 PM
Well, after 3 years of service, I retired my first 31 and slapped in another. So far so good, 16-ish hrs each day now are below 10% humidity and seems to be performing nicely. Will have to wait for the monsoon to see about the other end. I also finally replaced my original 12 year old C123, it was down to 2.3v. Certainly got my money's worth outta it. =D&gt;

Nice! The Sensirion humidity sensors are better suited for your neck of the woods.
No doubt. The old one was dropping off the table below 5% or so. This one so far has not. I'll watch it like a hawk as usual, maybe Davis has figured out how to mount it properly. Maybe not....I'll keep y'all advised.

Not sure mounting will help much except to reduce wet bias.
Well hell's bells, whaddaya want? That's exactly what we've been bitchin' about since Davis debuted the 31 three years ago. Granted, the 31 is "supposedly" only meant for indoor use, but Acurite uses a 31 for their little hand held device and works very well. IF, I repeat, IF Davis has solved this problem, then a lot of folks here go  =D&gt;. Frankly, I'm not holding my breath. We'll see.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on June 23, 2019, 07:48:53 PM
Tell ya what, this new 31 has my attention. Granted, I've only been between 5 and 37% since installing it, but the dew has been rock solid, so much so I took out my -2% correction factor from the old 31 and now run it at flat 0. Obviously still early, but really like what I'm seeing, at least on the dry end.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on June 24, 2019, 08:01:07 PM
Tell ya what, this new 31 has my attention. Granted, I've only been between 5 and 37% since installing it, but the dew has been rock solid, so much so I took out my -2% correction factor from the old 31 and now run it at flat 0. Obviously still early, but really like what I'm seeing, at least on the dry end.

Nice! Can you tell any difference the way Davis mounted the sensor?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on June 24, 2019, 08:21:26 PM
Tell ya what, this new 31 has my attention. Granted, I've only been between 5 and 37% since installing it, but the dew has been rock solid, so much so I took out my -2% correction factor from the old 31 and now run it at flat 0. Obviously still early, but really like what I'm seeing, at least on the dry end.

Nice! Can you tell any difference the way Davis mounted the sensor?
I had Ryan send me two, so I just compared the old and the other new one. With that, to my eye, absolutely no mounting difference. On the sensor side they look exactly identical, revision number and all. Only noticeable difference is on the back side with the attached white sticker, it's completely different, could just be a stock number for all I know. So if it is different, Davis is keeping it under the cuff.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on June 25, 2019, 05:56:45 PM
Tell ya what, this new 31 has my attention. Granted, I've only been between 5 and 37% since installing it, but the dew has been rock solid, so much so I took out my -2% correction factor from the old 31 and now run it at flat 0. Obviously still early, but really like what I'm seeing, at least on the dry end.

Nice! Can you tell any difference the way Davis mounted the sensor?
I had Ryan send me two, so I just compared the old and the other new one. With that, to my eye, absolutely no mounting difference. On the sensor side they look exactly identical, revision number and all. Only noticeable difference is on the back side with the attached white sticker, it's completely different, could just be a stock number for all I know. So if it is different, Davis is keeping it under the cuff.


Gotcha  Let me know how it performs in the long run.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on June 25, 2019, 06:43:39 PM
Tell ya what, this new 31 has my attention. Granted, I've only been between 5 and 37% since installing it, but the dew has been rock solid, so much so I took out my -2% correction factor from the old 31 and now run it at flat 0. Obviously still early, but really like what I'm seeing, at least on the dry end.

Nice! Can you tell any difference the way Davis mounted the sensor?
I had Ryan send me two, so I just compared the old and the other new one. With that, to my eye, absolutely no mounting difference. On the sensor side they look exactly identical, revision number and all. Only noticeable difference is on the back side with the attached white sticker, it's completely different, could just be a stock number for all I know. So if it is different, Davis is keeping it under the cuff.


Gotcha  Let me know how it performs in the long run.
I'm still tickled with it. Even though the monsoon is supposedly right around the corner, kinda wish one of you "wet" guys would give it another try. Not saying this sensor is a "new and improved version", but it's certainly performing better than my last one.
Who knows....
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on June 25, 2019, 08:25:38 PM
I'm still tickled with it. ...Not saying this sensor is a "new and improved version", but it's certainly performing better than my last one.
Who knows....
[/quote]

Could this be 8 times better than zero but that result is still zero?! I remember when the SHT31 was released, a few years ago, and we thought everything would be great. Unemployment in the US would disappear, the crime rate throughout the world would drop to near zero, inflation in Venezuela would disappear, and no further marriages would ever result in divorce! Ok, I may have gotten carried away but the point is it was a major disappointment—it was not the enhancement we were led to believe it was. Stupid me—I upgraded my two VP2s with the new sensor and ordered another VP2 with this godsend. Oh well...
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on June 25, 2019, 08:33:44 PM
I'm still tickled with it. ...Not saying this sensor is a "new and improved version", but it's certainly performing better than my last one.
Who knows....

Could this be 8 times better than zero but that result is still zero?! I remember when the SHT31 was released, a few years ago, and we thought everything would be great. Unemployment in the US would disappear, the crime rate throughout the world would drop to near zero, inflation in Venezuela would disappear, and no further marriages would ever result in divorce! Ok, I may have gotten carried away but the point is it was a major disappointment—it was not the enhancement we were led to believe it was. Stupid me—I upgraded my two VP2s with the new sensor and ordered another VP2 with this godsend. Oh well...
Quote
Damn Ron, whatta ya want? All I can do is tell you what I observe, and I like what I see. If it turns out to be another POS, oh well, but not so far.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on June 25, 2019, 08:36:37 PM
I'm still tickled with it. ...Not saying this sensor is a "new and improved version", but it's certainly performing better than my last one.
Who knows....

Could this be 8 times better than zero but that result is still zero?! I remember when the SHT31 was released, a few years ago, and we thought everything would be great. Unemployment in the US would disappear, the crime rate throughout the world would drop to near zero, inflation in Venezuela would disappear, and no further marriages would ever result in divorce! Ok, I may have gotten carried away but the point is it was a major disappointment—it was not the enhancement we were led to believe it was. Stupid me—I upgraded my two VP2s with the new sensor and ordered another VP2 with this godsend. Oh well...
Quote
Damn Ron, whatta ya want? All I can do is tell you what I observe, and I like what I see. If it turns out to be another POS, oh well, but not so far.

I truly hope you are right, but as you stated, this newbie needs a good test in higher humidities before I send Ryan my credit card!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on June 25, 2019, 08:41:03 PM
I'm still tickled with it. ...Not saying this sensor is a "new and improved version", but it's certainly performing better than my last one.
Who knows....

Could this be 8 times better than zero but that result is still zero?! I remember when the SHT31 was released, a few years ago, and we thought everything would be great. Unemployment in the US would disappear, the crime rate throughout the world would drop to near zero, inflation in Venezuela would disappear, and no further marriages would ever result in divorce! Ok, I may have gotten carried away but the point is it was a major disappointment—it was not the enhancement we were led to believe it was. Stupid me—I upgraded my two VP2s with the new sensor and ordered another VP2 with this godsend. Oh well...
Quote
Damn Ron, whatta ya want? All I can do is tell you what I observe, and I like what I see. If it turns out to be another POS, oh well, but not so far.

I truly hope you are right, but as you stated, the newbie needs a good test in higher humidities before I send Ryan my credit card!
I've sent three back to him so far, the fourth is coming. His return policy is as good as it gets.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 01, 2019, 11:04:17 AM
Tell ya what, this new 31 has my attention. Granted, I've only been between 5 and 37% since installing it, but the dew has been rock solid, so much so I took out my -2% correction factor from the old 31 and now run it at flat 0. Obviously still early, but really like what I'm seeing, at least on the dry end.

Nice! Can you tell any difference the way Davis mounted the sensor?
I had Ryan send me two, so I just compared the old and the other new one. With that, to my eye, absolutely no mounting difference. On the sensor side they look exactly identical, revision number and all. Only noticeable difference is on the back side with the attached white sticker, it's completely different, could just be a stock number for all I know. So if it is different, Davis is keeping it under the cuff.


Gotcha  Let me know how it performs in the long run.
I'm still tickled with it. Even though the monsoon is supposedly right around the corner, kinda wish one of you "wet" guys would give it another try. Not saying this sensor is a "new and improved version", but it's certainly performing better than my last one.
Who knows....

Let me know how the sensor performs when the monsoon hits. I’m assuming that it’s still working good right now.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 01, 2019, 03:40:15 PM
Well, I can tell you this, it's been in service 2 weeks now, my dew point spread has been from 12 to 53F, the humidity from 5 to 40%. Still obviously on the dry side of things, but I'm very pleased with what I'm seeing and am still happily using a zero correction factor. This is a big deal as the dew point is showing a much more linear response and hopefully will continue as the humidity rises in the coming weeks. As I said before, it would be nice for one of you more humid folks to pick one up and give it a whirl. If you don't agree with my preliminary findings, send it back, mine's going nowhere... =D&gt;
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 06, 2019, 03:34:48 PM
Had a little MCS south of the border finally push some BL moisture our way, dew's been in the 50's to low 60's and this new 31 is still rockin' it. There is zero doubt in my mind this is an improvement over my last one. Now, whether that's coincidence or by design, I have no idea, but it works. If I'm lucky, I may see some actual rain today and will continue to hawk it, cause we're gonna dry right the bleep out again after today as the non-soon continues.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 09, 2019, 11:06:28 AM
Had a little MCS south of the border finally push some BL moisture our way, dew's been in the 50's to low 60's and this new 31 is still rockin' it. There is zero doubt in my mind this is an improvement over my last one. Now, whether that's coincidence or by design, I have no idea, but it works. If I'm lucky, I may see some actual rain today and will continue to hawk it, cause we're gonna dry right the bleep out again after today as the non-soon continues.

Nice!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 09, 2019, 03:32:06 PM
Had a little MCS south of the border finally push some BL moisture our way, dew's been in the 50's to low 60's and this new 31 is still rockin' it. There is zero doubt in my mind this is an improvement over my last one. Now, whether that's coincidence or by design, I have no idea, but it works. If I'm lucky, I may see some actual rain today and will continue to hawk it, cause we're gonna dry right the bleep out again after today as the non-soon continues.

Nice!
I managed a whopping 0.04" of rain on Sunday with the dew in the mid-upper 60's for a while, briefly touched 71, still looking good. Need to see some much better/longer saturation though, I imagine that's what most are waiting to see me report, myself included...
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on July 13, 2019, 05:32:58 PM
I guess I need to order a new SHT31 (already replaced it once because of unacceptable results) for my VP2 with the 24 hour fan. See attached screenshot from CWOP showing the data from my 3 Davis VP2s. The newest one and the unit that has the 24 hour fan is FW3075. My older units, which have the daytime fan together with the SHT31, are stations CW5020 and FW4350. The ISSs are located within 25 feet or so of each other, same height on the same fence. I verified that the fans in all three VP2s are working—receiving a lot of sun today! Humidity looks reasonable on station FW3075 but not the temperature and three nearby airports confirm these results. How disappointing to say the least.

 [ You are not allowed to view attachments ]
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: galfert on July 13, 2019, 06:30:25 PM
If you are replacing a sensor it would be informative to know how long your sensor lasted.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 13, 2019, 06:31:56 PM
I guess I need to order a new SHT31 (already replaced it once because of unacceptable results) for my VP2 with the 24 hour fan. See attached screenshot from CWOP showing the data from my 3 Davis VP2s. The newest one and the unit that has the 24 hour fan is FW3075. My older units, which have the daytime fan together with the SHT31, are stations CW5020 and FW4350. The ISSs are located within 25 feet or so of each other, same height on the same fence. I verified that the fans in all three VP2s are working—receiving a lot of sun today! Humidity looks reasonable on station FW3075 but not the temperature and three nearby airports confirm these results. How disappointing to say the least.

 [ You are not allowed to view attachments ]

Remember humidity is relative to temperature is why the hot station had the lower %. What are the dewpoints running on the 3 stations?
Did you do the -.9F if using an older ISS and the 31? Still doesn't explain the large temperature difference. Almost like something is not working on the 24hr fars. I would swap sensor with one of the Dfars units and see if it still reads that high.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 13, 2019, 06:38:11 PM
I guess I need to order a new SHT31 (already replaced it once because of unacceptable results) for my VP2 with the 24 hour fan. See attached screenshot from CWOP showing the data from my 3 Davis VP2s. The newest one and the unit that has the 24 hour fan is FW3075. My older units, which have the daytime fan together with the SHT31, are stations CW5020 and FW4350. The ISSs are located within 25 feet or so of each other, same height on the same fence. I verified that the fans in all three VP2s are working—receiving a lot of sun today! Humidity looks reasonable on station FW3075 but not the temperature and three nearby airports confirm these results. How disappointing to say the least.

 [ You are not allowed to view attachments ]
What are the dewpoints running on the 3 stations?

Yes. The only time knowing the humidity that's worth a damn is 100%.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 13, 2019, 06:58:23 PM
I whipped out my handy-dandy calculator http://www.dpcalc.org/ and this is what it is:
94F/54%=75F dew
93/50=72
98/43=72
Now what??
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on July 13, 2019, 07:30:02 PM
Humidity looks good so the hot sensor issue I lean toward shield location or fan issue. If this was a replacement sensor for the same reason especially because I've never got a new 31 out of calibration with temperature before. Guess it could happen but 2 in a row? 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on July 13, 2019, 07:38:24 PM
Humidity looks good so the hot sensor issue I lean toward shield location or fan issue.
Hey Ron, don't laugh, but I recall not too long ago that some stock fans were running backwards. Sure yours is sucking from the bottom and not blowing?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on July 13, 2019, 08:23:59 PM
Humidity looks good so the hot sensor issue I lean toward shield location or fan issue.
Hey Ron, don't laugh, but I recall not too long ago that some stock fans were running backwards. Sure yours is sucking from the bottom and not blowing?

I remember reading in this Forum that some Davis units were blowing rather than sucking but mine is a sucker.   [tup] Dew points were 71.6 on stations CW5020 and FW3075, but was 74.8 on station FW4350 at the time the temperatures shown previously were recorded (4:10 pm CDT). CW2274–our dew point numbers (rounded) agree—but I guess they should as I don’t believe uploads to CWOP include dew points so MesoWest, my data source, used your formula!

I have not used any offsets on my consoles as the console firmware is sufficiently current as well as communicating with up-to-date PCBAs per Davis guidelines.

Maybe I should find another retirement hobby as I am confused but am open to suggestions on what might be going on. Probably the safest answer for my station (FW3075) that has the 24 hour fan model (Davis part no. 6153) is to replace the SHT31-again, the fan and batteries and call it a day. Sending about $100 to Scaled Instruments for parts to “fix” a station that is 14 months old, seems a bit drastic, but it is easier to do this all at once than to cherry pick replacing the parts one-by-one. If I was 100% convinced the new spoon tipper was worth $50, I might replace that part too.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: galfert on July 14, 2019, 02:22:31 AM
Remember some VP2 consoles need a 0.9 ºF calibration offset. Read the notes on this page:
https://www.scaledinstruments.com/shop/davis-instruments/parts/vantage-pro2-parts/temperature-and-humidity-sensor-parts/davis-7346-070-pro2-digital-temperature-humidity-sensor-sensiron-sht31/
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Tensor on July 19, 2019, 08:27:50 AM
I made a small test and compared 4 years old SHT21 against 1.5 years old SHT31. Both have been outside for one year, but now they are indoors (they were not outside at the same time period).
Both sensors have been exposed to temperature ranges from -30 °C to +30 °C and very high humidities.

The results of the test are quite clear. Both sensors have excellent temperature measurement precision and accuracy. SHT31 suffers from notable dry bias, which practically renders SHT31 useless for humidity measuring. Despite from being older, SHT21 has only slight wet bias, as Sensirion SHTxx sensors often have. SHT21's humidity readings are pretty close a calibrated traditional hair hygrometer. In the other words, the real humidity level was at the test within 52-54 %.

SHT31:
Humidity: 43.35 %, Temperature: +24.70 °C

SHT21:
Humidity: 55.18 %, Temperature: +24.68 °C

When the SHT31 sensor was last time outdoors, it barely climbed above 90 % even at the most humid days. It followed very well the nearby station' humidity levels in the beginning, but after 6 months it lost completely its accuracy. Setting an empirical calibration offset could be an option, but the readings depend nonlinearly from the real humidity adn temperature. During cold weather, the dry bias seemed to be even worse.

Does anyone have experience from HYT 221? The only piece of information from Arduino forum claims that it's better than SHTs. I am considering buying one or two of HYTs for further tests.

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: galfert on July 19, 2019, 09:07:36 AM
Does anyone have experience from HYT 221? The only piece of information from Arduino forum claims that it's better than SHTs. I am considering buying one or two of HYTs for further tests.

You can't use the HYT 221 because it is I2C protocol. The Davis ISS is not compatible with I2C sensors. The Davis ISS uses LSS (Legacy Sensirion Sensibus) protocol. Which is why you can't use the SHT35/SHT85 since they only come with I2C protocol support. There are different versions of previous SHTxx to respectively support either LSS or I2C. Meaning you can't use just any SHT31 for example.

Interesting that you find that the SHT31 has a dry bias, as some others have reported a wet bias.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Tensor on July 19, 2019, 01:30:42 PM
You can't use the HYT 221 because it is I2C protocol. The Davis ISS is not compatible with I2C sensors. The Davis ISS uses LSS (Legacy Sensirion Sensibus) protocol. Which is why you can't use the SHT35/SHT85 since they only come with I2C protocol support. There are different versions of previous SHTxx to respectively support either LSS or I2C. Meaning you can't use just any SHT31 for example.

Interesting that you find that the SHT31 has a dry bias, as some others have reported a wet bias.
I didn't express this in my previous post, but I use Arduino and use it with sensors directly, since I don't have a Davis. This thread is just the only one in the internet, which deals with SHT31 aging and bias problems. I hope my observations are helpful. I will tell how HYT 221 works, if I happen to buy a couple of them.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 19, 2019, 03:44:36 PM
You can't use the HYT 221 because it is I2C protocol. The Davis ISS is not compatible with I2C sensors. The Davis ISS uses LSS (Legacy Sensirion Sensibus) protocol. Which is why you can't use the SHT35/SHT85 since they only come with I2C protocol support. There are different versions of previous SHTxx to respectively support either LSS or I2C. Meaning you can't use just any SHT31 for example.

Interesting that you find that the SHT31 has a dry bias, as some others have reported a wet bias.
I didn't express this in my previous post, but I use Arduino and use it with sensors directly, since I don't have a Davis. This thread is just the only one in the internet, which deals with SHT31 aging and bias problems. I hope my observations are helpful. I will tell how HYT 221 works, if I happen to buy a couple of them.

Please get them and let us know. It’s a sensor that I recommended Davis to consider switching too. Would love to see how the HYT would match up with a nearby ASOS or a Psychro-Dyne
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on July 19, 2019, 03:45:25 PM
You can't use the HYT 221 because it is I2C protocol. The Davis ISS is not compatible with I2C sensors. The Davis ISS uses LSS (Legacy Sensirion Sensibus) protocol. Which is why you can't use the SHT35/SHT85 since they only come with I2C protocol support. There are different versions of previous SHTxx to respectively support either LSS or I2C. Meaning you can't use just any SHT31 for example.

Interesting that you find that the SHT31 has a dry bias, as some others have reported a wet bias.
I didn't express this in my previous post, but I use Arduino and use it with sensors directly, since I don't have a Davis. This thread is just the only one in the internet, which deals with SHT31 aging and bias problems. I hope my observations are helpful. I will tell how HYT 221 works, if I happen to buy a couple of them.

Please get them and let us know. It’s a sensor that I recommended Davis to consider switching too. Would love to see how the HYT would match up with a nearby ASOS or a Psychro-Dyne

A Kestrel DROP sensor would also be another good standard to compare too
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on August 09, 2019, 06:56:03 PM
Nice easterly wave moving through late tonight and well into tomorrow with lots of PWAT, so heavy rainers are likely. This should finally be a good saturation test (I hope) for this new 31. My impression to this point hasn't changed, it's doing a very nice job and what saturation I have had (up to 93% and mid 70's dew), it has recovered just fine.   
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on August 10, 2019, 12:05:01 PM
Nice easterly wave moving through late tonight and well into tomorrow with lots of PWAT, so heavy rainers are likely. This should finally be a good saturation test (I hope) for this new 31. My impression to this point hasn't changed, it's doing a very nice job and what saturation I have had (up to 93% and mid 70's dew), it has recovered just fine.


Good deal!  Keep me updated!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Tensor on September 21, 2019, 11:09:26 AM
Please get them and let us know. It’s a sensor that I recommended Davis to consider switching too. Would love to see how the HYT would match up with a nearby ASOS or a Psychro-Dyne

I bought a couple HYT 221s and now one of them has been outside for over 2 weeks.
The first observation is related to their mountings. SHT21 is inside a proper humidity probe, so it has good ventilation. HYT 221 is inside a punctured Kinder Surprise Egg. Also, I don't have a FARS, so HYT 221's humidity level may lag behind the truth at calm evenings. At days, the lag doesn't seem to be so large, because the days are rarely calm here.

The second thing is that HYT 221 measures nearly always lower humidity levels than the old SHT21. This is kinda expected, since almost all Sensirion sensors tend to have a small wet bias (but my SHT31 had bad dry bias). Their temperatures are within 0.1-0.3 degrees of Celsius, so both have good precision at temperature measurement.

For the third, dew point of HYT 221 seems to be more correlated with a nearby meteorological institute station (less than 1km away) than SHT21 is. Dewpoint correlation between HYT and the official station is 0.9802, whereas dewpoint correlation SHT and official is 0.9716.

At windy days, dew points of HYT221 are very close to the official station. At wet days, HYT 221 doesn't climb so easily to 99 % as SHT21 does (HYT 221 has climbed up to 99.4% so far, if anyone wonders). I don't think that HYT 221's poor casing can explain the slow climb speed fully, but I can be wrong. Maybe I should remove the Egg completely, because HYT221 has a protective grid against droplets by itself.
Both HYT221 and SHT21 report too high humidities after rain, but that's because my decrepit radiation shield absorbs water and is located near to bushes and grass.
I also think that the "official" station has a slight wet bias at 90%+ humidity levels, because it looks to report 100% even if there are no fog at all, but who knows.

Edit: I still underline, that the distributions of TD differences maybe lie a little bit: there have been many wet days recently, and then TD is almost the same as T. SHT looks to the have edge over HYT at those times, but not during dry weather. For instance: the weather is cloudy, cool and dryish now, and the difference HYT TD - official TD is +0.2 C while SHT TD - official TD is +1.3 C. In conclusion, I have to examine high level humidity measurement capabilities of HYT 221 more and of course its aging over time. SHT31 was also superb accurate at first, but became complete trash within months or so.



Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on September 21, 2019, 09:39:26 PM
Please get them and let us know. It’s a sensor that I recommended Davis to consider switching too. Would love to see how the HYT would match up with a nearby ASOS or a Psychro-Dyne

I bought a couple HYT 221s and now one of them has been outside for over 2 weeks.
The first observation is related to their mountings. SHT21 is inside a proper humidity probe, so it has good ventilation. HYT 221 is inside a punctured Kinder Surprise Egg. Also, I don't have a FARS, so HYT 221's humidity level may lag behind the truth at calm evenings. At days, the lag doesn't seem to be so large, because the days are rarely calm here.

The second thing is that HYT 221 measures nearly always lower humidity levels than the old SHT21. This is kinda expected, since almost all Sensirion sensors tend to have a small wet bias (but my SHT31 had bad dry bias). Their temperatures are within 0.1-0.3 degrees of Celsius, so both have good precision at temperature measurement.

For the third, dew point of HYT 221 seems to be more correlated with a nearby meteorological institute station (less than 1km away) than SHT21 is. Dewpoint correlation between HYT and the official station is 0.9802, whereas dewpoint correlation SHT and official is 0.9716.

At windy days, dew points of HYT221 are very close to the official station. At wet days, HYT 221 doesn't climb so easily to 99 % as SHT21 does (HYT 221 has climbed up to 99.4% so far, if anyone wonders). I don't think that HYT 221's poor casing can explain the slow climb speed fully, but I can be wrong. Maybe I should remove the Egg completely, because HYT221 has a protective grid against droplets by itself.
Both HYT221 and SHT21 report too high humidities after rain, but that's because my decrepit radiation shield absorbs water and is located near to bushes and grass.
I also think that the "official" station has a slight wet bias at 90%+ humidity levels, because it looks to report 100% even if there are no fog at all, but who knows.

Edit: I still underline, that the distributions of TD differences maybe lie a little bit: there have been many wet days recently, and then TD is almost the same as T. SHT looks to the have edge over HYT at those times, but not during dry weather. For instance: the weather is cloudy, cool and dryish now, and the difference HYT TD - official TD is +0.2 C while SHT TD - official TD is +1.3 C. In conclusion, I have to examine high level humidity measurement capabilities of HYT 221 more and of course its aging over time. SHT31 was also superb accurate at first, but became complete trash within months or so.

Seems to me that your set up might be affecting your results (more specifically the HYT-221 sensor). From what I’ve read on the HYT sensors, it shouldn’t age as quickly as the Sensirion sensors. Please continue on in reporting your results with the two sensors. I think the HYT-221 sensor is the best low cost sensor available for Davis to switch too.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on September 23, 2019, 07:23:12 PM
Please get them and let us know. It’s a sensor that I recommended Davis to consider switching too. Would love to see how the HYT would match up with a nearby ASOS or a Psychro-Dyne

I bought a couple HYT 221s and now one of them has been outside for over 2 weeks.
The first observation is related to their mountings. SHT21 is inside a proper humidity probe, so it has good ventilation. HYT 221 is inside a punctured Kinder Surprise Egg. Also, I don't have a FARS, so HYT 221's humidity level may lag behind the truth at calm evenings. At days, the lag doesn't seem to be so large, because the days are rarely calm here.

The second thing is that HYT 221 measures nearly always lower humidity levels than the old SHT21. This is kinda expected, since almost all Sensirion sensors tend to have a small wet bias (but my SHT31 had bad dry bias). Their temperatures are within 0.1-0.3 degrees of Celsius, so both have good precision at temperature measurement.

For the third, dew point of HYT 221 seems to be more correlated with a nearby meteorological institute station (less than 1km away) than SHT21 is. Dewpoint correlation between HYT and the official station is 0.9802, whereas dewpoint correlation SHT and official is 0.9716.

At windy days, dew points of HYT221 are very close to the official station. At wet days, HYT 221 doesn't climb so easily to 99 % as SHT21 does (HYT 221 has climbed up to 99.4% so far, if anyone wonders). I don't think that HYT 221's poor casing can explain the slow climb speed fully, but I can be wrong. Maybe I should remove the Egg completely, because HYT221 has a protective grid against droplets by itself.
Both HYT221 and SHT21 report too high humidities after rain, but that's because my decrepit radiation shield absorbs water and is located near to bushes and grass.
I also think that the "official" station has a slight wet bias at 90%+ humidity levels, because it looks to report 100% even if there are no fog at all, but who knows.

Edit: I still underline, that the distributions of TD differences maybe lie a little bit: there have been many wet days recently, and then TD is almost the same as T. SHT looks to the have edge over HYT at those times, but not during dry weather. For instance: the weather is cloudy, cool and dryish now, and the difference HYT TD - official TD is +0.2 C while SHT TD - official TD is +1.3 C. In conclusion, I have to examine high level humidity measurement capabilities of HYT 221 more and of course its aging over time. SHT31 was also superb accurate at first, but became complete trash within months or so.

BTW: How exactly close are the Dew Points between the HYT sensors and the official observations?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on September 23, 2019, 08:08:34 PM
Along those lines, I'm still waiting here for some summer storms to wet us up for more than a couple of hours, but it has been futile this year. This next few days could be some nice boundary layer saturation, if not, I'll almost certainly have to wait for the winter. By then, my "new" 31 will be 9ish months old. However, as of now, it's still performing like a champ.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on October 09, 2019, 06:56:46 PM
Well, now that non-soon 2019 has left the building, I thought I'd state my final-ish opinion (see what the winter brings as well) on this latest 31 and it's performance to date. I would not hesitate for one second to buy this again. No idea if this 31 is a fluke, or Davis has finally figured out what was occurring with their mounting technique, but it's been fantastic. One other possible reason that I've found this 31 to be so good is that it has had the SF2 cap on it from day one, I've never done that before. It acts as a barrier to an extent and may very well help with over-saturation and what not, and it recovers perfectly fine. Me having a case fan may also help with the "process".
All of this is not scientific, just my relentless study. Thing is, as much as I've preached this sensor from day one, not one, single person that I'm aware of (and I've lobbied) has tried to see whether I'm full of it or it's truly legit.

Guess we'll never know...I'll be enjoying it regardless.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Old Tele man on October 09, 2019, 07:08:01 PM
Old Japanese proverb: "No FIXEE what ain't BROKEE."



P.S. -- I'm still using my 31's (yep, I've got more than one in service).
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on October 09, 2019, 07:14:20 PM
Old Japanese proverb: "No FIXEE what ain't BROKEE."
Well, that's just it, everybody and their brother says it sucks...and there's been tons of data that backs that conclusion. All I've been saying is that perhaps the issue has been dealt with.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on October 09, 2019, 08:20:57 PM
Old Japanese proverb: "No FIXEE what ain't BROKEE."
Well, that's just it, everybody and their brother says it sucks...and there's been tons of data that backs that conclusion. All I've been saying is that perhaps the issue has been dealt with.

