Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 85140 times)

0 Members and 1 Guest are viewing this topic.

Offline jgentry

  • Forecaster
  • *****
  • Posts: 529
The weather station market is so small that companies are not going to manufacture a humidity sensor with specs just for weather stations. Most humidity sensors are designed for indoor use. One of their biggest uses is for HVAC duct testing. That includes the Sensirion sensors.

As I said earlier in the thread, the SHT-31 was not designed for meteorological use and that should be obvious by its limited normal operating temperature and humidity ranges. But those limited operating ranges are normal for humidity sensors because again, they are not designed for meteorological applications. Davis must have felt that this was the best sensor to do the job even though it was not designed for meteorological applications.

I only know of one humidity sensor that is designed for meteorological applications. That is the one I use at home, the Vaisala HMT337. From the Vaisala literature it states:

HMT337 Configurations

Vaisala HUMICAP Humidity and Temperature Transmitter

HMT337 is ideal for the most demanding process and meteorological measurements in high-humidity condensing environments.


It is a rock solid sensor across the board. If Davis were to use it in the VP2 the cost of the station would probably double.

Another note, the RainWise MK III stations use the Sensirion SHT-15 sensor and as far as I know, they do not have the same issues as Davis does.
Davis Vantage Pro2 & WeatherFlow Tempest. WU: KXALJEMI2, KALJEMIS7, KFLPANAM363 & KALTHORS2. CWOP/APRS: C6353 & E6358

Offline ValentineWeather

  • Forecaster
  • *****
  • Posts: 6230
    • LiVE WEATHER
Saw this on Sparkfun today.

Hygrometers: A Cautionary Tale
Our (kinda) epic journey about learning the proper way to handle humidity sensors.

https://www.sparkfun.com/news/1090


Humidity Sensors make use of a conductive polymer to measure relative humidity. If that polymer
gets too dry (or over-saturated) the sensor won't function properly, but that can be reversed.
Whenever we use one of these sensors on our designs, we put them through a re-conditioning
procedure to ensure that they keep their factory calibration
. If you expose your sensor to a really dry
environment for a prolonged period of time (or saturate it with water) you may have to run it through
the same process.

https://cdn.sparkfun.com/datasheets/Sensors/Weather/Hygrometer_ReCon.pdf
Randy

Offline Old Tele man

  • Singing in the rain...
  • Forecaster
  • *****
  • Posts: 1365
Saw this on Sparkfun today.

Hygrometers: A Cautionary Tale
Our (kinda) epic journey about learning the proper way to handle humidity sensors.

https://www.sparkfun.com/news/1090


Humidity Sensors make use of a conductive polymer to measure relative humidity. If that polymer
gets too dry (or over-saturated) the sensor won't function properly, but that can be reversed.
Whenever we use one of these sensors on our designs, we put them through a re-conditioning
procedure to ensure that they keep their factory calibration
. If you expose your sensor to a really dry
environment for a prolonged period of time (or saturate it with water) you may have to run it through
the same process.

https://cdn.sparkfun.com/datasheets/Sensors/Weather/Hygrometer_ReCon.pdf

...and, that was written FIVE (5) YEARS ago.
« Last Edit: July 22, 2018, 09:01:16 AM by Old Tele man »
SYS: Davis VP2 Vue/WL-IP & Envoy8X/WL-USB;
DBX2 & DBX1 Precision Digital Barographs
CWOP: DW6988 - 2 miles NNE of Cortaro, AZ
WU - KAZTUCSO202, Countryside

Offline jgentry

  • Forecaster
  • *****
  • Posts: 529
How did your reconditioned humidity sensor do this morning?
Davis Vantage Pro2 & WeatherFlow Tempest. WU: KXALJEMI2, KALJEMIS7, KFLPANAM363 & KALTHORS2. CWOP/APRS: C6353 & E6358

Offline ValentineWeather

  • Forecaster
  • *****
  • Posts: 6230
    • LiVE WEATHER
How did your reconditioned humidity sensor do this morning?

We had rain so didn't hit 98% for any.. The newest now over week old SHT31 hit 97% multiple times the reconditioned unit ran lower as it did before even new made only 96%.  What I did notice even though the reconditioned didn't peak well it averaged about 1 dewpoint lower yesterday so I'm baking 3 more units today after reading Sparks Fun conditions all sensors in designs for top performance.

They use to skip the hydration at 75% 20 hours and said it was a mistake in the link above. They bake their units at 125c, any higher I'm thinking things start melting.
 
 
« Last Edit: July 22, 2018, 10:55:31 AM by ValentineWeather »
Randy

Offline Jim's Weather

  • Forecaster
  • *****
  • Posts: 330
The weather station market is so small that companies are not going to manufacture a humidity sensor with specs just for weather stations. Most humidity sensors are designed for indoor use. One of their biggest uses is for HVAC duct testing. That includes the Sensirion sensors.

