Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 40965 times)

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Offline openvista

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I should add that all these sensors come with a calibration curve of some sort based on inherent non-linearities the manufacturer identified during initial testing. But that's something quite apart from solving an inconsistent wet bias discovered in the field.
« Last Edit: July 22, 2018, 03:13:02 PM by openvista »
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Offline ValentineWeather

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This is what WD... Weather Display humidity calibration looks like.
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Randy

Offline dendrite

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Randy...

I brought it up before, but have you ever tried an SHT75? If you're really suspicious about Davis' handling of the sensors it may be worth giving a sensor soldered at the Sensirion factory a try. I'm sure you have an old Davis SHTxx that you can sacrifice to solder directly to the 75 pins or insert into an acceptable connector.

Offline openvista

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Randy...

I brought it up before, but have you ever tried an SHT75? If you're really suspicious about Davis' handling of the sensors it may be worth giving a sensor soldered at the Sensirion factory a try. I'm sure you have an old Davis SHTxx that you can sacrifice to solder directly to the 75 pins or insert into an acceptable connector.

Correct me if I'm wrong, Randy, but didn't you say that your 15s were fried too when it came to humidity?
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Offline ValentineWeather

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Randy...

I brought it up before, but have you ever tried an SHT75? If you're really suspicious about Davis' handling of the sensors it may be worth giving a sensor soldered at the Sensirion factory a try. I'm sure you have an old Davis SHTxx that you can sacrifice to solder directly to the 75 pins or insert into an acceptable connector.

So remove sht15 chip and solder SHT75 in its place? How is the 75 with humidity? Does it have the same high bias even after reconditioning the 31 nothing really changed.


Yes
Randy...

I brought it up before, but have you ever tried an SHT75? If you're really suspicious about Davis' handling of the sensors it may be worth giving a sensor soldered at the Sensirion factory a try. I'm sure you have an old Davis SHTxx that you can sacrifice to solder directly to the 75 pins or insert into an acceptable connector.

Correct me if I'm wrong, Randy, but didn't you say that your 15s were fried too when it came to humidity?

Yes it's not much better.
Randy

Offline drew1021

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Here's a thought, maybe Davis could eliminate soldering the sensors directly to the board by installing sockets directly on the pcb. It may not be physically possible though, I have not seen the shtxx pinout.
VP2 with 24 hour FARS. SHT-75. WU: KNCLEWIS2. CWOP/APRS: DW4712, COCORAHS: NC-FR-7

Offline jgentry

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This is interesting...drier air moved into central Alabama today.

Airport that is 15 miles N of me. 88/64

Acurite Temp/Hum sensor with lightning detector 89/65.5

Davis VP 2 with -8% offset 88/67.7
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline openvista

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This is interesting...drier air moved into central Alabama today.

Airport that is 15 miles N of me. 88/64

Acurite Temp/Hum sensor with lightning detector 89/65.5

Davis VP 2 with -8% offset 88/67.7

Yikes! By my calculations that's a >20% absolute humidity error, assuming the dew point of 67.7 is actually 75.7 without the offset.
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Offline ValentineWeather

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This is interesting...drier air moved into central Alabama today.

Airport that is 15 miles N of me. 88/64

Acurite Temp/Hum sensor with lightning detector 89/65.5

Davis VP 2 with -8% offset 88/67.7

So chasing humidity too.... :sad:
Randy

Offline Old Tele man

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This is interesting...drier air moved into central Alabama today.

Airport that is 15 miles N of me. 88/64

Acurite Temp/Hum sensor with lightning detector 89/65.5

Davis VP 2 with -8% offset 88/67.7

So chasing humidity too.... :sad:

Dewpoint (DP) is calculated from the TEMP and RH values, so if either one is off, so too will be the calculated DP value. However, TEMP is usually far more accurate than RH, so DP errors can usually be traced back to errant RH values more than TEMP values.
« Last Edit: July 22, 2018, 08:05:56 PM by Old Tele man »
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Offline dendrite

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Randy...

I brought it up before, but have you ever tried an SHT75? If you're really suspicious about Davis' handling of the sensors it may be worth giving a sensor soldered at the Sensirion factory a try. I'm sure you have an old Davis SHTxx that you can sacrifice to solder directly to the 75 pins or insert into an acceptable connector.

So remove sht15 chip and solder SHT75 in its place? How is the 75 with humidity? Does it have the same high bias even after reconditioning the 31 nothing really changed.


Yes
Randy...

I brought it up before, but have you ever tried an SHT75? If you're really suspicious about Davis' handling of the sensors it may be worth giving a sensor soldered at the Sensirion factory a try. I'm sure you have an old Davis SHTxx that you can sacrifice to solder directly to the 75 pins or insert into an acceptable connector.

Correct me if I'm wrong, Randy, but didn't you say that your 15s were fried too when it came to humidity?

Yes it's not much better.
The 75 is basically a 15 with slightly better RH specs between 10-90% (1.8% versus 2.0%). The difference is that it is presoldered by Sensirion. Solder the correct wires to an old 6-conductor Davis temp/hum and it becomes plug and play. With your FARS the temp response will be through the roof because of the low thermal mass...especially if you run it with the filter off.

http://www.wxforum.net/index.php?topic=14175.msg189484#msg189484

http://www.mouser.com/ds/2/682/Sensirion_Humidity_SHT7x_Datasheet_V5-469726.pdf

Offline ValentineWeather

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This is interesting...drier air moved into central Alabama today.