Possibly dealt with but based on prior experience unlikely. Davis has such a great tool if they would only pay attention to details and feedback from us enthusiasts. The SHT31 and spoon tipper just to name two. I can live with a horribly dated and ugly console but these two weaknesses are fundamental to the integrity of a pws.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on October 09, 2019, 08:30:32 PM
Old Japanese proverb: "No FIXEE what ain't BROKEE."
Well, that's just it, everybody and their brother says it sucks...and there's been tons of data that backs that conclusion. All I've been saying is that perhaps the issue has been dealt with.

Possibly dealt with but based on prior experience unlikely.
And frankly tired of saying this over and over, you'll never know til you try.

Damn people, it's not like I'm asking y'all to buy a $1000 piece of equipment that you can't return, it's $40....and you can return it. I'm sick and tired of being the only who's TRYING here. If it still sucks, oh well, send the GD thing back. Mine works fine. :roll:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on October 09, 2019, 08:33:47 PM
Old Japanese proverb: "No FIXEE what ain't BROKEE."
Well, that's just it, everybody and their brother says it sucks...and there's been tons of data that backs that conclusion. All I've been saying is that perhaps the issue has been dealt with.

Possibly dealt with but based on prior experience unlikely.
And frankly tired of saying this over and over, you'll never know til you try.

Damn people, it's not like I'm asking y'all to buy a $1000 piece of equipment that you can't return, it's $40....and you can return it. I'm sick and tired of being the only who's TRYING here. If it still sucks, oh well, send the GD thing back. Mine works fine. :roll:

I have tried three times with this sensor and failed each time. I really don’t like returning items. Adds too much complexity to a small business like Ryan and other honest business people run.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on October 09, 2019, 08:41:16 PM
Old Japanese proverb: "No FIXEE what ain't BROKEE."
Well, that's just it, everybody and their brother says it sucks...and there's been tons of data that backs that conclusion. All I've been saying is that perhaps the issue has been dealt with.

Possibly dealt with but based on prior experience unlikely.
And frankly tired of saying this over and over, you'll never know til you try.

Damn people, it's not like I'm asking y'all to buy a $1000 piece of equipment that you can't return, it's $40....and you can return it. I'm sick and tired of being the only who's TRYING here. If it still sucks, oh well, send the GD thing back. Mine works fine. :roll:

I have tried three times and failed. I really don’t like returning items too. Adds too much complexity to a small business like Ryan and other honest business people run.
Fine. Ryan has stated to me all his returns go back to Davis, that's his business. You'd think after countless returns to Davis, they'd start to get the picture....that's the whole point of returns here.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on October 09, 2019, 08:52:32 PM
As I've said before I'm sensor poor 5-SHT31's and 10+ SHT 75's and in no need of another for years. The last one I purchased even properly packaged displayed the same high humidity bias as before.  I use the SHT75 during the spring, summer and the 31 during the fall, winter when dewpoints are well below the trouble point 60's +.   
I'm sure someone in one of the chronic high dewpoint regions year-round will eventually purchase and hopefully give you/us some feedback.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on October 09, 2019, 08:59:51 PM
dewpoints trouble point 60's +
No trouble for me...as in none...zip, zero, zilch.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on October 10, 2019, 02:44:49 AM
I wish my experience and enthusiasm matched those of CW2274. Unfortunately, they do not.

Despite good sense from Johnd and others, I couldn’t resist purchasing a Davis 6834 mostly because I assumed it was an SHT31 and because their spec sheet notes it is for indoor use only as the sensor is unshielded. https://www.davisinstruments.com/product_documents/weather/spec_sheets/6834%20Temp%20Hum%20Sheet.pdf

My hope was that the unshielded sensor would be mounted differently and therefore might produce better results (along the lines of the Acurite SHT31). Upon receipt, however, I found that it is literally the exact same outdoor 31 and filter with just a plastic casing for mounting. The date sticker on the sensor is June 2019. Without even having been brought outdoors, the sensor has a high humidity bias compared to my Belfort 566 and my SHT75 that has been outdoors for 6 mos.

No doubt these sensors do better in arid areas (which unfortunately doesn’t describe western Washington), but even straight out of the packaging, I see no improvement with a newly manufactured Davis 31, although I’m happy to see CW2274 getting improved results. For me, I’m done hoping for an improvement in these Davis sensors, unless they acknowledge a change or someone posts a demonstrable change in these sensors. Unfortunately, as many have noted, that is very unlikely.

Finally, for those tinkerers, I’ve mounted my SHT75 in a sintered metal temperature humidity probe casing I found on Amazon (from China, of course) and I’ve had good results. It’s an AM2305. https://www.ebay.com/i/382584872953?rt=nc&_trkparms=aid%3D222007%26algo%3DSIM.MBE%26ao%3D2%26asc%3D20160908110712%26meid%3Db49d3475180242579b83524be24642eb%26pid%3D100677%26rk%3D11%26rkt%3D30%26sd%3D283587140916%26itm%3D382584872953%26pmt%3D0%26noa%3D1%26pg%3D2386202  I’ve noticed most high end temp/hum sensor manufacturers (Vaisala etc) use sintered filters. I’ve removed the included sensor and wires and inserted my soldered SHT75.

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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on October 10, 2019, 03:54:21 AM
Thanks for the update sorry to hear there was no difference. I still use a silk material for a filter and works well.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Tensor on October 10, 2019, 05:00:13 AM
BTW: How exactly close are the Dew Points between the HYT sensors and the official observations?

I'm still running the comparisons. So far HYT221 seems to rise slower to higher humidity levels than the official one. It has also maxed out to 99.99% at super humid day. Dew points are very close to each other, if it's windy and humidity is less than 90.

And for everybody who are interested, here's another guy's realtime comparison of Vaisala and SHT31: https://www.mrsoft.fi/current.htm
I bet his Vaisala sensors are of type HMP60 or HMP110 (the cheapest ones you can buy from Vaisala). If I had spare money, I would buy also one of those and compare SHT31, HYT221 and HMP60 in the exactly same conditions.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on October 10, 2019, 10:27:05 PM
Ok so i just ordered another 31 for testing from Ryan at S.I. now that we are getting into the high humidity season down here on the coast near the Gulf of Mexico. I sometimes get humidity readings above 95% for days at a time during the winter months. I think i am wasting my money but just have to try one more time to see if i can get a good long run of good readings. My accurite meter has been outside a lot over the last year and is still working just fine so i know the 31 can work real well over a long period of time. Will get it set up and running for the winter and will give updates as needed.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on October 11, 2019, 11:30:11 AM
Ok so i just ordered another 31 for testing from Ryan at S.I. now that we are getting into the high humidity season down here on the coast near the Gulf of Mexico. I sometimes get humidity readings above 95% for days at a time during the winter months. I think i am wasting my money but just have to try one more time to see if i can get a good long run of good readings. My accurite meter has been outside a lot over the last year and is still working just fine so i know the 31 can work real well over a long period of time. Will get it set up and running for the winter and will give updates as needed.

Nice Jerry, now we will know for sure  =D&gt;
I like you seriously doubt there are any changes other than packaging. I think our conclusion was the molding process onto the board was the main problem. The Accurite doesn't use this process and even sensirion warns VOC's can harm the humidity sensor. 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on October 11, 2019, 05:02:10 PM
Ok so i just ordered another 31 for testing from Ryan at S.I. now that we are getting into the high humidity season down here on the coast near the Gulf of Mexico. I sometimes get humidity readings above 95% for days at a time during the winter months. I think i am wasting my money but just have to try one more time to see if i can get a good long run of good readings. My accurite meter has been outside a lot over the last year and is still working just fine so i know the 31 can work real well over a long period of time. Will get it set up and running for the winter and will give updates as needed.
Nice Jerry, now we will know for sure  =D&gt;
Granted, I don't live in the most humid of places, but I've spent the last two months with dew points consistently in the 60's. The monsoon doesn't always bring rain, but it always brings humidity and the 31 did fine. Just sayin'.......... :roll:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: krojan on October 12, 2019, 02:36:31 PM
Today I came across such a page comparing different sensors:

https://wiki.liutyi.info/display/ARDUINO/Test+0+v6+room+RH35+with+fan+unstable+board

Maybe it will be useful to someone.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on October 12, 2019, 06:53:21 PM
BTW: How exactly close are the Dew Points between the HYT sensors and the official observations?

I'm still running the comparisons. So far HYT221 seems to rise slower to higher humidity levels than the official one. It has also maxed out to 99.99% at super humid day. Dew points are very close to each other, if it's windy and humidity is less than 90.

And for everybody who are interested, here's another guy's realtime comparison of Vaisala and SHT31: https://www.mrsoft.fi/current.htm
I bet his Vaisala sensors are of type HMP60 or HMP110 (the cheapest ones you can buy from Vaisala). If I had spare money, I would buy also one of those and compare SHT31, HYT221 and HMP60 in the exactly same conditions.

Do you think the slowness of HYT’s humidity responsiveness is due to the siting of that sensor? I remember you saying that it wasn’t ideally sited.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on October 12, 2019, 07:07:19 PM
Please get them and let us know. It’s a sensor that I recommended Davis to consider switching too. Would love to see how the HYT would match up with a nearby ASOS or a Psychro-Dyne
I don't have a FARS, so HYT 221's humidity level may lag behind the truth at calm evenings. At days, the lag doesn't seem to be so large, because the days are rarely calm here.
I agree with this assessment, no fan. Sitting has nothing to do with responsiveness, only accuracy. 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on October 19, 2019, 06:30:10 PM
Ok got my new sensor in this afternoon and i removed the filter and wrapped it in a damp washcloth for a couple of hours to get it moistened up. It is now outside and under test and it is reading the same as my other two stations so now for the long haul testing.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on October 19, 2019, 06:52:25 PM
wrapped it in a damp washcloth for a couple of hours to get it moistened up.
Not saying it's wrong or right, but I didn't do that. The only thing I did different was use the SF2 cap from the get-go.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on October 19, 2019, 07:16:36 PM
wrapped it in a damp washcloth for a couple of hours to get it moistened up.
Not saying it's wrong or right, but I didn't do that. The only thing I did different was use the SF2 cap from the get-go.

I must have missed the post. What is a SF2 cap and where do I get one?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on October 19, 2019, 07:23:50 PM
https://www.mouser.com/ProductDetail/Sensirion/SF2?qs=wWC4CIiyLaNCAejbFUK5dw%3D%3D
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on October 20, 2019, 09:27:07 AM
It should be a good test looking at Jerry's dewpoint this morning. 
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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on October 20, 2019, 10:54:20 AM
Short first report, the humidity with the two 75 sensors maxed at 97% and the new 31 maxed at 98% and went down to 97% after 30 minutes. The wind was calm and one of the 75 sensors is in a Barani standard passive shield while the other 75 as well as the 31 are in modified Davis 24 hour shields that run with fan on during the day and off at night. The mod makes the shield a passive shield at night and fan added during the day and has been giving good results. When the fans turned on the 31 humidity began to drop first with the 75 with fan on lagging behind by a few minutes and the Barani passive did not start to drop until wind started to blow at 2 to 3 mph. Will update later when humidity drops a lot more to see how the 31 does against the 75.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on October 20, 2019, 01:50:24 PM
Update, i just moved the 31 to my website so i can monitor it from anywhere. It is working just fine right now and now have to see if it keeps on going.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on October 20, 2019, 04:20:31 PM
It is working just fine right now and now have to see if it keeps on going.
Maybe.....just maybe....I was right. Let alone the obvious that I want this sensor to perform to humidity spec, I want some vindication as well because not a damn soul here has believed a word I've said. ](*,)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on October 20, 2019, 07:18:34 PM
Update, i just moved the 31 to my website so i can monitor it from anywhere. It is working just fine right now and now have to see if it keeps on going.

Seems like it’s working good. Now the question becomes, does it develop a wet bias offset?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on October 21, 2019, 01:58:04 PM
It is working just fine right now and now have to see if it keeps on going.
Maybe.....just maybe....I was right. Let alone the obvious that I want this sensor to perform to humidity spec, I want some vindication as well because not a damn soul here has believed a word I've said. ](*,)

Definitely not a belief or a trust thing, it’s just that I’ve had a different result from the side by side comparisons I’ve done. But I will say this is definitely one of those circumstances where I look forward to be proven wrong. Here’s hoping Jerry sees something different ...
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on October 21, 2019, 02:58:27 PM
 Jerry, Looks like you are still in the China mountains.   :lol:  at 11,000 feet.


https://mesowest.utah.edu/cgi-bin/droman/mesomap.cgi?lat=28.77950&lon=96.83033&radius=25&rawsflag=290&site=AT358&unit=0&time=LOCAL&product=&year1=&month1=&day1=00&hour1=00&currTimeChecked=
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on October 21, 2019, 04:34:18 PM
It is working just fine right now and now have to see if it keeps on going.
Maybe.....just maybe....I was right. Let alone the obvious that I want this sensor to perform to humidity spec, I want some vindication as well because not a damn soul here has believed a word I've said. ](*,)

Definitely not a belief or a trust thing, it’s just that I’ve had a different result from the side by side comparisons I’ve done. But I will say this is definitely one of those circumstances where I look forward to be proven wrong. Here’s hoping Jerry sees something different ...
All I have been wanting for several months is someone to step up and see if my findings are unique or that an actual "change" to the sensor was successfully accomplished. Obviously my word alone isn't good enough....and I get that, just gets tiring spouting my findings that are being dismissed because it's "just me" with no corroboration. Whether Jerry or anyone else agrees or doesn't agree with my findings, so be it, at least we know someone else gave it a shot. Regardless, mine's a keeper, and hopefully others will find this as well.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on October 21, 2019, 05:02:03 PM
Nobody wants it to work more than me, i really like the 31 response time to changes in humidity. I got three of them when they first came out and they worked really well for awhile but then one died and another did the old get up high in humidity and started reading backwards til it hit zero. The third one still works but tops out at 96% and on the bottom end reads about 5% high. The new one has worked real well and with the front moving through this morning the humidity has gone down to 43% and went real smooth from 95% so it is still new and may not got the bad stuff going yet or maybe it is going to work just fine. I know it works fine out of the package and now only time will tell. I am really waiting for the time of year when the fog and coastal trofs get going and the humidity stays on the high end for days at a time, that will be the real test.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on October 21, 2019, 05:09:32 PM
I am really waiting for the time of year when the fog and coastal trofs get going and the humidity stays on the high end for days at a time, that will be the real test.
No doubt. Mine spent almost all of the last two months with dew points in the 60's and the occasional 70's, but nothing like the "days on end" of wetness you guys get.
Thank you for trying.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on October 21, 2019, 06:26:38 PM
I know it works fine out of the package and now only time will tell.
I forgot to ask, did you use the SF2 cap on this new one? Like I said earlier, I put the cap on out-of-the-box for the first time with this 31. I think it may actually help with keeping the "saturation" from the sensor itself. I have no proof of this, only an "educated" speculation. If I had my druthers, I'd like to see two 31 compared together without a cap for a base line, then one with and the other remaining without to see if there's a difference worth noting.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on October 21, 2019, 06:30:28 PM
I have ordered a bunch of SF2 caps and since I have 3 Davis VP2s with the world-class (???) SHT31 sensor, I will try the SF2 caps on two of my stations and not the other and see what kind of results I get.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on October 21, 2019, 06:41:54 PM
I have ordered a bunch of SF2 caps and since I have 3 Davis VP2s with the world-class SHT sensor, I will try the SF2 caps on two of my stations and not the other and see what kind of results I get.
Thank you. Not that you still can't compare, but all your 31's are "older", are they not? What I'm getting at is perhaps this "protection" is necessary from the get-go so the sensor isn't permanently "spoiled".
BTW, PM will be forth coming.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on October 21, 2019, 07:12:56 PM
I used the wet towel to moisten up the sensor before putting it outside. I replaced the Davis filter material with one i cut out of the silk material which gives better air flow when not using a fan. I have and have used the sf2 on the ones that went bad, i hope to get some good use out of the sensor in comparison to the 75's i have been running. I will be happy if i can get a good year or so out of it before replacement is necessary. I am glad to see it working so well these first couple of days and got my fingers crossed for the long haul.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on October 21, 2019, 07:18:43 PM
I am glad to see it working so well these first couple of days and got my fingers crossed for the long haul.
Once again, not your climate, but my current 31 went into service exactly four months ago today.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on October 21, 2019, 07:27:16 PM
I have ordered a bunch of SF2 caps and since I have 3 Davis VP2s with the world-class SHT sensor, I will try the SF2 caps on two of my stations and not the other and see what kind of results I get.
Thank you. Not that you still can't compare, but all your 31's are "older", are they not? What I'm getting at is perhaps this "protection" is necessary from the get-go so the sensor isn't permanently "spoiled".
BTW, PM will be forth coming.

I have swapped out so many sensors with Ryan, the post man knows the issue and when I get a new one. Seriously, my latest one is probably 18 months old. Is that too old?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on October 21, 2019, 07:36:40 PM
I have ordered a bunch of SF2 caps and since I have 3 Davis VP2s with the world-class SHT sensor, I will try the SF2 caps on two of my stations and not the other and see what kind of results I get.
Thank you. Not that you still can't compare, but all your 31's are "older", are they not? What I'm getting at is perhaps this "protection" is necessary from the get-go so the sensor isn't permanently "spoiled".
BTW, PM will be forth coming.
my latest one is probably 18 months old. Is that too old?
I have no idea. All I know is I bought this latest one in June.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on October 30, 2019, 01:23:27 PM
Over the past week I've been comparing my primary SHT75 sensor with my new Davis 31. The Davis 31 has a manufacture date of June 2019. The sensors are co-located and the Davis 31 has been outdoors only for this week. The temperature of the two sensors is nearly identical, so my comparison is in RH.

The SHT75 has been in use outdoors for 6+ months. For most of the week, humidity has been high up near saturation as is typical this time of year in western Washington state. The Davis 31 has reached as high as 98%. The 75 will hit 100%. From about 87-88% and above, the 75 reads about 1-2% higher.

Yesterday we had an unusual down-sloping east wind off the Cascades and this dried out the air very quickly after prolonged days of high humidity and some fog. The humidity dropped from a high of 89% just after midnight to the upper teens with dewpoints in the single digits to around 0°F. Temperatures were in the upper 30s to mid 40s.

I don't quite have the elaborate set up as some of you, but I have data in Cumulus for both sensors every 10 minutes. The comparisons are based on the raw readings of both sensors. After analyzing the data, the Davis 31 read on average 3-4% higher than the SHT75 in the humidity range of the upper teens to the 30s. The range in difference was the Davis 31 reading from -1%RH to +8%RH, but +3% to +5% was typical. I did notice that over time in the dry air, the Davis 31 did seem to improve a bit reading only 2-3% higher on average. The lowest humidity both sensors reached was 21% for the SHT75 and 23% for the Davis 31. The highest humidity for the day for both sensors was 89% for the SHT75 and 88% for the Davis 31.

The SHT75 read consistently about 3-4% high in the same range compared to the Belfort 566 I use for the best true reading I can afford. What I report online has a humidity calibration in Cumulus of 1.061x-5 which matches well with the Belfort. So the Davis 31 is about +6% to +7% RH from true as best I can tell in this humidity range, sometimes a little better, sometimes a little worse.

Just thought I'd pass along what I saw and it is consistent with past comparisons I've done as well as the Prodata studies. Not sure if Jerry is still using the SHT75 for comparison as well or what he is seeing more generally. I just may have bad luck with these Davis sensors.

The picture was fairly typical. Sensor 2 data is the Davis 31. The Belfort at this time was reading about 19%.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on October 30, 2019, 01:43:25 PM
So the Davis 31 is in the +6% to +7% from true as best I can tell in this humidity range.



Those numbers are what I came in with while testing also. Even when comparing to the accurite version of the 31. This is the high bias that bothers me the most. Jerry I think is more worried about the over-saturation issue because his sensors will drip water out when using a FARS unit at night down on the gulf.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on October 31, 2019, 05:31:50 PM
Well i finally got a run on high humidity, the humidity stayed between 95% and 100% for nearly two days and the new 31 worked just fine when the humidity dropped today down to 35%. There was no sign of wet bias but this is just the first time with high humidity and things could change as i get more of the really wet days so will keep updating when conditions warrant.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on October 31, 2019, 05:46:34 PM
Well i finally got a run on high humidity, the humidity stayed between 95% and 100% for nearly two days and the new 31 worked just fine when the humidity dropped today down to 35%. There was no sign of wet bias but this is just the first time with high humidity and things could change as i get more of the really wet days so will keep updating when conditions warrant.
Well I know the new one I have will hit 1% cause I did it several times yesterday with an accompanying negative mid 30's dew point, which I thought was ridiculous til I saw two RAWS sites in the negative 50's. The air mass is just a tiny bit dry... 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on October 31, 2019, 06:30:36 PM
Well i finally got a run on high humidity, the humidity stayed between 95% and 100% for nearly two days and the new 31 worked just fine when the humidity dropped today down to 35%. There was no sign of wet bias but this is just the first time with high humidity and things could change as i get more of the Rreally wet days so will keep updating when conditions warrant.

So what’s your highest RH so far with the new 31?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on October 31, 2019, 06:52:36 PM
The new sensor topped out at 99%, doing just fine on top and bottom and this reading was without heavy fog just like fog and drizzle.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on October 31, 2019, 08:49:30 PM
The new sensor topped out at 99%, doing just fine on top and bottom and this reading was without heavy fog just like fog and drizzle.

Awesome!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: weather34 on November 02, 2019, 01:40:13 PM
FWIW i installed a spare one early february i bought end last year around end of october . nothing wrong with original was just curious of how easy is installing replacement so i left it installed and put the original supplied in a box . being at sea level some 150 metres from sea front the spread for this year has been 12% to 99% i doubt i will see single digit readings unless ultra dry and extreme heat nearing the 40c mark but 99% has been noticed numerous times of dense fog periods . so SHT31 is certainly and been acceptable to my needs and was quite pleasantly suprised to see a sub 20% reading being this close to the sea front. rain events produce anywhere in the 94% to 97% mark..
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on November 17, 2019, 12:42:33 PM
Update time new 31 is working real well, no signs of wet bias. I have had several days with the humidity staying in the 95 to 98 percent range for nearly 24 hrs at a go and the new 31  showed the same tracking as my other sensors when the humidity finally went down, i saw no signs of it staying higher for a time at any period of the dropping humidity. I am comparing it to an older 31, a new 75 and three new 15's. All my sensors i am testing are in 24 hr fars shield except for one of the 15's which is in a Barani standard passive shield. Will keep updating at times or if something goes south with the new 31. 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on November 17, 2019, 02:28:33 PM
 =D&gt; We've got a wet week ahead, this will probably be my best test yet in the 5 months since install. :roll:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on November 17, 2019, 03:32:49 PM
Update time new 31 is working real well, no signs of wet bias.
If I asked this before, I apologize, but are you using the SF2 cap one your new 31? My new 31 has had it on from day one, don't know whether it helps for sure (I actually believe it does in wetter conditions), but it certainly doesn't hurt.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on November 17, 2019, 04:23:27 PM
Yes and no, i had the sf2 on the old and new for a few days and then put Davis filter on the new one to see if i could see any difference but could not detect any. Now today i just replaced both filters with the silk to test further. I am trying out several things right now out of curiosity if nothing else  :grin:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on November 17, 2019, 04:56:51 PM
Yes and no, i had the sf2 on the old and new for a few days and then put Davis filter on the new one to see if i could see any difference but could not detect any. Now today i just replaced both filters with the silk to test further. I am trying out several things right now out of curiosity if nothing else  :grin:
Thanks. No doubt SF2 provides better protection than the stock filter does, but as far as the silk...? I think the SF2 keeps the sensor itself from becoming saturated as it would unprotected or with the stock filter. I truly believe it helps with recovery.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: WheatonRon on November 17, 2019, 05:15:14 PM
Yes and no, i had the sf2 on the old and new for a few days and then put Davis filter on the new one to see if i could see any difference but could not detect any. Now today i just replaced both filters with the silk to test further. I am trying out several things right now out of curiosity if nothing else  :grin:
Thanks. No doubt SF2 provides better protection than the stock filter does, but as far as the silk...? I think the SF2 keeps the sensor itself from becoming saturated as it would unprotected or with the stock filter. I truly believe it helps with recovery.

Isn’t goofy that we have to put “band-aids” on Davis products to make them work correctly? Oh well, there isn’t much (any?) competition in their product price range so I guess we have keep developing and using our band aids.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on November 17, 2019, 05:35:45 PM
Yes and no, i had the sf2 on the old and new for a few days and then put Davis filter on the new one to see if i could see any difference but could not detect any. Now today i just replaced both filters with the silk to test further. I am trying out several things right now out of curiosity if nothing else  :grin:
Thanks. No doubt SF2 provides better protection than the stock filter does, but as far as the silk...? I think the SF2 keeps the sensor itself from becoming saturated as it would unprotected or with the stock filter. I truly believe it helps with recovery.

Isn’t goofy that we have to put “band-aids” on Davis products to make them work correctly? Oh well, there isn’t much (any?) competition in their product price range so I guess we have keep developing and using our band aids.
Not sure I'd call the SF2 a band aid, it's a Sensirion made option. Regardless, if it makes my PWS better without breaking the bank, I'm all for it. Suppose that's why I graduated from an SHT11 to a 15 to a 31, put in a case fan, use a Rainwise 111 as my hard wired tipper, and have my anny separate from my ISS.  All good. =D&gt;
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on November 17, 2019, 06:29:05 PM
Jerryg, have you seen a manufacture date on your sensor?

C2274, you’ve probably mentioned this before, but what do you use to mount the SF2?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on November 17, 2019, 07:00:55 PM
Jerryg, have you seen a manufacture date on your sensor?

C2274, you’ve probably mentioned this before, but what do you use to mount the SF2?
I know the question was not directed at me, but I know of no way of to determine the manufacture date other than go to Davis or Sensirion (I guess). There is labeling, but not with an apparent date.

As far as mounting, our board is not holed for the four feet of the cap (besides, I wanted nothing to do with specialized glue involved) so I trimmed the four feet closely and used very fine bare wire to cinch it down onto the "gummy" substance surrounding the sensor itself with the two holes provided in the board while leaving the membrane unrestricted. I can't guarantee a water tight seal, but I'll bet I have one.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on November 17, 2019, 07:20:39 PM
The newest Davis 31 I have has a sticker on the back with the part number and ”190610 DS” on it. I just assumed that’s either a manufacture or inspection date and initials. Total assumption obviously, but seemed reasonable.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on November 17, 2019, 07:34:37 PM
The newest Davis 31 I have has a sticker on the back with the part number and ”190610 DS” on it. I just assumed that’s either a manufacture or inspection date and initials. Total assumption obviously, but seemed reasonable.
My old 31 from 3+ years ago has I60216SB. There is a "16" in it....
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on November 18, 2019, 11:59:29 PM
Yes and no, i had the sf2 on the old and new for a few days and then put Davis filter on the new one to see if i could see any difference but could not detect any. Now today i just replaced both filters with the silk to test further. I am trying out several things right now out of curiosity if nothing else  :grin:

I’m curious to what your results are.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on November 19, 2019, 01:07:29 AM
Yes and no, i had the sf2 on the old and new for a few days and then put Davis filter on the new one to see if i could see any difference but could not detect any. Now today i just replaced both filters with the silk to test further. I am trying out several things right now out of curiosity if nothing else  :grin:

I’m curious to what your results are.
I think testing would have to be somewhat long term, not just a few days, but IMO a properly sealed SF2 cap is without a doubt the way to best protect the sensor. That's exactly what it's designed to do.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on November 19, 2019, 03:57:43 PM
What i was looking at is if there is any difference in response time between the types of filters. The silk has the best time of the three with the sf2 being next and the Davis last which my be due to the large size of the filter rather than the material. The factory filters fit down tight on the sensor so have very little air space to moisten up and better response. Now the silk filter will not let water pass just vapor and where i live out in the country the only thing i really need to worry about is dust which is pretty heavy when the farmers are plowing for planting and harvesting the crops. No real gas vapors to worry about. We had very dense fog this am and the 31 did not show any bias once again, so looking good still.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on November 19, 2019, 07:15:14 PM
What i was looking at is if there is any difference in response time between the types of filters. The silk has the best time of the three with the sf2 being next and the Davis last which my be due to the large size of the filter rather than the material. The factory filters fit down tight on the sensor so have very little air space to moisten up and better response. Now the silk filter will not let water pass just vapor and where i live out in the country the only thing i really need to worry about is dust which is pretty heavy when the farmers are plowing for planting and harvesting the crops. No real gas vapors to worry about. We had very dense fog this am and the 31 did not show any bias once again, so looking good still.

Nice!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on November 29, 2019, 06:08:35 PM
Fingers crossed on some pea soup tonight! Phfttt, I'll even take some chicken broth...
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on November 29, 2019, 08:00:56 PM
 :grin: well i have had a bunch of upper 90's on the humidity side and the sensor has worked just fine so far no sign of any bias at all. I am still waiting for one of the long drawn out periods where the humidity is in the high 90's for several days.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on November 29, 2019, 08:09:17 PM
:grin: well i have had a bunch of upper 90's on the humidity side and the sensor has worked just fine so far no sign of any bias at all. I am still waiting for one of the long drawn out periods where the humidity is in the high 90's for several days.
Well to be sure I'll never see that, just looking for some true saturation for several hours.  Thanks for checking in and bringing more hopeful news!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: tweatherman on November 29, 2019, 11:26:20 PM
Yes and no, i had the sf2 on the old and new for a few days and then put Davis filter on the new one to see if i could see any difference but could not detect any. Now today i just replaced both filters with the silk to test further. I am trying out several things right now out of curiosity if nothing else  :grin:
Thanks. No doubt SF2 provides better protection than the stock filter does, but as far as the silk...? I think the SF2 keeps the sensor itself from becoming saturated as it would unprotected or with the stock filter. I truly believe it helps with recovery.

What case fan are you using and is it solar powered?

