Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 41030 times)

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Offline CW2274

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I think its more of a moisture problem than a dust problem.
Concur.

Offline CW2274

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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.

Offline ValentineWeather

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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.
Randy

Offline hwcorder

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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.

Offline Jim's Weather

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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.

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Offline CW2274

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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.

Offline jgentry

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

  

Offline jgentry

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The RW MK III specs are +/-2% from 10-90% and +/-4% from 90-100% if my memory serves me correctly.
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline CW2274

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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

Offline jerryg

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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.

Offline jerryg

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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.

Offline ValentineWeather

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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
Randy

Offline jgentry

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

  

Offline jgentry

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BTW: What’s your station’s ID on WU jerryg?
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline jerryg

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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.

Offline Jim's Weather

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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.
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Offline WheatonRon

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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?
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 jgentry

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

  

Offline CW2274

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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

Offline openvista

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

Offline CW2274

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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:

Offline openvista

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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/. 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.
« Last Edit: July 16, 2018, 07:03:26 PM by openvista »
Davis Vantage Pro2 AC FARS | https://marquetteweather.com | EW7933

Offline CW2274

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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/. 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.....

Offline openvista

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

Offline CW2274

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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.