Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 31647 times)

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Offline openvista

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1175 on: September 03, 2018, 06:54:22 PM »
From my experience with any of these sensors whether it be the sht 15, 75, or 31 if it doesn't have a wet bias in the low to mid RH it will usually not read above 98%.  If it does reach the 99% to 100% it tends to read too wet a lower values. Very rare does it seem like I find a sensor that does both.  My preference is to have better accuracy in the mid ranges than to have a sensor that reads 100%, but that"s just me.

Yes, my 31 which is normally accurate in the mid ranges (when it hasn't spent extended time above 80% and gets out of calibration for awhile), will no longer read above 95% and even then only briefly. Practically speaking, 94% is the top end. Originally, I believe 97% was the top with maybe flashes of 98% in extended foggy periods. This is a sensor first installed at the end of November last year. Not even a year old!

Feeling pretty frustrated right now because we had some wet weather recently and the sensor got out of calibration. This afternoon humidity was in the 70s and the sensor wouldn't recalibrate. Last time it took almost 48 hours to return to normal. More rain is on the way tonight thru Wednesday. So... thinking to myself... do I yank the -8% offset I put in this afternoon (after testing with my Kestrel)? That's probably the only course of action I have or it will run dry when it's raining and immediately afterward.

It's getting quite ridiculous the lengths we all have to go to so we can have even somewhat accurate humidity readings.

UPDATE 11PM: Once the sun set and humidity started climbing into the 80s, the biases in the 31 converged. So I was able to safely remove the offset... at least until humidity drops below 80% again. 
« Last Edit: September 03, 2018, 11:14:12 PM by openvista »
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Offline ValentineWeather

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1176 on: September 03, 2018, 06:57:33 PM »
Something different, have the SHT75 back in service but this time inside FARS. FARS sensor chamber is big enough for the skinny 75 and big 31 at same time. So should I feel a need to swap over to 31 it will be quick and easy.

Added extension on the 75 also so can reach TX1 easily without moving transmitters around.

Has anybody tried using a short 1-2 foot RJ45 male/female extension cable between the ISS and sensor to eliminate having to open the ISS each time while changing between sensors?

If yes, where did you get it from?

Good Idea.
PIA feeding those cables inside ISS but I'm getting good at it now..  :-)
In my case I would need to protect the unused exposed connectors from elements. Same with any extension, couldn't leave exposed unless  weather proofed with something like sticky Tacky tape. Wrap in wax paper and sticky Tacky tape works great on camera connectors also making everything waterproof. Reason I wrap in paper first it's much easier to get off and doesn't adhere to connections making a mess.
 
I had the sht31 30' cable going into fars shield already which reached TX1 primary ISS.  Also needed a 30' cable for sht75 to reach so spliced in 4 wire flat cable I already had on hand. (Only takes 4 wires) for temperature sensor.  Now won't need to keep jocking stuff around every time.  \:D/
« Last Edit: September 03, 2018, 07:02:26 PM by ValentineWeather »
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Offline CW2274

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1177 on: September 03, 2018, 07:17:18 PM »
I picked up three more 31's from Ryan to see if I could get lucky.... ummmm, no. All three ran exactly 3% and 1.9 to 2.0F higher than my 8 month old 31, which is still running "only" 2% wet, not 5% like these new ones. Seems like the older 31's (I have two, both over two years old) are more accurate. They're all wrong, but the 3 new ones are more wrong, but consistently so. :???:

Offline Old Tele man

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1178 on: September 03, 2018, 07:30:29 PM »
Something different, have the SHT75 back in service but this time inside FARS. FARS sensor chamber is big enough for the skinny 75 and big 31 at same time. So should I feel a need to swap over to 31 it will be quick and easy.

Added extension on the 75 also so can reach TX1 easily without moving transmitters around.

Has anybody tried using a short 1-2 foot RJ45 male/female extension cable between the ISS and sensor to eliminate having to open the ISS each time while changing between sensors?

If yes, where did you get it from?

Good Idea.
PIA feeding those cables inside ISS but I'm getting good at it now..  :-)
In my case I would need to protect the unused exposed connectors from elements. Same with any extension, couldn't leave exposed unless  weather proofed with something like sticky Tacky tape. Wrap in wax paper and sticky Tacky tape works great on camera connectors also making everything waterproof. Reason I wrap in paper first it's much easier to get off and doesn't adhere to connections making a mess.
 