Thanks,
tweatherman
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on November 29, 2019, 11:38:39 PM
Yes and no, i had the sf2 on the old and new for a few days and then put Davis filter on the new one to see if i could see any difference but could not detect any. Now today i just replaced both filters with the silk to test further. I am trying out several things right now out of curiosity if nothing else  :grin:
Thanks. No doubt SF2 provides better protection than the stock filter does, but as far as the silk...? I think the SF2 keeps the sensor itself from becoming saturated as it would unprotected or with the stock filter. I truly believe it helps with recovery.

What case fan are you using and is it solar powered?

Thanks,
tweatherman
No solar. It's 120vac from the house then has a transformer included that converts it to 12vdc for the fan. The size is 80x80x25mm and there's a ton of choices. I suggest no less than 20CFM.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: fkapp on December 08, 2019, 12:08:13 AM
I just read the whole thread to understand this whole SHT 31 situation.
Seems like Jerry has had better luck with this newest sensor.
How is it going for others out there with recent new SHT 31 sensors.
I know time will tell, but trying to understand if the quality of these has changed recently, or maybe Davis is doing something differently as part of build process

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on December 08, 2019, 12:32:11 AM
I just read the whole thread to understand this whole SHT 31 situation.
Seems like Jerry has had better luck with this newest sensor.
How is it going for others out there with recent new SHT 31 sensors.
Mine's fine too... :roll:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: weather34 on December 08, 2019, 02:44:00 AM
Mine is fine too installed January 2019  :roll:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: fkapp on December 08, 2019, 10:01:22 AM
ok. am on the fence about purchasing VP2 after reading this thread.
I mean sounds like a known defect that will be triggered by our damp winters here in Buffalo Ny.
In effect need to change sensors like tires i guess if you sign up for that by going VP2. ](*,)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: DaleReid on December 08, 2019, 12:20:48 PM
I'm not an engineer, but I would think from the reading that there are a limited number of ways for a humidity sensor,  of the types that electronic stations use and excluding blonde hair or horse hair gizmos of the 40s, can work.

That being said, why would a sensor be substantially different from others, if the design were nearly the same.  Why would one be more sensitive to high humidity damage than others?

I know that Young and Vaisala have some very long life, extreme conditions, types of humidity sensors.  Are their construction that much more expensive than the glut of low cost things that are like the SHT stuff or the little almost pennies, ones for Arduino etc?

Anyone know the science behind it?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: fkapp on December 08, 2019, 01:20:18 PM
I think part of it comes down to manufacturing and assembly process quality.
The weather monitor II I had for over 15 years was great, but had the different type of sensor pin installed vs this on board install.
Davis has their own custom sensor which again raises quality questions.
They seem to not being in a hurry to evolve and more troublesome is seems quality focus is not what used to be.

All the same the NWS changes sensors every 18 months, but my guess this is driven by precaution vs necessity beacause sensor is shot from humidity exposure
Not the science answer you were looking for, but seems to be cost driven IMHO.
What is the life expectancy and cost of these other stations?
Go there because total cost of ownership equation needs to include a new $50 SHT 10 times over ten years, all of a sudden Davis is not the bargain it used to be and further complicates and maybe changes decision process.
No news from Davis is confusing. Did they change something to address the issue or is it a lost cause no one knows.

And one more thing is peace of mind, is my sensor shot or is this the accurate reading. in the end that is why we are so passionate re this topic. 
Yes i dont own a VP2 yet, but this is all going through my mind as a prospective buyer.  Sharing so the group and Davis sees what this quality issue is doing and creates real question if VP2 is correct investment based on current state of things.


Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on December 08, 2019, 03:20:28 PM
I'm not an engineer, but I would think from the reading that there are a limited number of ways for a humidity sensor,  of the types that electronic stations use and excluding blonde hair or horse hair gizmos of the 40s, can work.

That being said, why would a sensor be substantially different from others, if the design were nearly the same.  Why would one be more sensitive to high humidity damage than others?

I know that Young and Vaisala have some very long life, extreme conditions, types of humidity sensors.  Are their construction that much more expensive than the glut of low cost things that are like the SHT stuff or the little almost pennies, ones for Arduino etc?

Anyone know the science behind it?

I am no engineer and am no expert on the science, but I am familiar with the history for Davis. Johnd and Pro Data used to have this very well documented, but I am not sure they still do. I also know that the old analog film capacitor element Davis used to use was superior to the current capacitor/digital technology, despite what the specs say. Why that is is a great question.

My first VP2 was circa 2005 and had the analog temp/hum sensor. On paper, specs weren't as good as they promote with the current SHT31. The specs for the temp sensor (a platinum wire thermistor) weren't as good and at the extremes, resolution in readings degraded. But in practice, I found these sensors to be superior in the humidity realm. The specs are still available online as +/-3% in humidity and +/-4% above 90%. In real world conditions, these sensors did the 2 things most people complain about with the current Sensirion lineup: they routinely hit 100% humidity and did not exhibit a wet bias as they "dried out" better when humidity dropped.

In 2006 I bought a second VP2 and noticed almost immediately that the humidity and dew point readings weren't as accurate (it would never hit 100% humidity and at lower humidity levels it had a considerable wet bias).  This new station had the Sensirion SHT11 and Davis has been using Sensirion ever since. As far as I can tell, the weaknesses of these sensors continue now nearly 14 years later. As I also understand it, the reason for the switch was the obsolescence of the humidity sensing element for the analog sensor. Initially, the SHT11 did not have any coating protecting the soldering joints of the digital sensor and they had a rash of failures. Davis then began protecting the solder joints with a coating. Davis then briefly offered the improved-spec SHT15 shortly before going to the SHT31. By all accounts, the temperature performance of this sensor is outstanding, which is not insignificant. On the humidity side however, there is no way it lives up to the +/-2% humidity spec. The sensors are not designed for high humidity environments for prolonged periods of time creating an offset in the reading. Each chip is supposed to have a temperature and linear calibration correction for humidity loaded onto the chip. Maybe these are extremely accurate in a dry, laboratory setting, but these corrections are seemingly insufficient in a weather station. Plus many of us have experienced a further wet bias offset present specifically in Davis mounted sensors compared to homemade Sensirion sensors we've begun using.

As johnd has pointed out numerous times, aside from a few of us who are frustrated by the humidity performance, most users are probably either oblivious or indifferent to the humidity performance, so if it hasn't been improved in 14 years, I can't imagine many resources are being put into improving accuracy. Plus the other issue is compared to what? The VP2 is still a great weather station for the price and even the Rainwise brand has gone to the Sensirion sensor, so it does seem that the options for accurate, low cost humidity sensors designed for meteorological monitoring is quite limited.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on December 08, 2019, 03:42:09 PM
I may be completely in left field, but my 31 is about 6 months old and I still think using the SF2 cap from the get go (which I did, never have before) may aid in not letting the sensor get too "wet" and delaying proper drying. Regardless, I'm still happy with it....that's my story and I'm sticking to it.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: DaleReid on December 08, 2019, 03:45:09 PM
Excellent discussion, some history and some fine real-world information.

Yes, we spend a lot of time reading the specs and hitching our wagon to them.

I know there must be a reason company engineers choose to go down a path, especially if the design and layout and internal programming are already done.

I know that even the expensive (for its time) ID-5001 had major problems with the humidity sensor.  Yet temperature on this, and the now-defunct TWI was usually extremely close to my precious mercury thermometers with gradations fine enough to resolve about 1/2 a degree.

Its interesting to hear the platinum wire sensor wasn't as accurate in those settings (I know we're talking humidity here) but a R M Young device has a platinum wire sensor and is very expensive so one would think that if there weren't accuracy behind it, even the US Government and Universities with deeper pockets would opt for the method that gives best results.

Anyway, I know the precision of some of the accurite home monitors has been exceptional (hard to know accuracy since I don't have any way of testing them in a controlled setting like a salt controlled humidity environment like the calibrators for Vaisala and otherss.  But over all, through humid summer and now fall and entering into heating season indoors with the floor creaking low humidity, two of them are always the same or withing digit bobble of the same.

But I'm not sure what technology they are using.   

Are all the Arduino sensors using the same technology as the SHT uses?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: johnd on December 08, 2019, 03:49:14 PM
...As johnd has pointed out numerous times, aside from a few of us who are frustrated by the humidity performance, most users are probably either oblivious or indifferent to the humidity performance, so if it hasn't been improved in 14 years, I can't imagine many resources are being put into improving accuracy.

That's a good summary and I don't really have anything new to add. TBH I doubt that this barely registers as an issue at Davis, who will respond to two main influences - the incidence of reported user issues with RH and then, if this started to become significant, the availability of any reasonably cost-effective alternative sensor.

All I can reiterate is that for our users in the UK (which is of course a relatively cool 8-) but damp climate) it is close to a non-issue. Of the several hundred Davis ISS units we will have sold in recent years, I can scarcely recall a single complaint about the T/H sensor within say 3 years. The sensor certainly does have a finite service life, but most of our users typically get 5-10 years from their sensors before significant issues are reported. NB I'm certainly not trying to argue that the SHT31 is a perfect sensor, but it may be as good as it gets in the VP2 price range. But it's also a factor that RH is not a reading that ranks as a priority for most users, in contrast say to rainfall which can generate a lot of support calls (most of which are down to issues like poor exposure or inappropriate comparisons, but Davis are clearly taking steps to improve the consistency of the rainfall readings).

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on December 08, 2019, 03:52:31 PM


Its interesting to hear the platinum wire sensor wasn't as accurate in those settings (I know we're talking humidity here) but a R M Young device has a platinum wire sensor and is very expensive so one would think that if there weren't accuracy behind it, even the US Government and Universities with deeper pockets would opt for the method that gives best results.


I can't speak to the real world accuracy of the platinum wire sensor on the analog Davis sensor because I never tested it. It may very well have performed better than spec.

I may be completely in left field, but my 31 is about 6 months old and I still think using the SF2 cap from the get go (which I did, never have before) may aid in not letting the sensor get too "wet" and delaying proper drying. Regardless, I'm still happy with it....that's my story and I'm sticking to it.

I don't think you're in left field. I have just had a completely different experience as I've documented here. You do bring up a good additional point though that the board the SHT31 is mounted to and the big filter they use are holdovers from the old analog sensor. The filter had to be that big for the size of the components. I'm guessing they stuck with the board and filter out of convenience, not maximizing performance.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on December 08, 2019, 03:59:11 PM
The sensor certainly does have a finite service life, but most of our users typically get 5-10 years from their sensors before significant issues are reported.

That is also a good point. For any prospective VP2 purchaser, I don't think service life of the sensor is much of an issue as it was with the SHT11. From what I have seen, they will be reliable for many years, but with the inherent (and frustrating for me anyway!) limitations of the real world performance of the humidity side of their SHT31 mounted sensor.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on December 08, 2019, 04:43:05 PM
I know that WeatherFlow is using the SHT-31 for their upcoming Tempest station. They did it for cost reasons and they know that they can calibrate the sensor readings if needed. Even though it would raise the cost of the VP2 stations, I do think Davis needs to make a switchover to the HYT-221 sensor. Especially since their target buyers are those in agriculture, WeatherSTEM, colleges/universities, Earth Networks, etc.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: fkapp on December 08, 2019, 05:15:28 PM
Where can one buy the SF2 cap? what is the part number called?
How are these installed and where on the board do these go? (would it be possible for someone who has done this to post a pic of where to install SF2 Cap on the board?) 
Trying to understand so that can be ready to add this when order my station and appreciate guidance.
Guess going to have to do a mod straight out of the box in the theme of accuracy.
Extra work and not sure where to add but like to do things right.

Thanks for the help,
Frank
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on December 08, 2019, 05:29:56 PM
Where can one buy the SF2 cap?
https://www.mouser.com/ProductDetail/Sensirion/SF2?qs=wWC4CIiyLaNCAejbFUK5dw%3D%3D

Since our board does not have the holes required for the four feet, I cut them off and used the two holes available on the board to tightly cinch it down with very thin bare wire while not obstructing the membrane. The gummy substance around the sensor makes for what I believe is a water tight fit.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: fkapp on December 08, 2019, 06:07:12 PM
Thanks. Guess this is a must where I am located with damp foggy snow melt days etc.
Did you notice if this slowed down temperature or humidity response?
I am planning on 24 hour FARS so i guess more reason CAP is a must

One more question, did you have to remove the board from the radiation shield to install, or just take apart radiation shield?

Thanks
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on December 08, 2019, 06:24:53 PM
Thanks. Guess this is a must where I am located with damp foggy snow melt days etc.
Did you notice if this slowed down temperature or humidity response?
I am planning on 24 hour FARS so i guess more reason CAP is a must

One more question, did you have to remove the board from the radiation shield to install, or just take apart radiation shield?

Thanks
Whether the cap is a "must" for you or anyone else, I honestly can't say, it merely seemed like a logical step to try when I installed my latest 31.
 
As far as a slow response, no, if anything I'd think it's better than the stock filter.

You'll need to pull the sensor board out of the sensor chamber to expose the sensor itself, I suggest going in from the bottom of the ISS vice the top. You'll need to pull the fan out. It may sound complicated, but once you get it apart, it's pretty much child's play. The VP2 design is a PWS's dream to work on.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: fkapp on December 08, 2019, 06:52:59 PM
Thanks.
So if I understand correctly the stock filter (circled blue) in below image must be removed and the SF2 CAP placed stuck into the black tar like stuff and wire run between the 2 holes circled red in image below?
Any tips on removing? Prying this off brand new board will be nerve wracking.
Guess some thin insulated eletric wire to avoid rust or something is what you used?
[ You are not allowed to view attachments ]
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on December 08, 2019, 07:09:15 PM
Thanks.
So if I understand correctly the stock filter (circled blue) in below image must be removed and the SF2 CAP placed stuck into the black tar like stuff and wire run between the 2 holes circled red in image below?
Any tips on removing? Prying this off brand new board will be nerve wracking.
Guess some thin insulated eletric wire to avoid rust or something is what you used?
[ You are not allowed to view attachments ]
The stock cap merely snaps out. The holes are inside the cap next to the sensor, you'll see. As far as rust, never crossed my mind, however I live in one of the most dry climates on the planet. Just check it occasionally after install.
BTW, read about handling the SF2 on the link I sent, it can make it a little challenging to keep ones fingers off the membrane, I suggest buying at least several just in case. Plan on little needle nose pliers for the assist.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: fkapp on December 08, 2019, 07:15:58 PM
Thanks again appreciate your guidance and time.
Any Feedback on how the CAP works in more humid climate (how are response times, does this delay the wet bias, based on your experience recommend this extra work/Mod) from those that have tried this would be helpful?

Appreciate it as plan to go VP2 route for my new station, now want to fine tune best can.
Frank
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on December 08, 2019, 09:00:37 PM
Thanks.
So if I understand correctly the stock filter (circled blue) in below image must be removed and the SF2 CAP placed stuck into the black tar like stuff and wire run between the 2 holes circled red in image below?
Any tips on removing? Prying this off brand new board will be nerve wracking.
Guess some thin insulated eletric wire to avoid rust or something is what you used?
[ You are not allowed to view attachments ]
The stock cap merely snaps out. The holes are inside the cap next to the sensor, you'll see.
Upon reflection, I'm pretty sure the holes the stock filter snaps into are the holes I used for the wire. Merely trying to avoid confusion.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: galfert on December 09, 2019, 12:45:16 PM
I'm not an engineer, but I would think from the reading that there are a limited number of ways for a humidity sensor,  of the types that electronic stations use and excluding blonde hair or horse hair gizmos of the 40s, can work.

That being said, why would a sensor be substantially different from others, if the design were nearly the same.  Why would one be more sensitive to high humidity damage than others?

I know that Young and Vaisala have some very long life, extreme conditions, types of humidity sensors.  Are their construction that much more expensive than the glut of low cost things that are like the SHT stuff or the little almost pennies, ones for Arduino etc?

Anyone know the science behind it?

I wrote to Sensirion and asked them exactly what you were wondering. Here is their response.

Quote from: Sensirion Technical Support
Thank you for your inquiry.

There are many things that can influence the accuracy and lifetime of the sensor, especially in outdoor applications.

We used to have a definition of normal operating range with the SHT2x:

 [ You are not allowed to view attachments ]
Quote

Working outside of the normal range you may have to consider additional effects.

One important document is our handling instructions.
http://www.sensirion.com/file/handling_instructions_rht

This document is listing some points to consider, but they're not too relevant for outdoor conditions. The VOC levels are usually very low.

The biggest challenge outdoor are the dew cycles that can lead to corrosion. The electronics need to be protected with conformal coating and the manufacturers have to protect the sensor during that process.

Additionally some regions can be more harmful to the sensor. Airports, heavy traffic areas or proximity to the coast is also known to have adverse effects on sensor lifetime.

I haven't heard many reports about those failures. Do you have more in-depth knowledge if those products applied conformal coating or not?

Are there weather stations equipped with Vaisala sensors that last longer?

Does anyone know what Vaisala uses for temp/hum sensors so that I can respond back to Sensirion?

How about knowledge about what type of conformal coating chemistry Davis uses? (Acrylic, Epoxy, Polyurethane, Silicone, Fluorinated or non Fluorinated - Poly-Para-Xylylene (Parylene), or Amorphous Fluoropolymer)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Old Tele man on December 09, 2019, 12:52:14 PM
What? "Normal" range only goes up to 90% RH? Apparently SENSIRION avoids FOG!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on December 09, 2019, 01:20:55 PM
What? "Normal" range only goes up to 90% RH? Apparently SENSIRION avoids FOG!

Another way of saying our sensors aren't for the serious enthusiast.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on December 09, 2019, 01:28:13 PM


The biggest challenge outdoor are the dew cycles that can lead to corrosion. The electronics need to be protected with conformal coating and the manufacturers have to protect the sensor during that process.


I haven't heard many reports about those failures. Do you have more in-depth knowledge if those products applied conformal coating or not?

Are there weather stations equipped with Vaisala sensors that last longer?


Does anyone know what Vaisala uses for temp/hum sensors so that I can respond back to Sensirion?

How about knowledge about what type of conformal coating chemistry Davis uses? (Acrylic, Epoxy, Polyurethane, Silicone, Fluorinated or non Fluorinated - Poly-Para-Xylylene (Parylene), or Amorphous Fluoropolymer)

I find the part he mentions about conformal coating interesting, specifically that the manufacturer must protect the sensor during the process. Those of us unhappy with humidity performance of the SHT31 compared to say Acurite's use of the SHT31 have long suspected that it might be something with the conformal coating in Davis' manufacturing process. I know Acurite does not have a conformal coating on their sensor as it is designed as an indoor sensor. And I get better performance with the SHT75 than the Davis SHT31 and that obviously has no conformal coating. I'm wondering if Davis does not take whatever precautions Sensirion recommends during the process if it may induce an offset and if Sensirion's reconditioning procedure of baking the sensor might help. (Maybe partly why folks in really hot, really dry environments seem so much happier with the performance). Unfortunately, Sensirion would need to know what Davis uses for the conformal coating to answer that question as you ask and I'm not sure Davis will be willing to publicly release that info. I also believe Valentineweather tried the baking procedure without much success.

Here is what Vaisala uses in their humidity sensors ...
https://www.vaisala.com/sites/default/files/documents/HUMICAP-Sensors-Datasheet-B211748EN.pdf

Looks kind of like the old analog sensor, but much more robust ... [ You are not allowed to view attachments ]  [ You are not allowed to view attachments ]
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: fkapp on December 09, 2019, 03:30:37 PM
 deleted as mis read chart earlier and dont want to create confusion by my misunderstanding.

Thanks guys

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on December 09, 2019, 03:45:16 PM
Seeing 90% is normal range cap, have to rethink things...  unreal they would go with 90% as normal cap sensor.
I don't even know what that means. Every properly working 31 will easily get RH's in the upper 90's, my last would reached 99% all the time, it was the below 5% range that caused me to replace it with this current 31. :roll:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: galfert on December 09, 2019, 03:48:38 PM
:shock:or is it  #-o
Is the above normal range chart for the SHT31?

Wow if yes. or perhaps the Davis custom sensor was wider range, I dont  know anymore guys...

It is the same for the SHT3x series. They just don't call it normal range. Instead the spec sheet shows that it looses accuracy (max tolerance) above 90%.

 [ You are not allowed to view attachments ]
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: fkapp on December 09, 2019, 03:55:30 PM
That makes more sense. above %90 get less accuracy which matches 90% is upper limit of normal variance in above chart.
Thanks for clarifying I mis read/ miss understood pervious chart.

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: galfert on December 10, 2019, 08:18:27 AM
I responded to Sensirion and told them that I had reached out to Davis inquiring about their conformal coating strategy. I have not heard back from Davis regarding this yet, but if I do I will share it and also forward it on to my new contact at Sensirion. I also shared with Sensirion the pictures from the Vaisala station. The purpose being to identify technological differences in something a Vaisala station would use that as a complete station costs thousands of dollars. I then mentioned the frustration Davis users have in that Davis doesn't support I2C sensors and that we wonder what the Davis strategy is now that the LSS type of SHT31 sensor is no longer in production, and to see if and when we might see a Davis I2C type station. I inquired if there was a contract between Davis and Sensirion to keep providing these LSS type sensors past the end of general availability and you can see the response below that basically he didn't know what the Davis strategy is.

Below is the additional response back from Sensirion:

Quote from: Sensirion Technical Support
I do have some additional comments.

The normal range doesn't tell you it's not possible to operate the sensor outside that range. There are just side effects that have to be considered. For instance at high RH levels the RH reading may drift and that's a reversible effect. It's just getting more complicated when operated outside of normal range.

I've heard of people wanting to replace the sensor of Davis stations. And as you state correctly that's not possible because the LSS version isn't available.

I don't know about Davis strategy and why they don't change to the DIS [I2C] version. It might involve a lot of extra work and compatibility issues making it hard to support previous versions after changing the design.

From what I've experienced so far this doesn't seem to be a huge issue with people wanting to replace the sensor themselves. I've only received a few requests so far and haven't been concerned. But I understand that for those affected by a non-working sensor it would be much easier to replace it themselves.

Looking at the Vaisala picture I think it only shows the sensing element. It seems to be porous material and most likely based on capacitive technology as well.

Given the fact that the sensing element is much bigger than the thin material layer on a CMOS chip, an eventual degradation process would take longer to affect the readings.

As it's still capacitive technology it would still be affected by molecules of a similar size as water such as VOCs.

If you receive feedback from Davis I'd be happy to hear about it. But I doubt they'll provide detailed information about their strategy and eventual change to the I2C interface.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on December 10, 2019, 10:40:19 AM
I responded to Sensirion and told them that I had reached out to Davis inquiring about their conformal coating strategy. I have not heard back from Davis regarding this yet, but if I do I will share it and also forward it on to my new contact at Sensirion.

Hopefully Davis have read this ...
https://www.sensirion.com/fileadmin/user_upload/customers/sensirion/Dokumente/2_Humidity_Sensors/Sensirion_Humidity_Sensors_Handling_Instructions.pdf

Sensirion note that volatile chemicals in many epoxies and adhesives cause an irreversible offset specifically in humidity readings. Something no reconditioning procedure will fix. They even go on to recommend 3 types of conformal coatings to use with their sensors.

For those with the SF2 or considering it, they also recommend adhesives that can be safely used including Loctite 401
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on December 10, 2019, 02:26:41 PM
I responded to Sensirion and told them that I had reached out to Davis inquiring about their conformal coating strategy. I have not heard back from Davis regarding this yet, but if I do I will share it and also forward it on to my new contact at Sensirion.
For those with the SF2 or considering it, they also recommend adhesives that can be safely used including Loctite 401
I considered using approved glue but frankly decided it was too difficult to do properly without making a sensor destroying mess (damn thing is soooo small), hence my decision to use wire to seal it down.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on December 10, 2019, 02:48:20 PM
Well, after 6 months, I finally got my wish last night. It went W0X0F about 2am and stayed that way for at least several hours. My 31 would not report above 98% which equates to a temp/dew spread of 0.5F (0.4F is 99%). So, although it didn't reach the magical 100%, it does, as far as I can tell, conform to the stated 2% of accuracy from 1-100%, and in between, as I've been to 1% as well. Am I happy about it not pulling the 100% trigger, obviously not (at least my last 31 would hit 99%), but it does do it's job as specified. As far as recovery, I see no issues.
It is what it is...

Testing done....well at least til I buy another.... #-o
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on December 10, 2019, 02:53:04 PM
Oh, BTW, I unplugged my fan for a length of time to see if that would make a difference...none, the 0.5F spread remained unchanged.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: fkapp on December 12, 2019, 07:15:34 PM
How is humidity variance past few days after sensor went above normal range limit of 90%?
did 5% drift effect start?

Curious as this will indicate how fast humid climate sensors accuracy impacts will start.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on December 12, 2019, 08:08:34 PM
How is humidity variance past few days after sensor went above normal range limit of 90%?
did 5% drift effect start?
I'll assume your question is for me. There is no drift, the sensor is just as "in-line" before as it was after.
I think you're over borrowing some of the hype here.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: fkapp on December 12, 2019, 08:58:14 PM
Okay. I would say there are enough field data tests on this 60 pager to say hype may be a bit bold.
anyways. good to hear. the SF2 seems to help and the dry climate seems to help.  so could be a frequency of sensor getting exposed above normal range eventually starts it, or maybe  Davis has found way to correct (assumption: as none of us unless inside the walls can know for sure)

keep us posted on how it goes.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on December 14, 2019, 12:59:08 PM
So I decided to do a little holiday baking ... with my temp/hum sensors. My hope was to remove the wet bias in drier conditions but also found surprisingly better performance at saturation after the bake.

I have a new Davis SHT31 (which I believe was manufactured in June 2019) that I've reported on in this forum with disappointing results. I also have a few SHT75s. I baked them all per Sensirion's datasheet for 10hrs at 220°F. I did not bother with the rehydration procedure because it's the Puget Sound in December ... they will get plenty of hydration.

The SHT75 I used for the test had been outside for about 9mos. When I brought it in it read about 4% high at room temperature compared to my Belfort pyschrometer @ 40% RH. The new Davis SHT31 has been inside aside from a week of testing outside a few weeks ago. It was 6% high and +3°F dew point compared to the Belfort even having been acclimated to the dry conditions of a heated house.

After baking I tested them against a Belfort psychrometer at room temperature. Temperature results were so accurate at 72°F among all sensors that there is nothing to note.

The Belfort results were:
72.3°F Dry Bulb/59.0°F Wet Bulb giving: Humidity 45% Dew Point 50°F

Davis SHT31:
72.3°F  43%  DP: 49°F

SHT75:
72.3°F  42%  DP: 48°F

Not necessarily surprising after 10hrs at 220°F. I then put the sensors outside in what has been near saturation conditions. I expected the sensors to carry a dry bias at high humidity. Surprisingly, I found the opposite.

Before the bake, I could not get the SHT31 above 98%. Now it goes to 100% and actually stays there. Not the "Sensirion jump" to 100% for one update cycle. The SHT75 before the bake would get to 98% and stay there with a jump periodically to 99 or 100%. Now it goes to 99% and stays there. It will jump to 100% but not sustain it like the 31.

Obviously the real test will be as conditions dry out. Unfortunately, that happens infrequently here this time of year. We've only gotten as low as the 80%s and there are signs of the SHT75 drying out better, but only by a percent or two and when you consider it being 1% low at 100%, I would say that's negligible.

Just thought I'd pass along as I was surprised by the high humidity results. I have a feeling that if it does dry out, I will be right back to where I was before the bake with all this high humidity, but if we get a dry day, I'll post the results.



Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on December 14, 2019, 01:15:34 PM
 [tup]
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: fkapp on December 14, 2019, 01:31:24 PM
Thanks
So you bake the whole circuit board in the oven?
Or do you unsolder the sensor and bake?

what do you place on, cookie sheet or something?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on December 14, 2019, 02:21:03 PM
I had tested with an older sensor without negative effects, so I baked the whole thing with circuit board, wire, connector etc on a baking sheet.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on December 14, 2019, 02:38:56 PM
Not the "Sensirion jump" to 100% for one update cycle.
Hadn't heard that one before, but it's spot on. My last 31 would indeed hit 100%, but if you blinked, you missed it. This current one won't even do that, but it's still a better overall performer.
Thanks for the test!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on December 25, 2019, 03:23:03 PM
So I decided to do a little holiday baking ... with my temp/hum sensors. My hope was to remove the wet bias in drier conditions but also found surprisingly better performance at saturation after the bake.

I have a new Davis SHT31 (which I believe was manufactured in June 2019) that I've reported on in this forum with disappointing results. I also have a few SHT75s. I baked them all per Sensirion's datasheet for 10hrs at 220°F. I did not bother with the rehydration procedure because it's the Puget Sound in December ... they will get plenty of hydration.

The SHT75 I used for the test had been outside for about 9mos. When I brought it in it read about 4% high at room temperature compared to my Belfort pyschrometer @ 40% RH. The new Davis SHT31 has been inside aside from a week of testing outside a few weeks ago. It was 6% high and +3°F dew point compared to the Belfort even having been acclimated to the dry conditions of a heated house.

After baking I tested them against a Belfort psychrometer at room temperature. Temperature results were so accurate at 72°F among all sensors that there is nothing to note.

The Belfort results were:
72.3°F Dry Bulb/59.0°F Wet Bulb giving: Humidity 45% Dew Point 50°F

Davis SHT31:
72.3°F  43%  DP: 49°F

SHT75:
72.3°F  42%  DP: 48°F

Not necessarily surprising after 10hrs at 220°F. I then put the sensors outside in what has been near saturation conditions. I expected the sensors to carry a dry bias at high humidity. Surprisingly, I found the opposite.

Before the bake, I could not get the SHT31 above 98%. Now it goes to 100% and actually stays there. Not the "Sensirion jump" to 100% for one update cycle. The SHT75 before the bake would get to 98% and stay there with a jump periodically to 99 or 100%. Now it goes to 99% and stays there. It will jump to 100% but not sustain it like the 31.

