Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 40940 times)

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Online ValentineWeather

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The flat black (not Gloss)  is meant to minimize this reflective radiation not sure why Davis doesn't manufacturer shield like this. If you look at most modern passive shields flat black inside is pretty much how all look now.  This is just example how I mounted the sensor. This shield is going to another location with different SHT31 so screws are not tightened down. 
Randy

Offline jgentry

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I personally think the Davis stock shield does well. There is lagging but as along as you sited over grass and have a nice breeze on a consistent basis, it does good.
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline CW2274

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I personally think the Davis stock shield does well. There is lagging but as along as you sited over grass and have a nice breeze on a consistent basis, it does good.
Problem is, you can control the siting, not the breeze, especially on a consistent basis.

Offline jgentry

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I personally think the Davis stock shield does well. There is lagging but as along as you sited over grass and have a nice breeze on a consistent basis, it does good.
Problem is, you can control the siting, not the breeze, especially on a consistent basis.

Definitely true. Where one of my stations is located, it gets a good breeze most of the time. The other location is pretty much enclosed. Only really good breeze that sensor would’ve got us from the south. That’s why I have the 24hr FARS for that site.
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline Old Tele man

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It's all about LOCATION, LOCATION, LOCATION:

• LOCATION where you physically site it.

• LOCATION where weather can reach it.

• LOCATION where YOU can reach it for service/repair.
« Last Edit: July 27, 2018, 10:06:13 AM by Old Tele man »
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Online ValentineWeather

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This was what the Davis 7714 shield looked like today. The red dot was when test started. Blue #1 is the passive shield, grey FARS.
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« Last Edit: July 26, 2018, 08:40:50 PM by ValentineWeather »
Randy

Offline Old Tele man

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I ordered a Davis #07747 DFARS "kit" and will DIY it to be my "secondary/back-up" TEMP/RH system using an "old" spare ISS I have. Plan is for easy (hopefully) sensor access/change-out; it'll be mounted on same pole as current VP2, only pointing North so as NOT to physically interfere with the VP2. Should enable me to simultaneously test/play with older SHT-15 and current SHT-31 sensors (old & new) that I have stock-piled.
« Last Edit: July 27, 2018, 12:59:28 PM by Old Tele man »
• SYS: Davis VP2 Vue/WL-IP & Envoy8X/WL-USB;
• DBX2 & DBX1 Precision Digital Barographs
• CWOP: DW6988 - 2 miles NNE of Cortaro, AZ
• WU - KAZTUCSO202, Countryside

Offline Bobvelle

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I ordered a Davis DFARS "kit" and will DIY'ing it to be my "secondary/back-up" TEMP/RH system using an "old" spare ISS I have. Plan is for easy (hopefully) sensor access/change-out; it'll be mounted on same pole as current VP2, only pointing North so as NOT to physically interfere with the VP2. Should  enable me to simultaneously test/play with older SHT-15 and current SHT-31 sensors (old & new) that I have stock-piled.

Ha, I'm actually planning to do the exact same thing. Except my second Temp/Hum will be on another pole about 50ft away. Just waiting on a new ISS cover and some other odds and ends.

Online ValentineWeather

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No more FARS for me if these 7714's work out. I really hope we get some hot weather before slipping into fall.  Seems like all of a sudden only 70-80 & no 90's in sight so hard to confirm what I'm seeing. For sure I'll continue side by side test even into winter. Once the grass has turned and all leaves are gone, with different sun angles results could differ but very encouraging now in my heavy foliage humid environment.
Randy

Offline drew1021

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No more FARS for me if these 7714's work out. I really hope we get some hot weather before slipping into fall.  Seems like all of a sudden only 70-80 & no 90's in sight so hard to confirm what I'm seeing. For sure I'll continue side by side test even into winter. Once the grass has turned and all leaves are gone, with different sun angles results could differ but very encouraging now in my heavy foliage humid environment.

No way I'm giving up FARS. Even though I 'm frustrated to no end over the wet bias, the temperature is spot on. I will just have to wait for Davis to come up with a solution to the humidity issues we've all been experiencing.
VP2 with 24 hour FARS. SHT-75. WU: KNCLEWIS2. CWOP/APRS: DW4712, COCORAHS: NC-FR-7

Online ValentineWeather

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No more FARS for me if these 7714's work out. I really hope we get some hot weather before slipping into fall.  Seems like all of a sudden only 70-80 & no 90's in sight so hard to confirm what I'm seeing. For sure I'll continue side by side test even into winter. Once the grass has turned and all leaves are gone, with different sun angles results could differ but very encouraging now in my heavy foliage humid environment.

No way I'm giving up FARS. Even though I 'm frustrated to no end over the wet bias, the temperature is spot on. I will just have to wait for Davis to come up with a solution to the humidity issues we've all been experiencing.

