Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 40335 times)

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Offline jgentry

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Tell ya what, this new 31 has my attention. Granted, I've only been between 5 and 37% since installing it, but the dew has been rock solid, so much so I took out my -2% correction factor from the old 31 and now run it at flat 0. Obviously still early, but really like what I'm seeing, at least on the dry end.

Nice! Can you tell any difference the way Davis mounted the sensor?
I had Ryan send me two, so I just compared the old and the other new one. With that, to my eye, absolutely no mounting difference. On the sensor side they look exactly identical, revision number and all. Only noticeable difference is on the back side with the attached white sticker, it's completely different, could just be a stock number for all I know. So if it is different, Davis is keeping it under the cuff.


Gotcha  Let me know how it performs in the long run.
I'm still tickled with it. Even though the monsoon is supposedly right around the corner, kinda wish one of you "wet" guys would give it another try. Not saying this sensor is a "new and improved version", but it's certainly performing better than my last one.
Who knows....

Let me know how the sensor performs when the monsoon hits. I’m assuming that it’s still working good right now.
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline CW2274

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Well, I can tell you this, it's been in service 2 weeks now, my dew point spread has been from 12 to 53F, the humidity from 5 to 40%. Still obviously on the dry side of things, but I'm very pleased with what I'm seeing and am still happily using a zero correction factor. This is a big deal as the dew point is showing a much more linear response and hopefully will continue as the humidity rises in the coming weeks. As I said before, it would be nice for one of you more humid folks to pick one up and give it a whirl. If you don't agree with my preliminary findings, send it back, mine's going nowhere... =D>

Offline CW2274

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Had a little MCS south of the border finally push some BL moisture our way, dew's been in the 50's to low 60's and this new 31 is still rockin' it. There is zero doubt in my mind this is an improvement over my last one. Now, whether that's coincidence or by design, I have no idea, but it works. If I'm lucky, I may see some actual rain today and will continue to hawk it, cause we're gonna dry right the bleep out again after today as the non-soon continues.

Offline jgentry

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Had a little MCS south of the border finally push some BL moisture our way, dew's been in the 50's to low 60's and this new 31 is still rockin' it. There is zero doubt in my mind this is an improvement over my last one. Now, whether that's coincidence or by design, I have no idea, but it works. If I'm lucky, I may see some actual rain today and will continue to hawk it, cause we're gonna dry right the bleep out again after today as the non-soon continues.

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

  

Offline CW2274

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Had a little MCS south of the border finally push some BL moisture our way, dew's been in the 50's to low 60's and this new 31 is still rockin' it. There is zero doubt in my mind this is an improvement over my last one. Now, whether that's coincidence or by design, I have no idea, but it works. If I'm lucky, I may see some actual rain today and will continue to hawk it, cause we're gonna dry right the bleep out again after today as the non-soon continues.

Nice!
I managed a whopping 0.04" of rain on Sunday with the dew in the mid-upper 60's for a while, briefly touched 71, still looking good. Need to see some much better/longer saturation though, I imagine that's what most are waiting to see me report, myself included...

Offline WheatonRon

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I guess I need to order a new SHT31 (already replaced it once because of unacceptable results) for my VP2 with the 24 hour fan. See attached screenshot from CWOP showing the data from my 3 Davis VP2s. The newest one and the unit that has the 24 hour fan is FW3075. My older units, which have the daytime fan together with the SHT31, are stations CW5020 and FW4350. The ISSs are located within 25 feet or so of each other, same height on the same fence. I verified that the fans in all three VP2s are working—receiving a lot of sun today! Humidity looks reasonable on station FW3075 but not the temperature and three nearby airports confirm these results. How disappointing to say the least.

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« Last Edit: July 13, 2019, 06:06:18 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

Online galfert

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If you are replacing a sensor it would be informative to know how long your sensor lasted.
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Offline ValentineWeather

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I guess I need to order a new SHT31 (already replaced it once because of unacceptable results) for my VP2 with the 24 hour fan. See attached screenshot from CWOP showing the data from my 3 Davis VP2s. The newest one and the unit that has the 24 hour fan is FW3075. My older units, which have the daytime fan together with the SHT31, are stations CW5020 and FW4350. The ISSs are located within 25 feet or so of each other, same height on the same fence. I verified that the fans in all three VP2s are working—receiving a lot of sun today! Humidity looks reasonable on station FW3075 but not the temperature and three nearby airports confirm these results. How disappointing to say the least.

