Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 68715 times)

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Offline Jester

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CW, how did you modify your SF2 cap to work on the SHT31 Davis PCB?
I wasn't thrilled about the glue option so I trimmed the four feet down to be flush with the base and used very thin stainless steel wire to wrap around the cap and cinch it down with pliers twisting from the back side. The wire doesn't come in contact with the membrane but arches over. With the "gooey" substance that surrounds the sensor it allows the cap to seat firmly in what I believe is a true seal.

CW, do you have a source for the stainless wire you used?
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Offline CW2274

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CW, how did you modify your SF2 cap to work on the SHT31 Davis PCB?
I wasn't thrilled about the glue option so I trimmed the four feet down to be flush with the base and used very thin stainless steel wire to wrap around the cap and cinch it down with pliers twisting from the back side. The wire doesn't come in contact with the membrane but arches over. With the "gooey" substance that surrounds the sensor it allows the cap to seat firmly in what I believe is a true seal.

CW, do you have a source for the stainless wire you used?
All I use is braided wire that one might use to hang a picture with. It has like twenty or so strands and I peel off three of them. It's strong enough not to break when cinching it down (judgement required here, have extra  ;)) while snugging down the cap without covering too much membrane.

Offline CW2274

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BTW, if you were unaware, Sensirion advises not to touch the membrane itself. I used small needle nose pliers to center the cap. It can be a little tricky...

Offline Jester

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BTW, if you were unaware, Sensirion advises not to touch the membrane itself. I used small needle nose pliers to center the cap. It can be a little tricky...

Awesome! Thanks!
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Offline Jester

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BTW, if you were unaware, Sensirion advises not to touch the membrane itself. I used small needle nose pliers to center the cap. It can be a little tricky...

Installed the CAP yesterday. So far so good. I did a bake and then a refrigeration on a fresh sensor. Waiting for high humid days to see how well this mod works.
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Offline CW2274

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BTW, if you were unaware, Sensirion advises not to touch the membrane itself. I used small needle nose pliers to center the cap. It can be a little tricky...

Installed the CAP yesterday. So far so good. I did a bake and then a refrigeration on a fresh sensor. Waiting for high humid days to see how well this mod works.
Good, hopefully it wasn't too much of a pain. Are you going to use the stock filter as well? The reason I ask is that the SF2 has done such an outstanding job by itself, I haven't been out to service my ISS in over a year and a half and I know the sensor chamber is layered in dust/dirt from the case fan. That's three times longer than I have ever gone because the cap does such a good job that I see absolutely no degradation in sensor performance. That said, I was thinking of adding the stock filter to slow, yes I said slow, the response time in the summer heat. But that means climbing over my wall and out to the desert to service the ISS more like I use to. Ahhh, decisions...

Offline CW7491

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I'm using the Davis stock filter with stainless steel mesh because I have one analog temp/hum sensor that still works and gives better results than the Sensirion. The size of the humidity element requires a large filter. For a while I was using the Sensirion filter on a Sensirion sensor and liked the fast temperature response. The trouble is it can be a bit much and ultimately becomes noise, which I think is why most official stations use some sort of averaging algorithm for temperature. The other problem I ran into is the different sensing intervals for temperature (~10-12s) and humidity (~50-60s) with the VP2 can result in some crazy (and I would argue less than accurate) swings in dew point. If the humidity and temperature always updated together, it would be less problematic. I find the stock filter helps act as a buffer with some of the noise, yet it's responsive enough with the AC fan.

Offline CW2274

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I think is why most official stations use some sort of averaging algorithm for temperature.
They all do (at least ASOS's) and not just temp, but other parameters as well. If they didn't, then the data would potentially be all over the place just like a PWS, so they "even it out."

I think I'll try the stock filter as well and see what's what. If it's not to my satisfaction, so be it...but the SF2 will definitely stay.

