Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 18041 times)

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Offline Evening thunder

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I thought Iíd share my experience here, this topic confirms my long-held suspicions of wet bias with Davis sensors (not just the SHT31). I've not read the entire thing though.

I'm in the UK, in a valley, where on many nights the humidity rises above 95% with radiation mist/fog quite common. My set up is a 24h FARS and an older passive ISS, running two SHT31 sensors (18 and 24 months old)

These two sensors both drifted at the top end within 6 months (and in opposite directions for a while), The first went from reaching 98-99% maximum to only 95-96% (thick fog). I thought it may be a dodgy sensor so bought the 2nd sensor, but that also drifted down while by summer 2017 the first sensor was back to reaching 98-99% lol.
Currently, they reach about 97-98%, with the 2nd sensor 1-2% wetter through the mid-range.

Concerning the mid-range, and yep.. they seem to have a wet bias. I upload to a website that allows an easy comparison between two stations. This is me compared to the nearby official Met Office station (5 miles away) yesterday (a cooler, fresher day than many this summer with a well-mixed environment until about 9pm).

 [ You are not allowed to view attachments ]

My dew point is typically 2-3C higher (x1.8 for difference in F) Of course these are only hourly observation points so brief fluctuations could skew the graph, however this is a typical story.

Humidity for August so far

 [ You are not allowed to view attachments ]

As you can see I'm usually approaching 10% more humid. We are at virtually the same elevation with similar surroundings and I canít see any genuine reasons for this when the wind is in the direction that it was yesterday. It does seem with me at least the problem is not just with warm humid airmasses.

Not happy with that but what can I do.. I don't really have a controlled environment to recondition sensors (and if they have to be removed from the Davis board? that's a no-go for me anyway).
Nearby Davis stations do read similar to me but probably have the same issue.


Also, a brief history of the previous sensors I've used in case it's of interest:

My 1st SHT11 failed in August 2009, I think it had less of a wet bias, but only reached 95-96%.
The 2nd SHT11 had a significant wet bias in the mid-range, and reached 98-99%.
I had a SHT75 'Davis ready' sensor from a seller on Ebay. That possibly had little bias but it only reached about 95% so think I only used it in the old/backup ISS.
I also had a SHT15 (3rd party), and it reached 97-98%. It still had some degree of wet-bias.

So as you can see wet biased mid-range and no 100% readings seems to have been a general theme for me.

I still have the 15 and 75 lying around so am going to plug them into my backup ISS and see what they say compared to the SHT31.

I still like the SHT31 for temperature and prefer its humidity response to the 11's I used. If a stable 'corrected' sensor came out I'd buy that, although my records won't be consistent but they aren't anyway with those sensor changes and drift lol.
« Last Edit: August 10, 2018, 08:36:03 AM by Evening thunder »

Offline jgentry

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I thought Iíd share my experience here, this topic confirms my long-held suspicions of wet bias with Davis sensors (not just the SHT31). I've not read the entire thing though.

I'm in the UK, in a valley, where on many nights the humidity rises above 95% with radiation mist/fog quite common. My set up is a 24h FARS and an older passive ISS, running two SHT31 sensors (18 and 24 months old)

These two sensors both drifted at the top end within 6 months (and in opposite directions for a while), The first went from reaching 98-99% maximum to only 95-96% (thick fog). I thought it may be a dodgy sensor so bought the 2nd sensor, but that also drifted down while by summer 2017 the first sensor was back to reaching 98-99% lol.
Currently, they reach about 97-98%, with the 2nd sensor 1-2% wetter through the mid-range.

Concerning the mid-range, and yep.. they seem to have a wet bias. I upload to a website that allows an easy comparison between two stations. This is me compared to the nearby official Met Office station (5 miles away) yesterday (a cooler, fresher day than many this summer with a well-mixed environment until about 9pm).

 [ You are not allowed to view attachments ]

My dew point is typically 2-3C higher (x1.8 for difference in F) Of course these are only hourly observation points so brief fluctuations could skew the graph, however this is a typical story.

