Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 40967 times)

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Offline miraculon

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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

I ordered one of those Sensirion SHT-31 eval boards from Mouser.

Greg H.
 



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Offline openvista

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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.
« Last Edit: July 21, 2018, 12:18:50 PM by openvista »
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Offline jgentry

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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

I ordered one of those Sensirion SHT-31 eval boards from Mouser.

Greg H.

Hmm. Interesting. Let us know the results using the 31
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline ValentineWeather

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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.
« Last Edit: July 21, 2018, 01:27:11 PM by ValentineWeather »
Randy

Offline jgentry

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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?
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline openvista

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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.
Davis Vantage Pro2 AC FARS | https://marquetteweather.com | EW7933

Offline CW2274

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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.

Offline ValentineWeather

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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.

Randy

Offline jgentry

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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.
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline CW2274

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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

Offline Old Tele man

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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?$PDF$
• SYS: Davis VP2 Vue/WL-IP & Envoy8X/WL-USB;
• DBX2 & DBX1 Precision Digital Barographs
• CWOP: DW6988 - 2 miles NNE of Cortaro, AZ
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Offline ValentineWeather

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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... :-(
Randy

Offline ValentineWeather

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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?$PDF$

I have a sneaking suspicion they don't.  ;)
Randy

Offline CW2274

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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?$PDF$
Don't care.....

Offline CW2274

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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.

Offline openvista

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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.
Davis Vantage Pro2 AC FARS | https://marquetteweather.com | EW7933

Offline Old Tele man

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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.
« Last Edit: July 21, 2018, 07:26:50 PM by Old Tele man »
• SYS: Davis VP2 Vue/WL-IP & Envoy8X/WL-USB;
• DBX2 & DBX1 Precision Digital Barographs
• CWOP: DW6988 - 2 miles NNE of Cortaro, AZ
• WU - KAZTUCSO202, Countryside

Offline jgentry

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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.
« Last Edit: July 21, 2018, 07:10:35 PM by jgentry »
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline jgentry

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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

« Last Edit: July 21, 2018, 07:22:54 PM by jgentry »
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline Jim's Weather

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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.

« Last Edit: July 21, 2018, 09:35:58 PM by kcidwx »
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Offline WheatonRon

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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!
« Last Edit: July 21, 2018, 11:26:45 PM by WheatonRon »
Davis VP2 with SHT31 (3 complete VP2 systems—2 with a daytime fan and 1 that has a 24 hour fan); CWOP--CW5020, FW3075 and FW4350; WU--KILWHEAT17, KILWHEAT36 and KILWHEAT39; WeatherCloud.net; CoCoRaHS--IL-DP-132; and Weatherlink 2.0

Offline Old Tele man

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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."
• SYS: Davis VP2 Vue/WL-IP & Envoy8X/WL-USB;
• DBX2 & DBX1 Precision Digital Barographs
• CWOP: DW6988 - 2 miles NNE of Cortaro, AZ
• WU - KAZTUCSO202, Countryside

Offline jgentry

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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
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline openvista

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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.
Davis Vantage Pro2 AC FARS | https://marquetteweather.com | EW7933

Offline jgentry

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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.

Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358