Author Topic: SHT31 vs ASOS with fog  (Read 3526 times)

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Offline ValentineWeather

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SHT31 vs ASOS with fog
« on: April 13, 2017, 07:36:42 AM »
I've always felt the airport dewpoint was reading low. Today I would say visibility is less than 50 feet the Davis SHT31 is reading between 97-98% humidity while the airport is 87-93%. Which instrument is more accurate?

I'm surprised the SHT31 doesn't go to 100% but more surprised at the airport dewpoint.

Randy

Offline jerryg

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Re: SHT31 vs ASOS with fog
« Reply #1 on: April 13, 2017, 02:07:17 PM »
I saw some interesting readings from the two metars near me, one nw about 9 miles and one se about 12 miles near the coast. I thought when air temp and dew point were the same then the humidity was 100% but the station nw reports every hour and with the temp running 1 degree higher than the dp but with fog the readings were 96 and 97%. When the temp was 2 degrees higher than the dp the humidity was 93% but fog reported each time. The station se of me reports every 20 minutes and they were showing a 100% and fog just before midnight and up to around 9am then they showed four readings with the temp and dp the same first was 100% then 99% and the last two of 97% while still showing foggy conditions. It sure looks like you can have fog and lower than 100% humidity. My two stations with sht31s showed mainly 99 percent with a few periods of 100% before the fog started to lift.

Offline CW2274

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Re: SHT31 vs ASOS with fog
« Reply #2 on: April 13, 2017, 03:23:08 PM »
I've stated this before here, TUS and DMA are only 4 miles apart, but when it gets "wet" here, DMA regularly hits 100%, TUS never does. ASOS sensors are no different on having difficulty pulling the 100% trigger at times too. Sounds like Randy's airport may be jacked. :-) The 31 rocks. (think how good the 35 would be) ;)

Offline ValentineWeather

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Re: SHT31 vs ASOS with fog
« Reply #3 on: April 13, 2017, 03:26:24 PM »
I've always noticed the airport DP reading lower than the SHT31's which I trust more than airport. When its soupy fog like today you would think humidity would be upper 90's at least if it was calibrated correctly. I'm thinking now its actually the temperature reading high at ASOS 1-2° and the DP at 37 was most likely correct.
Randy

Offline ValentineWeather

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Re: SHT31 vs ASOS with fog
« Reply #4 on: April 13, 2017, 03:28:48 PM »
I've stated this before here, TUS and DMA are only 4 miles apart, but when it gets "wet" here, DMA regularly hits 100%, TUS never does. ASOS sensors are no different on having difficulty pulling the 100% trigger at times too. Sounds like Randy's airport may be jacked. :-) The 31 rocks. (think how good the 35 would be) ;)

Agree I think its jacked, it was reading 1° for several hours couple weeks ago and they just haven't got it calibrated correctly.
Randy

Offline dalecoy

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Re: SHT31 vs ASOS with fog
« Reply #5 on: April 13, 2017, 05:13:52 PM »
I'm surprised the SHT31 doesn't go to 100%

Have you ever seen 100% on your setup?

And how are you seeing the measurement?  [The point being that what you "see" is a result of the sensor, associated hardware circuitry, and firmware]

Offline Old Tele man

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Re: SHT31 vs ASOS with fog
« Reply #6 on: April 13, 2017, 05:17:50 PM »
I believe he's referring to looking out the window and actually seeing fog cloud, but NOT seeing 100%RH on the console display, ie: "eyeball vs. console."
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Offline CW2274

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Re: SHT31 vs ASOS with fog
« Reply #7 on: April 13, 2017, 05:32:48 PM »
I personally have run the table with my 31, 1-100%, albeit both were very shortly lived.

Offline dalecoy

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Re: SHT31 vs ASOS with fog
« Reply #8 on: April 13, 2017, 05:43:54 PM »
I believe he's referring to looking out the window and actually seeing fog cloud, but NOT seeing 100%RH on the console display, ie: "eyeball vs. console."

I understood that.  But it doesn't answer my question.

I personally have run the table with my 31, 1-100%, albeit both were very shortly lived.