As I said earlier in the thread, the SHT-31 was not designed for meteorological use and that should be obvious by its limited normal operating temperature and humidity ranges. But those limited operating ranges are normal for humidity sensors because again, they are not designed for meteorological applications. Davis must have felt that this was the best sensor to do the job even though it was not designed for meteorological applications.

I only know of one humidity sensor that is designed for meteorological applications. That is the one I use at home, the Vaisala HMT337. From the Vaisala literature it states:

HMT337 Configurations

Vaisala HUMICAP Humidity and Temperature Transmitter

HMT337 is ideal for the most demanding process and meteorological measurements in high-humidity condensing environments.


It is a rock solid sensor across the board. If Davis were to use it in the VP2 the cost of the station would probably double.

I got some PM's about this comment so let me clarify. Manufacturers don't design humidity sensors specifically for weather stations. However, they will often say their sensor is ideal for weather station use. Then they slap you with the fine print about the normal operating temperature and humidity range and what you can expect if the sensor is subjected to conditions outside those ranges. They are simply saying you can use our sensor in a weather station but don't expect optimal performance outside the normal operating temperature and humidity range. I'm sure Davis tested these sensors thoroughly before shipping them in the VP and concluded that the sensor had acceptable performance outside the sensors normal operating temperature and humidity range. Now what Davis thought was acceptable versus what you are expecting may be two different things  :lol: Also, I'm not saying this is the cause of the Davis sensor issue as the problem seems to occur even within the normal operating temperature and humidity range.

If you have high expectations of a particular weather station then before you buy that weather station, you should read the entire sensor datasheet from the sensor manufacturer. Read them for every single sensor on the weather station to see if it's acceptable to you. Just reading the sensors specifications doesn't tell the whole story.

I like to use the analogy of the webcam, security camera. It's funny how when I shop for a camera and it says "Outdoor Security Camera" (emphasis on outdoor) and then I check the operating specifications and it says operating temperature 4F to 120F. Outdoor? Really? Not in a Midwest winter! If you call the manufacturer, they will say, we're not saying you can't use it outside that temperature range. We just don't guarantee optimal performance outside that range and you'll have to try it outside the operating temperature range and see if it's acceptable to you.

Again, I'm not saying this has anything to do with the Davis sensor issue. I'm also not saying there is nothing wrong with the Davis sensor. It's more of a general comment on humidity sensors and why they don't have a wide operating temperature and humidity range when the manufacturer lists "weather stations" as one of their sensors uses.
Meteorological Technician
NWS Certified Aviation Weather Observer
B.S. Meteorology

Offline openvista

  • Forecaster
  • *****
  • Posts: 470
    • marquetteweather.com
As stated before, variablilty is the norm with the SHT31. Currently (8pm):

New station (24 hour fan)T 71, H 84
2 year old station (daytime fan)T 72, H 79
2.5 year old station (daytime fan)T 72, H 82.

So your new station runs the wettest?

If so, then Randy's 2 new out-of-spec sensors are not a fluke. That would begin to point toward a raft of defective sensors currently for sale.

Or was the point that all 3 sensors trade places in terms of accuracy?
Davis Vantage Pro2 AC FARS | https://marquetteweather.com | EW7933

Offline WheatonRon

  • Forecaster
  • *****
  • Posts: 1124
    • WUnderground
As stated before, variablilty is the norm with the SHT31. Currently (8pm):

New station (24 hour fan)T 71, H 84
2 year old station (daytime fan)T 72, H 79
2.5 year old station (daytime fan)T 72, H 82.

So your new station runs the wettest?

If so, then Randy's 2 new out-of-spec sensors are not a fluke. That would begin to point toward a raft of defective sensors currently for sale.

Or was the point that all 3 sensors trade places in terms of accuracy?

Yes, my newest station is running wet compared to my others. Originally wasnt doing that but this past week it shifted! Go figure.
Davis VP2 with SHT31 (3 complete VP2 systems2 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 openvista

  • Forecaster
  • *****
  • Posts: 470
    • marquetteweather.com
The Atlas uses a Sensirion SHT31 temp/humidity chip.  This is an upgrade from the SHT21 used in the 5n1.

OK, this is interesting. That's from the Acurite Atlas thread (https://www.wxforum.net/index.php?topic=33456.msg348911#msg348911).  Can anyone else confirm this?
Davis Vantage Pro2 AC FARS | https://marquetteweather.com | EW7933

Offline openvista

  • Forecaster
  • *****
  • Posts: 470
    • marquetteweather.com
Found an Atlas station online: https://www.wunderground.com/personal-weather-station/dashboard?ID=KCADUBLI16#history/tgraphs/s20180722/e20180722/mdaily. It's reporting 69/60 (temp/dp). Nearby airports are mostly reporting 57 dp, so it would appear to be running high as well (by over 6% humidity).
Davis Vantage Pro2 AC FARS | https://marquetteweather.com | EW7933

Offline openvista

  • Forecaster
  • *****
  • Posts: 470
    • marquetteweather.com
Found another Atlas (https://www.wunderground.com/personal-weather-station/dashboard?ID=KOHBEDFO15). It's reporting dew point of 69 and nearby airports are reporting 60-63.
Davis Vantage Pro2 AC FARS | https://marquetteweather.com | EW7933

Offline ValentineWeather

  • Forecaster
  • *****
  • Posts: 6230
    • LiVE WEATHER
Maybe using the SHT31?
Someone needs to find a better sensor that's accurate, I'm not sold on any of the Sensirion sensors for meteorological use now even the 15 had high bias just not as bad but also never went above 95%.