Offline CW7491

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Yeah, if you can work it out, I donít think the SF2 inside the stock filter will cost you much in response time with the fan. I know Sensirion now puts the SF2 membrane right onto the sensor. Campbell Scientific uses it in their Hygrovue sensors with a larger filter, so it utilizes both ...

https://www.campbellsci.com/hygrovue10
https://www.campbellsci.asia/35219
« Last Edit: February 25, 2021, 07:02:29 PM by CW7491 »

Offline CW7491

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I also thought this was an interesting read. Especially that even Campbell Scientific is frustrated that all humidity sensors arenít fundamentally designed for meteorological applications ...

https://www.campbellsci.com/blog/air-temp-rh-sensors-need-to-know

Offline CW2274

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Yeah, if you can work it out, I donít think the SF2 inside the stock filter will cost you much in response time with the fan.
What I'm trying to help alleviate is the wild temp swings that I see during max insolation as the heat eddies across the desert floor. With a 60CFM fan, those are sucked up like nobody's business. I have a 40CFM backup, but am not inclined to swap them at this time. When the stock filter starts to "dirty up", maybe I'll be more satisfied. Yep, dirtier may be better...well at least for me.

Offline CW2274

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I also thought this was an interesting read. Especially that even Campbell Scientific is frustrated that all humidity sensors arenít fundamentally designed for meteorological applications ...

https://www.campbellsci.com/blog/air-temp-rh-sensors-need-to-know
Good read. Humidity has always been the "rub" in sensors, but by far in the extremes.

Offline jgentry

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I also thought this was an interesting read. Especially that even Campbell Scientific is frustrated that all humidity sensors arenít fundamentally designed for meteorological applications ...

https://www.campbellsci.com/blog/air-temp-rh-sensors-need-to-know


Yep. And the Sensirion 4x isnít no better than its predecessors...

ď 2.3 Recommended Operating Conditions
The sensor shows best performance when operated within the recommended normal temperature and humidity range of 5 įC ... 60 įC and 20 %RH ... 80 %RH, respectively. Long term exposure to conditions outside recommended normal range, especially at high relative humidity, may temporarily offset the RH signal (e.g. +3 %RH after 60 h at > 80 %RH). After returning into the recommended normal temperature and humidity range the sensor will recover to within specifications by itself. Prolonged exposure to extreme conditions may accelerate ageing.Ē
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Offline CW2274

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I also thought this was an interesting read. Especially that even Campbell Scientific is frustrated that all humidity sensors arenít fundamentally designed for meteorological applications ...

https://www.campbellsci.com/blog/air-temp-rh-sensors-need-to-know


Yep. And the Sensirion 4x isnít no better than its predecessors...

ď 2.3 Recommended Operating Conditions
The sensor shows best performance when operated within the recommended normal temperature and humidity range of 5 įC ... 60 įC and 20 %RH ... 80 %RH, respectively. Long term exposure to conditions outside recommended normal range, especially at high relative humidity, may temporarily offset the RH signal (e.g. +3 %RH after 60 h at > 80 %RH). After returning into the recommended normal temperature and humidity range the sensor will recover to within specifications by itself. Prolonged exposure to extreme conditions may accelerate ageing.Ē
Not that it isn't frustrating, but as always, ya pay to play folks. Davis sees it as 99.9% satisfaction rate to their "basic" customer. If you want better, then move up and pay up. Simple as that.

Offline Jester

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BTW, if you were unaware, Sensirion advises not to touch the membrane itself. I used small needle nose pliers to center the cap. It can be a little tricky...

Installed the CAP yesterday. So far so good. I did a bake and then a refrigeration on a fresh sensor. Waiting for high humid days to see how well this mod works.
Good, hopefully it wasn't too much of a pain. Are you going to use the stock filter as well? The reason I ask is that the SF2 has done such an outstanding job by itself, I haven't been out to service my ISS in over a year and a half and I know the sensor chamber is layered in dust/dirt from the case fan. That's three times longer than I have ever gone because the cap does such a good job that I see absolutely no degradation in sensor performance. That said, I was thinking of adding the stock filter to slow, yes I said slow, the response time in the summer heat. But that means climbing over my wall and out to the desert to service the ISS more like I use to. Ahhh, decisions...

The installation wasn't bad since I do HO scale model railroading and model planes as a hobby, so working small isn't to much of an issue.  I didn't put the Davis filter back on, just letting the cap do it's thing. Day two and I'm not sure what to think right now. My humidity topped at 93% while other stations around me were 73%-85%. My closest ASOS, KNTU max humidity was 80%. My old sensor was closer to the aforementioned but was starting to wet bias after only three months of service.
« Last Edit: February 26, 2021, 08:14:50 AM by Jester »
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Offline CW7491

