Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 40968 times)

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Offline openvista

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No sir, bottom left, two pics, one good siting (according to them), one bad. Frankly, the gravel is white and I'd think of minimal effect.

Ah, I see now. Yes, that is a violation of the guidelines. I'm guessing they list that as "good" relative to the other horror shows that are catalogued there.
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Offline jgentry

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Aight gang, Iím going to try something on both of my stations. One being a passive shield with a month old SHT 31 and another one with a year old SHT-31 inside of a 24hr FARS. Both stations are around 6miles apart from each other. I decided to go on to the WL software and calibrated the humidity sensor by putting in the correct value based off of the current air temperature reported by the station itself and the DP temp from the nearby airport. Going to see how that goes and especially when the humidities reach near 100%
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Offline jerryg

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wu id  is KTXVICTO6

Offline jgentry

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Offline CW2274

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Aight gang, Iím going to try something on both of my stations. One being a passive shield with a month old SHT 31 and another one with a year old SHT-31 inside of a 24hr FARS. Both stations are around 6miles apart from each other.
With that distance, you'll need to study obs for quite a while to get a "feel" for what and how you're comparing, probably days at least.

Offline Dj1225

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Well after 2.96" of rain today here just north of Gulfport with rain rates 7.5-9.5" at certain points my three day old SHT31 made it to 99%  \:D/  I haven't seen 99% in almost 2 years! I wonder if I'll get the elusive 100% overnight or early in the morning?  :-k Do you guys think the sf2 filter along with the davis filter might calm down the wet bias the sensor has? My heat indices have come down from the 120+ range to a more respectable 110, but that still seems a tad bit to high.

Offline ValentineWeather

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If that were the case my strongly aspirated ISS and dirty two year old 31 would not have hit and stayed at both 99% and 1%. I'm sure the filter is there to help protect from dirt/dust, but it's becoming much more evident moisture is the culprit in sensor degradation, not aspiration. As a matter of fact, I'm using the two year old 31 now and it's just as good as the newer one.

I'm having a hard time finding 99% when did this occur?  Last 30 days which includes the humid season monson highest is low 90's.

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Randy

Offline Aardvark

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420 opinions so far,  One question?   Have you determined the reliability of the humidity sensor and have you the psychometer in possession for that?
Davis VP2 Plus; 24h  FARS; Extra Temp Humid sensor (2); Extra Temp Station (2); Soil Moisture/Temp  https://www.weatherlink.com/embeddablePage/show/1c484adbfa914d1aa10a58ad53ccd563/summary;Weatherlink IP;USB; MAC Mojave; Webcam;

Offline openvista

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420 opinions so far,  One question?   Have you determined the reliability of the humidity sensor and have you the psychometer in possession for that?

A drive by question deserves a drive by answer. The humidity sensor is unreliable and there are only a few decent psychrometers.  If you want to know more than that you're going to have to EXPEND EFFORT like the rest of us did.
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Offline Aardvark

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420 opinions so far,  One question?   Have you determined the reliability of the humidity sensor and have you the psychometer in possession for that?

A drive by question deserves a drive by answer. The humidity sensor is unreliable and there are only a few decent psychrometers.  If you want to know more than that you're going to have to EXPEND EFFORT like the rest of us did.

And a snarky answer deserves a  better answer.  I would assume then, if one sensor is fer-cocked,  then the whole unit might be similar.  I'd then send the whole dang thing back and buy something more professional and accurate.

However...  I must have been blessed, born under a happy cloud, lived right, prayed harder , danced the Wautse better than anyone else in my prime, because my VP2 is spot on.   I check it often and it is spot on.

 Yup, I'd save my pennies and buy something around the quality of a  WeatherHawk 620 Wireless Weather Station.  Now for the price, I'd assume it has to be accurate.

There are many variables that can cause fluctuations in temperature and humidity readings on a moment to moment situation.  the  consoles, Envoys,  can be off and have to be set in the software or on the logger, and in 10 minutes they are off again.

