Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 68493 times)

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Offline jgentry

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Re: Which sling psychrometer should I buy?
« Reply #100 on: July 08, 2018, 06:38:04 PM »
Surprised that johnd hasn’t chimed in yet on this discussion thread. Wouldn’t mind in hearing his take on the Sensirion humidity sensor issues.
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Offline ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #101 on: July 08, 2018, 07:00:03 PM »
It was kcidwx what he posted about the Gadget with SHT31.

One thing that seriously concerned me with these SHT31 evaluation boards was the long term shift in accuracy on the humidity side. I only test a particular type of sensor in minimum groups of three. When a sensor company is wanting to bid on a government contract it's usually pretty easy for me to secure at least three for testing. Although that's not the case with these evaluation boards, it was more for my own curiosity. The issue is approximately 3 weeks after receiving all three of these evaluation boards, I ran a test at 50% RH. All three were reading low by -1.2%, -1.7% and -1.9%. Almost a year later they were reading +0.5% to +0.8%. I'm at a loss to explain why the large shift over time. I'm not quite ready to call it "drift" because that's an insane amount of drift. I was so concerned, I sent my NIST reference instrument back to Washington D.C. to get it certified again after it was just certified 6 months before. It came back having tested just fine. In the hundreds of sensors I've tested over 20+ years, I never seen this much shift before over the course of a year. I was thinking of getting a fourth one and seeing how it compares right out of the box against these three units that have been running for almost a year. Some sensors do require a conditioning period when first used or what I refer to as exercising the sensor. I know that with some sensor manufacturers when they send you a sample for evaluation, sometimes these are not their best spec'ed sensors. Often they are sensors that are usable but don't quite pass QC 100% for some reason. I wonder if that's what these evaluation boards are. I never did ask Sensirion. The only thing I can think of is since these SHT31 sensors don't have a filter over them, they have become contaminated with dust over time.

https://www.wxforum.net/index.php?topic=32439.msg328209#msg328209
« Last Edit: July 08, 2018, 07:04:53 PM by ValentineWeather »
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Offline Old Tele man

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Re: Which sling psychrometer should I buy?
« Reply #102 on: July 08, 2018, 07:16:47 PM »
YIKES -- IF dust can cause that kind of error, then what the heck will dirty outdoor (and far aspiration) air do?!?
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Offline jgentry

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Re: Which sling psychrometer should I buy?
« Reply #103 on: July 08, 2018, 07:37:34 PM »
It was kcidwx what he posted about the Gadget with SHT31.

One thing that seriously concerned me with these SHT31 evaluation boards was the long term shift in accuracy on the humidity side. I only test a particular type of sensor in minimum groups of three. When a sensor company is wanting to bid on a government contract it's usually pretty easy for me to secure at least three for testing. Although that's not the case with these evaluation boards, it was more for my own curiosity. The issue is approximately 3 weeks after receiving all three of these evaluation boards, I ran a test at 50% RH. All three were reading low by -1.2%, -1.7% and -1.9%. Almost a year later they were reading +0.5% to +0.8%. I'm at a loss to explain why the large shift over time. I'm not quite ready to call it "drift" because that's an insane amount of drift. I was so concerned, I sent my NIST reference instrument back to Washington D.C. to get it certified again after it was just certified 6 months before. It came back having tested just fine. In the hundreds of sensors I've tested over 20+ years, I never seen this much shift before over the course of a year. I was thinking of getting a fourth one and seeing how it compares right out of the box against these three units that have been running for almost a year. Some sensors do require a conditioning period when first used or what I refer to as exercising the sensor. I know that with some sensor manufacturers when they send you a sample for evaluation, sometimes these are not their best spec'ed sensors. Often they are sensors that are usable but don't quite pass QC 100% for some reason. I wonder if that's what these evaluation boards are. I never did ask Sensirion. The only thing I can think of is since these SHT31 sensors don't have a filter over them, they have become contaminated with dust over time.

https://www.wxforum.net/index.php?topic=32439.msg328209#msg328209

Thanks for posting! Reading through that thread tells me that the SHT-31 isn’t as good of a humidity sensor as the Sensirion specs makes them out to be.
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #104 on: July 08, 2018, 07:58:46 PM »
It was kcidwx what he posted about the Gadget with SHT31.