Obviously the real test will be as conditions dry out. Unfortunately, that happens infrequently here this time of year. We've only gotten as low as the 80%s and there are signs of the SHT75 drying out better, but only by a percent or two and when you consider it being 1% low at 100%, I would say that's negligible.

Just thought I'd pass along as I was surprised by the high humidity results. I have a feeling that if it does dry out, I will be right back to where I was before the bake with all this high humidity, but if we get a dry day, I'll post the results.

How are the results going so far with your baked sensors?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on December 31, 2019, 07:10:19 PM
So I decided to do a little holiday baking ... with my temp/hum sensors. My hope was to remove the wet bias in drier conditions but also found surprisingly better performance at saturation after the bake.

I have a new Davis SHT31 (which I believe was manufactured in June 2019) that I've reported on in this forum with disappointing results. I also have a few SHT75s. I baked them all per Sensirion's datasheet for 10hrs at 220°F. I did not bother with the rehydration procedure because it's the Puget Sound in December ... they will get plenty of hydration.

The SHT75 I used for the test had been outside for about 9mos. When I brought it in it read about 4% high at room temperature compared to my Belfort pyschrometer @ 40% RH. The new Davis SHT31 has been inside aside from a week of testing outside a few weeks ago. It was 6% high and +3°F dew point compared to the Belfort even having been acclimated to the dry conditions of a heated house.

After baking I tested them against a Belfort psychrometer at room temperature. Temperature results were so accurate at 72°F among all sensors that there is nothing to note.

The Belfort results were:
72.3°F Dry Bulb/59.0°F Wet Bulb giving: Humidity 45% Dew Point 50°F

Davis SHT31:
72.3°F  43%  DP: 49°F

SHT75:
72.3°F  42%  DP: 48°F

Not necessarily surprising after 10hrs at 220°F. I then put the sensors outside in what has been near saturation conditions. I expected the sensors to carry a dry bias at high humidity. Surprisingly, I found the opposite.

Before the bake, I could not get the SHT31 above 98%. Now it goes to 100% and actually stays there. Not the "Sensirion jump" to 100% for one update cycle. The SHT75 before the bake would get to 98% and stay there with a jump periodically to 99 or 100%. Now it goes to 99% and stays there. It will jump to 100% but not sustain it like the 31.

Obviously the real test will be as conditions dry out. Unfortunately, that happens infrequently here this time of year. We've only gotten as low as the 80%s and there are signs of the SHT75 drying out better, but only by a percent or two and when you consider it being 1% low at 100%, I would say that's negligible.

Just thought I'd pass along as I was surprised by the high humidity results. I have a feeling that if it does dry out, I will be right back to where I was before the bake with all this high humidity, but if we get a dry day, I'll post the results.

How are the results going so far with your baked sensors?

Glad to see that the forum is back online. I’m thinking about baking a old SHT-31 sensor. But I’m beginning to think that if I bought a new one, baking it before installing it would ensure me that any offset that the sensor has would be rid of.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on December 31, 2019, 07:26:29 PM
So I decided to do a little holiday baking ... with my temp/hum sensors. My hope was to remove the wet bias in drier conditions but also found surprisingly better performance at saturation after the bake.

I have a new Davis SHT31 (which I believe was manufactured in June 2019) that I've reported on in this forum with disappointing results. I also have a few SHT75s. I baked them all per Sensirion's datasheet for 10hrs at 220°F. I did not bother with the rehydration procedure because it's the Puget Sound in December ... they will get plenty of hydration.

The SHT75 I used for the test had been outside for about 9mos. When I brought it in it read about 4% high at room temperature compared to my Belfort pyschrometer @ 40% RH. The new Davis SHT31 has been inside aside from a week of testing outside a few weeks ago. It was 6% high and +3°F dew point compared to the Belfort even having been acclimated to the dry conditions of a heated house.

After baking I tested them against a Belfort psychrometer at room temperature. Temperature results were so accurate at 72°F among all sensors that there is nothing to note.

The Belfort results were:
72.3°F Dry Bulb/59.0°F Wet Bulb giving: Humidity 45% Dew Point 50°F

Davis SHT31:
72.3°F  43%  DP: 49°F

SHT75:
72.3°F  42%  DP: 48°F

Not necessarily surprising after 10hrs at 220°F. I then put the sensors outside in what has been near saturation conditions. I expected the sensors to carry a dry bias at high humidity. Surprisingly, I found the opposite.

Before the bake, I could not get the SHT31 above 98%. Now it goes to 100% and actually stays there. Not the "Sensirion jump" to 100% for one update cycle. The SHT75 before the bake would get to 98% and stay there with a jump periodically to 99 or 100%. Now it goes to 99% and stays there. It will jump to 100% but not sustain it like the 31.

Obviously the real test will be as conditions dry out. Unfortunately, that happens infrequently here this time of year. We've only gotten as low as the 80%s and there are signs of the SHT75 drying out better, but only by a percent or two and when you consider it being 1% low at 100%, I would say that's negligible.

Just thought I'd pass along as I was surprised by the high humidity results. I have a feeling that if it does dry out, I will be right back to where I was before the bake with all this high humidity, but if we get a dry day, I'll post the results.

How are the results going so far with your baked sensors?
I’m thinking about baking a old SHT-31 sensor. But I’m beginning to think that if I bought a new one, baking it before installing it would ensure me that any offset that the sensor has would be rid of.
I'd bake the old one, compare it to it's "pre-baked" old self and leave the new one untouched, at least for now to compare.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on December 31, 2019, 08:30:32 PM
How are the results going so far with your baked sensors?

Unfortunately I can’t go into detail at the moment due to not having the time and I will follow up with more info as I’m able, but I’ve baked (another) brand new Davis 31 with positive results. Sensor goes to 100% AND this one follows the SHT75 when humidity drops which I’ve never seen before. This one has a clear potting not the black potting which my other “new” sensor had. I don’t know if that is part of the reason for the difference. The clear potting WILL MELT at 220F. It won’t turn completely into a liquid, but you have to be careful. More to follow as I’m able, but I’m cautiously optimistic from what I’ve seen so far and as you know I’ve been pretty critical ...
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on December 31, 2019, 09:36:15 PM
How are the results going so far with your baked sensors?

Unfortunately I can’t go into detail at the moment due to not having the time and I will follow up with more info as I’m able, but I’ve baked (another) brand new Davis 31 with positive results. Sensor goes to 100% AND this one follows the SHT75 when humidity drops which I’ve never seen before. This one has a clear potting not the black potting which my other “new” sensor had. I don’t know if that is part of the reason for the difference. The clear potting WILL MELT at 220F. It won’t turn completely into a liquid, but you have to be careful. More to follow as I’m able, but I’m cautiously optimistic from what I’ve seen so far and as you know I’ve been pretty critical ...

Please let us know when you’re able!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: galfert on January 01, 2020, 09:49:38 AM
Oooh sounds like someone may be bound to an NDA and have some test hardware. How exciting.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on January 01, 2020, 09:24:33 PM
How are the results going so far with your baked sensors?

Unfortunately I can’t go into detail at the moment due to not having the time and I will follow up with more info as I’m able, but I’ve baked (another) brand new Davis 31 with positive results. Sensor goes to 100% AND this one follows the SHT75 when humidity drops which I’ve never seen before. This one has a clear potting not the black potting which my other “new” sensor had. I don’t know if that is part of the reason for the difference. The clear potting WILL MELT at 220F. It won’t turn completely into a liquid, but you have to be careful. More to follow as I’m able, but I’m cautiously optimistic from what I’ve seen so far and as you know I’ve been pretty critical ...

I’m baking an older SHT 31 with the black coating and see how it does. Planning on installing it in my remote unit tomorrow morning as long it’s not raining. You can view data from that 31 here:

https://www.wunderground.com/dashboard/pws/KALTHORS2?cm_ven=localwx_pwsdash

I’ll let everyone know when the 31 goes live. Right now, I have the 75 installed.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on January 02, 2020, 09:53:29 AM
How are the results going so far with your baked sensors?

Unfortunately I can’t go into detail at the moment due to not having the time and I will follow up with more info as I’m able, but I’ve baked (another) brand new Davis 31 with positive results. Sensor goes to 100% AND this one follows the SHT75 when humidity drops which I’ve never seen before. This one has a clear potting not the black potting which my other “new” sensor had. I don’t know if that is part of the reason for the difference. The clear potting WILL MELT at 220F. It won’t turn completely into a liquid, but you have to be careful. More to follow as I’m able, but I’m cautiously optimistic from what I’ve seen so far and as you know I’ve been pretty critical ...

I’m baking an older SHT 31 with the black coating and see how it does. Planning on installing it in my remote unit tomorrow morning as long it’s not raining. You can view data from that 31 here:

https://www.wunderground.com/dashboard/pws/KALTHORS2?cm_ven=localwx_pwsdash

I’ll let everyone know when the 31 goes live. Right now, I have the 75 installed.

Aight. I have the baked 31 installed on my remote station. Shouldn’t left the filter cap on when I baked it. lol. It melted some. I took it off and installed the sensor without a filter. Going to see how long it lasts. Lol.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on January 02, 2020, 10:25:44 AM
How are the results going so far with your baked sensors?

Unfortunately I can’t go into detail at the moment due to not having the time and I will follow up with more info as I’m able, but I’ve baked (another) brand new Davis 31 with positive results. Sensor goes to 100% AND this one follows the SHT75 when humidity drops which I’ve never seen before. This one has a clear potting not the black potting which my other “new” sensor had. I don’t know if that is part of the reason for the difference. The clear potting WILL MELT at 220F. It won’t turn completely into a liquid, but you have to be careful. More to follow as I’m able, but I’m cautiously optimistic from what I’ve seen so far and as you know I’ve been pretty critical ...

Curious where did you order the clear potting sensor from? Davis or Ryan at Scaled?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on January 02, 2020, 11:33:23 AM
BTW. How quick did your baked 31 go up to 100%?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on January 02, 2020, 12:32:30 PM
Oooh sounds like someone may be bound to an NDA and have some test hardware. How exciting.

I’m not sure if this was for me, but I wish that were the case. The truth is I apparently just enjoy wasting my own time and money ...
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on January 02, 2020, 12:34:36 PM
How are the results going so far with your baked sensors?

Unfortunately I can’t go into detail at the moment due to not having the time and I will follow up with more info as I’m able, but I’ve baked (another) brand new Davis 31 with positive results. Sensor goes to 100% AND this one follows the SHT75 when humidity drops which I’ve never seen before. This one has a clear potting not the black potting which my other “new” sensor had. I don’t know if that is part of the reason for the difference. The clear potting WILL MELT at 220F. It won’t turn completely into a liquid, but you have to be careful. More to follow as I’m able, but I’m cautiously optimistic from what I’ve seen so far and as you know I’ve been pretty critical ...

Curious where did you order the clear potting sensor from? Davis or Ryan at Scaled?

From Ryan. I’ve ordered another and will let you know if it’s the clear kind.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: fkapp on January 02, 2020, 12:42:11 PM
How are the results going so far with your baked sensors?

Unfortunately I can’t go into detail at the moment due to not having the time and I will follow up with more info as I’m able, but I’ve baked (another) brand new Davis 31 with positive results. Sensor goes to 100% AND this one follows the SHT75 when humidity drops which I’ve never seen before. This one has a clear potting not the black potting which my other “new” sensor had. I don’t know if that is part of the reason for the difference. The clear potting WILL MELT at 220F. It won’t turn completely into a liquid, but you have to be careful. More to follow as I’m able, but I’m cautiously optimistic from what I’ve seen so far and as you know I’ve been pretty critical ...

Curious where did you order the clear potting sensor from? Davis or Ryan at Scaled?

While not a direct answer to the replacement sensor question, my new VP2 arrived the day after Christmas from Scaled Instruments.  I modified the filter with the SF2 Cap.  The picture of the SHT that came with the new VP2 is shown below (appears to be the clear coating mentioned above). Posting to help with what a recent SHT from Davis looks like to help with the questions.  So far been very happy and am running the Davis 7714 passive shield. When station went online was a .85" of rain and 96% humidity day/night. Today is 42% Humidity and sunny day, and has been +/- 2% to the NWS ASOS 2.6 miles away since online, and currently am 2% below the NWS ASOS, so fingers crossed wet bias does not rear its head.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on January 02, 2020, 12:43:24 PM
BTW. How quick did your baked 31 go up to 100%?

Pretty quickly. Within a day or two of being put outside. Keep in mind though I’m in Western Washington. We haven’t been below 70% humidity in probably close to a month. The sensor reaches 100% and stays there. It stays right with my SHT75 as humidity drops. Unfortunately we’ve only been as low as 76% for the raw humidity reading since I’ve had it out there. I will prob pull it inside and check it at room temp and humidity. A bit busy with other things at the moment, so will post more details as I can. The high Sensirion bias is still there, but it doesn’t seem exacerbated by Davis after the bake with this one. Finally, Davis has gone back and forth with the clear and black potting. I had 2 SHT11s when they used that sensor and one was clear and one was black. It may just be luck of the draw and not necessarily some improvement.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on January 02, 2020, 03:06:23 PM
How are the results going so far with your baked sensors?

Unfortunately I can’t go into detail at the moment due to not having the time and I will follow up with more info as I’m able, but I’ve baked (another) brand new Davis 31 with positive results. Sensor goes to 100% AND this one follows the SHT75 when humidity drops which I’ve never seen before. This one has a clear potting not the black potting which my other “new” sensor had. I don’t know if that is part of the reason for the difference. The clear potting WILL MELT at 220F. It won’t turn completely into a liquid, but you have to be careful. More to follow as I’m able, but I’m cautiously optimistic from what I’ve seen so far and as you know I’ve been pretty critical ...

Curious where did you order the clear potting sensor from? Davis or Ryan at Scaled?
96% humidity day/night. Today is 42% Humidity and sunny day, and has been +/- 2% to the NWS ASOS 2.6 miles away since online, and currently am 2% below the NWS ASOS, so fingers crossed wet bias does not rear its head.
You should actually compare dew points. I've seen as much as a 4% difference between stations when they show the same temp and dew point due to the particular spread at that time. Humidity AFAIC is only worth watching when very high or very low.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on January 02, 2020, 03:08:48 PM
How are the results going so far with your baked sensors?

Unfortunately I can’t go into detail at the moment due to not having the time and I will follow up with more info as I’m able, but I’ve baked (another) brand new Davis 31 with positive results. Sensor goes to 100% AND this one follows the SHT75 when humidity drops which I’ve never seen before. This one has a clear potting not the black potting which my other “new” sensor had. I don’t know if that is part of the reason for the difference. The clear potting WILL MELT at 220F. It won’t turn completely into a liquid, but you have to be careful. More to follow as I’m able, but I’m cautiously optimistic from what I’ve seen so far and as you know I’ve been pretty critical ...

I’m baking an older SHT 31 with the black coating and see how it does. Planning on installing it in my remote unit tomorrow morning as long it’s not raining. You can view data from that 31 here:

https://www.wunderground.com/dashboard/pws/KALTHORS2?cm_ven=localwx_pwsdash

I’ll let everyone know when the 31 goes live. Right now, I have the 75 installed.
Shouldn’t left the filter cap on when I baked it. lol. It melted some.
Hopefully it didn't emit any of the dreaded VOC's.... 8-[
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: fkapp on January 02, 2020, 03:24:58 PM
How are the results going so far with your baked sensors?

Unfortunately I can’t go into detail at the moment due to not having the time and I will follow up with more info as I’m able, but I’ve baked (another) brand new Davis 31 with positive results. Sensor goes to 100% AND this one follows the SHT75 when humidity drops which I’ve never seen before. This one has a clear potting not the black potting which my other “new” sensor had. I don’t know if that is part of the reason for the difference. The clear potting WILL MELT at 220F. It won’t turn completely into a liquid, but you have to be careful. More to follow as I’m able, but I’m cautiously optimistic from what I’ve seen so far and as you know I’ve been pretty critical ...

Curious where did you order the clear potting sensor from? Davis or Ryan at Scaled?
96% humidity day/night. Today is 42% Humidity and sunny day, and has been +/- 2% to the NWS ASOS 2.6 miles away since online, and currently am 2% below the NWS ASOS, so fingers crossed wet bias does not rear its head.
You should actually compare dew points. I've seen as much as a 4% difference between stations when they show the same temp and dew point due to the particular spread at that time. Humidity AFAIC is only worth watching when very high or very low.
Thanks for the tip.
This past Hour comparsion at 3pm ET update (CWOP setup is still under quality check I guess) My dewpoint was 1 degree higher than the 3 Mile away to the South NWS station and 1 Degree Lower than the 10 Miles away to NW NWS station.  I guess overall this is pretty good. will see how trends go long term.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on January 02, 2020, 03:32:20 PM
Good. Always check the time of the stations your comparing to as dew points can vacillate quite a bit compared to the more steady temp.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on January 02, 2020, 05:16:30 PM
Good. Always check the time of the stations your comparing to as dew points can vacillate quite a bit compared to the more steady temp.

This is more than anyone needs to know, so sorry if your eyes glaze over, but it is also important to understand the difference of how ASOS calculates dew point and RH. It isn't apples to apples with how our weather stations do it. ASOS uses a Vaisala sensor that uses its own temperature and humidity sensors to calculate a dew point, just like our Sensirion sensors. This dew point, however, is then used with a different (and official) temperature sensor to calculate a relative humidity. (This allows the NWS to heat their humidity sensor without affecting the official temperature reading and even though the humidity sensor's temperature sensor reads too high, the relative aspect of humidity with respect to temperature results in an accurate dew point calculation.)

This difference is further complicated by the fact that ASOS rounds their raw temperature and dew point temperatures BEFORE calculating the humidity. So be especially careful when comparing to a NWS ASOS using 5 min observations as these observations for temperature and dew point are rounded to the nearest degree C. Hourly (ie typically at :51, :53, or :56) and special observations are rounded to the nearest degree F and make for better comparisons.

This is more than just academic based on how the NWS ASOS uses dew point and temperature to calculate RH. For an extreme example, if the ASOS reads a temp of 9.4°C and a dew point of 8.5°C (an actual RH of 94%), the 5 min observation will be 9°C/9°C (48°F/48°F) and a relative humidity of 100%. If the temp goes up to 9.5°C and the dew point down to 8.4°C (an actual RH of 93%), the 5 min observation will be 10°C/8°C (50°F/46°F) and a RH of 87%. So a 13% humidity change and a 2°C increase in the temperature/dew point spread for only a 1 percentage point change in humidity and a 0.2­°C actual increase in the temp/DP spread!

Hourly observations tend to have less rounding error because they convert the raw °C temp and dew point to a raw °F and round to the nearest °F. So in both these cases the temp would convert and round to 49°F and the dew point to 47°F for a RH of 93%.

This explains why the ASOS humidity and dew point can jump around more and also why you never see a NWS ASOS report 98% or 99% humidity. ASOSs at Air Force bases use a different vendor and methodology, so they will act a little different.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on January 02, 2020, 05:28:22 PM
Good. Always check the time of the stations your comparing to as dew points can vacillate quite a bit compared to the more steady temp.
you never see a NWS ASOS report 98% or 99% humidity.
Not disagreeing with anything you've said (ASOS's averaging algorithm is just part of thing we must deal with when comparing) except, I've seen the ASOS at TUS report 99% more times than I can count. 98%, not sure, but definitely 99.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on January 02, 2020, 05:36:55 PM
Good. Always check the time of the stations your comparing to as dew points can vacillate quite a bit compared to the more steady temp.
you never see a NWS ASOS report 98% or 99% humidity.
Not disagreeing with anything you've said (ASOS's averaging algorithm is just part of thing we must deal with when comparing) except, I've seen the ASOS at TUS report 99% more times than I can count. 98%, not sure, but definitely 99.

Interesting. TUS or DMA? The reason I ask is because DMA being an Air Force base probably uses the FMQ19 or latest variation from Coastal Environmental Systems up here in Seattle and it works like an AWOS and like our stations, not like how I described above. I think it uses a temperature/humidity sensor in an aspirated RM Young. When I lived in DC, Andrews AFB was always at 98 or 99, but I've never seen a NWS ASOS do it.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on January 02, 2020, 05:51:09 PM
Good. Always check the time of the stations your comparing to as dew points can vacillate quite a bit compared to the more steady temp.
you never see a NWS ASOS report 98% or 99% humidity.
Not disagreeing with anything you've said (ASOS's averaging algorithm is just part of thing we must deal with when comparing) except, I've seen the ASOS at TUS report 99% more times than I can count. 98%, not sure, but definitely 99.

Interesting. TUS or DMA?
TUS. They're only 4 miles apart, but DMA will always hit 100% before TUS will. TUS will eventually get there, but in the mean time, it'll sit on 99%. Whether DMA will display 99%, I honestly can't say, but you now have me looking.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on January 02, 2020, 06:11:26 PM
Here is 01JAN19 for both. This is what I'm used to seeing with how the NWS ASOS rounds.

https://mesowest.utah.edu/cgi-bin/droman/meso_base_dyn.cgi?product=&past=1&stn=KDMA&unit=0&time=LOCAL&day1=2&month1=01&year1=2019&hour1=0

https://mesowest.utah.edu/cgi-bin/droman/meso_base_dyn.cgi?product=&past=1&stn=KTUS&unit=0&time=LOCAL&day1=2&month1=01&year1=2019&hour1=0
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on January 02, 2020, 06:31:55 PM
How are the results going so far with your baked sensors?

Unfortunately I can’t go into detail at the moment due to not having the time and I will follow up with more info as I’m able, but I’ve baked (another) brand new Davis 31 with positive results. Sensor goes to 100% AND this one follows the SHT75 when humidity drops which I’ve never seen before. This one has a clear potting not the black potting which my other “new” sensor had. I don’t know if that is part of the reason for the difference. The clear potting WILL MELT at 220F. It won’t turn completely into a liquid, but you have to be careful. More to follow as I’m able, but I’m cautiously optimistic from what I’ve seen so far and as you know I’ve been pretty critical ...

I’m baking an older SHT 31 with the black coating and see how it does. Planning on installing it in my remote unit tomorrow morning as long it’s not raining. You can view data from that 31 here:

https://www.wunderground.com/dashboard/pws/KALTHORS2?cm_ven=localwx_pwsdash

I’ll let everyone know when the 31 goes live. Right now, I have the 75 installed.
Shouldn’t left the filter cap on when I baked it. lol. It melted some.
Hopefully it didn't emit any of the dreaded VOC's.... 8-[

That’s my thought too.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on January 02, 2020, 06:42:30 PM
Here is 01JAN19 for both. This is what I'm used to seeing with how the NWS ASOS rounds.

https://mesowest.utah.edu/cgi-bin/droman/meso_base_dyn.cgi?product=&past=1&stn=KDMA&unit=0&time=LOCAL&day1=2&month1=01&year1=2019&hour1=0

https://mesowest.utah.edu/cgi-bin/droman/meso_base_dyn.cgi?product=&past=1&stn=KTUS&unit=0&time=LOCAL&day1=2&month1=01&year1=2019&hour1=0
I am familiar. I compare here for ease but all the data is rounded compared to Meso. However, this page doesn't update timely until after business hours in the States...as stupid as that is...  :roll: but obviously still comparable.

https://www.wrh.noaa.gov/total_forecast/other_obs.php?wfo=twc&zone=AZZ504
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on January 02, 2020, 06:50:23 PM
To follow up on more details of my holiday baking, the Davis SHT31 that I originally baked would get to 100% after the bake as I've described. The SHT75 I baked did the same. The SHT31 before would only get to 98%. As humidity dropped into the 80s and 70s, the Davis 31 started to show a wet bias compared to the SHT75 by about 3%. This was consistent with what I have been frustrated with with these Davis sensors. This sensor had black potting and I believe it was manufactured in June 2019.

I got another Davis 31 with clear potting that I believe was manufactured in November 2019. Before the bake, this sensor ran high on humidity at room temperature like all the rest. After the bake, it performed significantly better at room temperature. After the bake it also consistently hit 100%. Most importantly, this sensor will follow the SHT75 almost exactly as the humidity dries out into the 70s. Unfortunately I haven't seen below that. I have never seen a Davis 31 follow the 75. It almost always has a high bias as it dries out, so this is significant to me. I have one more sensor coming and will follow up. I will also follow up if I see lower humidity levels or if I test the current one inside.

With this next sensor I plan to try to shield the potting better during the bake if it is the clear potting as it does melt some. If anyone plans on baking theirs with clear potting, be sure to be careful about melting. I may simply try to wrap most of the next one in aluminum foil. I should have mentioned that I do remove the filter for baking (sorry jgentry). If this new sensor has black potting I will probably return it.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on January 02, 2020, 06:59:31 PM
Funny thing is, I specifically remember calling Sensirion (I think in San Fran) when I went from an 11 to the 15 and asked the "spoke person" if I needed to "bake" my 15 for proper operation and got a resounding "no". So much for that... :roll:
Thanks for the work.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on January 02, 2020, 07:00:04 PM
To follow up on more details of my holiday baking, the Davis SHT31 that I originally baked would get to 100% after the bake as I've described. The SHT75 I baked did the same. The SHT31 before would only get to 98%. As humidity dropped into the 80s and 70s, the Davis 31 started to show a wet bias compared to the SHT75 by about 3%. This was consistent with what I have been frustrated with with these Davis sensors. This sensor had black potting and I believe it was manufactured in June 2019.

I got another Davis 31 with clear potting that I believe was manufactured in November 2019. Before the bake, this sensor ran high on humidity at room temperature like all the rest. After the bake, it performed significantly better at room temperature. After the bake it also consistently hit 100%. Most importantly, this sensor will follow the SHT75 almost exactly as the humidity dries out into the 70s. Unfortunately I haven't seen below that. I have never seen a Davis 31 follow the 75. It almost always has a high bias as it dries out, so this is significant to me. I have one more sensor coming and will follow up. I will also follow up if I see lower humidity levels or if I test the current one inside.

With this next sensor I plan to try to shield the potting better during the bake if it is the clear potting as it does melt some. If anyone plans on baking theirs with clear potting, be sure to be careful about the melting. I may simply try to wrap most of the next one in aluminum foil. I should have mentioned that I do remove the filter for baking (sorry jgentry). If this new sensor has black potting I will probably return it.

No problem. Lol. How fast does the baked sensor goes up to 100%? It has been raining all day and my remote station with the baked sensor has peaked at 98% so far.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on January 02, 2020, 07:02:04 PM
To follow up on more details of my holiday baking, the Davis SHT31 that I originally baked would get to 100% after the bake as I've described. The SHT75 I baked did the same. The SHT31 before would only get to 98%. As humidity dropped into the 80s and 70s, the Davis 31 started to show a wet bias compared to the SHT75 by about 3%. This was consistent with what I have been frustrated with with these Davis sensors. This sensor had black potting and I believe it was manufactured in June 2019.

I got another Davis 31 with clear potting that I believe was manufactured in November 2019. Before the bake, this sensor ran high on humidity at room temperature like all the rest. After the bake, it performed significantly better at room temperature. After the bake it also consistently hit 100%. Most importantly, this sensor will follow the SHT75 almost exactly as the humidity dries out into the 70s. Unfortunately I haven't seen below that. I have never seen a Davis 31 follow the 75. It almost always has a high bias as it dries out, so this is significant to me. I have one more sensor coming and will follow up. I will also follow up if I see lower humidity levels or if I test the current one inside.

With this next sensor I plan to try to shield the potting better during the bake if it is the clear potting as it does melt some. If anyone plans on baking theirs with clear potting, be sure to be careful about the melting. I may simply try to wrap most of the next one in aluminum foil. I should have mentioned that I do remove the filter for baking (sorry jgentry). If this new sensor has black potting I will probably return it.

No problem. Lol. How fast does the baked sensor goes up to 100%? It has been raining all day and my remote station with the baked sensor has peaked at 98% so far.
Rain itself does not necessarily equate to full saturation, as a matter of fact, usually doesn't.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on January 02, 2020, 07:03:42 PM
Funny thing is, I specifically remember calling Sensirion (I think in San Fran) when I went from an 11 to the 15 and asked the "spoke person" if I needed to "bake" my 15 for proper operation and got a resounding "no". So much for that... :roll:
Thanks for the work.

Yeah, I'm not sure. Maybe it just helps with any potential contaminants in the Davis manufacturing process? Although my SHT75 performs better at the high end from it too. I haven't noticed any real improvement for the SHT75 on the lower end.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on January 02, 2020, 07:07:25 PM
To follow up on more details of my holiday baking, the Davis SHT31 that I originally baked would get to 100% after the bake as I've described. The SHT75 I baked did the same. The SHT31 before would only get to 98%. As humidity dropped into the 80s and 70s, the Davis 31 started to show a wet bias compared to the SHT75 by about 3%. This was consistent with what I have been frustrated with with these Davis sensors. This sensor had black potting and I believe it was manufactured in June 2019.

I got another Davis 31 with clear potting that I believe was manufactured in November 2019. Before the bake, this sensor ran high on humidity at room temperature like all the rest. After the bake, it performed significantly better at room temperature. After the bake it also consistently hit 100%. Most importantly, this sensor will follow the SHT75 almost exactly as the humidity dries out into the 70s. Unfortunately I haven't seen below that. I have never seen a Davis 31 follow the 75. It almost always has a high bias as it dries out, so this is significant to me. I have one more sensor coming and will follow up. I will also follow up if I see lower humidity levels or if I test the current one inside.

With this next sensor I plan to try to shield the potting better during the bake if it is the clear potting as it does melt some. If anyone plans on baking theirs with clear potting, be sure to be careful about the melting. I may simply try to wrap most of the next one in aluminum foil. I should have mentioned that I do remove the filter for baking (sorry jgentry). If this new sensor has black potting I will probably return it.