I just wanted to know if the person who started this thread had got the whole thing solved yet.  Sheesh.. You remind me of a certain world leader
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Offline openvista

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I would assume then, if one sensor is fer-cocked,  then the whole unit might be similar.  I'd then send the whole dang thing back and buy something more professional and accurate.

However...  I must have been blessed, born under a happy cloud, lived right, prayed harder , danced the Wautse better than anyone else in my prime, because my VP2 is spot on.   I check it often and it is spot on.

 Yup, I'd save my pennies and buy something around the quality of a  WeatherHawk 620 Wireless Weather Station.  Now for the price, I'd assume it has to be accurate.

If only we could trade your assumptions for something valuable!

No, people CAN'T just send it back for a refund. That's not how it works anywhere except for a short period after purchase.

And, no, just because something is expensive doesn't make it better. Spec sheets don't always reflect reality as this thread demonstrates.

Got any more cheap wise cracks, or are you ready to contribute to the thread? Perhaps start by telling us how you verify your VP2 is accurate. Maybe you have a psychrometer or hygrometer you trust? And, if so, how do you know that device is accurate?

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Offline Aardvark

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I would assume then, if one sensor is fer-cocked,  then the whole unit might be similar.  I'd then send the whole dang thing back and buy something more professional and accurate.

However...  I must have been blessed, born under a happy cloud, lived right, prayed harder , danced the Wautse better than anyone else in my prime, because my VP2 is spot on.   I check it often and it is spot on.

 Yup, I'd save my pennies and buy something around the quality of a  WeatherHawk 620 Wireless Weather Station.  Now for the price, I'd assume it has to be accurate.

If only we could trade your assumptions for something valuable!

No, people CAN'T just send it back for a refund. That's not how it works anywhere except for a short period after purchase.

And, no, just because something is expensive doesn't make it better. Spec sheets don't always reflect reality as this thread demonstrates.

Got any more cheap wise cracks, or are you ready to contribute to the thread? Perhaps start by telling us how you verify your VP2 is accurate. Maybe you have a psychrometer or hygrometer you trust? And, if so, how do you know that device is accurate?
  I did.  I suggested a sling psychrometer. Inexpensive and works fine .   

One could use something like a Kestrel, if they have one or access  and measure the temperature variance as well, assuming that the Kestrel is calibrated.  But a sling psychrometer, as ancient as they are , is fairly accurate.    I used one years ago and I mean years ago.   Wet the wick with distilled water, and spin away.  then consult tables for the humidity at that point.

Yes, I suggested that early on. 


How do I know it is correct.  Well, I have a couple of Kestrels that are calibrated  (which isn't fun to do).  I compared them with  the sling psychrometer and it was fairly within limits. I mean the Sling doesn't do decimals, but it worked for me.

 Now as far a cheap wise cracks,  there are none.  I am deeply offended by your insinuations and hope the moderator gets after you.

No,  my question was simple,  has the issue been resolved?  How far off was the SHT-31 unit from whatever was used?  And you got your panties in a knot after well beyond 400 responses?   My lord.    I am done offering any suggestion.

OH   I have had a weather station since 1990.  I originally and a WMII,  then the VP1 and now the VP2.   I have had the VP2 since 1993 and other than having to replace the ISS transmitter board once,  replace the super cap once, and have a fully populated station,soil station and so on,  I think I speak with some experience,   OH and the anemometer on the top of my mast is the original vp1 which is mysteriously working for over 20 years.
« Last Edit: July 17, 2018, 01:53:36 PM by Aardvark »
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Offline jerryg

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Ok just to put some info out there for those who want some accuracy in their sensor i asked Ryan over at Scaled Inst. about a calibrated sensor and he got word back from Davis they offer an nist traceable with paperwork for 100 dollars plus the price of a sensor. That would be the best way to have a known reference to work with.

Offline jerryg

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Yep that's what i would do, use the sensor for cal. purposes only. When you think of the price for some of the humidity gear to check a sensor for humidity accuracy it really isn't that much.