One thing that seriously concerned me with these SHT31 evaluation boards was the long term shift in accuracy on the humidity side. I only test a particular type of sensor in minimum groups of three. When a sensor company is wanting to bid on a government contract it's usually pretty easy for me to secure at least three for testing. Although that's not the case with these evaluation boards, it was more for my own curiosity. The issue is approximately 3 weeks after receiving all three of these evaluation boards, I ran a test at 50% RH. All three were reading low by -1.2%, -1.7% and -1.9%. Almost a year later they were reading +0.5% to +0.8%. I'm at a loss to explain why the large shift over time. I'm not quite ready to call it "drift" because that's an insane amount of drift. I was so concerned, I sent my NIST reference instrument back to Washington D.C. to get it certified again after it was just certified 6 months before. It came back having tested just fine. In the hundreds of sensors I've tested over 20+ years, I never seen this much shift before over the course of a year. I was thinking of getting a fourth one and seeing how it compares right out of the box against these three units that have been running for almost a year. Some sensors do require a conditioning period when first used or what I refer to as exercising the sensor. I know that with some sensor manufacturers when they send you a sample for evaluation, sometimes these are not their best spec'ed sensors. Often they are sensors that are usable but don't quite pass QC 100% for some reason. I wonder if that's what these evaluation boards are. I never did ask Sensirion. The only thing I can think of is since these SHT31 sensors don't have a filter over them, they have become contaminated with dust over time.

https://www.wxforum.net/index.php?topic=32439.msg328209#msg328209

Thanks for posting! Reading through that thread tells me that the SHT-31 isn’t as good of a humidity sensor as the Sensirion specs makes them out to be.

Agree
kcidwx does Government testing and had never seen that kind of drift ever over a short period.
I'm afraid his findings were legit.
Not sure what we can do  :?: Hope the price drops more and replace sensor every 6 months if humidity is important... :evil:
Randy

Offline jgentry

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Re: Which sling psychrometer should I buy?
« Reply #105 on: July 08, 2018, 08:24:26 PM »
I think he needs to send Davis an email or something and talk them into changing their sensor element.
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Offline ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #106 on: July 08, 2018, 08:29:30 PM »
I think he needs to send Davis an email or something and talk them into changing their sensor element.

Don't think he even owns a Davis his stuff was RM Young if memory serves.
Randy

Offline jgentry

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Re: Which sling psychrometer should I buy?
« Reply #107 on: July 08, 2018, 08:36:26 PM »
I think he needs to send Davis an email or something and talk them into changing their sensor element.

Don't think he even owns a Davis his stuff was RM Young if memory serves.


Darn...I wished I was born rich instead of “good looking.”
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Offline ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #108 on: July 08, 2018, 08:42:52 PM »
I think he needs to send Davis an email or something and talk them into changing their sensor element.

Don't think he even owns a Davis his stuff was RM Young if memory serves.


Darn...I wished I was born rich instead of “good looking.”

 :lol: ....Well his job has something to do with his toys I'm almost positive. Like sending the SHT31 to Washington for recalibration... ;)
Randy

Offline openvista

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Re: Which sling psychrometer should I buy?
« Reply #109 on: July 09, 2018, 10:10:10 AM »
It was kcidwx what he posted about the Gadget with SHT31.