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Remember humidity is relative to temperature is why the hot station had the lower %. What are the dewpoints running on the 3 stations?
Did you do the -.9F if using an older ISS and the 31? Still doesn't explain the large temperature difference. Almost like something is not working on the 24hr fars. I would swap sensor with one of the Dfars units and see if it still reads that high.
Randy

Offline CW2274

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I guess I need to order a new SHT31 (already replaced it once because of unacceptable results) for my VP2 with the 24 hour fan. See attached screenshot from CWOP showing the data from my 3 Davis VP2s. The newest one and the unit that has the 24 hour fan is FW3075. My older units, which have the daytime fan together with the SHT31, are stations CW5020 and FW4350. The ISSs are located within 25 feet or so of each other, same height on the same fence. I verified that the fans in all three VP2s are working—receiving a lot of sun today! Humidity looks reasonable on station FW3075 but not the temperature and three nearby airports confirm these results. How disappointing to say the least.

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What are the dewpoints running on the 3 stations?

Yes. The only time knowing the humidity that's worth a damn is 100%.

Offline CW2274

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I whipped out my handy-dandy calculator http://www.dpcalc.org/ and this is what it is:
94F/54%=75F dew
93/50=72
98/43=72
Now what??

Offline ValentineWeather

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Humidity looks good so the hot sensor issue I lean toward shield location or fan issue. If this was a replacement sensor for the same reason especially because I've never got a new 31 out of calibration with temperature before. Guess it could happen but 2 in a row? 
Randy

Offline CW2274

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Humidity looks good so the hot sensor issue I lean toward shield location or fan issue.
Hey Ron, don't laugh, but I recall not too long ago that some stock fans were running backwards. Sure yours is sucking from the bottom and not blowing?

Offline WheatonRon

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Humidity looks good so the hot sensor issue I lean toward shield location or fan issue.
Hey Ron, don't laugh, but I recall not too long ago that some stock fans were running backwards. Sure yours is sucking from the bottom and not blowing?

I remember reading in this Forum that some Davis units were blowing rather than sucking but mine is a sucker.   [tup] Dew points were 71.6 on stations CW5020 and FW3075, but was 74.8 on station FW4350 at the time the temperatures shown previously were recorded (4:10 pm CDT). CW2274–our dew point numbers (rounded) agree—but I guess they should as I don’t believe uploads to CWOP include dew points so MesoWest, my data source, used your formula!

I have not used any offsets on my consoles as the console firmware is sufficiently current as well as communicating with up-to-date PCBAs per Davis guidelines.

Maybe I should find another retirement hobby as I am confused but am open to suggestions on what might be going on. Probably the safest answer for my station (FW3075) that has the 24 hour fan model (Davis part no. 6153) is to replace the SHT31-again, the fan and batteries and call it a day. Sending about $100 to Scaled Instruments for parts to “fix” a station that is 14 months old, seems a bit drastic, but it is easier to do this all at once than to cherry pick replacing the parts one-by-one. If I was 100% convinced the new spoon tipper was worth $50, I might replace that part too.
« Last Edit: July 15, 2019, 09:00:03 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

Online galfert

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Offline Tensor

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I made a small test and compared 4 years old SHT21 against 1.5 years old SHT31. Both have been outside for one year, but now they are indoors (they were not outside at the same time period).
Both sensors have been exposed to temperature ranges from -30 °C to +30 °C and very high humidities.

The results of the test are quite clear. Both sensors have excellent temperature measurement precision and accuracy. SHT31 suffers from notable dry bias, which practically renders SHT31 useless for humidity measuring. Despite from being older, SHT21 has only slight wet bias, as Sensirion SHTxx sensors often have. SHT21's humidity readings are pretty close a calibrated traditional hair hygrometer. In the other words, the real humidity level was at the test within 52-54 %.

SHT31:
Humidity: 43.35 %, Temperature: +24.70 °C

SHT21:
Humidity: 55.18 %, Temperature: +24.68 °C

When the SHT31 sensor was last time outdoors, it barely climbed above 90 % even at the most humid days. It followed very well the nearby station' humidity levels in the beginning, but after 6 months it lost completely its accuracy. Setting an empirical calibration offset could be an option, but the readings depend nonlinearly from the real humidity adn temperature. During cold weather, the dry bias seemed to be even worse.