Offline openvista

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  I did.  I suggested a sling psychrometer. Inexpensive and works fine .   

...

Yes, I suggested that early on. 

Where? I only show one other contribution to this thread under your screen name. In it you stated that you'd rather not have to read the proceeding 163 posts and asked the O.P. whether he solved his problem. And he answered you.

Then you return hundreds of posts later and imply that you're too important to be bothered reading a bunch of "opinions" and that, once again, someone should furnish you the information you desire upon request?

Should we just put the report on your desk when we're *finally* done or would you like it emailed?

I am done offering any suggestion.

Indeed, there is no reason for you to be here since your station is always "spot on". I mean the rest of us can't even comprehend that the weather is constantly changing and, therefore, how difficult it is to calibrate our stations properly. Only YOU understand that, Aardvark.
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Offline CW2274

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If that were the case my strongly aspirated ISS and dirty two year old 31 would not have hit and stayed at both 99% and 1%. I'm sure the filter is there to help protect from dirt/dust, but it's becoming much more evident moisture is the culprit in sensor degradation, not aspiration. As a matter of fact, I'm using the two year old 31 now and it's just as good as the newer one.

I'm having a hard time finding 99% when did this occur?  Last 30 days which includes the humid season monson highest is low 90's.
This was back on 2/16, we were WOXOF (super rare here), I hit 99% starting at 10pm and stayed there til, wait for it.......
10:30am the next morning. Yes, 12 and half hours, the spread between temp and dew stayed at 0.3F the whole time. The service time on the 31 then was 22 months. Two months later I put in the other 31 because I was getting gigged for having too low of a dew when I was at 1%. The TUS ASOS and a RAWS both hit 1% also, just didn't stay there as long as I did. Obviously at that time the 31 was a full two years old.

Offline jerryg

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I just ordered a nist sensor so i will have a calibrated reference to check my working sensor. Ryan over at Scaled Inst. now has the sensor with calibration pricing on his site.

Offline Aardvark

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I just ordered a nist sensor so i will have a calibrated reference to check my working sensor. Ryan over at Scaled Inst. now has the sensor with calibration pricing on his site.
   Ryan has good things at a reasonable price.   
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Offline jgentry

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I'm testing three of these evaluation boards right now. RH spec: 1.5%

https://www.idt.com/products/sensor-products/humidity-sensors

I'll run them against my three SHT-31 evaluation boards as well.

Were you able to test those IDT sensors out?  How did they do?
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Offline openvista

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Ok just to put some info out there for those who want some accuracy in their sensor i asked Ryan over at Scaled Inst. about a calibrated sensor and he got word back from Davis they offer an nist traceable with paperwork for 100 dollars plus the price of a sensor. That would be the best way to have a known reference to work with.

If you check Davis' NIST reference publications (https://www.davisinstruments.com/product_documents/weather/spec_sheets/NIST_Traceable_Sensors.pdf), you'll see they only guarantee +/- 3% even after calibration. That's worse then the stated specs between 10 and 90%!

If you need a reference device, you can get the psychrometer that jgentry mentioned (http://www.iisusa.com/mobile/Psychro-Dyne-Battery-Operated-Psychrometer-Fahrenheit-BLACK-ORGANIC/productinfo/22014/) that's accurate to <1% for $175. That's only $25 more than Davis sensor + calibration, and is much more accurate. If you want 2% accuracy, you can get a Kestrel 5000 for $260 from Ryan. Those are pre-calibrated at the factory and have a 5 year warranty, including the calibration, and they measure more than just temp/humidity.

Please don't reward Davis for not standing behind their products. Davis could easily have cut this thread short by stepping up and promising to better protect the sensors in the future (installing the SF2 cap) and offering warranty replacements for anyone experiencing abnormally high humidity readings. They chose not to do that and, instead, put their retailers, like Ryan, in the line of fire.
« Last Edit: July 17, 2018, 04:23:30 PM by openvista »
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Offline jgentry

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Jerryg,

Iíve been keeping an eye on your station, the RW station at the EOC, and two nearby airport stations. I do know that the RW is installed on the roof with pavement and concrete around the general area. Airport has a similar setup except it isnít on the rooftop. Lol.

