Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 54737 times)

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Offline andro700

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I was thinking about springing for it anyways. Since it has much better specs than the one I have now.

Thanks,
Chuck

Offline jgentry

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Noticed that the Davis EnviroMonitor can take I2C sensors

https://www.davisinstruments.com/product_documents/weather/EM-sensors.pdf
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

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Offline jerryg

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Update on new 31, we had three days of very humid weather. The first day the lowest humidity was 86% and went up to 95%, day two stayed in the low to middle 90's all day and the third day the humidity was in the upper 90's all day and night. The sun came out today and dry air moved in and the 31 had no wet bias at all had a nice smooth drop in humidity and tracked the other two stations with no problems seen. So first long period of humidity was a winner. Now just have to see if anything shows up in the long run.

Offline jerryg

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Another update sensor still working fine, had dense fog this am and sensor hit solid 99% and no sign of wet bias so far.

Offline CW2274

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Another update sensor still working fine, had dense fog this am and sensor hit solid 99% and no sign of wet bias so far.
This one you threw in the Easy Bake Oven, right?

Offline jerryg

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Yes it is, baking made it work much better.

Offline CW2274

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baking made it work much better.
I just use my climate. ;)

Offline andro700

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I ordered a new fan and batteries and SHT-31 sensor. Got them installed and offset the temp -0.9. My 24 hour fan was not working at all. So going to see how temps and humidity react with new sensor and fan. Like I stated before I had humidity saturation. The date on the old temp/humidity sensor was 2009. The one I installed had the clear podding.

Chuck

Offline Tornado12

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Im new here on the forum, and am having to replace my humidity sensor in my VP2 as mine started reporting bad data. Ive tried to track through this long thread to get some info but its so long. Anyone care to give me a synopsis of the SHT-31 sensor? Is this a bad sensor? Good sensor? is the reliability poor? Im coming into this blind. Looking at my historical data it seems my snesor began going bad just 3 months or so after I bought my VP2 brand new. It is a 2017 unit so it came with the newer SHT-31 sensor. It reports humidty 10-20% too low compared to other stations. Today its reporting 73% and airport 20 miles away reporting 95% . Rain is looming today so very humid and cloudy. My new sensor arrives tomorrow and I will be installing it. Trying to gain some knowledge that may be helpful.

Offline ValentineWeather

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Im new here on the forum, and am having to replace my humidity sensor in my VP2 as mine started reporting bad data. Ive tried to track through this long thread to get some info but its so long. Anyone care to give me a synopsis of the SHT-31 sensor? Is this a bad sensor? Good sensor? is the reliability poor? Im coming into this blind. Looking at my historical data it seems my snesor began going bad just 3 months or so after I bought my VP2 brand new. It is a 2017 unit so it came with the newer SHT-31 sensor. It reports humidty 10-20% too low compared to other stations. Today its reporting 73% and airport 20 miles away reporting 95% . Rain is looming today so very humid and cloudy. My new sensor arrives tomorrow and I will be installing it. Trying to gain some knowledge that may be helpful.

Humidity is only relative to the temperature so saying your humidity is running low doesn't give a full picture of what's really going on. Where is the dewpoint in relation to the airport?  Like I said these SHT31 sensors run higher than airports is my experience so if anything they have a wet bias. This was the reason I was asking for a link to your station.
Randy

Offline Tornado12

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Im new here on the forum, and am having to replace my humidity sensor in my VP2 as mine started reporting bad data. Ive tried to track through this long thread to get some info but its so long. Anyone care to give me a synopsis of the SHT-31 sensor? Is this a bad sensor? Good sensor? is the reliability poor? Im coming into this blind. Looking at my historical data it seems my snesor began going bad just 3 months or so after I bought my VP2 brand new. It is a 2017 unit so it came with the newer SHT-31 sensor. It reports humidty 10-20% too low compared to other stations. Today its reporting 73% and airport 20 miles away reporting 95% . Rain is looming today so very humid and cloudy. My new sensor arrives tomorrow and I will be installing it. Trying to gain some knowledge that may be helpful.

