Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 44964 times)

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Offline CW2274

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It is working just fine right now and now have to see if it keeps on going.
Maybe.....just maybe....I was right. Let alone the obvious that I want this sensor to perform to humidity spec, I want some vindication as well because not a damn soul here has believed a word I've said. ](*,)
« Last Edit: October 20, 2019, 07:28:23 PM by CW2274 »

Offline jgentry

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Update, i just moved the 31 to my website so i can monitor it from anywhere. It is working just fine right now and now have to see if it keeps on going.

Seems like itís working good. Now the question becomes, does it develop a wet bias offset?
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline CW7491

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It is working just fine right now and now have to see if it keeps on going.
Maybe.....just maybe....I was right. Let alone the obvious that I want this sensor to perform to humidity spec, I want some vindication as well because not a damn soul here has believed a word I've said. ](*,)

Definitely not a belief or a trust thing, itís just that Iíve had a different result from the side by side comparisons Iíve done. But I will say this is definitely one of those circumstances where I look forward to be proven wrong. Hereís hoping Jerry sees something different ...


Offline CW2274

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It is working just fine right now and now have to see if it keeps on going.
Maybe.....just maybe....I was right. Let alone the obvious that I want this sensor to perform to humidity spec, I want some vindication as well because not a damn soul here has believed a word I've said. ](*,)

Definitely not a belief or a trust thing, itís just that Iíve had a different result from the side by side comparisons Iíve done. But I will say this is definitely one of those circumstances where I look forward to be proven wrong. Hereís hoping Jerry sees something different ...
All I have been wanting for several months is someone to step up and see if my findings are unique or that an actual "change" to the sensor was successfully accomplished. Obviously my word alone isn't good enough....and I get that, just gets tiring spouting my findings that are being dismissed because it's "just me" with no corroboration. Whether Jerry or anyone else agrees or doesn't agree with my findings, so be it, at least we know someone else gave it a shot. Regardless, mine's a keeper, and hopefully others will find this as well.

Offline jerryg

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Nobody wants it to work more than me, i really like the 31 response time to changes in humidity. I got three of them when they first came out and they worked really well for awhile but then one died and another did the old get up high in humidity and started reading backwards til it hit zero. The third one still works but tops out at 96% and on the bottom end reads about 5% high. The new one has worked real well and with the front moving through this morning the humidity has gone down to 43% and went real smooth from 95% so it is still new and may not got the bad stuff going yet or maybe it is going to work just fine. I know it works fine out of the package and now only time will tell. I am really waiting for the time of year when the fog and coastal trofs get going and the humidity stays on the high end for days at a time, that will be the real test.

Offline CW2274

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I am really waiting for the time of year when the fog and coastal trofs get going and the humidity stays on the high end for days at a time, that will be the real test.
No doubt. Mine spent almost all of the last two months with dew points in the 60's and the occasional 70's, but nothing like the "days on end" of wetness you guys get.
Thank you for trying.

Offline CW2274

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I know it works fine out of the package and now only time will tell.
I forgot to ask, did you use the SF2 cap on this new one? Like I said earlier, I put the cap on out-of-the-box for the first time with this 31. I think it may actually help with keeping the "saturation" from the sensor itself. I have no proof of this, only an "educated" speculation. If I had my druthers, I'd like to see two 31 compared together without a cap for a base line, then one with and the other remaining without to see if there's a difference worth noting.

Offline WheatonRon

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I have ordered a bunch of SF2 caps and since I have 3 Davis VP2s with the world-class (???) SHT31 sensor, I will try the SF2 caps on two of my stations and not the other and see what kind of results I get.
« Last Edit: October 21, 2019, 06:32:34 PM by WheatonRon »
Davis VP2 with SHT31 (3 complete VP2 systemsó2 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 CW2274

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I have ordered a bunch of SF2 caps and since I have 3 Davis VP2s with the world-class SHT sensor, I will try the SF2 caps on two of my stations and not the other and see what kind of results I get.
Thank you. Not that you still can't compare, but all your 31's are "older", are they not? What I'm getting at is perhaps this "protection" is necessary from the get-go so the sensor isn't permanently "spoiled".
BTW, PM will be forth coming.

Offline jerryg

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I used the wet towel to moisten up the sensor before putting it outside. I replaced the Davis filter material with one i cut out of the silk material which gives better air flow when not using a fan. I have and have used the sf2 on the ones that went bad, i hope to get some good use out of the sensor in comparison to the 75's i have been running. I will be happy if i can get a good year or so out of it before replacement is necessary. I am glad to see it working so well these first couple of days and got my fingers crossed for the long haul.

Offline CW2274

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I am glad to see it working so well these first couple of days and got my fingers crossed for the long haul.
Once again, not your climate, but my current 31 went into service exactly four months ago today.

Offline WheatonRon

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I have ordered a bunch of SF2 caps and since I have 3 Davis VP2s with the world-class SHT sensor, I will try the SF2 caps on two of my stations and not the other and see what kind of results I get.
Thank you. Not that you still can't compare, but all your 31's are "older", are they not? What I'm getting at is perhaps this "protection" is necessary from the get-go so the sensor isn't permanently "spoiled".
BTW, PM will be forth coming.