No problem. Lol. How fast does the baked sensor goes up to 100%? It has been raining all day and my remote station with the baked sensor has peaked at 98% so far.

Within a day or two. It did take a little time for it to rehydrate a little, so it was initially slow to come up, but once it did, it hit 100% like never before. But we have lots of saturated conditions here in fog and mist this time of year. It's been raining for hours here right now and it is at 95%, so as CW2274 says, rain doesn't necessarily mean saturated air.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on January 02, 2020, 07:48:12 PM
To follow up on more details of my holiday baking, the Davis SHT31 that I originally baked would get to 100% after the bake as I've described. The SHT75 I baked did the same. The SHT31 before would only get to 98%. As humidity dropped into the 80s and 70s, the Davis 31 started to show a wet bias compared to the SHT75 by about 3%. This was consistent with what I have been frustrated with with these Davis sensors. This sensor had black potting and I believe it was manufactured in June 2019.

I got another Davis 31 with clear potting that I believe was manufactured in November 2019. Before the bake, this sensor ran high on humidity at room temperature like all the rest. After the bake, it performed significantly better at room temperature. After the bake it also consistently hit 100%. Most importantly, this sensor will follow the SHT75 almost exactly as the humidity dries out into the 70s. Unfortunately I haven't seen below that. I have never seen a Davis 31 follow the 75. It almost always has a high bias as it dries out, so this is significant to me. I have one more sensor coming and will follow up. I will also follow up if I see lower humidity levels or if I test the current one inside.

With this next sensor I plan to try to shield the potting better during the bake if it is the clear potting as it does melt some. If anyone plans on baking theirs with clear potting, be sure to be careful about the melting. I may simply try to wrap most of the next one in aluminum foil. I should have mentioned that I do remove the filter for baking (sorry jgentry). If this new sensor has black potting I will probably return it.

No problem. Lol. How fast does the baked sensor goes up to 100%? It has been raining all day and my remote station with the baked sensor has peaked at 98% so far.
Rain itself does not necessarily equate to full saturation, as a matter of fact, usually doesn't.

True.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on January 02, 2020, 07:48:48 PM
To follow up on more details of my holiday baking, the Davis SHT31 that I originally baked would get to 100% after the bake as I've described. The SHT75 I baked did the same. The SHT31 before would only get to 98%. As humidity dropped into the 80s and 70s, the Davis 31 started to show a wet bias compared to the SHT75 by about 3%. This was consistent with what I have been frustrated with with these Davis sensors. This sensor had black potting and I believe it was manufactured in June 2019.

I got another Davis 31 with clear potting that I believe was manufactured in November 2019. Before the bake, this sensor ran high on humidity at room temperature like all the rest. After the bake, it performed significantly better at room temperature. After the bake it also consistently hit 100%. Most importantly, this sensor will follow the SHT75 almost exactly as the humidity dries out into the 70s. Unfortunately I haven't seen below that. I have never seen a Davis 31 follow the 75. It almost always has a high bias as it dries out, so this is significant to me. I have one more sensor coming and will follow up. I will also follow up if I see lower humidity levels or if I test the current one inside.

With this next sensor I plan to try to shield the potting better during the bake if it is the clear potting as it does melt some. If anyone plans on baking theirs with clear potting, be sure to be careful about the melting. I may simply try to wrap most of the next one in aluminum foil. I should have mentioned that I do remove the filter for baking (sorry jgentry). If this new sensor has black potting I will probably return it.

No problem. Lol. How fast does the baked sensor goes up to 100%? It has been raining all day and my remote station with the baked sensor has peaked at 98% so far.

Within a day or two. It did take a little time for it to rehydrate a little, so it was initially slow to come up, but once it did, it hit 100% like never before. But we have lots of saturated conditions here in fog and mist this time of year. It's been raining for hours here right now and it is at 95%, so as CW2274 says, rain doesn't necessarily mean saturated air.

Gotcha. It’s up to 99% now.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on January 02, 2020, 08:56:33 PM
To follow up on more details of my holiday baking, the Davis SHT31 that I originally baked would get to 100% after the bake as I've described. The SHT75 I baked did the same. The SHT31 before would only get to 98%. As humidity dropped into the 80s and 70s, the Davis 31 started to show a wet bias compared to the SHT75 by about 3%. This was consistent with what I have been frustrated with with these Davis sensors. This sensor had black potting and I believe it was manufactured in June 2019.

I got another Davis 31 with clear potting that I believe was manufactured in November 2019. Before the bake, this sensor ran high on humidity at room temperature like all the rest. After the bake, it performed significantly better at room temperature. After the bake it also consistently hit 100%. Most importantly, this sensor will follow the SHT75 almost exactly as the humidity dries out into the 70s. Unfortunately I haven't seen below that. I have never seen a Davis 31 follow the 75. It almost always has a high bias as it dries out, so this is significant to me. I have one more sensor coming and will follow up. I will also follow up if I see lower humidity levels or if I test the current one inside.

With this next sensor I plan to try to shield the potting better during the bake if it is the clear potting as it does melt some. If anyone plans on baking theirs with clear potting, be sure to be careful about the melting. I may simply try to wrap most of the next one in aluminum foil. I should have mentioned that I do remove the filter for baking (sorry jgentry). If this new sensor has black potting I will probably return it.

No problem. Lol. How fast does the baked sensor goes up to 100%? It has been raining all day and my remote station with the baked sensor has peaked at 98% so far.

Within a day or two. It did take a little time for it to rehydrate a little, so it was initially slow to come up, but once it did, it hit 100% like never before. But we have lots of saturated conditions here in fog and mist this time of year. It's been raining for hours here right now and it is at 95%, so as CW2274 says, rain doesn't necessarily mean saturated air.

Gotcha. It’s up to 99% now.


Now it’s flipping back and forth from 99% to 100%. I like it.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on January 02, 2020, 09:21:52 PM
To follow up on more details of my holiday baking, the Davis SHT31 that I originally baked would get to 100% after the bake as I've described. The SHT75 I baked did the same. The SHT31 before would only get to 98%. As humidity dropped into the 80s and 70s, the Davis 31 started to show a wet bias compared to the SHT75 by about 3%. This was consistent with what I have been frustrated with with these Davis sensors. This sensor had black potting and I believe it was manufactured in June 2019.

I got another Davis 31 with clear potting that I believe was manufactured in November 2019. Before the bake, this sensor ran high on humidity at room temperature like all the rest. After the bake, it performed significantly better at room temperature. After the bake it also consistently hit 100%. Most importantly, this sensor will follow the SHT75 almost exactly as the humidity dries out into the 70s. Unfortunately I haven't seen below that. I have never seen a Davis 31 follow the 75. It almost always has a high bias as it dries out, so this is significant to me. I have one more sensor coming and will follow up. I will also follow up if I see lower humidity levels or if I test the current one inside.

With this next sensor I plan to try to shield the potting better during the bake if it is the clear potting as it does melt some. If anyone plans on baking theirs with clear potting, be sure to be careful about the melting. I may simply try to wrap most of the next one in aluminum foil. I should have mentioned that I do remove the filter for baking (sorry jgentry). If this new sensor has black potting I will probably return it.

No problem. Lol. How fast does the baked sensor goes up to 100%? It has been raining all day and my remote station with the baked sensor has peaked at 98% so far.

Within a day or two. It did take a little time for it to rehydrate a little, so it was initially slow to come up, but once it did, it hit 100% like never before. But we have lots of saturated conditions here in fog and mist this time of year. It's been raining for hours here right now and it is at 95%, so as CW2274 says, rain doesn't necessarily mean saturated air.

Gotcha. It’s up to 99% now.


Now it’s flipping back and forth from 99% to 100%. I like it.
Great to hear. My baked SHT75 generally gets to 100% quicker. But that they both get there at all with consistency is a win for me.

It also helps with the lower end due to software. Because it will go to 100%, I've put a -2% offset in my Envoy through Weatherlink so that a raw reading of 100% is 98% in Cumulus which I tell to report 98 as 100%. So my humidity will jump from 97% to 100%, but that equates to about a 1°F dew point depression and mimics what I've seen from the NWS ASOS in my earlier post. (Plus Sensirion's datasheet says 99% should be reported as saturated air anyway.) Then the whole rest of the scale I benefit from a -2% offset to help with the high humidity bias. Last really dry spell we had, the SHT75 was reading 3-4% high at about 20% humidity. This will cut that error in half and if I need more, I simply can use the calibration in Cumulus to make further adjustments in lower humidity. I think this will work much more accurately in the mid and low humidity ranges. Before it was a compromise of using a calibration slope that helped enough in the mid range without completely distorting the low range humidity with too much of a correction. Unfortunately, probably not helpful for folks like CW2274 who see extreme low humidity.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on January 02, 2020, 10:10:42 PM
To follow up on more details of my holiday baking, the Davis SHT31 that I originally baked would get to 100% after the bake as I've described. The SHT75 I baked did the same. The SHT31 before would only get to 98%. As humidity dropped into the 80s and 70s, the Davis 31 started to show a wet bias compared to the SHT75 by about 3%. This was consistent with what I have been frustrated with with these Davis sensors. This sensor had black potting and I believe it was manufactured in June 2019.

I got another Davis 31 with clear potting that I believe was manufactured in November 2019. Before the bake, this sensor ran high on humidity at room temperature like all the rest. After the bake, it performed significantly better at room temperature. After the bake it also consistently hit 100%. Most importantly, this sensor will follow the SHT75 almost exactly as the humidity dries out into the 70s. Unfortunately I haven't seen below that. I have never seen a Davis 31 follow the 75. It almost always has a high bias as it dries out, so this is significant to me. I have one more sensor coming and will follow up. I will also follow up if I see lower humidity levels or if I test the current one inside.

With this next sensor I plan to try to shield the potting better during the bake if it is the clear potting as it does melt some. If anyone plans on baking theirs with clear potting, be sure to be careful about the melting. I may simply try to wrap most of the next one in aluminum foil. I should have mentioned that I do remove the filter for baking (sorry jgentry). If this new sensor has black potting I will probably return it.

No problem. Lol. How fast does the baked sensor goes up to 100%? It has been raining all day and my remote station with the baked sensor has peaked at 98% so far.

Within a day or two. It did take a little time for it to rehydrate a little, so it was initially slow to come up, but once it did, it hit 100% like never before. But we have lots of saturated conditions here in fog and mist this time of year. It's been raining for hours here right now and it is at 95%, so as CW2274 says, rain doesn't necessarily mean saturated air.

Gotcha. It’s up to 99% now.


Now it’s flipping back and forth from 99% to 100%. I like it.
Unfortunately, probably not helpful for folks like CW2274 who see extreme low humidity.
Other than not pulling the 100% trigger, I'm still pleased with this sensor's overall performance. I considered the next time I pulled the ISS into the house I'd bake the sensor...but the problem with that is I have zero idea how long it would be before I see a "100%" day. The sensor could be completely back to "normal" by then. Oh well..
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on January 03, 2020, 07:56:07 AM
Well i decided to bake my new 31 which was showing about 1 to 2 percent high on the bottom end and on the top end in pretty good fog it was a solid 97 percent and bounced up to 98 percent. Well after baking it now shows to be right on on the lower end and in moderate fog shows a good 98 percent and goes to 99 at times. Now will have to wait to see how it does in dense fog but baking seems to have got it back in calibration.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on January 03, 2020, 12:59:44 PM
I just wanted to add this note about baking the sensor, i had an old sht15 and a sht 11 in a draw of poor sensors, they both maxed at 97%. I figured why not try and see if baking them would work and it worked really well with both of them maxing on my test setup at 99% which is uncommon, most max at 98% but do better when installed outdoors. So i am going to dig around in my sensor draw and see if i can get anymore to come alive. Oh yes the response time is much improved too.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: AWL on January 03, 2020, 07:43:31 PM
Thanks
So you bake the whole circuit board in the oven?
Or do you unsolder the sensor and bake?

what do you place on, cookie sheet or something?

Valid questions  :-k  At what temperature for how long? Hoping one of you "Bakers" might help me out?  Thinking of doing this also. Have a 31 I bought in January 2019 and put it to work in August.  Not very happy at all. Now will only get to 94% in heavy fog lasting for hours. Will sometimes jump up a couple of more percent but only for a few seconds. Have a new one I ordered a week ago on my desk so will change out next week. Have a spare VP2+ in the garage that I might put in service after hopefully baking the 31.

Doug
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on January 03, 2020, 07:57:25 PM
I followed the info in a prior post which said a temp. of 220F for 10 hours. I have been doing mine in a toaster over, got the temp set with a thermometer and it worked from about 215 to 225 degrees and has worked out real well.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: AWL on January 03, 2020, 08:01:09 PM
I followed the info in a prior post which said a temp. of 220F for 10 hours. I have been doing mine in a toaster over, got the temp set with a thermometer and it worked from about 215 to 225 degrees and has worked out real well.

Thank you so much for the information! 10hrs....wow! Looks like I might need a toaster oven also #-o  Long time to run the kitchen oven!

Thanks again, Doug
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on January 03, 2020, 11:52:08 PM
Finally had an extreme drop in humidity today after weeks of high humidity. Raw humidity for both my baked Davis 31 and SHT75 reached 26%. I have never seen the Davis 31 match the SHT75 at humidity levels this low. My Cumulus correction is likely excessive at this humidity level, the total correction I was using was -5%. Normally for my sensors about -5% for humidity in the mid to low 20s is about right compared to my Belfort and to match nearby ASOS stations.

To compare with ASOS, my raw data at 555p was:
Temp: 59°F Hum: 27%  DP: 25°F

KTIW at 553p:
Temp: 59°F Hum: 28% DP: 26°F

KSEA at 553p:
Temp: 61°F Hum: 24% DP: 24°F

My raw data for these sensors never compares well to these ASOS stations at this low humidity especially after long periods near saturation.

Here is the calibrated data for my station that is maybe overcorrected:
https://mesowest.utah.edu/cgi-bin/droman/meso_base_dyn.cgi?product=&past=1&stn=C7491&unit=0&time=LOCAL&day1=3&month1=01&year1=2020&hour1=18

To get 100% after the bake and now this kind of performance at the low end from a Davis 31 is really welcome news to me. I hope it continues and isn't an anomaly.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on January 04, 2020, 06:31:29 AM
Finally had an extreme drop in humidity today after weeks of high humidity. Raw humidity for both my baked Davis 31 and SHT75 reached 26%. I have never seen the Davis 31 match the SHT75 at humidity levels this low. My Cumulus correction is likely excessive at this humidity level, the total correction I was using was -5%. Normally for my sensors about -5% for humidity in the mid to low 20s is about right compared to my Belfort and to match nearby ASOS stations.

To compare with ASOS, my raw data at 555p was:
Temp: 59°F Hum: 27%  DP: 25°F

KTIW at 553p:
Temp: 59°F Hum: 28% DP: 26°F

KSEA at 553p:
Temp: 61°F Hum: 24% DP: 24°F

My raw data for these sensors never compares well to these ASOS stations at this low humidity especially after long periods near saturation.

Here is the calibrated data for my station that is maybe overcorrected:
https://mesowest.utah.edu/cgi-bin/droman/meso_base_dyn.cgi?product=&past=1&stn=C7491&unit=0&time=LOCAL&day1=3&month1=01&year1=2020&hour1=18

To get 100% after the bake and now this kind of performance at the low end from a Davis 31 is really welcome news to me. I hope it continues and isn't an anomaly.

The 31 you’re talking here is the one with the clear potting and not the black one right?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on January 04, 2020, 12:19:23 PM
The 31 you’re talking here is the one with the clear potting and not the black one right?

That's correct. Another Davis 31 came yesterday. It also has clear potting. Manufacture date also looks like November 2019. I baked it last night wrapped in aluminum foil with just the actual sensor exposed. Worked great. No melting. I will post the pre and post bake results later. I've actually put the other clear baked Davis 31 as my main sensor now. It has a -2% offset and then a Cumulus calibration of (1.021x-2). This produces a -2% correction at the top of the scale and max -4% correction at the lower end of the scale. I may take the Cumulus calibration out completely as I see how it performs. The SHT75 I was using as my main sensor is still outside for comparison. My CWOP ID is CW7491 of course for those that want to see how this baked Davis 31 does
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on January 04, 2020, 01:30:37 PM
Nothing surprising here, but passing along anyway. Here are the pre bake and post bake comparisons for the latest Davis clear potting SHT31:

Pre-bake:
Davis 31
Temp: 73.6°F  Hum: 47%  DP: 52°F

Belfort Psychrometer
Temp: 73.4°F  Hum: 42%  DP: 49°F

Post Bake:
Davis 31
Temp: 67.4°F  Hum: 42%  DP: 44°F

Belfort Psychrometer
Temp: 67.4°F  Hum: 43%  DP: 44°F


Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on January 04, 2020, 01:41:03 PM
Good results I just wonder how long it will last?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on January 04, 2020, 01:48:19 PM
Good results I just wonder how long it will last?
That is the question isn't it? I'm cautiously optimistic based on yesterday's humidity drop. The Davis 31 I am using now has been baked and outside in near saturated air for 2 weeks. I've never seen the raw data match the SHT75 or ASOS even after previous bakes, but I've been disappointed with these things before ... I would not be surprised to see it no longer hit 100% and/or for the high humidity bias to return.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on January 04, 2020, 03:08:15 PM
The 31 you’re talking here is the one with the clear potting and not the black one right?

That's correct. Another Davis 31 came yesterday. It also has clear potting. Manufacture date also looks like November 2019. I baked it last night wrapped in aluminum foil with just the actual sensor exposed. Worked great. No melting. I will post the pre and post bake results later. I've actually put the other clear baked Davis 31 as my main sensor now. It has a -2% offset and then a Cumulus calibration of (1.021x-2). This produces a -2% correction at the top of the scale and max -4% correction at the lower end of the scale. I may take the Cumulus calibration out completely as I see how it performs. The SHT75 I was using as my main sensor is still outside for comparison. My CWOP ID is CW7491 of course for those that want to see how this baked Davis 31 does

I baked one with the black potting. So far, it’s performing well. But I have no filter over it and the sensor is receiving good airflow in a passive shield.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on January 04, 2020, 03:38:35 PM
The 31 you’re talking here is the one with the clear potting and not the black one right?

That's correct. Another Davis 31 came yesterday. It also has clear potting. Manufacture date also looks like November 2019. I baked it last night wrapped in aluminum foil with just the actual sensor exposed. Worked great. No melting. I will post the pre and post bake results later. I've actually put the other clear baked Davis 31 as my main sensor now. It has a -2% offset and then a Cumulus calibration of (1.021x-2). This produces a -2% correction at the top of the scale and max -4% correction at the lower end of the scale. I may take the Cumulus calibration out completely as I see how it performs. The SHT75 I was using as my main sensor is still outside for comparison. My CWOP ID is CW7491 of course for those that want to see how this baked Davis 31 does
I have no filter over it and the sensor is receiving good airflow in a passive shield.
Even without a fan, I'd think particulates will eventually get into that tiny sensor hole without some kind of filter.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on January 04, 2020, 04:19:16 PM
The 31 you’re talking here is the one with the clear potting and not the black one right?

That's correct. Another Davis 31 came yesterday. It also has clear potting. Manufacture date also looks like November 2019. I baked it last night wrapped in aluminum foil with just the actual sensor exposed. Worked great. No melting. I will post the pre and post bake results later. I've actually put the other clear baked Davis 31 as my main sensor now. It has a -2% offset and then a Cumulus calibration of (1.021x-2). This produces a -2% correction at the top of the scale and max -4% correction at the lower end of the scale. I may take the Cumulus calibration out completely as I see how it performs. The SHT75 I was using as my main sensor is still outside for comparison. My CWOP ID is CW7491 of course for those that want to see how this baked Davis 31 does
I have no filter over it and the sensor is receiving good airflow in a passive shield.
Even without a fan, I'd think particulates will eventually get into that tiny sensor hole without some kind of filter.

True. I “melted” my filter some when I baked the sensor. Should’ve taken the filter cap off. Lol. But I’m going to see how long it lasts without a filter.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on January 07, 2020, 11:11:22 PM
So far, I’m still liking the results of the baked sensor. Anybody else still liking their results?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on January 08, 2020, 11:57:03 AM
So far, I’m still liking the results of the baked sensor. Anybody else still liking their results?

Same here. My baked clear potting Davis 31 is in an AC aspirated shield. Baked SHT75 is in a passive 7714 now. They're side by side still though. I've noticed that the Davis 31 is now running a little lower as humidity drops, which is a first. At the high end, it also reads a little lower. Haven't had real heavy saturation conditions since I have had it as my primary sensor, which has been less than a week. We'll see if this drier performance will prohibit it from getting to 100%. Since it has been my primary sensor, max RH has been 98%. I'll update when we get conditions that are full saturation.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: fkapp on January 10, 2020, 03:59:43 PM
Hi,
Just wanted to update that still am very happy with the SHT with the clear coat that came with my VP2 that installed last week of 2019.

This is in passive 7714 shield, will get FARS going before summer, but results are very close to nearby NWS stations.
I have not gotten int baking the sensor (to be honest a bit concerned on doing this first time) but this has the SF2 Cap mod right out of box before going into outside air.

CWOP analysis has been completed and figured would post link to the results here in case anyone was curious how this compares
https://weather.gladstonefamily.net/site/F6381

Thanks for all the tips
Frank
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on January 26, 2020, 04:14:56 PM
Anyone still having good results with the baked sensor?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on January 27, 2020, 12:29:14 PM
Anyone still having good results with the baked sensor?

I am still having great results. My clear potting baked Davis 31 actually reads about 2-3% drier side by side with a baked SHT75 in the mid to upper humidity range. I have still yet to see really heavy saturation, but the 31 has gotten to 99%. The SHT75 has hit 100% multiple times. I’m still waiting to get a heavy fog to see if the 31 will hit 100%.

I have also noticed fkapp is getting great results compared to KBUF ASOS without a bake. What is the max humidity you’ve seen Frank?

As I posted earlier in the thread, I saw an improvement in the performance at room temperature post bake, but I am not running an unbaked clear Davis 31 for comparison, so I can’t say for certain how much this improved performance is attributed to the bake and how much to a potential improvement by Davis. I’ve seen enough compared to the SHT75, so I may try a baked Davis 31 with black potting for comparison.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: fkapp on January 28, 2020, 11:26:37 AM
Hi guys,
Max humidity so far has been 96% back on Jan 12th.

I just checked against the NWS Buffalo daily climate summary and shows the Max as the NWS BUF was 96% on this day as well.

Overall still very happy

Thanks
Frank
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on January 30, 2020, 11:20:27 AM
Well i haven't updated in a while because the new 31 that i was testing went belly up after only two months and i was in mourning for some time because i took a chance and did a little reshaping of the board to fit my passive shield and thus no warranty. I just got a new 31 in yesterday and after wrapping it in a wet wash cloth for some time it seems to max out at only 96%. I put it outside last night so i could compare it with three other sensors i am testing and this morning the 3 sensors showed one at 92% and the other two at 91% while the new 31 was showing 86%, just lousy reading. I am now baking it to see if it will get back on track with a good reading. Will report tomorrow if it is going back to Ryan for another one.



Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on January 30, 2020, 02:54:25 PM
Well i haven't updated in a while because the new 31 that i was testing went belly up after only two months and i was in mourning for some time because i took a chance and did a little reshaping of the board to fit my passive shield and thus no warranty. I just got a new 31 in yesterday and after wrapping it in a wet wash cloth for some time it seems to max out at only 96%. I put it outside last night so i could compare it with three other sensors i am testing and this morning the 3 sensors showed one at 92% and the other two at 91% while the new 31 was showing 86%, just lousy reading. I am now baking it to see if it will get back on track with a good reading. Will report tomorrow if it is going back to Ryan for another one.

Did you happen to notice the color of the potting and/or the presumed date on the sticker on the back?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on January 30, 2020, 03:02:34 PM
I did notice the potting stuff was clear and seems to cover less of the board than the heavy black stuff. I didn't pay any attention to the sticker.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on January 31, 2020, 09:30:16 AM
Ok finished the 10 hour baking and testing of the sensor and so far all is great. The max reading so far is 99% and i just finished installing it as my main sensor and it is reading the same humidity and temp. as the two other sensors co-located  on the same mount.  One is in a passive shield and the other in a Davis fars shield. The response time is really very good, went from 42% to 99% in less than an hour under test. Most sensors i have tested top out at 98% after an hour and a half to two hours. Now if it will just keep working for a long time i will be a happy camper. Next check is for any kind of wet bias to show up. =D&gt;
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on February 04, 2020, 12:20:29 AM
Keep in mind that these two stations are around .08 miles apart from each other. One station has the baked 31 and the other one has a 15 inside of a Barani shield. Both recorded a high of 73° and the 15 read 2% lower than the 31 and that correlated with a 1° variance in DP
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on February 10, 2020, 11:41:09 PM
Not sure but I think my 31 has succumbed back to having a wet bias. But then again, it’s been raining a lot and when it’s dry, the sensor would only have one day to recalibrate itself.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: andro700 on February 12, 2020, 07:11:35 PM
Not sure if this has been asked. When you bake your sensor, do you put the wires and all in oven or take them off.

Chuck
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: galfert on February 12, 2020, 07:36:44 PM
Not sure if this has been asked. When you bake your sensor, do you put the wires and all in oven or take them off.

Chuck

Yup, the whole thing. https://www.wxforum.net/index.php?topic=34658.msg394802#msg394802
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: andro700 on February 12, 2020, 07:40:48 PM
I believe I have an SHT15. I bought my station around 2013 or 2014 I believe. Would I be better off getting an SHT31? I also live on the western Washington. A lot of humidity here.

Chuck
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on February 12, 2020, 07:56:37 PM
I believe I have an SHT15. I bought my station around 2013 or 2014 I believe. Would I be better off getting an SHT31?
If you bought it new and have not modified it, you have an SHT11. The 31 is a superior sensor, problem aside, which has "seemingly" been addressed. Personally, I would spring for a 31. If you do, don't forget the -0.9F offset for the temperature.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: andro700 on February 12, 2020, 07:59:11 PM
I was thinking about springing for it anyways. Since it has much better specs than the one I have now.

Thanks,
Chuck
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on February 13, 2020, 10:09:22 AM
Noticed that the Davis EnviroMonitor can take I2C sensors

https://www.davisinstruments.com/product_documents/weather/EM-sensors.pdf
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: galfert on February 13, 2020, 10:52:00 AM
Noticed that the Davis EnviroMonitor can take I2C sensors

https://www.davisinstruments.com/product_documents/weather/EM-sensors.pdf

Yes, johnd brought this to my attention a little while back.

https://www.wxforum.net/index.php?topic=38266.msg393736#msg393736
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on February 13, 2020, 07:26:48 PM
Update on new 31, we had three days of very humid weather. The first day the lowest humidity was 86% and went up to 95%, day two stayed in the low to middle 90's all day and the third day the humidity was in the upper 90's all day and night. The sun came out today and dry air moved in and the 31 had no wet bias at all had a nice smooth drop in humidity and tracked the other two stations with no problems seen. So first long period of humidity was a winner. Now just have to see if anything shows up in the long run.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on February 17, 2020, 04:12:00 PM
Another update sensor still working fine, had dense fog this am and sensor hit solid 99% and no sign of wet bias so far.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on February 17, 2020, 05:10:49 PM
Another update sensor still working fine, had dense fog this am and sensor hit solid 99% and no sign of wet bias so far.
This one you threw in the Easy Bake Oven, right?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on February 17, 2020, 05:26:37 PM
Yes it is, baking made it work much better.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on February 17, 2020, 05:42:17 PM
baking made it work much better.
I just use my climate. ;)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: andro700 on February 19, 2020, 07:47:16 PM
I ordered a new fan and batteries and SHT-31 sensor. Got them installed and offset the temp -0.9. My 24 hour fan was not working at all. So going to see how temps and humidity react with new sensor and fan. Like I stated before I had humidity saturation. The date on the old temp/humidity sensor was 2009. The one I installed had the clear podding.

Chuck
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Tornado12 on February 20, 2020, 09:35:12 AM
Im new here on the forum, and am having to replace my humidity sensor in my VP2 as mine started reporting bad data. Ive tried to track through this long thread to get some info but its so long. Anyone care to give me a synopsis of the SHT-31 sensor? Is this a bad sensor? Good sensor? is the reliability poor? Im coming into this blind. Looking at my historical data it seems my snesor began going bad just 3 months or so after I bought my VP2 brand new. It is a 2017 unit so it came with the newer SHT-31 sensor. It reports humidty 10-20% too low compared to other stations. Today its reporting 73% and airport 20 miles away reporting 95% . Rain is looming today so very humid and cloudy. My new sensor arrives tomorrow and I will be installing it. Trying to gain some knowledge that may be helpful.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on February 20, 2020, 10:33:17 AM
Im new here on the forum, and am having to replace my humidity sensor in my VP2 as mine started reporting bad data. Ive tried to track through this long thread to get some info but its so long. Anyone care to give me a synopsis of the SHT-31 sensor? Is this a bad sensor? Good sensor? is the reliability poor? Im coming into this blind. Looking at my historical data it seems my snesor began going bad just 3 months or so after I bought my VP2 brand new. It is a 2017 unit so it came with the newer SHT-31 sensor. It reports humidty 10-20% too low compared to other stations. Today its reporting 73% and airport 20 miles away reporting 95% . Rain is looming today so very humid and cloudy. My new sensor arrives tomorrow and I will be installing it. Trying to gain some knowledge that may be helpful.