Humidity is only relative to the temperature so saying your humidity is running low doesn't give a full picture of what's really going on. Where is the dewpoint in relation to the airport?  Like I said these SHT31 sensors run higher than airports is my experience so if anything they have a wet bias. This was the reason I was asking for a link to your station.

As of right now.

Airport:
TEMP: 61F
Dew Point: 56
Humidity: 84%
Pressure: 30.12

Ambient Weather WS-2902 3 miles from my station:
TEMP: 64F
Dew Point: 59
Humidity: 85%
Pressure: 30.17

My Station:
TEMP: 64F
Dew Point: 51
Humidity: 64%
Pressure: 30.15


The airport is ~20 miles north of me, and a cold front is approaching with lots of rain. The temperature difference is accurate and is reflected by all the weather stations in a 60 mile radius. The humidity im reporting (64%) is a lot lower than everyone right now. Dew Point obviously a little off as well. So dew point/humidty are innacurate. My temperature however is very accurate and has had no issues from what I can tell.

« Last Edit: February 20, 2020, 11:25:21 AM by Tornado12 »

Offline ValentineWeather

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Im new here on the forum, and am having to replace my humidity sensor in my VP2 as mine started reporting bad data. Ive tried to track through this long thread to get some info but its so long. Anyone care to give me a synopsis of the SHT-31 sensor? Is this a bad sensor? Good sensor? is the reliability poor? Im coming into this blind. Looking at my historical data it seems my snesor began going bad just 3 months or so after I bought my VP2 brand new. It is a 2017 unit so it came with the newer SHT-31 sensor. It reports humidty 10-20% too low compared to other stations. Today its reporting 73% and airport 20 miles away reporting 95% . Rain is looming today so very humid and cloudy. My new sensor arrives tomorrow and I will be installing it. Trying to gain some knowledge that may be helpful.

Humidity is only relative to the temperature so saying your humidity is running low doesn't give a full picture of what's really going on. Where is the dewpoint in relation to the airport?  Like I said these SHT31 sensors run higher than airports is my experience so if anything they have a wet bias. This was the reason I was asking for a link to your station.

As of right now.

Airport:
TEMP: 61F
Dew Point: 56
Humidity: 84%
Pressure: 30.12

Ambient Weather WS-2902 3 miles from my station:
TEMP: 64F
Dew Point: 59
Humidity: 85%
Pressure: 30.17

My Station:
TEMP: 64F
Dew Point: 51
Humidity: 64%
Pressure: 30.15


The airport is ~20 miles north of me, and a cold front is approaching with lots of rain. The temperature difference is accurate and is reflected by all the weather stations in a 60 mile radius. The humidity im reporting (64%) is a lot lower than everyone right now. Dew Point obviously a little off as well. So dew point/humidty are innacurate. My temperature however is very accurate and has had no issues from what I can tell.

With a cold front approaching hard to tell from a snapshot what's going on. You can remove the ambient from the picture. That would be my last choice of a reference station.  Need a station link?
Randy

Offline jgentry

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Even though a baked SHT-31 works better than a one that isnít baked, it seems to me that the 15 reads drier at the lower end of the humidity range compared to the 31. Any others noticed this?
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline jerryg

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Well we just got through with a strong high pressure system behind a cold front and the humidity of my 31 and my 15 both hit 18% which is very low for me down here and the new 31 has been doing real well from bottom to top on readings and still no sign of wet bias.  \:D/

Offline jgentry

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Well we just got through with a strong high pressure system behind a cold front and the humidity of my 31 and my 15 both hit 18% which is very low for me down here and the new 31 has been doing real well from bottom to top on readings and still no sign of wet bias.  \:D/

Itís been a wet winter here in Alabama. 15-16Ē in Central Alabama for this month. The 31s may have gain a wet bias but I donít know for certain.
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline jgentry

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Well we just got through with a strong high pressure system behind a cold front and the humidity of my 31 and my 15 both hit 18% which is very low for me down here and the new 31 has been doing real well from bottom to top on readings and still no sign of wet bias.  \:D/

But it is still performing well on the high end.
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline jerryg

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Yes it is, max has been 99% in dense fog and has matched real well with my other sensors.