Airport that is 15 miles N of me. 88/64

Acurite Temp/Hum sensor with lightning detector 89/65.5

Davis VP 2 with -8% offset 88/67.7

So chasing humidity too.... :sad:

Dewpoint (DP) is calculated from the TEMP and RH values, so if either one is off, so too will be the calculated DP value. However, TEMP is usually far more accurate than RH, so DP errors can usually be traced back to the errant RH values more than TEMP values.

I'm talking about his -8% HUM offset. You either do that or have outrageous heat index readings. 120°+ for some.
Move it down 8-9% during heat of day and  then back up in the evening so morning humidity doesn't peak at 91% when it's 100% (Gets real old)  ](*,)
Randy

Offline jgentry

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This is interesting...drier air moved into central Alabama today.

Airport that is 15 miles N of me. 88/64

Acurite Temp/Hum sensor with lightning detector 89/65.5

Davis VP 2 with -8% offset 88/67.7

Yikes! By my calculations that's a >20% absolute humidity error, assuming the dew point of 67.7 is actually 75.7 without the offset.

Probably needed to note that the VP2 is sited at a location where it doesn’t get great air mixing if the wind is from the North.
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline jerryg

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Interesting comparison with new sht31 and three about 2years old and an old sht11. New showed 86.8/69 old 1 86.8/72 old 2 86.8/71 old 3 86.9/71 and sht11 86.8/69  the temps are really something.  Interesting how the sht11 was right on with the new 31.

Offline jerryg

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I forgot to put in my little acurite is showing 86/64 humidity is way off for a 31 sensor chip.

Offline ValentineWeather

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I forgot to put in my little acurite is showing 86/64 humidity is way off for a 31 sensor chip.

Because it's not reading high? My 2 acurite units are normal on humidity. 
Randy

Offline jerryg

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No because it is reading lower than all the other sensors i have, by 5% or more. I guess i will have to put it in the bag and see how close it comes to 75%.

Offline openvista

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I forgot to put in my little acurite is showing 86/64 humidity is way off for a 31 sensor chip.

Because it's not reading high? My 2 acurite units are normal on humidity.

Have you thought about, perhaps, leaving one of them outside in the shade during days without precipitation? I wonder how long it would stay in calibration?
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Offline ValentineWeather

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No because it is reading lower than all the other sensors i have, by 5% or more. I guess i will have to put it in the bag and see how close it comes to 75%.

From my testing the Davis sensors all read high. I didn't use a bag but going from my other instruments and ASOS the acurite is correct.
That was my point how can I have 2 acurite sht31's read normal with instruments and ASOS and then have 3 Davis sensors all high.
That's where the idea of contamination came in with 2 different companies mounting differently onto boards.
Randy

Offline ValentineWeather

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I forgot to put in my little acurite is showing 86/64 humidity is way off for a 31 sensor chip.

Because it's not reading high? My 2 acurite units are normal on humidity.

Have you thought about, perhaps, leaving one of them outside in the shade during days without precipitation? I wonder how long it would stay in calibration?

I'll try it and see if they drift apart.

Also Jerry FYI both these acurites have hit 95% in morning when Davis only 1 of 3 managed.
Randy

Offline jerryg

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I put mine out when light fog was expected and it pegged out at 99 and main station was reporting 97 and that was the general high for around the area. Not too impressed with my cheap meter, i might have to take it apart and see if it does have a 31 and how it is mounted.

Offline ValentineWeather

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I put mine out when light fog was expected and it pegged out at 99 and main station was reporting 97 and that was the general high for around the area. Not too impressed with my cheap meter, i might have to take it apart and see if it does have a 31 and how it is mounted.

Wait so it reads lower when it should and normal 99% in fog and you think it's wrong because it doesn't match what? Your Davis sensor? I'm very confused unless for some odd reason you don't believe in the high bias Davis sensors that are junk. 
Randy

Offline openvista

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From my testing the Davis sensors all read high. I didn't use a bag but going from my other instruments and ASOS the acurite is correct.
That was my point how can I have 2 acurite sht31's read normal with instruments and ASOS and then have 3 Davis sensors all high.
That's where the idea of contamination came in with 2 different companies mounting differently onto boards.

Well, keep in mind that Ron observed some weird behavior in the PCB-mounted sensors he obtained from Sensirion. Remember when he put one outside in the shade for a mere 6 hours without a cap and after that it read 5% high? Those were brand new sensors. See: http://www.wxforum.net/index.php?topic=34658.msg353147#msg353147

Your indoor Acurite sensors might not have a cap which would make for a good test against the Davis sensors which are, effectively, uncapped as well.

Just trying to get a handle on whether exposure itself will cause issues. If so, how much does it take to knock it out of calibration permanently?

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Offline jerryg

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Well i am going to wait and see how it looks in the am after being in the bag overnight. I have two new sht15's due in the am and i also ordered a sht75 which comes mounted and sealed from the manufacturer to do some testing, it has a little better humidity spec than the 31.

Offline ValentineWeather

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I've had my acurite out for couple hours now cloudy day matching in temperature and humidity is spot on. All 3  Davis units continue to run the normal high bias.
I see nothing that says the acurite is off just the opposite.

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« Last Edit: July 22, 2018, 08:42:15 PM by ValentineWeather »
Randy