I had the sht31 30' cable going into fars shield already which reached TX1 primary ISS.  Also needed a 30' cable for sht75 to reach so spliced in 4 wire flat cable I already had on hand. (Only takes 4 wires) for temperature sensor.  Now won't need to keep jocking stuff around every time.  \:D/

I plan to slide a piece of "heat-shrink wrap" over the whole connector, leaving about 1" extra on each end, and then, after heat-shrinking, I'll tape both ends with Gorilla Tape to waterproof. But I wonder what others are using?
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Offline WheatonRon

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1180 on: September 03, 2018, 08:12:14 PM »
I have decided to ask Ryan for a new SHT31 to replace the one in my new 24 hour fan VP2, not because of being wet, rather temperature inaccuracies at dusk. I guess I can live with a wet hum-temp sensor but when the temperatures are inaccurate, even for 2-3 hours, that is unacceptable. My 2 year old VP2 with only a daytime fan has never had a temperature issue—always spot on.  I can easily compare results they are on the same fence 25 feet apart. 

Somewhere the QC on the Davis SHT has failed us.

Somewhat related to this, we had a huge downpour this afternoon. My new VP2 showed .66 and my older VP2 was .90 which matched my nearby CoCo gauge! Maybe I got a lemon but I will reserve judgment pending cleaning of the rainbase and tipper mechanism! Until this storm, my new VP2 was spot on with the CoCo gauge but it hadn’t experienced a rain like we got today.
« Last Edit: September 03, 2018, 08:37:11 PM by WheatonRon »
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Offline ValentineWeather

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1181 on: September 03, 2018, 08:16:11 PM »
This stuff is messier than hell, but boy do it work.
https://www.amazon.com/Gardner-Bender-LTB-400-Electrical-Waterproof/dp/B000FPAN2K

Yikes I don't like it. I prefer any of the the self fusing electrical tapes. Doesn't have to be scotch 70 others work just as well.
Randy

Offline ValentineWeather

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1182 on: September 03, 2018, 08:19:07 PM »
I have decided to ask Ryan for a new SHT31 to replace the one in my new 24 hour fan VP2, not because of being wet, rather temperature inaccuracies at dawn.

That would be a first for me. Never seen a 31 temperature go bad.
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Offline CW2274

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1183 on: September 03, 2018, 08:20:09 PM »
temperature inaccuracies at dawn.
:???: How is it only inaccurate at dawn?

Offline Old Tele man

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1184 on: September 03, 2018, 08:23:03 PM »
This stuff is messier than hell, but boy do it work.
https://www.amazon.com/Gardner-Bender-LTB-400-Electrical-Waterproof/dp/B000FPAN2K

Yikes I don't like it. I prefer any of the the self fusing electrical tapes. Doesn't have to be scotch 70 others work just as well.

Yes, the "volatiles" are probably hell on the RH% sensor! How close to sensor is too close?
« Last Edit: September 03, 2018, 08:24:50 PM by Old Tele man »
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Offline CW2274

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1185 on: September 03, 2018, 08:26:12 PM »
This stuff is messier than hell, but boy do it work.
https://www.amazon.com/Gardner-Bender-LTB-400-Electrical-Waterproof/dp/B000FPAN2K

Yikes I don't like it. I prefer any of the the self fusing electrical tapes. Doesn't have to be scotch 70 others work just as well.

Yes, the "volatiles" are probably hell on the RH% sensor!
Had no idea this was meant to be in the "smelling range" of the sensor. Obviously not meant for that.

Offline CW2274

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1186 on: September 03, 2018, 08:28:11 PM »
This stuff is messier than hell, but boy do it work.
https://www.amazon.com/Gardner-Bender-LTB-400-Electrical-Waterproof/dp/B000FPAN2K

Yikes I don't like it. I prefer any of the the self fusing electrical tapes. Doesn't have to be scotch 70 others work just as well.
I actually use fusing tape before the liquid stuff. Just me I guess...

Offline WheatonRon

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1187 on: September 03, 2018, 08:44:08 PM »
temperature inaccuracies at dawn.
:???: How is it only inaccurate at dawn?

My typo—should be dusk not dawn. The temperature after the sun sets doesn’t drop and stays higher than it should be for a couple of hours. The comparison that MADIS does on my two stations is crystal clear and it is obvious when viewing temperatures on fellow PWSs on WU shortly after sunset. It is like the sensor doesn’t want to cool off for a bit, then later in the night and all the next day, til dusk, it is fine—spot on. Never had a SHT31 do that!
« Last Edit: September 03, 2018, 08:52:05 PM by WheatonRon »
Davis VP2 with 24 hour FARS, SHT31 (3 complete systems-2 for uploading to the internet the other system for test and play); CWOP--CW5020 and FW3075; WU--KILWHEAT17 and KILWHEAT36; WeatherCloud.net; CoCoRaHS--IL-DP-132; Rainwise 111

Offline ValentineWeather

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1188 on: September 03, 2018, 08:45:21 PM »
Running a 75 and 31 in same fars sensor chamber. The SHT75 runs .2-.4 below the 31 mid 70's ambient outside. So looks very similar to the SHT15's I've tested. Be interesting if this continues or just in the 70's range.
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Offline CW2274

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1189 on: September 03, 2018, 09:01:37 PM »
temperature inaccuracies at dawn.
:???: How is it only inaccurate at dawn?