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As a Navy guy, Iíd use some caution comparing to Navy and even Air Force ASOS for fear of trying to get your good data to compare favorably with their bad. Your proximity to Oceana is great and their data looks good at the moment, but in my experience the military installations donít have the most robust routine maintenance on their ASOS, particularly for humidity/dew point. Whidbeyís dew point had been out to lunch for a few years. The Air Force uses a completely different ASOS from the FAA and McChordís humidity sensor was stuck near 100% for nearly a year and Gray AAF right next door is still stuck at near 100% going on a few years. Iíve found FAA maintained ASOS at airports with regular commercial passenger service are most reliable.
« Last Edit: February 26, 2021, 11:22:36 AM by CW7491 »

Offline jgentry

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I also thought this was an interesting read. Especially that even Campbell Scientific is frustrated that all humidity sensors arenít fundamentally designed for meteorological applications ...

https://www.campbellsci.com/blog/air-temp-rh-sensors-need-to-know


Yep. And the Sensirion 4x isnít no better than its predecessors...

ď 2.3 Recommended Operating Conditions
The sensor shows best performance when operated within the recommended normal temperature and humidity range of 5 įC ... 60 įC and 20 %RH ... 80 %RH, respectively. Long term exposure to conditions outside recommended normal range, especially at high relative humidity, may temporarily offset the RH signal (e.g. +3 %RH after 60 h at > 80 %RH). After returning into the recommended normal temperature and humidity range the sensor will recover to within specifications by itself. Prolonged exposure to extreme conditions may accelerate ageing.Ē
Not that it isn't frustrating, but as always, ya pay to play folks. Davis sees it as 99.9% satisfaction rate to their "basic" customer. If you want better, then move up and pay up. Simple as that.

True but Davis is also serving some bigger dogs such as WeatherSTEM and Earth Networks. They arenít just serving the needs/wants to a general weather hobbyist.
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline Jim_49

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I just got a new VP2 with the 24 hour fan-aspirated shield.  It's still in the box waiting for the frost to get out of the ground here in Kansas so it can be installed.  I haven't seen, or may have missed, any mention of SF2 maintenance.  I live in a rural area surrounded by grain fields.  The air gets real dusty during planting and harvest seasons.  I am considering getting an SF2 but was wondering what the dusty air will do to it.  I have searched Sensirion's web site relative to maintenance but all I could find was handling requirements as mentioned in an above post.  Can dust be removed with a soft brush or will it need to be replaced?  Or will use of the stock filter over the SF2 be sufficient to keep the dust off of it?

Offline jgentry

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I just got a new VP2 with the 24 hour fan-aspirated shield.  It's still in the box waiting for the frost to get out of the ground here in Kansas so it can be installed.  I haven't seen, or may have missed, any mention of SF2 maintenance.  I live in a rural area surrounded by grain fields.  The air gets real dusty during planting and harvest seasons.  I am considering getting an SF2 but was wondering what the dusty air will do to it.  I have searched Sensirion's web site relative to maintenance but all I could find was handling requirements as mentioned in an above post.  Can dust be removed with a soft brush or will it need to be replaced?  Or will use of the stock filter over the SF2 be sufficient to keep the dust off of it?

I would bake the sensor first at 220įF for 10hrs and use the SF2 filter (if youíre good at putting small stuff together).  You would most likely have to replace the filter.  The SF2 filters are cheap.
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Offline Jim_49

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Thanks.  I'll try that.

Offline CW7491

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Leave the actual sensor exposed while baking, but just be sure to protect the potting around the sensor during the bake to keep it from melting. Wrapping it in aluminum foil worked well for me.

Offline CW2274

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I am considering getting an SF2 but was wondering what the dusty air will do to it.
See reply #1655 up top.

Offline Jim_49

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Thanks.  After you pointed that out I remembered reading that.  I've read so much here that I've forgotten some of the information.

Offline Jim_49

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Leave the actual sensor exposed while baking, but just be sure to protect the potting around the sensor during the bake to keep it from melting. Wrapping it in aluminum foil worked well for me.

Another point that I've read but forgot.  Thanks for pointing that out.

Offline CW2274

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Now I remember why I go out to the ISS so infrequently, whata PITA... Can't count how many needles I've got stuck in me. Anyway, after about a 16 month hiatus, I can report that the SF2 did it's job, I literally could barely see the filter as it was completely encased in dust/dirt. No matter, it still worked, and well. Can't recommend this enough, especially for people like me. Also stuck in a new 31, although the old one was fine. Lets see how that goes.