Offline jgentry

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Iím going to have to rebake my 31 sensor. Even though itís performing well on the high end, itís showing a wet bias in the mid range. I didnít put any filter over my 31 so Iím sure that led to the wet bias issue. But Iím beginning to think that the Sensirion filter keeps the sensor from having a wet bias longer than the silk ďfilter.Ē
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Offline CW2274

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But Iím beginning to think that the Sensirion filter keeps the sensor from having a wet bias longer than the silk ďfilter.Ē
As Sensirion states, the SF2 cap, if properly installed, is a water tight solution that "consists of a single piece of PBT and an integrated water vapor-permeable filter membrane." I doubt silk was designed as such.

Offline Tensor

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I have been comparing HYT221 and SHT21 for about 6 months now. Lately, HYT221 started to have significant wet bias, if the real humidity level is above 70%. This means that if the near official station and SHT21 think the humidity is about 80%, HYT221 can show that RH is more than 90%. However, if the humidity is less than 70%, both SHT21 and HYT221 have similar readings. Actually, HYT221 is very close to the official station, if the humidity is around 20%-40%, while SHT21 has a slight wet bias all the time.

It is worth of mentioning that it has not rained or there has not been fog/frost for weeks now. Both sensors are now installed in the same radiation shield and HYT221 has no more a DIY plastic splash cover (but of course it has its own yellowish filter). I have no idea why HYT221 has started to act like this. In the winter, it performed very well. Also, HYT221 has +0.4 Celsius warm bias at cold temperatures (if outdoor temperature is less than 0 Celsius) compared to the SHT21. I trust SHT21 temperatures more, since I have other thermometers as well and they agree with SHT21.

In conclusion, SHT31 and HYT221 both failed within one winter. SHT31 has near constant -10% dry bias, HYT221 has +10% wet-bias at high humidity levels and SHT21 has not-so-bad wet-bias. I really consider buying a Vaisala sensor to stop this nightmare...
« Last Edit: March 26, 2020, 01:32:18 PM by Tensor »

Offline jgentry

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I have been comparing HYT221 and SHT21 for about 6 months now. Lately, HYT221 started to have significant wet bias, if the real humidity level is above 70%. This means that if the near official station and SHT21 think the humidity is about 80%, HYT221 can show that RH is more than 90%. However, if the humidity is less than 70%, both SHT21 and HYT221 have similar readings. Actually, HYT221 is very close to the official station, if the humidity is around 20%-40%, while SHT21 has a slight wet bias all the time.

It is worth of mentioning that it has not rained or there has not been fog/frost for weeks now. Both sensors are now installed in the same radiation shield and HYT221 has no more a DIY plastic splash cover (but of course it has its own yellowish filter). I have no idea why HYT221 has started to act like this. In the winter, it performed very well. Also, HYT221 has +0.4 Celsius warm bias at cold temperatures (if outdoor temperature is less than 0 Celsius) compared to the SHT21. I trust SHT21 temperatures more, since I have other thermometers as well and they agree with SHT21.

In conclusion, SHT31 and HYT221 both failed within one winter. SHT31 has near constant -10% dry bias, HYT221 has +10% wet-bias at high humidity levels and SHT21 has not-so-bad wet-bias. I really consider buying a Vaisala sensor to stop this nightmare...

Interesting. Wonder if the HYT sensor has gone bad? At least with the Sensirion sensors, you can bake them back into calibration if needed.
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline jgentry

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But Sensirion does make good temperature sensors (especially the 3x series like the 31).
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358