Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?

[dendrite]

I notice with my 75 that with the high temperature response the dewpoint has a lot of variation as well...in fact more than I think it should be. I think the problem has to do with the update interval. The temp is updating every 10-12s while the RH is 50-60s. If I see a 1-2F swing in temp in a minute, the RH doesn't have time to "catch up" and sync with the temp.

It would be ideal if the RH updated exactly when the temperature does. Therefore, for any update in time you know that the RH measurement corresponds to the temp.

Many of you tech guys would know better than me...how reasonable would it be to have a firmware update that sent a new RH ob with every temp measurement? I know we've complained about the integer resolution with 2nd temp sensors before and how it's a byte issue.

[ValentineWeather]

Cumulus has a setting to calculate dewpoint so any changes with temperature it should adjust. Can't confirm how often it adjust but it's there. 
 [ You are not allowed to view attachments ]

[ValentineWeather]

I can confirm with cumulus both DP and humidity adjust with temperature sometimes every couple seconds and doesn't wait for humidity change.

[dendrite]

I can confirm with cumulus both DP and humidity adjust with temperature sometimes every couple seconds and doesn't wait for humidity change.

Not to go OT, but I’ll have to look into Cumulus more. You confused me a bit there...with Cumulus, DP and RH change every few seconds, but doesn’t wait for an RH change? Does Cumulus have faster update intervals than the VP2 console? I think I once saw someone say here that you can get temperature in tenths resolution with temp/hum stations using it?

Now the slightly OT part..will it report wind in 0.1 mph resolution using a 3rd party anemometer that has the ability? The one I have coming in has 0.01m/s resolution capability so I’d love to get the max potential out of it.

[ValentineWeather]

I may of said that wrong the DP changes with temperature every update couple seconds sometimes, because both are in tenths, Rh is changing less often. 

[dendrite]

I may of said that wrong the DP changes with temperature every update couple seconds sometimes, because both are in tenths, Rh is changing less often.

Ah okay. I get that with weatherlink. But therein lies the problem. If the RH holds at say 74% for me in a 1-min update, my temp can fluctuate 1-2F during the course of that minute and the dewpoint follows it given the "frozen" RH value.

What I would like to do is keep my 75 in the FARS for temp, but use a Davis 31 in a 2nd temp/hum station with my old 7714 shield. Then I would take the temp from my FARS/75 and dew from the passive/31 to calculate RH and post to my site/CWOP. I think I saw that idea mentioned here before and that Cumulus would report 2nd temp readings in tenths. Basically it would be like an ASOS setup...FARS for temp and then a separate passive shield for the dewpoint.

[ValentineWeather]

I may of said that wrong the DP changes with temperature every update couple seconds sometimes, because both are in tenths, Rh is changing less often.

Ah okay. I get that with weatherlink. But therein lies the problem. If the RH holds at say 74% for me in a 1-min update, my temp can fluctuate 1-2F during the course of that minute and the dewpoint follows it given the "frozen" RH value.

What I would like to do is keep my 75 in the FARS for temp, but use a Davis 31 in a 2nd temp/hum station with my old 7714 shield. Then I would take the temp from my FARS/75 and dew from the passive/31 to calculate RH and post to my site/CWOP. I think I saw that idea mentioned here before and that Cumulus would report 2nd temp readings in tenths. Basically it would be like an ASOS setup...FARS for temp and then a separate passive shield for the dewpoint.

Interesting. I didn't realize Cumulus could do that.

[dendrite]

I may of said that wrong the DP changes with temperature every update couple seconds sometimes, because both are in tenths, Rh is changing less often.

Ah okay. I get that with weatherlink. But therein lies the problem. If the RH holds at say 74% for me in a 1-min update, my temp can fluctuate 1-2F during the course of that minute and the dewpoint follows it given the "frozen" RH value.

What I would like to do is keep my 75 in the FARS for temp, but use a Davis 31 in a 2nd temp/hum station with my old 7714 shield. Then I would take the temp from my FARS/75 and dew from the passive/31 to calculate RH and post to my site/CWOP. I think I saw that idea mentioned here before and that Cumulus would report 2nd temp readings in tenths. Basically it would be like an ASOS setup...FARS for temp and then a separate passive shield for the dewpoint.

Interesting. I didn't realize Cumulus could do that.

Scratch that. It's meteobridge/meteohub.

https://www.wxforum.net/index.php?topic=34658.msg354339#msg354339

[WheatonRon]

Well eating crow today, in fact several of us because we were convinced the ASOS at KVTN was off. Council member/ Coop manager here in Valentine got permission to check ASOS ourselves and the unexpected happened. It was running it's usual +2°F above my station and coop so as drove out there was thinking "we have them now proof and will take pictures for evidence".

Took the certified thermometer out up and stuck probe into vaisala shield intake and and to my  was dead on.  Spent 20 minutes waiting for any discrepancy and it was matching 5 minute average updates every time meanwhile in town it was 2° cooler still.
No conspiracy theory it's just warmer at airport. +2°
Icing last winter probably just didn't occur at airport. 

I feel better knowing.  I mentioned we may have a reverse of heat island going on because of intown foliage and vegetation.
 

In short, life is good again!

[ValentineWeather]

 



In short, life is good again!

