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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
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 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.
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

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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.