Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 31606 times)

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Offline jgentry

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Im 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!
« Last Edit: July 10, 2018, 09:25:00 AM by jgentry »
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Offline miraculon

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Re: Which sling psychrometer should I buy?
« Reply #1 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

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

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Online ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #2 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 (+/-) 1F.

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
Randy

Offline DaleReid

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Re: Which sling psychrometer should I buy?
« Reply #3 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
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Offline miraculon

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Re: Which sling psychrometer should I buy?
« Reply #4 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

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.


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Offline DaleReid

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Re: Which sling psychrometer should I buy?
« Reply #5 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.

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Online ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #6 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.
« Last Edit: July 04, 2018, 11:00:32 AM by ValentineWeather »
Randy

Offline klschmidt

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Re: Which sling psychrometer should I buy?
« Reply #7 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.

Offline jgentry

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Re: Which sling psychrometer should I buy?
« Reply #8 on: July 04, 2018, 05:11:08 PM »
Im 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
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline klschmidt

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Re: Which sling psychrometer should I buy?
« Reply #9 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.

Offline DaleReid

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Re: Which sling psychrometer should I buy?
« Reply #10 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
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Offline DaleReid

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Re: Which sling psychrometer should I buy?
« Reply #11 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
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Offline miraculon

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Re: Which sling psychrometer should I buy?
« Reply #12 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.



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Online ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #13 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.
Randy

Offline klschmidt

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Re: Which sling psychrometer should I buy?
« Reply #14 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.

Offline klschmidt

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Re: Which sling psychrometer should I buy?
« Reply #15 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.

Online ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #16 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/
Randy

Offline jgentry

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Re: Which sling psychrometer should I buy?
« Reply #17 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
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Online ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #18 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.
Randy

Offline AF6GL

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Re: Which sling psychrometer should I buy?
« Reply #19 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


Offline AF6GL

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Re: Which sling psychrometer should I buy?
« Reply #20 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.

Online ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #21 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 3F 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.
Randy

Offline jgentry

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Re: Which sling psychrometer should I buy?
« Reply #22 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 3F 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 doesnt reach to 100% like it should. I guess its safe to say that the SHT-31 doesnt 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?

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Offline CW2274

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Re: Which sling psychrometer should I buy?
« Reply #23 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.

Online ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #24 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.
Randy

 

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