One thing that seriously concerned me with these SHT31 evaluation boards was the long term shift in accuracy on the humidity side. I only test a particular type of sensor in minimum groups of three. When a sensor company is wanting to bid on a government contract it's usually pretty easy for me to secure at least three for testing. Although that's not the case with these evaluation boards, it was more for my own curiosity. The issue is approximately 3 weeks after receiving all three of these evaluation boards, I ran a test at 50% RH. All three were reading low by -1.2%, -1.7% and -1.9%. Almost a year later they were reading +0.5% to +0.8%. I'm at a loss to explain why the large shift over time. I'm not quite ready to call it "drift" because that's an insane amount of drift. I was so concerned, I sent my NIST reference instrument back to Washington D.C. to get it certified again after it was just certified 6 months before. It came back having tested just fine. In the hundreds of sensors I've tested over 20+ years, I never seen this much shift before over the course of a year. I was thinking of getting a fourth one and seeing how it compares right out of the box against these three units that have been running for almost a year. Some sensors do require a conditioning period when first used or what I refer to as exercising the sensor. I know that with some sensor manufacturers when they send you a sample for evaluation, sometimes these are not their best spec'ed sensors. Often they are sensors that are usable but don't quite pass QC 100% for some reason. I wonder if that's what these evaluation boards are. I never did ask Sensirion. The only thing I can think of is since these SHT31 sensors don't have a filter over them, they have become contaminated with dust over time.

https://www.wxforum.net/index.php?topic=32439.msg328209#msg328209

My guess on why ALL 3 units ran low initially is that the manufacturer KNOWS that they drift up quickly. So, at least initially and for a period of months after purchase, they'll be within specification. I bet most backyard weather enthusiasts don't even notice that their AWS humidity readings are moving upward because other fellow station owners are reporting similar humidities.
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Offline openvista

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Re: Which sling psychrometer should I buy?
« Reply #110 on: July 09, 2018, 10:24:39 AM »
I ordered a Kestrel 5000 meter (+/- 2% supposedly) and will be running some tests with it.

Kestrel not only calibrates and certifies every meter they sell, but they promise that if their unit drifts by more than 0.25% per year you can send the unit to them and they will recalibrate it for free within 5 years of purchase. Not sure how most people would determine which instrument is drifting excessively: their kestrel or the sensor they are checking -- but that's another matter.

They also offer a tune-up service for $75. They'll test and calibrate every weather parameter, clean it and install a new wind impeller & battery. Not bad.

I know the military uses these -- they've got a big contract with Kestrel. So do firefighters, athletic orgs, surveyers, shooting clubs and, of course, storm chasers who proudly display them on TV.

I figured it's probably the best you can do under $1000.
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Offline jgentry

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Re: Which sling psychrometer should I buy?
« Reply #111 on: July 09, 2018, 10:35:20 AM »
I ordered a Kestrel 5000 meter (+/- 2% supposedly) and will be running some tests with it.

Kestrel not only calibrates and certifies every meter they sell, but they promise that if their unit drifts by more than 0.25% per year you can send the unit to them and they will recalibrate it for free within 5 years of purchase. Not sure how most people would determine which instrument is drifting excessively: their kestrel or the sensor they are checking -- but that's another matter.

They also offer a tune-up service for $75. They'll test and calibrate every weather parameter, clean it and install a new wind impeller & battery. Not bad.

I know the military uses these -- they've got a big contract with Kestrel. So do firefighters, athletic orgs, surveyers, shooting clubs and, of course, storm chasers who proudly display them on TV.

I figured it's probably the best you can do under $1000.

Please let us know your results.
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline openvista

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Re: Which sling psychrometer should I buy?
« Reply #112 on: July 09, 2018, 10:44:10 AM »
I ordered a Kestrel 5000 meter (+/- 2% supposedly) and will be running some tests with it.

Kestrel not only calibrates and certifies every meter they sell, but they promise that if their unit drifts by more than 0.25% per year you can send the unit to them and they will recalibrate it for free within 5 years of purchase. Not sure how most people would determine which instrument is drifting excessively: their kestrel or the sensor they are checking -- but that's another matter.

They also offer a tune-up service for $75. They'll test and calibrate every weather parameter, clean it and install a new wind impeller & battery. Not bad.

I know the military uses these -- they've got a big contract with Kestrel. So do firefighters, athletic orgs, surveyers, shooting clubs and, of course, storm chasers who proudly display them on TV.

I figured it's probably the best you can do under $1000.