I have swapped out so many sensors with Ryan, the post man knows the issue and when I get a new one. Seriously, my latest one is probably 18 months old. Is that too old?
« Last Edit: October 21, 2019, 07:30:36 PM by WheatonRon »
Davis VP2 with SHT31 (3 complete VP2 systemsó2 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 CW2274

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I have ordered a bunch of SF2 caps and since I have 3 Davis VP2s with the world-class SHT sensor, I will try the SF2 caps on two of my stations and not the other and see what kind of results I get.
Thank you. Not that you still can't compare, but all your 31's are "older", are they not? What I'm getting at is perhaps this "protection" is necessary from the get-go so the sensor isn't permanently "spoiled".
BTW, PM will be forth coming.
my latest one is probably 18 months old. Is that too old?
I have no idea. All I know is I bought this latest one in June.

Offline CW7491

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Over the past week I've been comparing my primary SHT75 sensor with my new Davis 31. The Davis 31 has a manufacture date of June 2019. The sensors are co-located and the Davis 31 has been outdoors only for this week. The temperature of the two sensors is nearly identical, so my comparison is in RH.

The SHT75 has been in use outdoors for 6+ months. For most of the week, humidity has been high up near saturation as is typical this time of year in western Washington state. The Davis 31 has reached as high as 98%. The 75 will hit 100%. From about 87-88% and above, the 75 reads about 1-2% higher.

Yesterday we had an unusual down-sloping east wind off the Cascades and this dried out the air very quickly after prolonged days of high humidity and some fog. The humidity dropped from a high of 89% just after midnight to the upper teens with dewpoints in the single digits to around 0įF. Temperatures were in the upper 30s to mid 40s.

I don't quite have the elaborate set up as some of you, but I have data in Cumulus for both sensors every 10 minutes. The comparisons are based on the raw readings of both sensors. After analyzing the data, the Davis 31 read on average 3-4% higher than the SHT75 in the humidity range of the upper teens to the 30s. The range in difference was the Davis 31 reading from -1%RH to +8%RH, but +3% to +5% was typical. I did notice that over time in the dry air, the Davis 31 did seem to improve a bit reading only 2-3% higher on average. The lowest humidity both sensors reached was 21% for the SHT75 and 23% for the Davis 31. The highest humidity for the day for both sensors was 89% for the SHT75 and 88% for the Davis 31.

The SHT75 read consistently about 3-4% high in the same range compared to the Belfort 566 I use for the best true reading I can afford. What I report online has a humidity calibration in Cumulus of 1.061x-5 which matches well with the Belfort. So the Davis 31 is about +6% to +7% RH from true as best I can tell in this humidity range, sometimes a little better, sometimes a little worse.

Just thought I'd pass along what I saw and it is consistent with past comparisons I've done as well as the Prodata studies. Not sure if Jerry is still using the SHT75 for comparison as well or what he is seeing more generally. I just may have bad luck with these Davis sensors.

The picture was fairly typical. Sensor 2 data is the Davis 31. The Belfort at this time was reading about 19%.
« Last Edit: October 31, 2019, 11:26:53 AM by CW7491 »

Offline ValentineWeather

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So the Davis 31 is in the +6% to +7% from true as best I can tell in this humidity range.



Those numbers are what I came in with while testing also. Even when comparing to the accurite version of the 31. This is the high bias that bothers me the most. Jerry I think is more worried about the over-saturation issue because his sensors will drip water out when using a FARS unit at night down on the gulf.
Randy

Offline jerryg

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Well i finally got a run on high humidity, the humidity stayed between 95% and 100% for nearly two days and the new 31 worked just fine when the humidity dropped today down to 35%. There was no sign of wet bias but this is just the first time with high humidity and things could change as i get more of the really wet days so will keep updating when conditions warrant.
« Last Edit: October 31, 2019, 05:33:35 PM by jerryg »

Offline CW2274

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Well i finally got a run on high humidity, the humidity stayed between 95% and 100% for nearly two days and the new 31 worked just fine when the humidity dropped today down to 35%. There was no sign of wet bias but this is just the first time with high humidity and things could change as i get more of the really wet days so will keep updating when conditions warrant.
Well I know the new one I have will hit 1% cause I did it several times yesterday with an accompanying negative mid 30's dew point, which I thought was ridiculous til I saw two RAWS sites in the negative 50's. The air mass is just a tiny bit dry... 

Offline jgentry

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Well i finally got a run on high humidity, the humidity stayed between 95% and 100% for nearly two days and the new 31 worked just fine when the humidity dropped today down to 35%. There was no sign of wet bias but this is just the first time with high humidity and things could change as i get more of the Rreally wet days so will keep updating when conditions warrant.

So whatís your highest RH so far with the new 31?
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline jerryg

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The new sensor topped out at 99%, doing just fine on top and bottom and this reading was without heavy fog just like fog and drizzle.

Offline jgentry

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The new sensor topped out at 99%, doing just fine on top and bottom and this reading was without heavy fog just like fog and drizzle.

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

  

Offline weather34

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FWIW i installed a spare one early february i bought end last year around end of october . nothing wrong with original was just curious of how easy is installing replacement so i left it installed and put the original supplied in a box . being at sea level some 150 metres from sea front the spread for this year has been 12% to 99% i doubt i will see single digit readings unless ultra dry and extreme heat nearing the 40c mark but 99% has been noticed numerous times of dense fog periods . so SHT31 is certainly and been acceptable to my needs and was quite pleasantly suprised to see a sub 20% reading being this close to the sea front. rain events produce anywhere in the 94% to 97% mark..
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