But your 31 isn't his 31, and your other outside components aren't identical to his.  Etc.

Offline ValentineWeather

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Re: SHT31 vs ASOS with fog
« Reply #9 on: April 13, 2017, 05:44:16 PM »
I'm surprised the SHT31 doesn't go to 100%

Have you ever seen 100% on your setup?

And how are you seeing the measurement?  [The point being that what you "see" is a result of the sensor, associated hardware circuitry, and firmware]

I may have once, today both stations I run hit 99%. One station is east 2 miles in country located at friends.
Randy

Offline CW2274

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Re: SHT31 vs ASOS with fog
« Reply #10 on: April 13, 2017, 06:18:47 PM »
I believe he's referring to looking out the window and actually seeing fog cloud, but NOT seeing 100%RH on the console display, ie: "eyeball vs. console."
But your 31 isn't his 31, and your other outside components aren't identical to his.  Etc.
Of course. Just relaying that it is possible with the 31. Can be a little like pulling teeth tho....

Offline CW2274

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Re: SHT31 vs ASOS with fog
« Reply #11 on: April 13, 2017, 06:20:33 PM »
I'm surprised the SHT31 doesn't go to 100%

Have you ever seen 100% on your setup?

And how are you seeing the measurement?  [The point being that what you "see" is a result of the sensor, associated hardware circuitry, and firmware]

I may have once, today both stations I run hit 99%. One station is east 2 miles in country located at friends.
At 99%, what was your temp/dew spread? I'm guessing 0.4F.

Offline ValentineWeather

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Re: SHT31 vs ASOS with fog
« Reply #12 on: April 13, 2017, 06:30:23 PM »
Something like that..
Randy

Offline ValentineWeather

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Re: SHT31 vs ASOS with fog
« Reply #13 on: April 13, 2017, 06:57:03 PM »
Here is a video look at just how dense the fog was early AM.
[youtube]https://youtu.be/XVtK_sno06c[/youtube]
Randy

Offline jerryg

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Re: SHT31 vs ASOS with fog
« Reply #14 on: April 14, 2017, 08:53:05 AM »
Well this am the metar se of me has been showing 100% humidity for the last four hours but no fog, clear skies visibility 10 miles. So it looks like 100% does not mean fog and you can have fog and be at 97%. So it seems to me that fog or no fog is not a good way to judge how your sensor is working.

Offline pfletch101

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Re: SHT31 vs ASOS with fog
« Reply #15 on: April 14, 2017, 10:52:46 AM »
Well this am the metar se of me has been showing 100% humidity for the last four hours but no fog, clear skies visibility 10 miles. So it looks like 100% does not mean fog and you can have fog and be at 97%. So it seems to me that fog or no fog is not a good way to judge how your sensor is working.

I would agree that it is possible to have 100% humidity without visible fog, at least transiently, but I am not sure that I can see how you can have fog without 100% humidity. The best of stations can be wrong.
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Offline dalecoy

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Re: SHT31 vs ASOS with fog
« Reply #16 on: April 14, 2017, 11:42:24 AM »
Check the specifications for the sensor.

For instance, for the SHT31, at 100% actual humidity, the typical tolerance is +/- 2%, and the maximal tolerance is +/- 3.5%.

So, at 100% actual humidity, the sensor might be "saying"  97.5% humidity, or 103.5% humidity, or .......

And then, what is the external circuitry doing to adjust that measurement?  Calibration?

And then, what is the display mechanism doing to adjust (or limit) the indication?  Hint: most firmware-based displays won't show greater than 100% humidity.

Offline johnd

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Re: SHT31 vs ASOS with fog
« Reply #17 on: April 14, 2017, 11:59:56 AM »
I would agree that it is possible to have 100% humidity without visible fog, at least transiently, but I am not sure that I can see how you can have fog without 100% humidity.

I'm not so sure. I can accept that where the water droplets actually form is likely to have 100% RH. But then the droplets diffuse away from where they form and won't evaporate quickly and completely if they encounter air where the RH is say 97%.