Does anyone have experience from HYT 221? The only piece of information from Arduino forum claims that it's better than SHTs. I am considering buying one or two of HYTs for further tests.

« Last Edit: July 19, 2019, 08:33:13 AM by Tensor »

Online galfert

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Does anyone have experience from HYT 221? The only piece of information from Arduino forum claims that it's better than SHTs. I am considering buying one or two of HYTs for further tests.

You can't use the HYT 221 because it is I2C protocol. The Davis ISS is not compatible with I2C sensors. The Davis ISS uses LSS (Legacy Sensirion Sensibus) protocol. Which is why you can't use the SHT35/SHT85 since they only come with I2C protocol support. There are different versions of previous SHTxx to respectively support either LSS or I2C. Meaning you can't use just any SHT31 for example.

Interesting that you find that the SHT31 has a dry bias, as some others have reported a wet bias.
« Last Edit: July 19, 2019, 09:09:41 AM by galfert »
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Offline Tensor

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You can't use the HYT 221 because it is I2C protocol. The Davis ISS is not compatible with I2C sensors. The Davis ISS uses LSS (Legacy Sensirion Sensibus) protocol. Which is why you can't use the SHT35/SHT85 since they only come with I2C protocol support. There are different versions of previous SHTxx to respectively support either LSS or I2C. Meaning you can't use just any SHT31 for example.

Interesting that you find that the SHT31 has a dry bias, as some others have reported a wet bias.
I didn't express this in my previous post, but I use Arduino and use it with sensors directly, since I don't have a Davis. This thread is just the only one in the internet, which deals with SHT31 aging and bias problems. I hope my observations are helpful. I will tell how HYT 221 works, if I happen to buy a couple of them.

Offline jgentry

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You can't use the HYT 221 because it is I2C protocol. The Davis ISS is not compatible with I2C sensors. The Davis ISS uses LSS (Legacy Sensirion Sensibus) protocol. Which is why you can't use the SHT35/SHT85 since they only come with I2C protocol support. There are different versions of previous SHTxx to respectively support either LSS or I2C. Meaning you can't use just any SHT31 for example.

Interesting that you find that the SHT31 has a dry bias, as some others have reported a wet bias.
I didn't express this in my previous post, but I use Arduino and use it with sensors directly, since I don't have a Davis. This thread is just the only one in the internet, which deals with SHT31 aging and bias problems. I hope my observations are helpful. I will tell how HYT 221 works, if I happen to buy a couple of them.

Please get them and let us know. It’s a sensor that I recommended Davis to consider switching too. Would love to see how the HYT would match up with a nearby ASOS or a Psychro-Dyne
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline jgentry

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You can't use the HYT 221 because it is I2C protocol. The Davis ISS is not compatible with I2C sensors. The Davis ISS uses LSS (Legacy Sensirion Sensibus) protocol. Which is why you can't use the SHT35/SHT85 since they only come with I2C protocol support. There are different versions of previous SHTxx to respectively support either LSS or I2C. Meaning you can't use just any SHT31 for example.

Interesting that you find that the SHT31 has a dry bias, as some others have reported a wet bias.
I didn't express this in my previous post, but I use Arduino and use it with sensors directly, since I don't have a Davis. This thread is just the only one in the internet, which deals with SHT31 aging and bias problems. I hope my observations are helpful. I will tell how HYT 221 works, if I happen to buy a couple of them.

Please get them and let us know. It’s a sensor that I recommended Davis to consider switching too. Would love to see how the HYT would match up with a nearby ASOS or a Psychro-Dyne

A Kestrel DROP sensor would also be another good standard to compare too
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline CW2274

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Nice easterly wave moving through late tonight and well into tomorrow with lots of PWAT, so heavy rainers are likely. This should finally be a good saturation test (I hope) for this new 31. My impression to this point hasn't changed, it's doing a very nice job and what saturation I have had (up to 93% and mid 70's dew), it has recovered just fine.   

Offline jgentry

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Nice easterly wave moving through late tonight and well into tomorrow with lots of PWAT, so heavy rainers are likely. This should finally be a good saturation test (I hope) for this new 31. My impression to this point hasn't changed, it's doing a very nice job and what saturation I have had (up to 93% and mid 70's dew), it has recovered just fine.


Good deal!  Keep me updated!
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

 

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