Can't blame you I felt the same way until yesterday and this confirms what I saw. 2 year study by WMO gave the 7714 its highest 5 star rating running 97-99% with the reference shield.
I started a different thread since this is a shield subject not reliabilty of SHT.
https://www.wxforum.net/index.php?topic=34796.msg354556;topicseen#msg354556
Randy

Offline drew1021

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Thanks for the link. I didn't mean to stray from the topic. I personally don't believe the FARS is the culprit here. My personal feeling is the 31 sensor is the problem, not temperature, just humidity.
VP2 with 24 hour FARS. SHT-75. WU: KNCLEWIS2. CWOP/APRS: DW4712, COCORAHS: NC-FR-7

Offline openvista

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Thanks for the link. I didn't mean to stray from the topic. I personally don't believe the FARS is the culprit here. My personal feeling is the 31 sensor is the problem, not temperature, just humidity.

We've identified 3 problems with the SHT31 humidity sensor so far in this thread:

  • A wet bias that develops over time
  • A wet bias that ships with new sensors
  • A relatively small (3-5%) dry bias that's confined to readings 90% and above

Fan aspiration affects #1 and to a much lesser extent #3 too. In non-arid climates active ventilation bombs the sensor with moist air that leads to premature aging.  It also elevates nighttime temperatures on clear nights which leads to lower humidity readings. Although, I would argue that any good passive shield will have the same effect during radiational cooling so long as it's 2 meters off the ground.

The clearest and most frustrating aspect of the dry bias is during fog events when most SHTxx sensors won't achieve 100%. This has nothing to do with your shielding choice. It's simply a limitation of the sensor. If your sensor reads 96.5% humidity in saturated air it is considered within specification.

By far the biggest problem is the wet bias (worst between 40-70%). This thread contains reports of max humidity errors in the range of 10-20%! That leads to wildly inflated heat indices in the summer. A survey of other VP2's online compared to nearby airports reveals a widespread problem that appears to stretch back to the SHT11 sensor.

EDIT 10:52AM: As for when the wet bias develops, that's uncertain. I have one good SHT31 sensor that's 7 months old and one with the wet bias that's 16 months old. Both have been in a mixture of FARS and passive shields. Others have reported receiving new sensors recently and all of them have run wet.
« Last Edit: July 27, 2018, 10:53:55 AM by openvista »
Davis Vantage Pro2 AC FARS | https://marquetteweather.com | EW7933

Offline drew1021

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Excellent summary!  8-)
VP2 with 24 hour FARS. SHT-75. WU: KNCLEWIS2. CWOP/APRS: DW4712, COCORAHS: NC-FR-7

Offline dendrite

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I've been dealing with a stomach bug and a cold turkey, coffee quitting, caffeine withdrawal migraine so my new toys aren't getting installed until this weekend. I got a couple of new 12V FARS fans, new rad plates, aerocone, new uv/solar covers, new SHT75, and new connectors to make the 12 y/o VP2 look somewhat new again. I'll post some graphs later Sunday if I can get everything installed Saturday morning (or tonight if I'm up for it). I'll be able to show you the difference in my response times between the SHT31 (no cover) and SHT75 (no cover) with the same 40 CFM fan. We'll see how high the new 75's RH can reach too.

Offline Old Tele man

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In my best Cuba Gooding, Jr imitation voice, "...Show me US the money DATA..." (ha,ha)!
• SYS: Davis VP2 Vue/WL-IP & Envoy8X/WL-USB;
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• CWOP: DW6988 - 2 miles NNE of Cortaro, AZ
• WU - KAZTUCSO202, Countryside

Offline dendrite

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In my best Cuba Gooding, Jr imitation voice, "...Show me US the money DATA..." (ha,ha)!
You had my data at hello.

Offline drew1021

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What I've done temporarily is set a -1% bias on my console so at 100% I will max out at 97%
I would rather be off by 3% at the high end then suffer by 4to 6% mid range. I know it's not much but every little bit helps. I sure hope Davis is really working on this issue and not blowing smoke up our a**
VP2 with 24 hour FARS. SHT-75. WU: KNCLEWIS2. CWOP/APRS: DW4712, COCORAHS: NC-FR-7

Offline jgentry

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I've been dealing with a stomach bug and a cold turkey, coffee quitting, caffeine withdrawal migraine so my new toys aren't getting installed until this weekend. I got a couple of new 12V FARS fans, new rad plates, aerocone, new uv/solar covers, new SHT75, and new connectors to make the 12 y/o VP2 look somewhat new again. I'll post some graphs later Sunday if I can get everything installed Saturday morning (or tonight if I'm up for it). I'll be able to show you the difference in my response times between the SHT31 (no cover) and SHT75 (no cover) with the same 40 CFM fan. We'll see how high the new 75's RH can reach too.