Humidity is only relative to the temperature so saying your humidity is running low doesn't give a full picture of what's really going on. Where is the dewpoint in relation to the airport?  Like I said these SHT31 sensors run higher than airports is my experience so if anything they have a wet bias. This was the reason I was asking for a link to your station.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Tornado12 on February 20, 2020, 11:22:37 AM
Im new here on the forum, and am having to replace my humidity sensor in my VP2 as mine started reporting bad data. Ive tried to track through this long thread to get some info but its so long. Anyone care to give me a synopsis of the SHT-31 sensor? Is this a bad sensor? Good sensor? is the reliability poor? Im coming into this blind. Looking at my historical data it seems my snesor began going bad just 3 months or so after I bought my VP2 brand new. It is a 2017 unit so it came with the newer SHT-31 sensor. It reports humidty 10-20% too low compared to other stations. Today its reporting 73% and airport 20 miles away reporting 95% . Rain is looming today so very humid and cloudy. My new sensor arrives tomorrow and I will be installing it. Trying to gain some knowledge that may be helpful.

Humidity is only relative to the temperature so saying your humidity is running low doesn't give a full picture of what's really going on. Where is the dewpoint in relation to the airport?  Like I said these SHT31 sensors run higher than airports is my experience so if anything they have a wet bias. This was the reason I was asking for a link to your station.

As of right now.

Airport:
TEMP: 61F
Dew Point: 56
Humidity: 84%
Pressure: 30.12

Ambient Weather WS-2902 3 miles from my station:
TEMP: 64F
Dew Point: 59
Humidity: 85%
Pressure: 30.17

My Station:
TEMP: 64F
Dew Point: 51
Humidity: 64%
Pressure: 30.15


The airport is ~20 miles north of me, and a cold front is approaching with lots of rain. The temperature difference is accurate and is reflected by all the weather stations in a 60 mile radius. The humidity im reporting (64%) is a lot lower than everyone right now. Dew Point obviously a little off as well. So dew point/humidty are innacurate. My temperature however is very accurate and has had no issues from what I can tell.

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on February 20, 2020, 12:06:17 PM
Im new here on the forum, and am having to replace my humidity sensor in my VP2 as mine started reporting bad data. Ive tried to track through this long thread to get some info but its so long. Anyone care to give me a synopsis of the SHT-31 sensor? Is this a bad sensor? Good sensor? is the reliability poor? Im coming into this blind. Looking at my historical data it seems my snesor began going bad just 3 months or so after I bought my VP2 brand new. It is a 2017 unit so it came with the newer SHT-31 sensor. It reports humidty 10-20% too low compared to other stations. Today its reporting 73% and airport 20 miles away reporting 95% . Rain is looming today so very humid and cloudy. My new sensor arrives tomorrow and I will be installing it. Trying to gain some knowledge that may be helpful.

Humidity is only relative to the temperature so saying your humidity is running low doesn't give a full picture of what's really going on. Where is the dewpoint in relation to the airport?  Like I said these SHT31 sensors run higher than airports is my experience so if anything they have a wet bias. This was the reason I was asking for a link to your station.

As of right now.

Airport:
TEMP: 61F
Dew Point: 56
Humidity: 84%
Pressure: 30.12

Ambient Weather WS-2902 3 miles from my station:
TEMP: 64F
Dew Point: 59
Humidity: 85%
Pressure: 30.17

My Station:
TEMP: 64F
Dew Point: 51
Humidity: 64%
Pressure: 30.15


The airport is ~20 miles north of me, and a cold front is approaching with lots of rain. The temperature difference is accurate and is reflected by all the weather stations in a 60 mile radius. The humidity im reporting (64%) is a lot lower than everyone right now. Dew Point obviously a little off as well. So dew point/humidty are innacurate. My temperature however is very accurate and has had no issues from what I can tell.

With a cold front approaching hard to tell from a snapshot what's going on. You can remove the ambient from the picture. That would be my last choice of a reference station.  Need a station link?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on February 29, 2020, 04:09:59 PM
Even though a baked SHT-31 works better than a one that isn’t baked, it seems to me that the 15 reads drier at the lower end of the humidity range compared to the 31. Any others noticed this?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on February 29, 2020, 04:18:55 PM
Well we just got through with a strong high pressure system behind a cold front and the humidity of my 31 and my 15 both hit 18% which is very low for me down here and the new 31 has been doing real well from bottom to top on readings and still no sign of wet bias.  \:D/
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on February 29, 2020, 04:45:15 PM
Well we just got through with a strong high pressure system behind a cold front and the humidity of my 31 and my 15 both hit 18% which is very low for me down here and the new 31 has been doing real well from bottom to top on readings and still no sign of wet bias.  \:D/

It’s been a wet winter here in Alabama. 15-16” in Central Alabama for this month. The 31s may have gain a wet bias but I don’t know for certain.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on February 29, 2020, 04:45:46 PM
Well we just got through with a strong high pressure system behind a cold front and the humidity of my 31 and my 15 both hit 18% which is very low for me down here and the new 31 has been doing real well from bottom to top on readings and still no sign of wet bias.  \:D/

But it is still performing well on the high end.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on February 29, 2020, 05:55:01 PM
Yes it is, max has been 99% in dense fog and has matched real well with my other sensors.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on March 24, 2020, 08:41:36 PM
I’m going to have to rebake my 31 sensor. Even though it’s performing well on the high end, it’s showing a wet bias in the mid range. I didn’t put any filter over my 31 so I’m sure that led to the wet bias issue. But I’m beginning to think that the Sensirion filter keeps the sensor from having a wet bias longer than the silk “filter.”
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on March 24, 2020, 08:57:42 PM
But I’m beginning to think that the Sensirion filter keeps the sensor from having a wet bias longer than the silk “filter.”
As Sensirion states, the SF2 cap, if properly installed, is a water tight solution that "consists of a single piece of PBT and an integrated water vapor-permeable filter membrane." I doubt silk was designed as such.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Tensor on March 26, 2020, 01:26:23 PM
I have been comparing HYT221 and SHT21 for about 6 months now. Lately, HYT221 started to have significant wet bias, if the real humidity level is above 70%. This means that if the near official station and SHT21 think the humidity is about 80%, HYT221 can show that RH is more than 90%. However, if the humidity is less than 70%, both SHT21 and HYT221 have similar readings. Actually, HYT221 is very close to the official station, if the humidity is around 20%-40%, while SHT21 has a slight wet bias all the time.

It is worth of mentioning that it has not rained or there has not been fog/frost for weeks now. Both sensors are now installed in the same radiation shield and HYT221 has no more a DIY plastic splash cover (but of course it has its own yellowish filter). I have no idea why HYT221 has started to act like this. In the winter, it performed very well. Also, HYT221 has +0.4 Celsius warm bias at cold temperatures (if outdoor temperature is less than 0 Celsius) compared to the SHT21. I trust SHT21 temperatures more, since I have other thermometers as well and they agree with SHT21.

In conclusion, SHT31 and HYT221 both failed within one winter. SHT31 has near constant -10% dry bias, HYT221 has +10% wet-bias at high humidity levels and SHT21 has not-so-bad wet-bias. I really consider buying a Vaisala sensor to stop this nightmare...
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on March 26, 2020, 09:37:02 PM
I have been comparing HYT221 and SHT21 for about 6 months now. Lately, HYT221 started to have significant wet bias, if the real humidity level is above 70%. This means that if the near official station and SHT21 think the humidity is about 80%, HYT221 can show that RH is more than 90%. However, if the humidity is less than 70%, both SHT21 and HYT221 have similar readings. Actually, HYT221 is very close to the official station, if the humidity is around 20%-40%, while SHT21 has a slight wet bias all the time.

It is worth of mentioning that it has not rained or there has not been fog/frost for weeks now. Both sensors are now installed in the same radiation shield and HYT221 has no more a DIY plastic splash cover (but of course it has its own yellowish filter). I have no idea why HYT221 has started to act like this. In the winter, it performed very well. Also, HYT221 has +0.4 Celsius warm bias at cold temperatures (if outdoor temperature is less than 0 Celsius) compared to the SHT21. I trust SHT21 temperatures more, since I have other thermometers as well and they agree with SHT21.

In conclusion, SHT31 and HYT221 both failed within one winter. SHT31 has near constant -10% dry bias, HYT221 has +10% wet-bias at high humidity levels and SHT21 has not-so-bad wet-bias. I really consider buying a Vaisala sensor to stop this nightmare...

Interesting. Wonder if the HYT sensor has gone bad? At least with the Sensirion sensors, you can bake them back into calibration if needed.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on March 26, 2020, 09:38:31 PM
But Sensirion does make good temperature sensors (especially the 3x series like the 31).
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on April 06, 2020, 12:24:08 PM
Update, my 31 is still working just fine no hiccups seen yet. I bought another 31 and put it in another fars right out of the box, no mods are anything done to it to see how it does compared to the 31 that was baked and working fine.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: fkapp on April 20, 2020, 06:22:49 PM
Hi guys,
Hope everyone is doing well. Update as have been very busy and wanted to update on how things are going with the VP2 I setup end of last year.
Overall dewpoint and humidity has been good overall, but the clear coating on the temp sensor that shipped with the unit seems to be in the same boat as the 96% humidity cap.  last week had a 100% fog humid calm early morning at the buffalo NWS and even in fog mist my station only got to 96%.

https://weather.gladstonefamily.net/site/F6381

I am also going to be relocating my anemometer higher than the mast I mounted in back yard to get wind more accurate now that weather is getting better.

Overall happy and have some fine tuning to do, but a bummer on the humidity at max levels.  This is non FARS and in the Davis 7714 shield with Sensorian SF2 Cap mod i did
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on April 20, 2020, 06:39:01 PM
even in fog mist my station only got to 96%.
Well, I suppose you could try the "baking method" if you wanted.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: fkapp on April 20, 2020, 06:59:34 PM
Thanks. overall am happy as has been smooth operation. dialing in is the next chapter.
To be honest the baking the sensor process makes me nervous am going to damage the sensor.
If a mishap happens can always replace the sensor :).

What tips for baking do you have and I read a toaster oven is an option?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on April 20, 2020, 07:25:37 PM
Thanks. overall am happy as has been smooth operation. dialing in is the next chapter.
To be honest the baking the sensor process makes me nervous am going to damage the sensor.
If a mishap happens can always replace the sensor :).

What tips for baking do you have and I read a toaster oven is an option?
"Dialing in" humidity is almost always a two way street..rob Peter to pay Paul. You may fix one end, but trash the other. Then again, here I see the full range from 1% to 100%, so perhaps you can get it where you want it more so than I.

Read back in the thread for baking procedures, I've never done it.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on April 20, 2020, 07:27:34 PM
Well need to update my post too, the new 31 has been doing ok but like the other 31 it maxed out at 98% so i went ahead an baked it and now it hits a solid 99%. We have had a few nights of dense fog and it has been solid and shows no signs of any wet bias at all. I decided to bake the older 31 again because it was a little slower than the new one and it now follows the new one just fine plus with a dense fog and 99% showing it bumps 100% every so often so the baking did help it some. The old 31 is still trucking along with no problems seen.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: fkapp on April 21, 2020, 02:34:47 PM
Forgot to mention that I came across a used wireless weather monitor II that I setup for my son who is in elementary as his first weather station as a science project together.

The analog sensor in the WMII after good cleaning is working very well and on this same fog mist morning hit 100% for several hours.  Impressed that after all these years still working accurately.

Just wanted to mention that another WM II is still going strong
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on April 21, 2020, 04:33:37 PM
The analog sensor in the WMII after good cleaning is working very well and on this same fog mist morning hit 100% for several hours.  Impressed that after all these years still working accurately.
Yes, the old analog sensors seem to have little trouble pulling the 100% trigger unlike the current digital batch. No idea how they did/do in the other extreme. Being so old, there may be some drift to watch for if you care.

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on May 11, 2020, 09:44:08 PM
Update the newest 31 is still working fine with no bias seen and works good from low humidity to the max of 99%.  =D&gt;
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on May 16, 2020, 04:22:26 PM
On an interesting note....There is a guy on the WeatherFlow forum that received his WeatherFlow Tempest station yesterday and the SHT31 in the Tempest has performed really well so far.  The humidity sensor reached to 100% this morning.

Here is his station webpage. https://tempestwx.com/station/18140/ and https://smartweather.weatherflow.com/station/18140/grid

Here are the surrounding NWS/RAWS stations
 https://mesowest.utah.edu/cgi-bin/droman/trend_zip_mm.cgi?zipcode=33186&type=0&radius=25&noho=1&rawsflag=1&orderby=p&mode=
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: drew1021 on May 18, 2020, 11:42:14 AM
Sorry to break in to the conversation but I was hoping someone could point me in the right direction to the instructions on how to bake the 31 as so many have successfully accomplished. Thanks.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: galfert on May 18, 2020, 11:53:08 AM
Sorry to break in to the conversation but I was hoping someone could point me in the right direction to the instructions on how to bake the 31 as so many have successfully accomplished. Thanks.

From Sensirion's datasheet...

Reconditioning Procedure
The following reconditioning
procedure may bring the sensor back to calibration state:
Baking: 100 – 105°C at < 5%RH for 10h
Re-Hydration: 20 – 30°C at ~ 75%RH for 12h
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: drew1021 on May 18, 2020, 12:23:33 PM
So do you wrap the sensor with foil and place it on a baking sheet
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on May 18, 2020, 12:30:36 PM
What i use and it works real well is a cheap table top toaster oven that i set with a calibrated thermometer  to 220 degrees and it cycles between 215 to 225. It has worked real well on several sensors that i just put in the oven without any wrapping. I place it towards the front door so it doesn't get too close to the element. I have not had any type of melting problem with the temperature being that low.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: drew1021 on May 18, 2020, 04:55:49 PM
Great idea, not sure I want to buy a toaster oven just for this though.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on May 18, 2020, 04:58:26 PM
Great idea, not sure I want to buy a toaster oven just for this though.
Celebrate afterwards with some tater tots...
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Intheswamp on June 25, 2020, 09:32:12 AM
 :lol:
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on August 11, 2020, 08:05:08 AM
I’ve now have some experience with two WeatherFlow Tempest weather stations. Not sure how they did it, but the SHT-31 in their station doesn’t have any wet bias issues and it easily goes up to 100%. Keep in mind that this happens right out of the box and it’s before WF does any further calibrations to it.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on December 26, 2020, 02:38:29 PM
I thought about starting a new topic, but thought the input keeps with the continuity of this original topic.

I recently compared the SHT85 with the SHT75 in my Davis VP2 as well as a Davis mounted SHT31. I was able to get the data from the SHT85 with an old USB test kit I had from Sensirion and their RS485 Sensor Viewer 2.91 software. The siting of the SHT85 was not desirable due to cable length restrictions, but the sensor comes with its own sensor-mounted filter and I placed it in the Davis 7714 radiation shield. If temperatures between the sensors were the same, I was able to compare humidity values directly. In times when they weren't, I used dewpoint.

Initially, the SHT85 data seemed promising, running 6-8 percentage points lower in humidity than the SHT75 (2-3°F lower in dewpoint). Even after a period of high humidity, where both sensors reached 98%, the SHT85 would read a lower humidity when the air dried out again. Unfortunately, after a longer period of high humidity, the high humidity bias presented itself in the SHT85 and pretty much matched the SHT75 exactly. After the test, I brought the high bias SHT85 and SHT75 inside to compare with the Davis-mounted SHT31 which I have baked previously and has been inside in low humidity. Both the SHT75 and SHT85 perform about the same or a little better even with the high humidity bias present.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on December 26, 2020, 03:24:55 PM
I thought about starting a new topic, but thought the input keeps with the continuity of this original topic.

I recently compared the SHT85 with the SHT75 in my Davis VP2 as well as a Davis mounted SHT31. I was able to get the data from the SHT85 with an old USB test kit I had from Sensirion and their RS485 Sensor Viewer 2.91 software. The siting of the SHT85 was not desirable due to cable length restrictions, but the sensor comes with its own sensor-mounted filter and I placed it in the Davis 7714 radiation shield. If temperatures between the sensors were the same, I was able to compare humidity values directly. In times when they weren't, I used dewpoint.

Initially, the SHT85 data seemed promising, running 6-8 percentage points lower in humidity than the SHT75 (2-3°F lower in dewpoint). Even after a period of high humidity, where both sensors reached 98%, the SHT85 would read a lower humidity when the air dried out again. Unfortunately, after a longer period of high humidity, the high humidity bias presented itself in the SHT85 and pretty much matched the SHT75 exactly. After the test, I brought the high bias SHT85 and SHT75 inside to compare with the Davis-mounted SHT31 which I have baked previously and has been inside in low humidity. Both the SHT75 and SHT85 perform about the same or a little better even with the high humidity bias present.

I’m wondering if how Davis mounts their 31 on the PCB causes the 31 not to “air out” fully; which causes the wet bias issues to be “permanent.”
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on December 26, 2020, 06:29:07 PM
Yeah, I was disappointed with the SHT85. There are definitely two issues: a Sensirion sensor characteristic at prolonged high humidity and Davis’ application of the sensor which demonstrably worsens/makes permanent the sensor’s high humidity bias.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on December 26, 2020, 07:16:03 PM
Just to add to the mix i have an old 31 that i had out for some time and it showed the bias of 3% high which is keeping with the specs of the sensor which is the sensor will read high after being in a high humidity setting of more than 80% for three days are more and will only return to the normal reading after being below the 80% for some time as it says the return will be very slow. I put the new 31 out and it has been very good with the low and high end readings with a lot of 100% readings in heavy fog. I put the 31 back out in a different shield and sure enough after a long period of drying out it read 1% lower than the new 31 and has been holding that for several weeks. We haven't got into a run of high humidity yet so won't know what happens until i get a good run of above 80% to set the stage for the error to show up. Now i think the baking works well because of the very low humidity in the oven and makes for a faster recovery time but just keeping it below 80% for sometime works the same just slower. This time of year i can get periods of above 80% for a week at a time so i will just have to wait for that to happen.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on December 26, 2020, 08:35:15 PM
Just to add to the mix i have an old 31 that i had out for some time and it showed the bias of 3% high which is keeping with the specs of the sensor which is the sensor will read high after being in a high humidity setting of more than 80% for three days are more and will only return to the normal reading after being below the 80% for some time as it says the return will be very slow. I put the new 31 out and it has been very good with the low and high end readings with a lot of 100% readings in heavy fog. I put the 31 back out in a different shield and sure enough after a long period of drying out it read 1% lower than the new 31 and has been holding that for several weeks. We haven't got into a run of high humidity yet so won't know what happens until i get a good run of above 80% to set the stage for the error to show up. Now i think the baking works well because of the very low humidity in the oven and makes for a faster recovery time but just keeping it below 80% for sometime works the same just slower. This time of year i can get periods of above 80% for a week at a time so i will just have to wait for that to happen.

I’m curious if the 31 was mounted correctly, it would return back into calibration quicker than the Davis’ setup?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on December 26, 2020, 09:13:14 PM
Just to add to the mix i have an old 31 that i had out for some time and it showed the bias of 3% high which is keeping with the specs of the sensor which is the sensor will read high after being in a high humidity setting of more than 80% for three days are more and will only return to the normal reading after being below the 80% for some time as it says the return will be very slow. I put the new 31 out and it has been very good with the low and high end readings with a lot of 100% readings in heavy fog. I put the 31 back out in a different shield and sure enough after a long period of drying out it read 1% lower than the new 31 and has been holding that for several weeks. We haven't got into a run of high humidity yet so won't know what happens until i get a good run of above 80% to set the stage for the error to show up. Now i think the baking works well because of the very low humidity in the oven and makes for a faster recovery time but just keeping it below 80% for sometime works the same just slower. This time of year i can get periods of above 80% for a week at a time so i will just have to wait for that to happen.

I’m curious if the 31 was mounted correctly, it would return back into calibration quicker than the Davis’ setup?
If you recall, the Acurite device that utilized the 31 had no bias compared to the Davis mounted 31. I kinda thought we were done with this fiasco. Yes, I know, I'm in the desert, but my current 31 is 18 months old and I remain very happy with it. I still think a sealed SF2 cap is a difference maker in the big scheme of things.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on December 26, 2020, 10:03:57 PM
You certainly can be done with the fiasco if you wish, I just was providing input for anyone who is interested in the SHT85. For those of us in humid climates, the fiasco is real. The SHT85 is a Sensirion mounted SHT35 with a PTFE membrane right on the sensor. Unfortunately the 85 performs exactly like the SHT75 in a humid climate despite the hope it might do better. And yes, the one thing that hasn’t changed is in my experience and climate, the SHT75 and 85 are superior to the Davis mounted 31.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on December 26, 2020, 10:18:49 PM
You certainly can be done with the fiasco if you wish, I just was providing input for anyone who is interested in the SHT85. For those of us in humid climates, the fiasco is real. The SHT85 is a Sensirion mounted SHT35 with a PTFE membrane right on the sensor. Unfortunately the 85 performs exactly like the SHT75 in a humid climate despite the hope it might do better. And yes, the one thing that hasn’t changed is in my experience and climate, the SHT75 and 85 are superior to the Davis mounted 31.
I quoted and replied to 'jgentry'...not you.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on December 26, 2020, 10:40:25 PM
Understood. It’s an open forum and I’m replying to you.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on December 26, 2020, 10:49:13 PM
Understood. It’s an open forum and I’m replying to you.
Fine. My response was not meant for your issues as I have zero experience with either the 75 or 85. My "fiasco" comment was strictly meant in regards to the 31 issue, which I have plenty of experience with.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on January 18, 2021, 04:15:41 PM
Is there any way to make adjustments to the PCB of the Davis 31 temp/hum sensor that will allow maximum airflow to the sensor without damaging the sensor? My thinking is that if you bake the sensor to put it back into calibration, along with the sensor getting maximum airflow, it will fix any prolong wet bias issues of the sensor along with it struggling to reach 100%.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on January 18, 2021, 04:21:13 PM
Just to add to the mix i have an old 31 that i had out for some time and it showed the bias of 3% high which is keeping with the specs of the sensor which is the sensor will read high after being in a high humidity setting of more than 80% for three days are more and will only return to the normal reading after being below the 80% for some time as it says the return will be very slow. I put the new 31 out and it has been very good with the low and high end readings with a lot of 100% readings in heavy fog. I put the 31 back out in a different shield and sure enough after a long period of drying out it read 1% lower than the new 31 and has been holding that for several weeks. We haven't got into a run of high humidity yet so won't know what happens until i get a good run of above 80% to set the stage for the error to show up. Now i think the baking works well because of the very low humidity in the oven and makes for a faster recovery time but just keeping it below 80% for sometime works the same just slower. This time of year i can get periods of above 80% for a week at a time so i will just have to wait for that to happen.

I’m curious if the 31 was mounted correctly, it would return back into calibration quicker than the Davis’ setup?
If you recall, the Acurite device that utilized the 31 had no bias compared to the Davis mounted 31. I kinda thought we were done with this fiasco. Yes, I know, I'm in the desert, but my current 31 is 18 months old and I remain very happy with it. I still think a sealed SF2 cap is a difference maker in the big scheme of things.

I’m sure the cap helps. But I think the big issue is that the sensor isn’t getting maximum airflow, which causes moisture to collect for long periods of time and permanently offset the sensor. WeatherFlow Tempest doesn’t use the Sensirion cap for the 31 and humidity sensor is exposed. No prolong wet bias issues with the 31 for them either.

But having the Sensirion filter instead of the Davis one is a major plus.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on January 18, 2021, 04:52:40 PM
Just to add to the mix i have an old 31 that i had out for some time and it showed the bias of 3% high which is keeping with the specs of the sensor which is the sensor will read high after being in a high humidity setting of more than 80% for three days are more and will only return to the normal reading after being below the 80% for some time as it says the return will be very slow. I put the new 31 out and it has been very good with the low and high end readings with a lot of 100% readings in heavy fog. I put the 31 back out in a different shield and sure enough after a long period of drying out it read 1% lower than the new 31 and has been holding that for several weeks. We haven't got into a run of high humidity yet so won't know what happens until i get a good run of above 80% to set the stage for the error to show up. Now i think the baking works well because of the very low humidity in the oven and makes for a faster recovery time but just keeping it below 80% for sometime works the same just slower. This time of year i can get periods of above 80% for a week at a time so i will just have to wait for that to happen.

I’m curious if the 31 was mounted correctly, it would return back into calibration quicker than the Davis’ setup?
If you recall, the Acurite device that utilized the 31 had no bias compared to the Davis mounted 31. I kinda thought we were done with this fiasco. Yes, I know, I'm in the desert, but my current 31 is 18 months old and I remain very happy with it. I still think a sealed SF2 cap is a difference maker in the big scheme of things.

I’m sure the cap helps. But I think the big issue is that the sensor isn’t getting maximum airflow, which causes moisture to collect for long periods of time and permanently offset the sensor.
That may or may not be true. However, with an SF2 cap that is properly sealed, it is a water tight solution that only allows water vapor to penetrate. That an a case fan should alleviate any potential moisture build up over ambient AFAIC. One thing for certain, I'll never go without an SF2 cap again.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Jester on February 17, 2021, 09:08:57 AM
CW, how did you modify your SF2 cap to work on the SHT31 Davis PCB?

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: johnd on February 17, 2021, 09:17:40 AM
Not directly relevant, but I see that the Sensirion SHT4x series are starting to becoming available:

https://www.sensirion.com/en/environmental-sensors/humidity-sensors/humidity-sensor-sht4x/

Just SHT40 so far, but 41 and 45 apparently due later this year. Adafruit have stock of the SHT40 (true) I2C version in a STEMMA board:

https://www.adafruit.com/product/4885

Absolutely no word or hint from Davis about any changes, so this is just for general background info.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Jester on February 17, 2021, 12:07:04 PM
Not directly relevant, but I see that the Sensirion SHT4x series are starting to becoming available:

https://www.sensirion.com/en/environmental-sensors/humidity-sensors/humidity-sensor-sht4x/

Just SHT40 so far, but 41 and 45 apparently due later this year. Adafruit have stock of the SHT40 (true) I2C version in a STEMMA board:

https://www.adafruit.com/product/4885

Absolutely no word or hint from Davis about any changes, so this is just for general background info.


Interesting.  I kind of like this idea https://www.adafruit.com/product/4099 instead of the cap filter. What is the VCC operating range of the Davis SHT31?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: galfert on February 17, 2021, 12:29:47 PM
You'll likely never be able to run anything other than the SHT31 with Davis VP2. Unless Davis comes out with a new ISS. This is because Sensirion has stopped making sensor variant with the Legacy Sensirion Sensibus protocol. The SHT31 was the last sensor to be made with the required LSS version.

Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: johnd on February 17, 2021, 01:02:40 PM
You'll likely never be able to run anything other than the SHT31 with Davis VP2. Unless Davis comes out with a new ISS.

100%. But, and this is just an errant thought: If someone wanted to do so badly enough and assuming that the LSS protocol details are in the public domain, then it wouldn't it be reasonably easy for a suitably skilled person to write some code for a little microcontroller that sat in between an SHT35 or 4x and translated the true I2C data  into LSS to respond to the SIM boards requests for T/H data every 10 secs?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: galfert on February 17, 2021, 01:48:53 PM
Yes in theory reading in an I2C sensor and then replacing the protocol with LSS sounds interesting. This has been mentioned before but it never saw results.
https://www.wxforum.net/index.php?topic=33157.msg355110#msg355110

It doesn't mean someone else can't succeed.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on February 17, 2021, 05:01:06 PM
I'm almost positive that a member here got a 35 to work with their VP2 ISS. Obviously he had a very knowledgeable background, and I believe some of us said we would pay him for his service, but nothing ever came of it. That was like four years ago or so.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on February 17, 2021, 05:09:36 PM
CW, how did you modify your SF2 cap to work on the SHT31 Davis PCB?
I wasn't thrilled about the glue option so I trimmed the four feet down to be flush with the base and used very thin stainless steel wire to wrap around the cap and cinch it down with pliers twisting from the back side. The wire doesn't come in contact with the membrane but arches over. With the "gooey" substance that surrounds the sensor it allows the cap to seat firmly in what I believe is a true seal.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on February 17, 2021, 10:28:17 PM
Not directly relevant, but I see that the Sensirion SHT4x series are starting to becoming available:

https://www.sensirion.com/en/environmental-sensors/humidity-sensors/humidity-sensor-sht4x/

Just SHT40 so far, but 41 and 45 apparently due later this year. Adafruit have stock of the SHT40 (true) I2C version in a STEMMA board:

https://www.adafruit.com/product/4885

Absolutely no word or hint from Davis about any changes, so this is just for general background info.

My biggest pet peeve about the Sensirion sensor (and I’ve heard others do the same) is that after being exposed to humidities over 80% for a long period of time, it goes out of calibration until it recalibrates itself.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on February 17, 2021, 10:54:22 PM
Not directly relevant, but I see that the Sensirion SHT4x series are starting to becoming available:

https://www.sensirion.com/en/environmental-sensors/humidity-sensors/humidity-sensor-sht4x/

Just SHT40 so far, but 41 and 45 apparently due later this year. Adafruit have stock of the SHT40 (true) I2C version in a STEMMA board:

https://www.adafruit.com/product/4885

Absolutely no word or hint from Davis about any changes, so this is just for general background info.

My biggest pet peeve about the Sensirion sensor (and I’ve heard others do the same) is that after being exposed to humidities over 80% for a long period of time, it goes out of calibration until it recalibrates itself.
Is it high humidity or high dew point? If it's 0F outside and a dew point of -1F, that's 98ish% humidity, but bone dry air. I'm thinking high dew is the culprit, it's not fond of being "wet".
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on February 18, 2021, 12:02:33 AM
The datasheet specifies humidity. Sensirion’s documents say that prolonged exposure to high humidity will induce the reversible high bias which has been discussed ad nauseam here, but it also mentions extreme low humidity for prolonged periods will introduce a low bias, which hasn’t been discussed because most of us probably don’t experience it. Finally, the sensor outputs relative humidity with reference to saturated water vapor pressure over liquid water even for temperatures below freezing. The result is as temperatures get colder and colder below 0C, maximum humidity output by the sensor decreases. So at 0F (-18C) in your example, maximum humidity output from the sensor would be about 85% and that reading would indicate fully saturated air.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on February 18, 2021, 12:21:49 AM
it also mentions extreme low humidity for prolonged periods will introduce a low bias, which hasn’t been discussed because most of us probably don’t experience it.
Good post, I didn't know this. Obviously I do experience regular extreme low humidity, and sure enough, when I get to the 4% or lower range it starts to get a little over the top with tanking dew points, especially down at 1%, but even I don't get that but a few days a year.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Jester on February 18, 2021, 07:05:18 AM
CW, how did you modify your SF2 cap to work on the SHT31 Davis PCB?
I wasn't thrilled about the glue option so I trimmed the four feet down to be flush with the base and used very thin stainless steel wire to wrap around the cap and cinch it down with pliers twisting from the back side. The wire doesn't come in contact with the membrane but arches over. With the "gooey" substance that surrounds the sensor it allows the cap to seat firmly in what I believe is a true seal.