So eg often where the air may be coldest is the layer immediately next to the ground and this will be one place that the fog starts to form. Then the droplets start to rise and mix and maybe reach an air layer where the temp may be say a degree warmer and the RH somewhat <100%. The droplets will start to evaporate but not immediately and will to some extent be replaced by new droplets from below. The net effect over the whole population of droplets will be to reach a (temporary) steady state where both fog density and RH will decrease with height over eg the 0-50 feet air layer above ground level.
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Offline jerryg

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Re: SHT31 vs ASOS with fog
« Reply #18 on: April 14, 2017, 12:00:36 PM »
I don't pretend to understand all the info i have read about humidity measurement, but nearly all info says humidity is hard to measure. On the high end it gets more difficult and i would not think that a 50 dollar sensor is going to be as accurate as the instruments used by the metar stations. The Davis sensors seem to react to changes in humidity fairly well up to about 95% then they take quite some time to go above that reading. It can take several hours or more to catch up to the actual humidity reading and fog can come and go in bands. The metar that updates every 20 minutes can go from 95% to 100% in that 20 minute update time where the
Davis with its slower response time on the high end might never show the change if the humidity doesn't stay 100% long enough for it to catch up.

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Re: SHT31 vs ASOS with fog
« Reply #19 on: April 15, 2017, 08:58:04 AM »
I'm surprised the SHT31 doesn't go to 100%

Have you ever seen 100% on your setup?

And how are you seeing the measurement?  [The point being that what you "see" is a result of the sensor, associated hardware circuitry, and firmware]

The highest I have seen on my 31 was 99% (once) and that was after several hours of fog.  I expected better response form my 31.  Most ASOS sights, KNGU (1.5 mi away), and KORF read low vs other PWS in my area to include mine. I have seen KNGU read 100% while my 31 reads 96%-98%.
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Offline DA270

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Re: SHT31 vs ASOS with fog
« Reply #20 on: April 15, 2017, 11:25:06 AM »
The Vaisala sensors used by the NWS are designed with sensor heating to provide more accurate measurements in condensing conditions. The thin film capacitive humidity sensor is heated above ambient temperature to eliminate the hysteresis and recovery issues of the sensor substrate at the saturation point.

Offline ValentineWeather

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Re: SHT31 vs ASOS with fog
« Reply #21 on: April 15, 2017, 06:06:18 PM »
I've found the SHT31 to be accurate when I've checked with my old Weksler sling psychrometer. I'm sticking to its accuracy when we have fog the humidity isn't 87%.
Randy

Offline CW2274

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Re: SHT31 vs ASOS with fog
« Reply #22 on: April 15, 2017, 06:13:42 PM »
I'm surprised the SHT31 doesn't go to 100%

Have you ever seen 100% on your setup?

And how are you seeing the measurement?  [The point being that what you "see" is a result of the sensor, associated hardware circuitry, and firmware]
I expected better response form my 31. 
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Offline DA270

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Re: SHT31 vs ASOS with fog
« Reply #23 on: April 16, 2017, 10:24:54 AM »
I've found the SHT31 to be accurate when I've checked with my old Weksler sling psychrometer. I'm sticking to its accuracy when we have fog the humidity isn't 87%.

I'd have to agree on both counts! I would caution though, if the boundary layer was decoupled unless you were actually standing at the ASOS sensor taking an observation with your Weksler, it's only a guess as to what was going on there.

Offline ValentineWeather

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Re: SHT31 vs ASOS with fog
« Reply #24 on: April 16, 2017, 10:32:16 AM »
I've found the SHT31 to be accurate when I've checked with my old Weksler sling psychrometer. I'm sticking to its accuracy when we have fog the humidity isn't 87%.

I'd have to agree on both counts! I would caution though, if the boundary layer was decoupled unless you were actually standing at the ASOS sensor taking an observation with your Weksler, it's only a guess as to what was going on there.

I wasn't there but was going by visibility being reported <.13 mile and cameras around area. I think its just out of calibration because when I first moved here the airport was hitting 100% at times when I was only upper 90's and now 93% is highest I ever see the ASOS even in fog.
« Last Edit: April 16, 2017, 10:34:04 AM by ValentineWeather »
Randy