My typo—should be dusk not dawn.
Even still..... :?   Don't have an obstructed SF2 on the goofy one, do ya?

Offline WheatonRon

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1190 on: September 03, 2018, 09:04:23 PM »
temperature inaccuracies at dawn.
:???: How is it only inaccurate at dawn?

My typo—should be dusk not dawn.
Even still..... :?   Don't have an obstructed SF2 on the goofy one, do ya?

Nope!
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Offline jgentry

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1191 on: September 03, 2018, 09:17:45 PM »
From my experience with any of these sensors whether it be the sht 15, 75, or 31 if it doesn't have a wet bias in the low to mid RH it will usually not read above 98%.  If it does reach the 99% to 100% it tends to read too wet a lower values. Very rare does it seem like I find a sensor that does both.  My preference is to have better accuracy in the mid ranges than to have a sensor that reads 100%, but that"s just me.

The Acurite unit I have (https://www.acurite.com/rain-gauge-station-lightning-detection-01021m.html) does not have the humidity problems like the Davis SHT sensors have. Only issue that this Acurite unit has is lagging.
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Offline CW2274

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1192 on: September 03, 2018, 10:14:24 PM »
From my experience with any of these sensors whether it be the sht 15, 75, or 31 if it doesn't have a wet bias in the low to mid RH it will usually not read above 98%.  If it does reach the 99% to 100% it tends to read too wet a lower values. Very rare does it seem like I find a sensor that does both.  My preference is to have better accuracy in the mid ranges than to have a sensor that reads 100%, but that"s just me.

The Acurite unit I have (https://www.acurite.com/rain-gauge-station-lightning-detection-01021m.html) does not have the humidity problems like the Davis SHT sensors have.
Huh? It doesn't even have an outdoor ISS, or whatever acurite calls it. How would you know?

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1193 on: September 04, 2018, 08:44:12 AM »
I put a 31 back in use to do some comparing with the 75 and so far they have been tracking each other real well. Right now they are both showing 98% humidity and 75.7 temp. The temp is the same on both sensors and the humidity has been tracking real well. I am waiting for the humidity to get down into the mid ranges to see if the high bias shows up on the 31. Have had a lot of rain the last few days and the humidity yesterday stayed above 80% most of the day so sensors have been on the damp side for some time and will get a chance to see if the 31 lags the 75 when humidity goes down.

Offline ValentineWeather

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1194 on: September 04, 2018, 09:54:39 AM »
Removed the bulky 31 filter and added trimmed down version, now the 31 is running slightly lower on temperature.   Did a flip flop so the SHT31 stock filter appears to be causing about +.4 F swing. I'll keep a watch on this and log throughout day.
Both sensors are in same FARS sensor chamber so should be good indication.
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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1195 on: September 04, 2018, 10:09:53 AM »
Update, humidity drop was about the same for both sensors no wet bias seen.

Offline jgentry

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1196 on: September 04, 2018, 10:40:37 AM »
From my experience with any of these sensors whether it be the sht 15, 75, or 31 if it doesn't have a wet bias in the low to mid RH it will usually not read above 98%.  If it does reach the 99% to 100% it tends to read too wet a lower values. Very rare does it seem like I find a sensor that does both.  My preference is to have better accuracy in the mid ranges than to have a sensor that reads 100%, but that"s just me.

The Acurite unit I have (https://www.acurite.com/rain-gauge-station-lightning-detection-01021m.html) does not have the humidity problems like the Davis SHT sensors have.
Huh? It doesn't even have an outdoor ISS, or whatever acurite calls it. How would you know?

Simple. Put the sensor inside of Acurite’s radiation shield. Since their shielding isn’t great, I put it under a tree. Their monitor or “console” tells me the readings. During the days when temps are high and in humid conditions, the Acurite unit tracks well with the ASOS 15 miles north.  The Davis unit shows DPs around 2-4° higher than either two.
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline ValentineWeather

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1197 on: September 04, 2018, 11:37:23 AM »
Well didn't take long for the 75 and 31 temps to come back together after filter change and now 75 is running -.2F lower as before.

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1198 on: September 06, 2018, 09:28:18 AM »
Well my 75 was showing 99% this am without any fog just humidity. It has been reading just fine from low humidity to top end, much smoother in the readings than the 31 which at times would seem to get hung up on the higher side of what it should read.

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1199 on: September 09, 2018, 10:09:10 AM »
Ok, I still haven't installed the new (bought a year ago?) temp/humidity sensor.  Looks like I need to.  I looked this morning and was surprised to see a report of 21% humidity.  This is after it being so foggy I couldn't even see the ISS this morning at 220' distant.  Uh,...no.  So, this week I hope to swap the sensors out.

09909_2018_HumidityChart by Intheswamp, on Flickr

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