Please let us know your results.

Will do.

By the way, after swapping my sensors 36 hours ago my results have been consistent. The active shield remains lower in dew point by about 3-4 degrees compared to the passive shield with the older sensor in it.
Davis Vantage Pro2 AC FARS | https://marquetteweather.com | EW7933

Offline ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #113 on: July 09, 2018, 01:50:24 PM »
Proof it's all about age of sensor.
Ron sent me a PM with graphs that demonstrates what's been said here already. His new SHT31 is is running 6% lower humidity than 2 year old sensor only 10 feet distance away. I'll let him comment further if he chooses.

I'm thinking the lifespan on these SHT31 sensors for 2% humidity accuracy is between 6 months and 1 year.
Randy

Offline openvista

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Re: Which sling psychrometer should I buy?
« Reply #114 on: July 09, 2018, 02:03:48 PM »
I'm thinking the lifespan on these SHT31 sensors for 2% humidity accuracy is between 6 months and 1 year.

I'd concur with that estimate, particularly for aspirated sensors, judging by my own experimentation, analysis of past data and comparisons to nearby reliable sources.

There may be a reason Sensirion uses the prefix "SHT". Looks like they need to be flushed regularly (in non-arid regions). Not enjoying the thought of ordering $50 disposable kits 1-2x per year.
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Offline ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #115 on: July 09, 2018, 02:17:38 PM »


There may be a reason Sensirion uses the prefix "SHT".

 :lol: :lol:
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Offline ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #116 on: July 09, 2018, 03:02:07 PM »
I'll just add to my yearly list "to do" before summer hits. I have two stations online so couple $43 dollar sensors won't break the bank I guess each spring.... ](*,) Make though the summer is what's important, winter it's not as big a deal.
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Offline CW2274

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Re: Which sling psychrometer should I buy?
« Reply #117 on: July 09, 2018, 03:38:31 PM »
Proof it's all about age of sensor.
I'm thinking the lifespan on these SHT31 sensors for 2% humidity accuracy is between 6 months and 1 year.
My first 31 had almost two years on it, this current one about five months. I didn't notice the first one getting bad, I just replaced it to see if there was a noticeable difference even after two years of service, and I can honestly say, I did not. Not saying there isn't one, just not noticeable to me, and I hawk this stuff like nobody's business.

Offline WheatonRon

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Re: Which sling psychrometer should I buy?
« Reply #118 on: July 09, 2018, 05:12:54 PM »
SHT 31 drifts with age! I have two VP2 stations both have the SHT31 sensor--one station is about 2 months old and it has a 24 hour fan (CWOP FW3075) and the other station is about 2 years old and has a daytime fan (CWOP CW5020). Right now, in a western suburb of Chicago (where I live), the new station reports 91, humidity of 44 and dewpoint of 66 whereas the older station reports 91, humidity of 50 and dewpoint of 70. The stations are about 10 feet apart on the same fence and the sun is making both fans rumble today! The newer station always gets lower humidity and dewpoint readings as demonstrated by the attached QC graphs from CWOP.  I am not a fan (pun intended) of the QC checks on CWOP but this does raise some eyebrows!
« Last Edit: July 09, 2018, 05:17:56 PM by WheatonRon »
Davis VP2 with SHT31 (3 complete VP2 systems—2 with a daytime fan and 1 that has a 24 hour fan); CWOP--CW5020, FW3075 and FW4350; WU--KILWHEAT17, KILWHEAT36 and KILWHEAT39; WeatherCloud.net; CoCoRaHS--IL-DP-132; and Weatherlink 2.0

Offline CW2274

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Re: Which sling psychrometer should I buy?
« Reply #119 on: July 09, 2018, 05:48:33 PM »
SHT 31 drifts with age!
All sensors drift with age. The question is to what degree (pun intended, or not...) :roll: the 31 does, and does climate/aspiration/rate of aspiration/dust/dirt/pollen/phase of the moon.........