Much better   , the intake screen was 1/2 plugged also I'm heading back out to clean off.

[CW2274]

You didn't compare any dews while you were there???????????

[ValentineWeather]

You didn't compare any dews while you were there???????????

Negative on that.

[drew1021]

I would be suprised if it was a continuous 2 degree discrepancy. Even with well stirred air?
Also looks like a lot of metal around the sensor shield, could this effect the readings? Not trying to be negative, just my curiosity and observation.

[ValentineWeather]

I would be suprised if it was a continuous 2 degree discrepancy. Even with well stirred air?
Also looks like a lot of metal around the sensor shield, could this effect the readings? Not trying to be negative, just my curiosity and observation.

Yes lots of metal right there. Ended up being +4° over in town. The coop guy thinks something is still wrong he's been running it 30+ years and claims airport was never been +4-5 degrees warmer and he is less than mile away.
All I can say I went out there twice and it was running +2° both times. If I left thermometer and see what it records vs ASOS I guess maybe next.

[drew1021]

It would be nice if you could place a remote setup next to the the ASOS for a few weeks just to see if any of discrepancies occur. Good luck with that idea.

[ValentineWeather]

It would be nice if you could place a remote setup next to the the ASOS for a few weeks just to see if any of discrepancies occur. Good luck with that idea.

We may be doing a full day long test soon, just record high and low with portable certified thermometer. At least can see if the temperature spikes are real. Spot checking you may miss a issue because temps took off late 5:30-6:30 pm. One thing I noticed the air intake for blower seems rather low to ground, estimate about 4.3'.  Lower than my FARS intakes at home 5.0'.  I always thought between 5-6' was standard.
Next time I go out there I'll tape measure the intake.

[dendrite]

It would be nice if you could place a remote setup next to the the ASOS for a few weeks just to see if any of discrepancies occur. Good luck with that idea.

We may be doing a full day long test soon, just record high and low with portable certified thermometer. At least can see if the temperature spikes are real. Spot checking you may miss a issue because temps took off late 5:30-6:30 pm. One thing I noticed the air intake for blower seems rather low to ground, estimate about 4.3'.  Lower than my FARS intakes at home 5.0'.  I always thought between 5-6' was standard.
Next time I go out there I'll tape measure the intake.

Yeah...I think they generally run 1.5-2m so that would be a little low.

It wouldn't really explain 1-2F too high during low solar situations though. Something to keep in mind about the rime icing via FZFG though...the ground sfc and railings will be colder than the 2m temp on those radiational cooling nights. So, just like with a ground frost situation with a 2m temp of say 35F, you could have icing on the fence railings with the ASOS 2m temp showing 33F. It wouldn't explain why you, and other stations, were running 31-31.5F though.

[drew1021]

It would be nice if you could place a remote setup next to the the ASOS for a few weeks just to see if any of discrepancies occur. Good luck with that idea.

We may be doing a full day long test soon, just record high and low with portable certified thermometer. At least can see if the temperature spikes are real. Spot checking you may miss a issue because temps took off late 5:30-6:30 pm. One thing I noticed the air intake for blower seems rather low to ground, estimate about 4.3'.  Lower than my FARS intakes at home 5.0'.  I always thought between 5-6' was standard.
Next time I go out there I'll tape measure the intake.

Yeah I noticed that the intake looked a bit low. With that said if anything the nighttime temperatures would be cooler, especially on nights with maximum radiational cooling.

[CW2274]

Finally mounted my SF2 filter, left the Davis filter off. Response time and data looks fine, didn't really expect different.

[ValentineWeather]

Results of ASOS test are in.   Sunny light winds.  Started test early to catch the low temperature. Uncanny how high temperature captured were exact. No explanation on low temperature difference.
ASOS High- 93.2°        Test NIST certified thermometer -93.23°
ASOS Low  -55.0°        Test    "              "       "            -53.51°



In town heavy foliage, Passive 7714 SHT31    High-91.7°        Low-53.3°
                                 Davis FARS   SHT31    High-91.3          Low-53.4°
 
Measured the blower inlet from ground exactly 5'0"
Kept thermometer in worm bait box.

 [ You are not allowed to view attachments ]

[CW2274]

No explanation on low temperature difference.

If I missed this from earlier, I apologize, but is your probe inside the aspirated chamber while testing?

[ValentineWeather]

I'll need a drawing exactly where sensor is located I measured at inlet screen. Not exactly sure where the sensor chamber is located but it has to be close.

Found this. Red arrow was where measurements taken on outside of screen.
This sucker is really pulling some air.

[CW2274]

No explanation on low temperature difference.

If I missed this from earlier, I apologize, but is your probe inside the aspirated chamber while testing?

This sucker is really pulling some air.

Was pretty sure they did. My old 67CFM doesn't look so bad now, does it?

[jerryg]

Talking about sucking in air the rm young shield i have has a squirrel cage type fan and it sucks up anywhere from 5 to 11 m/s and use a 1 amp 12 volt module to power it compared to the davis which is a puny .86 m/s. I can hear it all over the yard and it will ruin a sensor in no time in high humidity. I plan on using it for temp probe only for comparison with the other shields, it is just too air flow.

[Evening thunder]

I thought I’d 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 can’t 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.