Humidity for August so far

 [ You are not allowed to view attachments ]

As you can see I'm usually approaching 10% more humid. We are at virtually the same elevation with similar surroundings and I canít see any genuine reasons for this when the wind is in the direction that it was yesterday. It does seem with me at least the problem is not just with warm humid airmasses.

Not happy with that but what can I do.. I don't really have a controlled environment to recondition sensors (and if they have to be removed from the Davis board? that's a no-go for me anyway).
Nearby Davis stations do read similar to me but probably have the same issue.


Also, a brief history of the previous sensors I've used in case it's of interest:

My 1st SHT11 failed in August 2009, I think it had less of a wet bias, but only reached 95-96%.
The 2nd SHT11 had a significant wet bias in the mid-range, and reached 98-99%.
I had a SHT75 'Davis ready' sensor from a seller on Ebay. That possibly had little bias but it only reached about 95% so think I only used it in the old/backup ISS.
I also had a SHT15 (3rd party), and it reached 97-98%. It still had some degree of wet-bias.

So as you can see wet biased mid-range and no 100% readings seems to have been a general theme for me.

I still have the 15 and 75 lying around so am going to plug them into my backup ISS and see what they say compared to the SHT31.

I still like the SHT31 for temperature and prefer its humidity response to the 11's I used. If a stable 'corrected' sensor came out I'd buy that, although my records won't be consistent but they aren't anyway with those sensor changes and drift lol.

If I were you, I would get a brand new 75 sensor and do the necessary work to fit it in with the transmitter. hwcorder has the 75 and it matches up well with the Airport stations. In fact, it reached up to 100% one morning. Jerryg also uses the 75.
Davis Vantage Pro2. SHT 31. 24hr FARS. WU: KXALJEMI2 & KALTHORS2 CWOP: C6353 & C6358

  

Offline dendrite

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I think the 75 may drift somewhat too. My old one was only maxing out at 96% when I plugged it back in a few weeks ago, but the new one is regularly hitting 98-100%. The caveat is that the old 75 was collecting dust for a couple of years so maybe there's another issue there. My 31 was only peaking at 93% so at least the old 75 was better than that.

Offline dendrite

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We also have our first consistent dry breeze in awhile today and my dewpoint is running right on par with the LCI AWOS and a little under the CON ASOS. With the 31, I was rarely lower than them during the day. With my 40CFM fan it can get a little jumpy with wet ground and light winds as we fluctuate between drier, mixed air and more moist, surface dewpoint pooling.

Offline ValentineWeather

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Are you using the 75 without a filter?
Randy

Offline dendrite

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Are you using the 75 without a filter?
Yup. Used the 31 without a filter too.

Offline openvista

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Just a warning for those planning to mount a 75 using the wire-to-board connector. Those things are flimsy crap. I got it working on my 4th try (after burning through 3 other connectors). I then moved the sensor, and it wouldn't read correctly after that.

For those wondering, yes, the wires were in there properly (you could pull on them and they wouldn't come out). The wire end of the connector was as seated as you could make it, but it always kinda moves, no matter what.

Even if I could seal it when it was connected properly, I have no confidence that connector would last long term.

I know dendrite got it working and that's great. I guess I'm saying I'm not sure how easily that success can be replicated.

Caveat emptor. YMMV.
Davis Vantage Pro2 | https://marquetteweather.com

Offline ValentineWeather

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Just a warning for those planning to mount a 75 using the wire-to-board connector. Those things are flimsy crap. I got it working on my 4th try (after burning through 3 other connectors). I then moved the sensor, and it wouldn't read correctly after that.

For those wondering, yes, the wires were in there properly (you could pull on them and they wouldn't come out). The wire end of the connector was as seated as you could make it, but it always kinda moves, no matter what.

Even if I could seal it when it was connected properly, I have no confidence that connector would last long term.