Offline WheatonRon

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Re: Which sling psychrometer should I buy?
« Reply #120 on: July 09, 2018, 06:01:00 PM »
Proof it's all about age of sensor.
Ron sent me a PM with graphs that demonstrates what's been said here already. His new SHT31 is is running 6% lower humidity than 2 year old sensor only 10 feet distance away. I'll let him comment further if he chooses.

I'm thinking the lifespan on these SHT31 sensors for 2% humidity accuracy is between 6 months and 1 year.

See my post below.
Davis VP2 with SHT31 (3 complete VP2 systems—2 with a daytime fan and 1 that has a 24 hour fan); CWOP--CW5020, FW3075 and FW4350; WU--KILWHEAT17, KILWHEAT36 and KILWHEAT39; WeatherCloud.net; CoCoRaHS--IL-DP-132; and Weatherlink 2.0

Offline ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #121 on: July 09, 2018, 06:08:08 PM »
Thanks Ron, good job... =D> it helps clarify what we mostly had figured out. Until they do something different with the 31 either update or go with a new version I'll just swap sensors in spring provided they don't jack price up.

For those living in arid climates sub 60 dew points you should be good for several years.
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Offline CW2274

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Re: Which sling psychrometer should I buy?
« Reply #122 on: July 09, 2018, 06:11:32 PM »
Yes, your two 31's have a big discrepancy, when I pulled my two year old one, they did not, hence the "does climate/aspiration/rate of aspiration/dust/dirt/pollen/phase of the moon........." tongue in cheek response.

Offline jgentry

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Re: Which sling psychrometer should I buy?
« Reply #123 on: July 09, 2018, 06:21:30 PM »
Here is some information I received from the good folks at the NWS

Hi Justin! Here is the response below about your weather equipment question which we received from our electronics technicians.  Hope this helps. 🙂                                                                                                         -----------------------------------------------------------------------                                   Our Dew Point Sensor is the Vaisala DTS1.  This sensor uses a HUMICAP® capacitive thin film polymer sensor to determine RH. The measured capacitance of the DTS1 probe will vary depending on the amount of moisture in the air. The resistance of the PT 100  temperature sensor is used to determine the sensor air temperature. During periods of high humidity, the sensor electronics will turn on the sensor heater element to improve sensor humidity resolution. The algorithm then computes the dew point temperature using the measured sensor temperature and capacitance values.

Our calibration procedures include verifying accuracy every 90 days, there are no adjustments.  We also replace the sensor every 18 months as a time change requirement. 

We also use a NWS 1088 hygrothermometer to determine ambient air temperature.  This sensor uses a fan to draw air in and across the temperature probe, thus the temperature is aspirated.  Our calibrations procedures consist of cleaning, adjusting, and verifying accuracy every 90 days.                                                                                                              ----------------------------------------------------------------------------
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Offline WheatonRon

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Re: Which sling psychrometer should I buy?
« Reply #124 on: July 09, 2018, 06:24:46 PM »
Thanks Ron, good job... =D> it helps clarify what we mostly had figured out. Until they do something different with the 31 either update or go with a new version I'll just swap sensors in spring provided they don't jack price up.

For those living in arid climates sub 60 dew points you should be good for several years.

Good news! Actually, I have 3 VP2s, but only 2 upload to the internet. I just checked my VP2 that doesn't upload to the internet and is 40 feet away from the other stations on the same fence, which also has the daytime fan and SHT31, but is probably 3 years old! The readings are almost identical to my two year old VP2, so the good news? The SHT31 sensor doesn't get much worse after two years! Celebrate and tear down the goal posts! Fortunately, Ryan Wilhour sells these at a reasonable price--but something is clearly wrong--unless you live in Arizona!
Davis VP2 with SHT31 (3 complete VP2 systems—2 with a daytime fan and 1 that has a 24 hour fan); CWOP--CW5020, FW3075 and FW4350; WU--KILWHEAT17, KILWHEAT36 and KILWHEAT39; WeatherCloud.net; CoCoRaHS--IL-DP-132; and Weatherlink 2.0

 

anything