Even though your station is sited over grass, Iím beginning to think that it shouldnít cause your station to report DPs that are 4-5į higher than the Airport & RW during the daytime hours. It would be more sensible if it was 1-3į higher at times during transpiration but that should be it I would think. The RW & your station matches up well along with the airport at night when it comes to DP. During the daytime, the RW seems to read the same or 1į higher or lower than the airport where you are reading 2-4į higher consistently. The RW air temps during the dayare very representative of your area, even though itís instalked on top of a roof.

Iím getting the feeling that it doesnít matter if you have the SF2 filter or not, the SHT-31 is going to have a wet bias during the daytime hours.

Here are some currents at 3:18p

Jerryg 94/77

RW station: 93/73

ASOS 91/72

The only other idea I have is to calibrate the humidity on the console. That will work fairly well until the humidities get closer to 100% (late 80s & 90s).

My thinking might be wrong here but it doesnít make sense to me when a RW (for example) humidity sensor would perform better than the Davis.

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Offline ValentineWeather

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After 4 days with 2 new sensors, my conclusion they aren't much better than the old still running high on DP at those critical levels when its important to be most accurate (high heat index).

So basically I have 2 new sensors both now reach 98%.. Whoopie!  I'm beginning to think these sensors are just off. They surely don't come close to meeting the claimed specs when it counts. 
Randy

Offline openvista

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Iím getting the feeling that it doesnít matter if you have the SF2 filter or not, the SHT-31 is going to have a wet bias during the daytime hours.

I'm starting to get that impression too. jerryg initally stated with such confidence that the SF2 had kept his sensors accurate. The readings you posted would suggest otherwise.

I also recall Ron's (kcidwx) statement about heat ultimately being the only way to prevent too much moisture sitting against the sensor.

However, that makes one wonder why it is Rainwise sensors don't fall victim to this same problem?

The only other idea I have is to calibrate the humidity on the console. That will work fairly well until the humidities get closer to 100% (late 80s & 90s).

My tests, purely from the field involving 2 sensors of differing age, indicate the sensor's bias point crosses over somewhere in the 70 percentiles. Meaning, the 70s are the range where the sensor is least positively or negatively biased. Below that, once a sensor is out of specification, I've observed a wet bias (at least down to the 30% range - haven't been able to test below that). Above that starting around 80%, there's a dry bias that worsens as you approach 100% (although technically within spec).

The departures both in DP and humidity are worse when it's warm (above 80), but shows up when it's cooler too. Currently, my good sensor reads 63/47 and my bad sensor reads 64/50. That's a 3% humidity difference in the mid 50 percentiles. Still out of spec on the older sensor, but not nearly as bad as when it gets warmer. When it's in the 80s/90s and midrange humidities, I'm seeing 5-8% and 4-5 deg DP departures between the two sensors.
« Last Edit: July 17, 2018, 05:39:46 PM by openvista »
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Offline jgentry

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I know IDT HS 3001 spec sheet says that it can go out of spec if itís in high humidity conditions for a long time but Iím curious if it would behave the same way as the Sensirion sensors would or not?

I do know that the VP2 canít take IDTís sensor but Iím curious to how it would perform compared to Sensirionís sensors.
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Offline openvista

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After 4 days with 2 new sensors, my conclusion they aren't much better than the old still running high on DP at those critical levels when its important to be most accurate (high heat index).

So basically I have 2 new sensors both now reach 98%.. Whoopie!  I'm beginning to think these sensors are just off. They surely don't come close to meeting the claimed specs when it counts.

I guess a key question is the temp at which Sensirion tests the sensors' humidity spectrum. My guess would be 25C (77F) since I see mention of it along with 63% in the notes on their datasheet. If they aren't testing beyond that, then, indeed, there could be discrepancies elsewhere.
« Last Edit: July 17, 2018, 05:54:43 PM by openvista »
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