Thanks!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on February 21, 2021, 04:07:05 PM
I think the wax stuff that Davis uses is the main culprit of their humidity sensor woes. The wax holds moisture and keeps the sensor from fully “drying” out.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on February 21, 2021, 04:32:07 PM
One of the things I noticed in the SHT4X datasheet is an onboard heater. This is incorporated into the SHT3X as well, but the SHT4X datasheet specifically mentions one of the functions of the heater is for condensing environments to eliminate creep, which is the name they give to the high bias we see and they mention in prolonged high humidity. They also mention that there will be dedicated application notes elaborating on the use of the heater for this purpose.

It would be interesting if this function could be utilized so the Sensirion sensor was used to calculate and output dew point and then incorporate a separate temperature measuring element (thermally isolated from any self-heating), maybe a standard thermistor, for air temperature. The console could then calculate RH based on the two temperatures instead of calculating dew point. I posted a bit ago about a new version .80 temp/hum sensor Rainwise uses for their MKIII that I was speculating might actually do this as it appears to have both a thermistor and a Sensirion sensor on their temp/hum board
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on February 21, 2021, 04:59:03 PM
One of the things I noticed in the SHT4X datasheet is an onboard heater. This is incorporated into the SHT3X as well, but the SHT4X datasheet specifically mentions one of the functions of the heater is for condensing environments to eliminate creep, which is the name they give to the high bias we see and they mention in prolonged high humidity. They also mention that there will be dedicated application notes elaborating on the use of the heater for this purpose.

It would be interesting if this function could be utilized so the Sensirion sensor was used to calculate and output dew point and then incorporate a separate temperature measuring element (thermally isolated from any self-heating), maybe a standard thermistor, for air temperature. The console could then calculate RH based on the two temperatures instead of calculating dew point. I posted a bit ago about a new version .80 temp/hum sensor Rainwise uses for their MKIII that I was speculating might actually do this as it appears to have both a thermistor and a Sensirion sensor on their temp/hum board

That might be the solution to the problem. Sensirion isn’t the only ones that have the issue. Bosch deals with a similar issue also.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on February 21, 2021, 05:00:30 PM
The wax holds moisture and keeps the sensor from fully “drying” out.
I really can't see that. If anything, wax is hydrophobic.  That's why you put it on your car's finish for example.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on February 21, 2021, 05:09:26 PM
The wax holds moisture and keeps the sensor from fully “drying” out.
I really can't see that. If anything, wax is hydrophobic.  That's why you put it on your car's finish for example.

It’s my guess and I could be totally wrong.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: galfert on February 21, 2021, 05:16:16 PM
Wax has both hydrophobic and hydrophilic properties. It is a little complicated. It is sort of like soap where it starts off being hydrophobic but then as it gets worn it becomes hydrophilic.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on February 21, 2021, 05:19:24 PM
Don't even know for a fact what the substance is, but it's certainly gummy. Well, just another reason to pimp the SF2. It's just big enough to cover the sensor itself.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on February 21, 2021, 07:21:54 PM
After playing with my spare 31, I think the substance isn't wax at all, but some form of rubber.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Jester on February 22, 2021, 06:33:21 AM
CW, how did you modify your SF2 cap to work on the SHT31 Davis PCB?
I wasn't thrilled about the glue option so I trimmed the four feet down to be flush with the base and used very thin stainless steel wire to wrap around the cap and cinch it down with pliers twisting from the back side. The wire doesn't come in contact with the membrane but arches over. With the "gooey" substance that surrounds the sensor it allows the cap to seat firmly in what I believe is a true seal.

CW, do you have a source for the stainless wire you used?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on February 22, 2021, 04:13:11 PM
CW, how did you modify your SF2 cap to work on the SHT31 Davis PCB?
I wasn't thrilled about the glue option so I trimmed the four feet down to be flush with the base and used very thin stainless steel wire to wrap around the cap and cinch it down with pliers twisting from the back side. The wire doesn't come in contact with the membrane but arches over. With the "gooey" substance that surrounds the sensor it allows the cap to seat firmly in what I believe is a true seal.

CW, do you have a source for the stainless wire you used?
All I use is braided wire that one might use to hang a picture with. It has like twenty or so strands and I peel off three of them. It's strong enough not to break when cinching it down (judgement required here, have extra  ;)) while snugging down the cap without covering too much membrane.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on February 22, 2021, 04:23:41 PM
BTW, if you were unaware, Sensirion advises not to touch the membrane itself. I used small needle nose pliers to center the cap. It can be a little tricky...
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Jester on February 23, 2021, 10:33:38 AM
BTW, if you were unaware, Sensirion advises not to touch the membrane itself. I used small needle nose pliers to center the cap. It can be a little tricky...

Awesome! Thanks!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Jester on February 25, 2021, 08:59:55 AM
BTW, if you were unaware, Sensirion advises not to touch the membrane itself. I used small needle nose pliers to center the cap. It can be a little tricky...

Installed the CAP yesterday. So far so good. I did a bake and then a refrigeration on a fresh sensor. Waiting for high humid days to see how well this mod works.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on February 25, 2021, 05:07:19 PM
BTW, if you were unaware, Sensirion advises not to touch the membrane itself. I used small needle nose pliers to center the cap. It can be a little tricky...

Installed the CAP yesterday. So far so good. I did a bake and then a refrigeration on a fresh sensor. Waiting for high humid days to see how well this mod works.
Good, hopefully it wasn't too much of a pain. Are you going to use the stock filter as well? The reason I ask is that the SF2 has done such an outstanding job by itself, I haven't been out to service my ISS in over a year and a half and I know the sensor chamber is layered in dust/dirt from the case fan. That's three times longer than I have ever gone because the cap does such a good job that I see absolutely no degradation in sensor performance. That said, I was thinking of adding the stock filter to slow, yes I said slow, the response time in the summer heat. But that means climbing over my wall and out to the desert to service the ISS more like I use to. Ahhh, decisions...
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on February 25, 2021, 06:29:09 PM
I'm using the Davis stock filter with stainless steel mesh because I have one analog temp/hum sensor that still works and gives better results than the Sensirion. The size of the humidity element requires a large filter. For a while I was using the Sensirion filter on a Sensirion sensor and liked the fast temperature response. The trouble is it can be a bit much and ultimately becomes noise, which I think is why most official stations use some sort of averaging algorithm for temperature. The other problem I ran into is the different sensing intervals for temperature (~10-12s) and humidity (~50-60s) with the VP2 can result in some crazy (and I would argue less than accurate) swings in dew point. If the humidity and temperature always updated together, it would be less problematic. I find the stock filter helps act as a buffer with some of the noise, yet it's responsive enough with the AC fan.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on February 25, 2021, 06:45:16 PM
I think is why most official stations use some sort of averaging algorithm for temperature.
They all do (at least ASOS's) and not just temp, but other parameters as well. If they didn't, then the data would potentially be all over the place just like a PWS, so they "even it out."

I think I'll try the stock filter as well and see what's what. If it's not to my satisfaction, so be it...but the SF2 will definitely stay.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on February 25, 2021, 06:56:27 PM
Yeah, if you can work it out, I don’t think the SF2 inside the stock filter will cost you much in response time with the fan. I know Sensirion now puts the SF2 membrane right onto the sensor. Campbell Scientific uses it in their Hygrovue sensors with a larger filter, so it utilizes both ...

https://www.campbellsci.com/hygrovue10
https://www.campbellsci.asia/35219
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on February 25, 2021, 07:07:25 PM
I also thought this was an interesting read. Especially that even Campbell Scientific is frustrated that all humidity sensors aren’t fundamentally designed for meteorological applications ...

https://www.campbellsci.com/blog/air-temp-rh-sensors-need-to-know
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on February 25, 2021, 07:08:27 PM
Yeah, if you can work it out, I don’t think the SF2 inside the stock filter will cost you much in response time with the fan.
What I'm trying to help alleviate is the wild temp swings that I see during max insolation as the heat eddies across the desert floor. With a 60CFM fan, those are sucked up like nobody's business. I have a 40CFM backup, but am not inclined to swap them at this time. When the stock filter starts to "dirty up", maybe I'll be more satisfied. Yep, dirtier may be better...well at least for me.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on February 25, 2021, 07:13:25 PM
I also thought this was an interesting read. Especially that even Campbell Scientific is frustrated that all humidity sensors aren’t fundamentally designed for meteorological applications ...

https://www.campbellsci.com/blog/air-temp-rh-sensors-need-to-know
Good read. Humidity has always been the "rub" in sensors, but by far in the extremes.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on February 26, 2021, 01:34:57 AM
I also thought this was an interesting read. Especially that even Campbell Scientific is frustrated that all humidity sensors aren’t fundamentally designed for meteorological applications ...

https://www.campbellsci.com/blog/air-temp-rh-sensors-need-to-know


Yep. And the Sensirion 4x isn’t no better than its predecessors...

“ 2.3 Recommended Operating Conditions
The sensor shows best performance when operated within the recommended normal temperature and humidity range of 5 °C ... 60 °C and 20 %RH ... 80 %RH, respectively. Long term exposure to conditions outside recommended normal range, especially at high relative humidity, may temporarily offset the RH signal (e.g. +3 %RH after 60 h at > 80 %RH). After returning into the recommended normal temperature and humidity range the sensor will recover to within specifications by itself. Prolonged exposure to extreme conditions may accelerate ageing.”
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on February 26, 2021, 02:15:25 AM
I also thought this was an interesting read. Especially that even Campbell Scientific is frustrated that all humidity sensors aren’t fundamentally designed for meteorological applications ...

https://www.campbellsci.com/blog/air-temp-rh-sensors-need-to-know


Yep. And the Sensirion 4x isn’t no better than its predecessors...

“ 2.3 Recommended Operating Conditions
The sensor shows best performance when operated within the recommended normal temperature and humidity range of 5 °C ... 60 °C and 20 %RH ... 80 %RH, respectively. Long term exposure to conditions outside recommended normal range, especially at high relative humidity, may temporarily offset the RH signal (e.g. +3 %RH after 60 h at > 80 %RH). After returning into the recommended normal temperature and humidity range the sensor will recover to within specifications by itself. Prolonged exposure to extreme conditions may accelerate ageing.”
Not that it isn't frustrating, but as always, ya pay to play folks. Davis sees it as 99.9% satisfaction rate to their "basic" customer. If you want better, then move up and pay up. Simple as that.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Jester on February 26, 2021, 07:13:53 AM
BTW, if you were unaware, Sensirion advises not to touch the membrane itself. I used small needle nose pliers to center the cap. It can be a little tricky...

Installed the CAP yesterday. So far so good. I did a bake and then a refrigeration on a fresh sensor. Waiting for high humid days to see how well this mod works.
Good, hopefully it wasn't too much of a pain. Are you going to use the stock filter as well? The reason I ask is that the SF2 has done such an outstanding job by itself, I haven't been out to service my ISS in over a year and a half and I know the sensor chamber is layered in dust/dirt from the case fan. That's three times longer than I have ever gone because the cap does such a good job that I see absolutely no degradation in sensor performance. That said, I was thinking of adding the stock filter to slow, yes I said slow, the response time in the summer heat. But that means climbing over my wall and out to the desert to service the ISS more like I use to. Ahhh, decisions...

The installation wasn't bad since I do HO scale model railroading and model planes as a hobby, so working small isn't to much of an issue.  I didn't put the Davis filter back on, just letting the cap do it's thing. Day two and I'm not sure what to think right now. My humidity topped at 93% while other stations around me were 73%-85%. My closest ASOS, KNTU max humidity was 80%. My old sensor was closer to the aforementioned but was starting to wet bias after only three months of service.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on February 26, 2021, 10:58:02 AM
As a Navy guy, I’d use some caution comparing to Navy and even Air Force ASOS for fear of trying to get your good data to compare favorably with their bad. Your proximity to Oceana is great and their data looks good at the moment, but in my experience the military installations don’t have the most robust routine maintenance on their ASOS, particularly for humidity/dew point. Whidbey’s dew point had been out to lunch for a few years. The Air Force uses a completely different ASOS from the FAA and McChord’s humidity sensor was stuck near 100% for nearly a year and Gray AAF right next door is still stuck at near 100% going on a few years. I’ve found FAA maintained ASOS at airports with regular commercial passenger service are most reliable.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on February 26, 2021, 12:13:17 PM
I also thought this was an interesting read. Especially that even Campbell Scientific is frustrated that all humidity sensors aren’t fundamentally designed for meteorological applications ...

https://www.campbellsci.com/blog/air-temp-rh-sensors-need-to-know


Yep. And the Sensirion 4x isn’t no better than its predecessors...

“ 2.3 Recommended Operating Conditions
The sensor shows best performance when operated within the recommended normal temperature and humidity range of 5 °C ... 60 °C and 20 %RH ... 80 %RH, respectively. Long term exposure to conditions outside recommended normal range, especially at high relative humidity, may temporarily offset the RH signal (e.g. +3 %RH after 60 h at > 80 %RH). After returning into the recommended normal temperature and humidity range the sensor will recover to within specifications by itself. Prolonged exposure to extreme conditions may accelerate ageing.”
Not that it isn't frustrating, but as always, ya pay to play folks. Davis sees it as 99.9% satisfaction rate to their "basic" customer. If you want better, then move up and pay up. Simple as that.

True but Davis is also serving some bigger dogs such as WeatherSTEM and Earth Networks. They aren’t just serving the needs/wants to a general weather hobbyist.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Jim_49 on February 26, 2021, 01:24:02 PM
I just got a new VP2 with the 24 hour fan-aspirated shield.  It's still in the box waiting for the frost to get out of the ground here in Kansas so it can be installed.  I haven't seen, or may have missed, any mention of SF2 maintenance.  I live in a rural area surrounded by grain fields.  The air gets real dusty during planting and harvest seasons.  I am considering getting an SF2 but was wondering what the dusty air will do to it.  I have searched Sensirion's web site relative to maintenance but all I could find was handling requirements as mentioned in an above post.  Can dust be removed with a soft brush or will it need to be replaced?  Or will use of the stock filter over the SF2 be sufficient to keep the dust off of it?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on February 26, 2021, 01:50:48 PM
I just got a new VP2 with the 24 hour fan-aspirated shield.  It's still in the box waiting for the frost to get out of the ground here in Kansas so it can be installed.  I haven't seen, or may have missed, any mention of SF2 maintenance.  I live in a rural area surrounded by grain fields.  The air gets real dusty during planting and harvest seasons.  I am considering getting an SF2 but was wondering what the dusty air will do to it.  I have searched Sensirion's web site relative to maintenance but all I could find was handling requirements as mentioned in an above post.  Can dust be removed with a soft brush or will it need to be replaced?  Or will use of the stock filter over the SF2 be sufficient to keep the dust off of it?

I would bake the sensor first at 220°F for 10hrs and use the SF2 filter (if you’re good at putting small stuff together).  You would most likely have to replace the filter.  The SF2 filters are cheap.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Jim_49 on February 26, 2021, 02:39:54 PM
Thanks.  I'll try that.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on February 26, 2021, 02:44:31 PM
Leave the actual sensor exposed while baking, but just be sure to protect the potting around the sensor during the bake to keep it from melting. Wrapping it in aluminum foil worked well for me.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on February 26, 2021, 03:33:02 PM
I am considering getting an SF2 but was wondering what the dusty air will do to it.
See reply #1655 up top.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Jim_49 on February 26, 2021, 04:39:21 PM
Thanks.  After you pointed that out I remembered reading that.  I've read so much here that I've forgotten some of the information.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Jim_49 on February 26, 2021, 04:40:51 PM
Leave the actual sensor exposed while baking, but just be sure to protect the potting around the sensor during the bake to keep it from melting. Wrapping it in aluminum foil worked well for me.

Another point that I've read but forgot.  Thanks for pointing that out.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on February 26, 2021, 05:02:48 PM
Now I remember why I go out to the ISS so infrequently, whata PITA... Can't count how many needles I've got stuck in me. Anyway, after about a 16 month hiatus, I can report that the SF2 did it's job, I literally could barely see the filter as it was completely encased in dust/dirt. No matter, it still worked, and well. Can't recommend this enough, especially for people like me. Also stuck in a new 31, although the old one was fine. Lets see how that goes.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on February 27, 2021, 05:58:42 PM
Granted, only been a day, and it's dry, but the new 31 is nailing the dew point. Very pleased!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: dendrite on February 28, 2021, 12:50:42 AM
Yeah, if you can work it out, I don’t think the SF2 inside the stock filter will cost you much in response time with the fan.
What I'm trying to help alleviate is the wild temp swings that I see during max insolation as the heat eddies across the desert floor. With a 60CFM fan, those are sucked up like nobody's business. I have a 40CFM backup, but am not inclined to swap them at this time. When the stock filter starts to "dirty up", maybe I'll be more satisfied. Yep, dirtier may be better...well at least for me.
Have you compared the graphs between your station and KTUS? If you’re not aware, you can get the raw 1 minute historical ASOS readings from here.

ftp://ftp.ncdc.noaa.gov/pub/data/asos-onemin/

Choose the annual folder that begins with 6406...those have the temp/dew data in them. They’re large files so you have a lot of scrolling to go through to find the station you’re looking for.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on February 28, 2021, 02:29:36 AM
Yeah, if you can work it out, I don’t think the SF2 inside the stock filter will cost you much in response time with the fan.
What I'm trying to help alleviate is the wild temp swings that I see during max insolation as the heat eddies across the desert floor. With a 60CFM fan, those are sucked up like nobody's business. I have a 40CFM backup, but am not inclined to swap them at this time. When the stock filter starts to "dirty up", maybe I'll be more satisfied. Yep, dirtier may be better...well at least for me.
Have you compared the graphs between your station and KTUS? If you’re not aware, you can get the raw 1 minute historical ASOS readings from here.
Yes I have, and I'm aware. Thank you.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Intheswamp on March 26, 2021, 07:38:02 PM
I hereby proclaim myself as "Procrastinator In Chief".  On April 24, 2016 I bought a Sensiron SHT31 from Ryan Wilhour and this afternoon I installed it!  :D  Not that I would take my own sweet time or anything.  It's nice to not be so dry around here...it had got down to 1% humidity and dewpoint down in the minus-twenties...really hazardous climate around here!!!  :lol: :lol:

Anyhow, it's "kind of" a dangerous procedure.  Here is a (semi) before and after picture...
(https://live.staticflickr.com/65535/51005461425_4d7fa7fe30_o.jpg) (https://flic.kr/p/2kHbjFk)IMG_0928a (Custom) (https://flic.kr/p/2kHbjFk) by Ed Welch (https://www.flickr.com/photos/71532701@N07/), on Flickr

Installed and ready to be remounted (a little cleanup job, too)...
(https://live.staticflickr.com/65535/51073945193_6cc8e7666d_o.jpg) (https://flic.kr/p/2kPejvX)IMG_0931a (Custom) (https://flic.kr/p/2kPejvX) by Ed Welch (https://www.flickr.com/photos/71532701@N07/), on Flickr

My gardening buddy might stop telling me I live in Death Valley now!!!!

It's interesting, but my anemometer stopped working and I had to do a little trouble-shooting.  What I found was something had bit(?) the anemometer cable.  I cut that section out and spliced the pieces back together and all is well with the anemometer, too!  But, fixing that got me on a roll so in between rains today I got the sensor replaced, too!  [tup]  Lazy that I am....  :grin: :grin:
(https://live.staticflickr.com/65535/51074635806_3152c00dc7_o.jpg) (https://flic.kr/p/2kPhRP5)IMG_2126a (Custom) (https://flic.kr/p/2kPhRP5) by Ed Welch (https://www.flickr.com/photos/71532701@N07/), on Flickr
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on March 26, 2021, 09:06:22 PM
Jeez dude, 'bout time. 1% in Alabama? :-P That said, did you apply the -0.9F temp offset?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Intheswamp on March 26, 2021, 10:35:00 PM
<chuckle> Well, I knew I'd get around to it sooner.............................................................or later!  :lol:

Umm, besides taking my own sweet time I've skipped a few classes along the way.  The, uh, -0.9F temp offset? :-s
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on March 26, 2021, 11:00:45 PM
Your VP2 came with an SHT11, you replaced it with a 31, same as me and many others here. Since the protocol is different with the PCB, you'll need to subtract 0.9F from your console to correct for that fact.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Intheswamp on March 27, 2021, 07:59:43 AM
Thanks!  I'll get'er done!!!!!!!!!!! 

Now to go scrub the moss out of the rain bucket....  :roll:  I will say that the new(er) bucket with the bird spikes have made a BIG difference in debris in the bucket...I mean, basically ZERO debris!   =D&gt;
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Intheswamp on March 27, 2021, 10:31:15 AM
It took me a couple of tries but I got the -0.9 offset keyed in.

I was interested in seeing changes in the humidity reading.  We had off-and-on rains yesterday so the humidity sensor was maxing out at around 95% after getting things put back together.  This morning was heavy fog with a saturated ground, the reading peaked at 98%.  Now it is finally dropping a bit...down to 92% at the moment.  I don't have to have perfection (ya think? :lol:) but it's good to have it back in a believable ballpark!  [tup]
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Intheswamp on March 27, 2021, 04:47:38 PM
I think I've got a new station!  :lol:

How long do most people get out of the SHT-31?  It looks like the original SHT-11 gave me around three years of service here in humid south Alabama.  Can I expect the SHT-31 give about the same service?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on March 27, 2021, 04:55:23 PM
Don't know whether you've followed this very thread or not, but it's basically about how the Davis derived SHT31 can develop a wet bias after being subjected to a lot of high humidity. Since your sensor is literally the first batch released, this is something you may want to watch for. Whether Davis has addressed this issue or not is debatable, but it seems that it may have been. I'm in a very dry climate, so I'm not the best at judging this.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Intheswamp on March 27, 2021, 05:04:47 PM
Yes, I followed along with some of it...lots of it over my head but I did get the drift of water-logged sensors.  I wonder whether the finer screen on the cover might help some...and that's probably been discussed in the thread.  I'm gonna go with it lasting at least as long as the OE one.  I was needing to order a VP2 solar cover from Scaled Instruments and was going to order a backup sensor while I was at it.  But, I think I'll skip the sensor and see what happens.  No quicker than I got then one installed it can sit on the shelf in Florida as well as in Alabama.  :grin:  The solar cover is showing it's age, though.  Part of the locking clip broke a couple or three years ago and the screen protector is all fogged and cracking up...but it keeps working and it stays dry inside.  I may wait a little longer on it, too.  Glad to have the station back at 100%...I just need to get a weather-cam up again.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on March 27, 2021, 05:19:31 PM
I know others probably think I'm full of it, but I swear by the SF2 cap. Is it an end-all-be-all solution, don't know, but I'll never go without it again.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Intheswamp on March 27, 2021, 08:09:49 PM
I'll see how it goes with the new cap, afterall...it's been 1% humidity around here for quiet a while...anything higher is improvement!! :lol:

I did save the old sensor and cap...something maybe to tinker with later...  [tup]
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on March 27, 2021, 08:24:53 PM
I'll see how it goes with the new cap
Understood, however, the stock filter will do nothing to alleviate any potential bias, as it's merely still just that...the stock filter. The SF2 isn't and performs a much more protective roll.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Intheswamp on March 27, 2021, 09:00:49 PM
It looks like it requires modification to the cap?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on March 27, 2021, 09:06:40 PM
This was reply #1636 I made a couple of weeks ago. You may want to read further after that.

"I wasn't thrilled about the glue option so I trimmed the four feet down to be flush with the base and used very thin stainless steel wire to wrap around the cap and cinch it down with pliers twisting from the back side. The wire doesn't come in contact with the membrane but arches over. With the "gooey" substance that surrounds the sensor it allows the cap to seat firmly in what I believe is a true seal."
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Intheswamp on March 27, 2021, 09:55:01 PM
Ok, I get it.  I may go that route.  Thanks.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on May 15, 2021, 11:14:49 AM
Aight gang… Davis may have fixed their humidity sensor issue. I ordered one from Ryan the other day for one of my stations and the sensor is performing well so far. Ryan says he just recently replaced his humidity sensor and it’s working well for him too. Anybody else having the same result?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on May 15, 2021, 02:02:03 PM
I recently purchased two new sensors and they have been working just fine, both hit 100%  and checked out just fine on low end. No sign so far of wet bias that i have seen.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on May 15, 2021, 02:17:28 PM
I recently purchased two new sensors and they have been working just fine, both hit 100%  and checked out just fine on low end. No sign so far of wet bias that i have seen.

Awesome! Planning on getting the one with the 25ft cable for the Barani shield. Hopefully it will perform the same. How much board did you have to cut off? I think with my setup I don’t have to worry about removing all the stuff that protects the wiring.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on May 15, 2021, 03:28:25 PM
I always remove as much board as i can to increase the response time of the sensor as fast as possible and i am using the sf2 filter too. I am running the newest sensor to get some good in warranty time before cutting the board down.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on May 15, 2021, 10:09:47 PM
Have you guys noticed any particular modification to the sensor or the potting, or does it just seem to perform better?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on May 16, 2021, 09:17:47 AM
No real changes that i can see, still the outdated large mounting board and filter cover and over use of the potting material. Could be an unseen thing like better handling of the mounting of the sensor and storage or even some changes at the factory but they seem to work better than in the past. It could be just got a good batch for a change. My oldest new sensor has been through several periods of over 90% humidity for days on end and when it did finally dry out the humidity did not show the hang up in the low 80% range that the other ones did, just a nice even drop as the humidity went down.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jerryg on May 18, 2021, 12:25:44 PM
This is the size i cut the sht31 down to compared to a nickel [ You are not allowed to view attachments ]
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on May 18, 2021, 12:29:12 PM
This is the size i cut the sht31 down to compared to a nickel [ You are not allowed to view attachments ]

Gotcha. Thanks!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on August 11, 2021, 12:42:11 AM
I’m interested to hear over time what people have resolved with the SHT31 issues. I know some have said they’ve seen an improvement in the newer Davis 31s. For those folks, do you still see improved performance as it relates to a wet bias and being able to reach 100% humidity in saturation? Have folks just accepted that the issues with the Davis 31 are what they are? Is anyone still using the SHT75 instead? I’m still using the SHT75 and periodically compare my Davis 31s (after baking the sensors etc) and still ultimately run into the same problems. I guess I’m wondering if the lack of activity in this thread over time is more a function of acceptance or a function of sensor improvement that jgentry and jerryg have noted.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on August 11, 2021, 12:50:35 AM
I’m interested to hear over time what people have resolved with the SHT31 issues. I know some have said they’ve seen an improvement in the newer Davis 31s. For those folks, do you still see improved performance as it relates to a wet bias and being able to reach 100% humidity in saturation? Have folks just accepted that the issues with the Davis 31 are what they are? Is anyone still using the SHT75 instead? I’m still using the SHT75 and periodically compare my Davis 31s (after baking the sensors etc) and still ultimately run into the same problems. I guess I’m wondering if the lack of activity in this thread over time is more a function of acceptance or a function of sensor improvement that jgentry and jerryg have noted.

The new Davis SHT-31s finally showed the same ole wet bias issues. I thought they were resolved but I ended up being wrong. My SHT-31s did not reach to 100%. It only reached to 99%. I’m just forced to accept the errors. The WeatherFlow Tempest SHT-31 doesn’t have this problem except for one (no wet bias but started topping off at 95% after a few months of use). 

The way Davis handles and mounts the sensor, it doesn’t take long for it to be out of calibration.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on August 11, 2021, 12:55:34 AM
I’m interested to hear over time what people have resolved with the SHT31 issues. I know some have said they’ve seen an improvement in the newer Davis 31s. For those folks, do you still see improved performance as it relates to a wet bias and being able to reach 100% humidity in saturation? Have folks just accepted that the issues with the Davis 31 are what they are? Is anyone still using the SHT75 instead? I’m still using the SHT75 and periodically compare my Davis 31s (after baking the sensors etc) and still ultimately run into the same problems. I guess I’m wondering if the lack of activity in this thread over time is more a function of acceptance or a function of sensor improvement that jgentry and jerryg have noted.

The new Davis SHT-31s finally showed the same ole wet bias issues. I thought they were resolved but I ended up being wrong. My SHT-31s did not reach to 100%. It only reached to 99%. I’m just forced to accept the errors. The WeatherFlow Tempest SHT-31 doesn’t have this problem except for one (no wet bias but started topping off at 95% after a few months of use). 

The way Davis handles and mounts the sensor, it doesn’t take long for it to be out of calibration.

Thanks jgentry. I was afraid of that …
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on August 11, 2021, 12:57:11 AM
I’m interested to hear over time what people have resolved with the SHT31 issues. I know some have said they’ve seen an improvement in the newer Davis 31s. For those folks, do you still see improved performance as it relates to a wet bias and being able to reach 100% humidity in saturation? Have folks just accepted that the issues with the Davis 31 are what they are? Is anyone still using the SHT75 instead? I’m still using the SHT75 and periodically compare my Davis 31s (after baking the sensors etc) and still ultimately run into the same problems. I guess I’m wondering if the lack of activity in this thread over time is more a function of acceptance or a function of sensor improvement that jgentry and jerryg have noted.