I know dendrite got it working and that's great. I guess I'm saying I'm not sure how easily that success can be replicated.

Caveat emptor. YMMV.

This sounds like fun. Not!  I think I ordered 6 for 2 sensors. Is the issue with the wire (plug) side or sensor? If on wire side I may be able to solder a better connection.
Randy

Offline openvista

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Wire side.

Yup, I'm going the solder route too.
Davis Vantage Pro2 | https://marquetteweather.com

Offline jerryg

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If it is the same ones i got they really suck, one fell apart and the other made connection about half the time even though it was seated right. El Stinko.

Offline openvista

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I have experience soldering... just not in such tight spaces. So I spread the leads apart vertically to give myself extra space.

Well... after shorting the leads with excess solder, I attempted to "fix" my mess and the sensor became too hot to touch. It's pretty fubar'd now.

And with that I'm out of the 75 project.

To those who plan to proceed, you will need good soldering skills/tools, a well-lit workspace, and some way to safely immobilize the sensor.

For the average VP2 owner who just wants accurate humidity readings, the wait continues for a viable solution.
Davis Vantage Pro2 | https://marquetteweather.com

Offline CW2274

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Don't know if anyone else has tried compensating by dialing in what humidity you want, but six days ago I dropped mine 2%. Since then I've been between 11 and 91%, 109 and 70įF, and have been tickled with the results. Of course I'm completely aware that this is certainly no fix and haven't "challenged" the upper and lower limit to boot, but in between, I'm very pleased. At least it's better than a sharp stick in the eye til there's a fix.

Offline dendrite

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Ugh. Sorry they didn't work well for you guys. I hate suggesting something and having them fail for everyone. Mine is working great, but they're definitely tough to work with.

Offline ValentineWeather

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Don't know if anyone else has tried compensating by dialing in what humidity you want, but six days ago I dropped mine 2%. Since then I've been between 11 and 91%, 109 and 70įF, and have been tickled with the results. Of course I'm completely aware that this is certainly no fix and haven't "challenged" the upper and lower limit to boot, but in between, I'm very pleased. At least it's better than a sharp stick in the eye til there's a fix.

Not really an option for most. Many have sensors that struggle to reach 98% with half stopping at 96% even in soupy fog so dialing back what most need around 6-8% (not 2%) would really look worse on foggy mornings and you are reporting 88-90% humidity. 

I would rather be reporting 48% during afternoon when it's really 41% and less noticeable unless it's in the high heat index region when that extra 6-8% starts sticking out like a sore thumb.   
Randy

Offline CW2274

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When I switched from the 11 to the 15 a few years back, I took the sensor board to a dude down the street that works on old stereo stuff. Not only did he nail the job, he did it with his naked eye :eek:, threw him a Jackson and was gone. Maybe some of you guys can find someone similar.

Offline dendrite

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I have experience soldering... just not in such tight spaces. So I spread the leads apart vertically to give myself extra space.

Well... after shorting the leads with excess solder, I attempted to "fix" my mess and the sensor became too hot to touch. It's pretty fubar'd now.

And with that I'm out of the 75 project.

To those who plan to proceed, you will need good soldering skills/tools, a well-lit workspace, and some way to safely immobilize the sensor.

For the average VP2 owner who just wants accurate humidity readings, the wait continues for a viable solution.
Sensirion doesn't recommend soldering to the 75 leads. These are the ones they recommend on the data sheet.

https://www.mouser.com/ProductDetail/Mill-Max/851-43-004-20-001000?qs=iJx90KzHWBrT6JXc8SwU%252bg%3D%3D

I used these and they work fine, but again...tight spaces.

Offline CW2274

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Don't know if anyone else has tried compensating by dialing in what humidity you want, but six days ago I dropped mine 2%. Since then I've been between 11 and 91%, 109 and 70įF, and have been tickled with the results. Of course I'm completely aware that this is certainly no fix and haven't "challenged" the upper and lower limit to boot, but in between, I'm very pleased. At least it's better than a sharp stick in the eye til there's a fix.