Nice!  Like I said earlier, I’m planning on switching over to the 75. Shoot, I might even change out the stock passive shield from my station at my friend’s farm and install the 7714 shield.
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline jgentry

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

  

Offline dendrite

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Would I be correct to say that the SHT-75 doesn’t have the wet bias like the SHT-31?  Because I don’t see it with the comparison with these two stations...

https://mesowest.utah.edu/cgi-bin/droman/meso_table_mesowest_m.cgi?stn=KLCI&unit=0&time=&hours=24&hour1=23&day1=26&month1=07&year1=2018&radius=25&past=0&order=1


SHT-75. https://mesowest.utah.edu/cgi-bin/droman/meso_table_mesowest_m.cgi?stn=C7324&unit=0&time=&hours=24&hour1=23&day1=26&month1=07&year1=2018&radius=25&past=0&order=1
I don't have the 75 in right now...that's the uncovered 31 with the 40CFM fan. LCI is an AWOS and I'm not sure what they're using for dewpoint sensing right now. Maybe someone else here knows if it's the Vaisala PTU307 or something else. CON is about 20 miles south of me and that's an ASOS.

I found this dewpoint accuracy chart for the Vaisala PTU307. I'm not sure if that is what is used in ASOS (as part of the DTS1) or if it is just a sensor with similar technology. It's interesting for you SW desert guys trying to compare dewpoints in situtations with high temperature and large dewpoint depressions. 40C (104F) with a dewpoint of -10C (14F) and an RH of 8% gives you an error of about 2.2C. Drop the dewpoint to -20C, for a 60C depression, and the error goes off the chart.

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« Last Edit: July 27, 2018, 02:42:44 PM by dendrite »

Online ValentineWeather

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Would I be correct to say that the SHT-75 doesn’t have the wet bias like the SHT-31?  Because I don’t see it with the comparison with these two stations...

https://mesowest.utah.edu/cgi-bin/droman/meso_table_mesowest_m.cgi?stn=KLCI&unit=0&time=&hours=24&hour1=23&day1=26&month1=07&year1=2018&radius=25&past=0&order=1


SHT-75. https://mesowest.utah.edu/cgi-bin/droman/meso_table_mesowest_m.cgi?stn=C7324&unit=0&time=&hours=24&hour1=23&day1=26&month1=07&year1=2018&radius=25&past=0&order=1

Ambient temperature is so different 6°, I hesitate to compare. Even DP will fluctuate some with temperature I've noticed and C7324 is 2° higher with ambient 6° lower so I think the wet bias is still there.
Randy

Offline dendrite

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Would I be correct to say that the SHT-75 doesn’t have the wet bias like the SHT-31?  Because I don’t see it with the comparison with these two stations...

https://mesowest.utah.edu/cgi-bin/droman/meso_table_mesowest_m.cgi?stn=KLCI&unit=0&time=&hours=24&hour1=23&day1=26&month1=07&year1=2018&radius=25&past=0&order=1


SHT-75. https://mesowest.utah.edu/cgi-bin/droman/meso_table_mesowest_m.cgi?stn=C7324&unit=0&time=&hours=24&hour1=23&day1=26&month1=07&year1=2018&radius=25&past=0&order=1

Ambient temperature is so different 6°, I hesitate to compare. Even DP will fluctuate some with temperature I've noticed and C7324 is 2° higher with ambient 6° lower so I think the wet bias is still there.
I wouldn’t compare my station to LCI. They’re on the water and I’m on a hill. There’s a lot of terrain effects out here so I gave up comparing to other stations.

Also, the wet bias is real here. My northern slope has trouble evaporating off surface moisture. So when the winds are light the dews pool here between the tree canopies.
« Last Edit: July 27, 2018, 04:31:51 PM by dendrite »

Offline dendrite

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I have the 75 almost ready to go. Here it is next to a Davis sensor for size comparison.

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« Last Edit: July 27, 2018, 07:09:27 PM by dendrite »

Offline jerryg

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Ran a little experiment over night and today, i put a sensor in a damp wash cloth an let it sit over night showing 98%. This am i removed it from the cloth and put it in a fars to see how it would respond, well it was running around 2% high to start and as the day went on the humidity on main station was running around 39 to 45% but the test sensor was running between 5 to 7% higher. So i removed the sensor and baked it for several hours and the error corrected itself and after awhile was checking with the other station. So what they say about prolonged exposure to high humidity sure showed up and with a fan running at night in high humidity you can image the error that shows up until it dries out. One thing it does correct itself but it takes awhile and your reported readings are going to be really screwed up for awhile. I had noticed for a long time that the humidity reading was slow to come down in the am after a humid night, sometimes taking two to three hours to get dried out.

 

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