The new Davis SHT-31s finally showed the same ole wet bias issues. I thought they were resolved but I ended up being wrong. My SHT-31s did not reach to 100%. It only reached to 99%. I’m just forced to accept the errors. The WeatherFlow Tempest SHT-31 doesn’t have this problem except for one (no wet bias but started topping off at 95% after a few months of use). 

The way Davis handles and mounts the sensor, it doesn’t take long for it to be out of calibration.

Thanks jgentry. I was afraid of that …

No problem.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 11, 2021, 03:38:26 AM
Well I have psychro-dyne now which is pretty accurate and a new SHT-31 I always purchase the SI 25 foot 31's due to my custom station setup and I'm finding the dew point is either right on or reading low 1 degree not high. I only use dew point chasing humidity around is a waste of time.  Anyway, the highest I've seen in DP is 71 and the lowest is around 48 so limited where I've checked but I no longer see those crazy high heat indices of old. Yes if you heavy aspirate it moves around a couple of degrees once the ambient gets above about 84 or so but if you don't heavily aspirate with a higher-powered AC fan the spiking movement is less.

Now yes it reads higher than the ASOS but it's correct where the ASOS is actually reading low humidity and actually is a POS itself they are force-feeding this warming by cranking the rural ASOS temperature up so it's hard to trust anything about it. 
This crap started around March of 2017 the local official coop station happens to be run by the radio station owner in Valentine too so everyone pretty much knows what's up with the Valentine KVTN ASOS. Just a short example without getting into full detail about being above freezing during icing events, the ASOS inability to reach full saturation so now the max humidity is 94% the AP ASOS has had 9-100+ days while all the other local Wx stations including the COOP-3 this summer and it goes on and on. Really rock-solid case they are tampering with the temperatures on these FAA weather stations likely inserting a multiplier because the hotter it gets the wider the spread from other thermometers.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Tensor on August 11, 2021, 06:12:31 AM
I have been using SHT85 for over a year now. Its accuracy is much better than that of a SHT21 (which has been outdoors for 5 years now). SHT21 has always had a small but not bad wet bias, and the bias has become gradually worse. I also tested HYT221 some time ago and the sensor turned out to be really poor. It lost its calibration in rather mild conditions and has a dry bias of about 10-15%.

Since SHT4x series is coming from Sensirion, I will report here if I happen to buy and test those.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on August 11, 2021, 07:42:10 AM
Well I have psychro-dyne now which is pretty accurate and a new SHT-31 I always purchase the SI 25 foot 31's due to my custom station setup and I'm finding the dew point is either right on or reading low 1 degree not high. I only use dew point chasing humidity around is a waste of time.  Anyway, the highest I've seen in DP is 71 and the lowest is around 48 so limited where I've checked but I no longer see those crazy high heat indices of old. Yes if you heavy aspirate it moves around a couple of degrees once the ambient gets above about 84 or so but if you don't heavily aspirate with a higher-powered AC fan the spiking movement is less.

Now yes it reads higher than the ASOS but it's correct where the ASOS is actually reading low humidity and actually is a POS itself they are force-feeding this warming by cranking the rural ASOS temperature up so it's hard to trust anything about it. 
This crap started around March of 2017 the local official coop station happens to be run by the radio station owner in Valentine too so everyone pretty much knows what's up with the Valentine KVTN ASOS. Just a short example without getting into full detail about being above freezing during icing events, the ASOS inability to reach full saturation so now the max humidity is 94% the AP ASOS has had 9-100+ days while all the other local Wx stations including the COOP-3 this summer and it goes on and on. Really rock-solid case they are tampering with the temperatures on these FAA weather stations likely inserting a multiplier because the hotter it gets the wider the spread from other thermometers.

That’s great you’re getting that kind of performance. I can’t think of a time I’ve had a Sensirion sensor run low on dewpoint compared to the psychro-dyne, let alone a Davis Sensirion sensor. Is that a raw reading from the sensor or are you running a software calibration?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 11, 2021, 08:38:46 AM
Raw and not just on 1 sensor I have two running.  Like I said I do the dewpoint calculation only. Make sure you get the exact temp as close to possible too. If it's 66.3 don't put 66.5 in the calculator.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on August 12, 2021, 01:03:41 AM
Well I have psychro-dyne now which is pretty accurate and a new SHT-31 I always purchase the SI 25 foot 31's due to my custom station setup and I'm finding the dew point is either right on or reading low 1 degree not high. I only use dew point chasing humidity around is a waste of time.  Anyway, the highest I've seen in DP is 71 and the lowest is around 48 so limited where I've checked but I no longer see those crazy high heat indices of old. Yes if you heavy aspirate it moves around a couple of degrees once the ambient gets above about 84 or so but if you don't heavily aspirate with a higher-powered AC fan the spiking movement is less.

Now yes it reads higher than the ASOS but it's correct where the ASOS is actually reading low humidity and actually is a POS itself they are force-feeding this warming by cranking the rural ASOS temperature up so it's hard to trust anything about it. 
This crap started around March of 2017 the local official coop station happens to be run by the radio station owner in Valentine too so everyone pretty much knows what's up with the Valentine KVTN ASOS. Just a short example without getting into full detail about being above freezing during icing events, the ASOS inability to reach full saturation so now the max humidity is 94% the AP ASOS has had 9-100+ days while all the other local Wx stations including the COOP-3 this summer and it goes on and on. Really rock-solid case they are tampering with the temperatures on these FAA weather stations likely inserting a multiplier because the hotter it gets the wider the spread from other thermometers.

I don’t have the psychrodyne right now but I was basically getting your results with the new SHT-31s until the latter part of July when finally the upper 70s to lower 80s DP airmass settled in. Then, the bias showed back up.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on August 12, 2021, 10:14:15 AM
I have both a psychro-dyne and an old Belfort 566 and read them to the nearest tenth of a degree as suggested. I’ve had Sensirion sensor raw dewpoint readings match the psychrometers, but I’ve never had them read low. I’ve also noticed that the psychrometers either match ASOS for dewpoint or read a bit high on dewpoint compared to ASOS when I’ve tested them co-located
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 12, 2021, 03:29:16 PM
I'm just saying what my experience is. Havn't had any high DP's above 71 and don't want any.   I did a check with the psychro-dyne but got distracted these were current snapshots at the time where everyone was at. . I'm not seeing anything but accuracy at the current DP, if anything it came in slightly low again. I attached a snapshot of the station I also run .67 mi east of here. The dewpoint is running lower but here in town over grass it matters. 

 [ You are not allowed to view attachments ]
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on August 12, 2021, 05:09:16 PM
I'm just saying what my experience is. Havn't had any high DP's above 71 and don't want any.   I did a check with the psychro-dyne but got distracted these were current snapshots at the time where everyone was at. . I'm not seeing anything but accuracy at the current DP, if anything it came in slightly low again. I attached a snapshot of the station I also run .67 mi east of here. The dewpoint is running lower but here in town over grass it matters. 

 [ You are not allowed to view attachments ]

That’s great news. Do you know how old the sensors are and how long you’ve been using them? Please let us know if you continue to get good performance. Maybe I’ll finally come around to spending another $50 trying to give the Davis 31 another chance …
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 12, 2021, 05:35:33 PM
I'm just saying what my experience is. Havn't had any high DP's above 71 and don't want any.   I did a check with the psychro-dyne but got distracted these were current snapshots at the time where everyone was at. . I'm not seeing anything but accuracy at the current DP, if anything it came in slightly low again. I attached a snapshot of the station I also run .67 mi east of here. The dewpoint is running lower but here in town over grass it matters. 

 [ You are not allowed to view attachments ]

That’s great news. Do you know how old the sensors are and how long you’ve been using them? Please let us know if you continue to get good performance. Maybe I’ll finally come around to spending another $50 trying to give the Davis 31 another chance …

They are new units only about a month in use, Ryan at Scaled Instruments may have them specially made, only because I haven't seen the 25' cable sold anywhere else.  I couldn't even find them on DI own site. All my stations are custom so I can't use the short cables that are getting shorter lately. The last one I tried I couldn't reach anywhere so had to do a splice.

I've purchased what he had in stock but sure he'll be getting more in. I've got 4, two are backups still in the bags. All the units I've got have the black rubber material I don't know if that matters some come in white. I'll send a pic with stock code number. 

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Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on August 12, 2021, 05:42:21 PM
I’m pretty sure the 210609QN is a manufacture/inspection date too, which means they are very recently coming from Davis.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 12, 2021, 06:00:11 PM
I don't know if the molding process has changed or maybe the material used or maybe nothing at all has changed other than I know Ryan uses low VOC packaging now. How Davis ships to him no clue.  I ordered two for replacements and noticed dew points were pretty dang good even at 70 degrees so I went ahead and ordered 2 more and that was what was left in stock.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: johnd on August 13, 2021, 03:54:46 AM
They are new units only about a month in use, Ryan at Scaled Instruments may have them specially made, only because I haven't seen the 25' cable sold anywhere else.  I couldn't even find them on DI own site. All my stations are custom so I can't use the short cables that are getting shorter lately. The last one I tried I couldn't reach anywhere so had to do a splice.

The T/H sensors with a 25ft cable are a standard Davis part (7346.221) and have been on the Davis spares list for a few years now. This is the sensor used in the 6830** T/H sensor with shield IIRC. Of course some online outlets only stock/sell the faster-moving Davis items, but any specialist Davis dealer worldwide should be able to source them. We certainly carry them for UK/EU customers.

** 6830 is the sensor introduced roughly along with Weatherlink Live and with the demise of the 6372/6382 packages. WLL can obviously receive data from any combination of up to 8 fully or partially populated ISS units and so rather than sell 6332 bundled with various other sensors as specific products, Davis just suggest that you buy 6332 (or 6331) plus whatever sensor you want. For T/H sensors a shield is obviously needed, so the relevant sensor parts are 6830 and 6832 (the FARS variant) .
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 13, 2021, 05:13:37 AM
The search doesn't show the part. I can't find any spares list either. The Davis website since the change a few years back is not user-friendly.  Can't even find a 25 foot anything using search. As far as I'm concerned SI was the only retailer in the US that carried the 25-foot sensor and still is.  If you know of another link it.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 13, 2021, 05:15:26 AM
 [ You are not allowed to view attachments ]
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: johnd on August 13, 2021, 05:29:14 AM
TBH Davis's own online listings for spare parts (which is what we're talking about with the .221 T/H sensor) is rather hit and miss and its Search isn't great either. Their Replacement Parts PDF is the definitive listing for readily available parts - not sure that Davis make this public but try a search online for PR67 Replacement Parts 2021 - someone may have uploaded it.  And also try a Google search for eg Davis 7346.221 and you'll see plenty of hits.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 13, 2021, 05:37:49 AM
TBH Davis's own online listings for spare parts (which is what we're talking about with the .221 T/H sensor) is rather hit and miss and its Search isn't great either. Their Replacement Parts PDF is the definitive listing for readily available parts - not sure that Davis make this public but try a search online for PR67 Replacement Parts 2021 - someone may have uploaded it.  And also try a Google search for eg Davis 7346.221 and you'll see plenty of hits.

I wondered about that. So they have a parts list they don't publish. The old website had every part they carried.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: johnd on August 13, 2021, 05:58:06 AM
I wondered about that. So they have a parts list they don't publish. The old website had every part they carried.

I don't know why they don't publish the Replacement Parts PDF - there's nothing obviously confidential about it - just a listing of all the standard spares and US list prices. But there are, give or take, around 200 parts on the list and to an extent you do need to know your way around the list to identify the right part. In the past, the Davis product line-up was much simpler so it was easier to make the website comprehensive. But now, especially with all the EM gateways, nodes and sensors the number of spares has grown considerably. I'm not sure how many casual users would welcome needing to plough through 200 or so options in an online shop to find the right part. But that's just IMO of course.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 13, 2021, 06:11:43 AM
 [tup] Agree, what's the big secret it can't be helping with sales. The only thing I can think of is like you said they keep adding parts and don't have an in-house website handler anymore.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: johnd on August 13, 2021, 06:50:47 AM
Actually, I see Ryan does have a downloadable PR67 copy on his website. There is a newer one available now with, I'm afraid, some price rises just announced, but which I haven't analysed in detail as yet, but eyeballing in the +5 to 7% band on average. So if you're considering a Davis station or part, now might be a good time to order before online prices increase. (Guessing September, but obviously dealer-dependent.)
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: drew1021 on August 14, 2021, 02:31:30 PM
Well I have psychro-dyne now which is pretty accurate and a new SHT-31 I always purchase the SI 25 foot 31's due to my custom station setup and I'm finding the dew point is either right on or reading low 1 degree not high. I only use dew point chasing humidity around is a waste of time.  Anyway, the highest I've seen in DP is 71 and the lowest is around 48 so limited where I've checked but I no longer see those crazy high heat indices of old. Yes if you heavy aspirate it moves around a couple of degrees once the ambient gets above about 84 or so but if you don't heavily aspirate with a higher-powered AC fan the spiking movement is less.

Now yes it reads higher than the ASOS but it's correct where the ASOS is actually reading low humidity and actually is a POS itself they are force-feeding this warming by cranking the rural ASOS temperature up so it's hard to trust anything about it. 
This crap started around March of 2017 the local official coop station happens to be run by the radio station owner in Valentine too so everyone pretty much knows what's up with the Valentine KVTN ASOS. Just a short example without getting into full detail about being above freezing during icing events, the ASOS inability to reach full saturation so now the max humidity is 94% the AP ASOS has had 9-100+ days while all the other local Wx stations including the COOP-3 this summer and it goes on and on. Really rock-solid case they are tampering with the temperatures on these FAA weather stations likely inserting a multiplier because the hotter it gets the wider the spread from other thermometers.

Totally agree with you regarding ASOS issue. The local site near me,KINT, has been reading higher temperatures than all surrounding ASOS and private stations going on 4 months now. Also their dew point is in my opinion way off, reading low. Max humidity I have seen over the past 4 months is 94%
I have contacted the NWS in Raleigh where their technicians are stationed and was told that they do a field spot check every so often and as long as it falls within 5 degrees it’s considered good. Bullshit I say!! Trust my VP2, you bet.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: ValentineWeather on August 14, 2021, 03:18:22 PM
Well I have psychro-dyne now which is pretty accurate and a new SHT-31 I always purchase the SI 25 foot 31's due to my custom station setup and I'm finding the dew point is either right on or reading low 1 degree not high. I only use dew point chasing humidity around is a waste of time.  Anyway, the highest I've seen in DP is 71 and the lowest is around 48 so limited where I've checked but I no longer see those crazy high heat indices of old. Yes if you heavy aspirate it moves around a couple of degrees once the ambient gets above about 84 or so but if you don't heavily aspirate with a higher-powered AC fan the spiking movement is less.

Now yes it reads higher than the ASOS but it's correct where the ASOS is actually reading low humidity and actually is a POS itself they are force-feeding this warming by cranking the rural ASOS temperature up so it's hard to trust anything about it. 
This crap started around March of 2017 the local official coop station happens to be run by the radio station owner in Valentine too so everyone pretty much knows what's up with the Valentine KVTN ASOS. Just a short example without getting into full detail about being above freezing during icing events, the ASOS inability to reach full saturation so now the max humidity is 94% the AP ASOS has had 9-100+ days while all the other local Wx stations including the COOP-3 this summer and it goes on and on. Really rock-solid case they are tampering with the temperatures on these FAA weather stations likely inserting a multiplier because the hotter it gets the wider the spread from other thermometers.

Totally agree with you regarding ASOS issue. The local site near me,KINT, has been reading higher temperatures than all surrounding ASOS and private stations going on 4 months now. Also their dew point is in my opinion way off, reading low. Max humidity I have seen over the past 4 months is 94%
I have contacted the NWS in Raleigh where their technicians are stationed and was told that they do a field spot check every so often and as long as it falls within 5 degrees it’s considered good. Bullshit I say!! Trust my VP2, you bet.

Looks like to me they are trying to tie up some loose ends when they were called out by critics rural areas weren't warming anywhere close to the same rate as cities and some areas actually lost ground. So now rural areas next to the climate classified ASOS stations get to live with these fake temperatures destroying our historic records.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on November 16, 2021, 10:15:56 PM
For years I have been very critical of Davis' use of the Sensirion SHT31 in this thread. I have been frustrated that I have been able to get considerably more accurate humidity and dew point results with a home-brew, older style SHT75. I've been convinced that there was something Davis was doing in their mounting of the SHT31 on it's board that somehow was causing a high humidity bias in addition to the high humidity bias of the Sensirion sensor itself. I want to pass along that after testing a new Davis SHT31, I observed a notable improvement in the performance of the Davis SHT31 that matches the results I get from the SHT75. This is the first time I have ever seen this kind of performance and I've similarly tested 6+ Davis SHT31s over the years.

After hearing from a few people on this forum, I purchased a new Davis SHT31 with a manufacture date in 2021 because they noticed what they thought was an improvement in the humidity/dew point performance of the sensor. The Davis SHT31 I bought has 25ft of cable. Out of the box, it actually read slightly LOWER in humidity and dew point than an SHT75 and SHT35 I have. Never have I observed this. Every other new Davis SHT31 I have ever bought typically ready 3-5% higher than these sensors at room temperature.

I started to use the new Davis sensor outside for a few weeks to let it acclimate and experience prolonged high humidity. Then I co-located my home-brew SHT75, inside the same radiation shield, and compared the readings for a series of days. Humidity during these days ranged from saturation to around 30%. The Davis SHT31 followed the performance of the SHT75 in a way I have not observed before. Previously, as humidity dropped from saturation, the SHT75 would read consistently lower, but with this new SHT31, it stayed right with the 75.

The sensor still suffers from the limitations inherent to the Sensirion sensor. It still is reluctant to reach 100% at saturation (although it has jumped to 100%) and it still has a wet bias in the mid and lower ranges in my climate. I use a slope and offset to help correct this bias and to match it to my Belfort Psychron.

I do not know if I am just lucky with this sensor or if Davis has changed something in their processes that improve performance. There is no obvious or even less than obvious modification to the sensor board that I can tell. We'll never know, but I'm going to be an optimist here and give Davis credit for improving their product. I noticed Sensirion published a document for manufacturers specifically addressing the dos and don'ts of mounting their sensors with conformal coatings as Davis does: https://www.sensirion.com/fileadmin/user_upload/customers/sensirion/Dokumente/2_Humidity_Sensors/Application_Note/Sensirion_AppNotes_Humidity_Sensors_Handling_Instructions.pdf.
I like to think that combined with feedback, maybe even from this forum and the people on it, Davis took action to improve a deficiency in their product.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on November 16, 2021, 11:01:38 PM
I like to think that combined with feedback, maybe even from this forum and the people on it, Davis took action to improve a deficiency in their product.
I have no doubt. If ValentineWeather hadn't discovered this flaw in the then new 31's, who knows if anyone of us (including Davis) would have since. I feel confident that Davis heard our squeaky wheel and oiled in a fix.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: drew1021 on November 18, 2021, 09:38:03 AM
So with this new information would it be best to order the 31 with 25’ cable? And did you order it from Davis directly?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on November 18, 2021, 09:55:24 AM
I got mine from Scaled Instruments. I wanted the 25' of cable and I thought I'd have a better chance at getting a more recently manufactured sensor since the 25' option is a relatively new option. I've heard great things about Mr. Wilhour at SI, so he may be able to provide information about the recency of his supply if the shorter cable is desired. If you do order the 25', I don't imagine you'd be getting a sensor older than mine ...
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: johnd on November 18, 2021, 09:57:09 AM
FWIW I'm not aware of any differences whatsoever between the sensor elements of the standard and the 25' parts. It's just a question of what cable length is fitted to the sensor PCBA.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: drew1021 on November 26, 2021, 01:38:09 PM
Well I was curious to see if there was any change so I purchased a 31 with the short cable from Ryan and checked the performance against my Belfort psychrometer and the results were disappointing. At room temperature I got the usual 5 to 6 percent difference to the high side. Temperature was .5 to the low side. I put in a offset to the humidity of -2% and a +.5 offset for the temperature. I’d rather have the readings to be accurate in the mid range than at saturation. I give up!
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on November 26, 2021, 01:41:02 PM
Well I was curious to see if there was any change so I purchased a 31 with the short cable from Ryan and checked the performance against my Belfort psychrometer and the results were disappointing. At room temperature I got the usual 5 to 6 percent difference to the high side. Temperature was .5 to the low side. I put in a offset to the humidity of -2% and a +.5 offset for the temperature. I’d rather have the readings to be accurate in the mid range than at saturation. I give up!

Probably means some of us got a good batch. Right now, all of mine have a wet bias.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on November 26, 2021, 09:19:07 PM
Well I was curious to see if there was any change so I purchased a 31 with the short cable from Ryan and checked the performance against my Belfort psychrometer and the results were disappointing. At room temperature I got the usual 5 to 6 percent difference to the high side. Temperature was .5 to the low side. I put in a offset to the humidity of -2% and a +.5 offset for the temperature. I’d rather have the readings to be accurate in the mid range than at saturation. I give up!

I’m sorry to hear that. I’ve been hopeful. If it’s accessible, can you provide the info or take a picture of the sticker on the back side of the sensor? Davis puts a manufacture/inspection sticker on it.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: drew1021 on November 26, 2021, 10:02:48 PM

I’m sorry to hear that. I’ve been hopeful. If it’s accessible, can you provide the info or take a picture of the sticker on the back side of the sensor? Davis puts a manufacture/inspection sticker on it.

Sorry to say I’ve already mounted it outside in the radiation shield
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: drew1021 on January 29, 2022, 08:25:19 AM
Original 31 purchased was returned. After watching the newest 31 I purchased for about 10 weeks now I have to say I’m impressed by the accuracy I’ve seen so far. Not only has this sensor on multiple occasions hit 100%, it also is performing much better in low dew points. Will have to see what happens when our Carolina summer weather takes hold. Also, I’m using the fs2 filter cap.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on January 29, 2022, 03:24:37 PM
Also, I’m using the fs2 filter cap.
=D&gt;
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: drew1021 on January 29, 2022, 05:14:29 PM
In full disclosure, I’ve been using the fs2 for a few years now
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on January 29, 2022, 05:46:46 PM
Oh, didn't realize that. I have as well. I figured all my preaching about it had fallen on deaf ears. I'm also using the stock mesh filter now (didn't before) as I think it helps with the over sensitivity from the strong fan and obviously keeps the SF2 much cleaner. The last SF2 was coated in dirt. Either way, the SF2 will always be in use for me.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: drew1021 on January 29, 2022, 08:16:18 PM
I'm also using the stock mesh filter now (didn't before) as I think it helps with the over sensitivity

So your using both sf2 and the stock filter cap?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on January 29, 2022, 08:24:19 PM
Yep.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Bashy on February 05, 2022, 08:02:34 AM
It looks like my UV doesnt go high enough or low enough when tracking with the 3  RAF bases nearby

https://weather.gladstonefamily.net/qchart/E9093?date=20220205&addnl=EGUL&addnl=EGUN&addnl=EGYM&Add+to+charts=Add+to+charts&.cgifields=addnl
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: William BAZ on February 07, 2022, 08:12:49 AM
Good morning guys, I'm reading the topic, and I had problems with the davis pro 2 and davis vue sensors, could you tell me if it's the same sensor in both? I just wanted to change the sensor chip without changing the whole part

  Sorry my english I'm from Brazil
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on June 14, 2022, 01:14:07 AM
Good morning guys, I'm reading the topic, and I had problems with the davis pro 2 and davis vue sensors, could you tell me if it's the same sensor in both? I just wanted to change the sensor chip without changing the whole part

  Sorry my english I'm from Brazil

It’s the same SHT-31 sensor.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on June 14, 2022, 01:15:46 AM
How is everyone’s experience with the Davis SHT-31 sensor now? Had to put a -5% offset for mine.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Jester on June 14, 2022, 12:41:52 PM
IMO it is still a piece of crap.  I would take a SHT-15 if I could find one.  Still having wet bias issues.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on June 14, 2022, 05:20:28 PM
IMO it is still a piece of crap.  I would take a SHT-15 if I could find one.  Still having wet bias issues.

The WF Tempest uses the SHT-31. Without any calibrations done by their CL system, it performs very well. Davis screws up the SHT-31 sensor.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: drew1021 on June 16, 2022, 05:53:45 AM
How is everyone’s experience with the Davis SHT-31 sensor now? Had to put a -5% offset for mine.

Still having mid range bias here. Would love to know what Davis is doing to the sensor to cause this.
On a positive note, the temperature is pretty much spot on.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on June 16, 2022, 11:43:25 AM
How is everyone’s experience with the Davis SHT-31 sensor now? Had to put a -5% offset for mine.

Still having mid range bias here. Would love to know what Davis is doing to the sensor to cause this.
On a positive note, the temperature is pretty much spot on.

Yep and why they choose not to fix the issue.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on December 05, 2022, 08:57:28 PM
After the last two days of rain, rare, heavy fog enveloped southern AZ last night. Didn't quite get to W0X0F (at least while I was awake), but very close. My SHT31 refused to get closer than 0.1F in temp/dew spread, resulting in 99% humidity for almost seven hours til 10 this morning. Not stating anything new, just wish it would pull that 100% trigger. This sensor is about two years old. Anyone getting consistent 100% out of newer ones?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on December 05, 2022, 10:35:13 PM
After the last two days of rain, rare, heavy fog enveloped southern AZ last night. Didn't quite get to W0X0F (at least while I was awake), but very close. My SHT31 refused to get closer than 0.1F in temp/dew spread, resulting in 99% humidity for almost seven hours til 10 this morning. Not stating anything new, just wish it would pull that 100% trigger. This sensor is about two years old. Anyone getting consistent 100% out of newer ones?

With the WeatherFlow Tempest SHT-31, I hit 100% easily. With the Davis, naw. But I haven’t used a brand new sensor. The last time I’ve texted Ryan, he told me Davis hasn’t changed anything so the new ones would probably perform about the same.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW2274 on December 05, 2022, 10:39:04 PM
Harrumph. That's what I figured. Thanks. 
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: drew1021 on December 06, 2022, 07:06:45 AM
Mine did but only for a short period after I installed new. Now 97% is the highest I’ve seen it since. Sensor is about a year old #-o
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: Mapantz on December 09, 2022, 05:45:31 PM
The SHT31-7346-070 has been bang on for me.

Now we've hit the damper end of the year, it will go to 100%. November several times, and December. Quite a few 99% days too.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on December 10, 2023, 07:24:36 PM
I just installed a brand new Davis VP2 station at Petals From The Past in Jemison, AL. So far the humidity sensor is performing well with no wet bias. 

You can see the comparison here with the nearby ASOS

Petals From The Past https://www.weatherlink.com/embeddablePage/show/66eab9495ce5496ba281d83872dcb2bf/summary



Shelby County Airport (ASOS) https://mesowest.utah.edu/cgi-bin/droman/meso_base_dyn.cgi?stn=KEET&unit=0&time=&hours=24&hour1=00&day1=11&month1=12&year1=2023&radius=25&past=0&order=1
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on December 11, 2023, 12:12:06 PM
That’s great to hear. Let us know how it does over time. I’ve been initially encouraged to only see the usual wet bias return. Seems it’s just a limit of the technology. Fortunately I have an old analog sensor 7346.029 that is really very accurate. It hits 100% easily and dries out very well. If anything it tends to go to saturation too easily.

I’ve written both customer service and tech support at Davis within the last month about a potential improved sensor. Unsurprisingly they would neither confirm or deny any potential new temp/humidity sensor product.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: jgentry on December 11, 2023, 12:29:10 PM
Would there be an improved sensor out on the market that Davis can use that wouldn’t cause a huge increase in the cost of the product?
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on December 11, 2023, 01:40:47 PM
That’s a great question I don’t have the answer to. I asked them in the same email if they had any information on the supplier of the analog humidity sensing element they used in the 7346.029 and they also declined to answer. Funny how sometimes the older technologies seem to work better than the new. I know there has been speculation that Davis could no longer get a supply of that part.

Sensirion is on to the SHT4X series now, but presumably the those sensors will suffer from the same shortcomings not to mention the communication protocol problem for Davis with I2C, which I presume will be the expensive aspect for them. Also, if you look at the Sensirion website, of all the applications they advertise using their humidity sensors for, weather observations/stations is not among them.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: JCA433 on December 11, 2023, 03:51:26 PM
CW, it is unfortunate Davis declines to answer your questions in your email.   The best solution right now is to calibrate the humidity sensor using the Cumulus software or other good calibration software. With Cumulus I can reduce the wet bias in humidity readings and get the dew point reading within +- 2F of my local reference.  It is not perfect though and there will still be error.


The trouble with the calibration is that my sensor reads humidity accurately between 75% and 93% and reads too low above 93% and too high below 75%. so my corrected humidity reading is too high between 75% and 93%.  It is quite good below 75% and above 93%.
Title: Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
Post by: CW7491 on December 11, 2023, 09:27:10 PM
Thanks, yes, that’s exactly what I would do when I had to use the Davis SHT31 sensor. I’d use offset and slope to deal with the inherent biases. It works fairly well, but it’s not a perfect fit. The trouble is in the midrange there is still a wet bias and when the humidity really drops, the correction can be excessive. I’d use 1.071x-6 and I found that to be about as good as you could do.

I found two of the older analog sensors and after some refurbishment they are superior to the SHT31. These sensors are almost 20 years old and had a dry bias. Davis would not help me when I asked if rinsing the humidity element in distilled water would bring it back into tolerance. I figured I had nothing to lose and tried it on one of my sensors. It brought it right back into spec. Amazing how well they work given how old they are. Unfortunately when these go, that’s it. Hopefully Davis has a better solution by then