Not really an option for most. Many have sensors that struggle to reach 98% with half stopping at 96% even in soupy fog so dialing back what most need around 6-8% (not 2%) would really look worse on foggy mornings and you are reporting 88-90% humidity. 
 
Yeah, figured as much since "only" 2% was needed for me to get in line.
I imagine I'm likely see less than 11% before there's a fix ](*,) and will obviously see how it does.

Offline dendrite

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Just a warning for those planning to mount a 75 using the wire-to-board connector. Those things are flimsy crap. I got it working on my 4th try (after burning through 3 other connectors). I then moved the sensor, and it wouldn't read correctly after that.

For those wondering, yes, the wires were in there properly (you could pull on them and they wouldn't come out). The wire end of the connector was as seated as you could make it, but it always kinda moves, no matter what.

Even if I could seal it when it was connected properly, I have no confidence that connector would last long term.

I know dendrite got it working and that's great. I guess I'm saying I'm not sure how easily that success can be replicated.

Caveat emptor. YMMV.

This sounds like fun. Not!  I think I ordered 6 for 2 sensors. Is the issue with the wire (plug) side or sensor? If on wire side I may be able to solder a better connection.
If you give it a shot...

Notice how the connector opens and closes. Use a little flat head precision screwdriver to gently open it all the way. There will be a subtle "click" into the fully open position. Cut the wires straight across, line them up, and insert them. They will freely and loosely go in to a certain point. Then you need to give them some extra force to push them through all the way to make the connection.

The next part is where they tend to break. When closing the connector over the wires, do it gently with even pressure. I don't recommend trying to close it all the way as there are multiple locations where the plastic can break apart. I found that out twice. Heck, just closing it with your thumb and index finger may be enough.

Offline ValentineWeather

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Just a warning for those planning to mount a 75 using the wire-to-board connector. Those things are flimsy crap. I got it working on my 4th try (after burning through 3 other connectors). I then moved the sensor, and it wouldn't read correctly after that.

For those wondering, yes, the wires were in there properly (you could pull on them and they wouldn't come out). The wire end of the connector was as seated as you could make it, but it always kinda moves, no matter what.

Even if I could seal it when it was connected properly, I have no confidence that connector would last long term.

I know dendrite got it working and that's great. I guess I'm saying I'm not sure how easily that success can be replicated.

Caveat emptor. YMMV.

This sounds like fun. Not!  I think I ordered 6 for 2 sensors. Is the issue with the wire (plug) side or sensor? If on wire side I may be able to solder a better connection.
If you give it a shot...

Notice how the connector opens and closes. Use a little flat head precision screwdriver to gently open it all the way. There will be a subtle "click" into the fully open position. Cut the wires straight across, line them up, and insert them. They will freely and loosely go in to a certain point. Then you need to give them some extra force to push them through all the way to make the connection.

The next part is where they tend to break. When closing the connector over the wires, do it gently with even pressure. I don't recommend trying to close it all the way as there are multiple locations where the plastic can break apart. I found that out twice. Heck, just closing it with your thumb and index finger may be enough.

Thanks for the tip.
I'll copy and keep handy.
Made the mistake ordering through Newark because sensor was cheaper vs Mouser. WOW! They sit on order for 3 days, I finally called and was told by someone with really bad english because I was first time customer they hold order. WHAT? Never heard of such BS...Anyway it will be next Wednesday before it gets here. I get stuff from Mouser shipped same day sometimes, incredibly different companies.
Randy

Offline CW2274

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I'd have cancel it and told them to pound sand, then happily told them you'd do business elsewhere.

Offline hwcorder

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I have experience soldering... just not in such tight spaces. So I spread the leads apart vertically to give myself extra space.

Well... after shorting the leads with excess solder, I attempted to "fix" my mess and the sensor became too hot to touch. It's pretty fubar'd now.

And with that I'm out of the 75 project.

To those who plan to proceed, you will need good soldering skills/tools, a well-lit workspace, and some way to safely immobilize the sensor.

For the average VP2 owner who just wants accurate humidity readings, the wait continues for a viable solution.

There are female pin connectors that you can solder instead of the actual sensor. You can find them at mouser or newark 14.

Also I made a bonehead mistake of not mentioning I  do use the humidity offset feature on weather display then add back in the offset on the high-end to reach 100%.
The raw readings on my 75 reach 98 tops.

Offline jgentry

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I have experience soldering... just not in such tight spaces. So I spread the leads apart vertically to give myself extra space.

Well... after shorting the leads with excess solder, I attempted to "fix" my mess and the sensor became too hot to touch. It's pretty fubar'd now.

And with that I'm out of the 75 project.

To those who plan to proceed, you will need good soldering skills/tools, a well-lit workspace, and some way to safely immobilize the sensor.

For the average VP2 owner who just wants accurate humidity readings, the wait continues for a viable solution.

There are female pin connectors that you can solder instead of the actual sensor. You can find them at mouser or newark 14.

Also I made a bonehead mistake of not mentioning I  do use the humidity offset feature on weather display then add back in the offset on the high-end to reach 100%.
The raw readings on my 75 reach 98 tops.

So thatís why your DPs is matching up well with the nearby airport stations?

BTW: I sent you a PM
Davis Vantage Pro2. SHT 31. 24hr FARS. WU: KXALJEMI2 & KALTHORS2 CWOP: C6353 & C6358

  

Offline ValentineWeather

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I have experience soldering... just not in such tight spaces. So I spread the leads apart vertically to give myself extra space.

Well... after shorting the leads with excess solder, I attempted to "fix" my mess and the sensor became too hot to touch. It's pretty fubar'd now.

And with that I'm out of the 75 project.

To those who plan to proceed, you will need good soldering skills/tools, a well-lit workspace, and some way to safely immobilize the sensor.

For the average VP2 owner who just wants accurate humidity readings, the wait continues for a viable solution.

There are female pin connectors that you can solder instead of the actual sensor. You can find them at mouser or newark 14.

Also I made a bonehead mistake of not mentioning I  do use the humidity offset feature on weather display then add back in the offset on the high-end to reach 100%.
The raw readings on my 75 reach 98 tops.

Makes a big difference. It would be nice if Davis offered this as a fix.  HINT
I make my numbers look better with Cumulus also but still need sensor to be capably of reaching 98% on its own. 
Randy

Offline dendrite

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Mine has regularly been reaching 99-100%. Weíll see how long that lasts.

Offline hwcorder

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I have experience soldering... just not in such tight spaces. So I spread the leads apart vertically to give myself extra space.

Well... after shorting the leads with excess solder, I attempted to "fix" my mess and the sensor became too hot to touch. It's pretty fubar'd now.

And with that I'm out of the 75 project.

To those who plan to proceed, you will need good soldering skills/tools, a well-lit workspace, and some way to safely immobilize the sensor.

For the average VP2 owner who just wants accurate humidity readings, the wait continues for a viable solution.

There are female pin connectors that you can solder instead of the actual sensor. You can find them at mouser or newark 14.

Also I made a bonehead mistake of not mentioning I  do use the humidity offset feature on weather display then add back in the offset on the high-end to reach 100%.
The raw readings on my 75 reach 98 tops.

So thatís why your DPs is matching up well with the nearby airport stations?

BTW: I sent you a PM

On Weather Display I use a -1 slope factor which would lower my value by 2% through the mid range but then use a +2 offset.  So in effect though the mid ranges my final reading and raw reading match. When I get to the higher end of the scale my final reading is slightly higher than the raw output.  Once my 75 reaches 98% the adjusted reading reaches 100%.


Quote
It would be nice if Davis offered this as a fix.  HINT

Yes and I would think it would not be too hard to add some kind of adjustment slope in a firmware update.  Of coarse if they were to do that they would be admitting that there is a problem.

 

anything