Randy

Offline openvista

  • Forecaster
  • *****
  • Posts: 470
    • marquetteweather.com
Maybe using the SHT31?

Looks like it based on the report from nincehelser. Because of NDAs signed by beta testers, it's going to be hard to confirm. Remember this is for a product that hasn't even been released and the prototypes are less than 1 year old.

I found exactly two of these units online and both had an out of spec, wet humidity bias. One of them was high by approx 15%! Anyone still salivating to get an Atlas?
Davis Vantage Pro2 AC FARS | https://marquetteweather.com | EW7933

Offline DoctorKnow

  • Forecaster
  • *****
  • Posts: 1860
Found another Atlas (https://www.wunderground.com/personal-weather-station/dashboard?ID=KOHBEDFO15). It's reporting dew point of 69 and nearby airports are reporting 60-63.

That is a Bloomsky. Atlas does not show UV

Offline ocala

  • Forecaster
  • *****
  • Posts: 4179
Maybe using the SHT31?
Someone needs to find a better sensor that's accurate, I'm not sold on any of the Sensirion sensors for meteorological use now even the 15 had high bias just not as bad but also never went above 95%.
Would the Vaisala sensor work with a Davis?
The blues had a baby and they named it Rock & Roll

Offline Old Tele man

  • Singing in the rain...
  • Forecaster
  • *****
  • Posts: 1365
Maybe using the SHT31?
Someone needs to find a better sensor that's accurate, I'm not sold on any of the Sensirion sensors for meteorological use now even the 15 had high bias just not as bad but also never went above 95%.

So...MAYBE...we're gonna have to chose between:
  • top-end limited, somewhat accurate, SHT-15
  • top-end capable, somewhat accurate, SHT-31
SYS: Davis VP2 Vue/WL-IP & Envoy8X/WL-USB;
DBX2 & DBX1 Precision Digital Barographs
CWOP: DW6988 - 2 miles NNE of Cortaro, AZ
WU - KAZTUCSO202, Countryside

Offline Old Tele man

  • Singing in the rain...
  • Forecaster
  • *****
  • Posts: 1365
Maybe using the SHT31?
Someone needs to find a better sensor that's accurate, I'm not sold on any of the Sensirion sensors for meteorological use now even the 15 had high bias just not as bad but also never went above 95%.
Would the Vaisala sensor work with a Davis?

Not currently...they use different communication protocols.
SYS: Davis VP2 Vue/WL-IP & Envoy8X/WL-USB;
DBX2 & DBX1 Precision Digital Barographs
CWOP: DW6988 - 2 miles NNE of Cortaro, AZ
WU - KAZTUCSO202, Countryside

Offline openvista

  • Forecaster
  • *****
  • Posts: 470
    • marquetteweather.com
Maybe using the SHT31?
Someone needs to find a better sensor that's accurate, I'm not sold on any of the Sensirion sensors for meteorological use now even the 15 had high bias just not as bad but also never went above 95%.

So...MAYBE...we're gonna have to chose between:
  • top-end limited, somewhat accurate, SHT-15
  • top-end capable, somewhat accurate, SHT-31

If you're referring to humidity readings between 95-100% I don't think the 31s are any better in that respect than the 15s. Besides, that's not the issue. Technically, if the 31 hits 96.5% or more during saturated conditions, it's within spec. That's a separate issue.

The main issue is the range from approx 40 - 70% where most humidity readings occur during the day in temperate, non-arid climates. Those readings, especially during the summer, are not "somewhat accurate" unless you think having a 10-15 degree heat index error is acceptable. Of course, being in the desert, that's not an issue for you generally.
« Last Edit: July 22, 2018, 02:00:39 PM by openvista »
Davis Vantage Pro2 AC FARS | https://marquetteweather.com | EW7933

Offline ValentineWeather

  • Forecaster
  • *****
  • Posts: 6230
    • LiVE WEATHER
I'm wondering if some software can't fix this issue.
I don't know enough about how they put code in sensor but I've got 2-SHT31's by Acurite that doesn't display the high bias just the opposite almost right with the airport all the time. 

What did they do different with these $15 dollar units vs their new Atlas running high also?  Maybe they didn't realize there was an issue. Looks like they are following Davis even having aspiration by fan, maybe they thought everything was good.
Randy

Offline DoctorKnow

  • Forecaster
  • *****
  • Posts: 1860
Currently...

Fine offset... 89 F DP 78 H 72
Acurite 5n1 91 82 76
Atlas 89 79 73
Acurite tower 89 76 67

Offline jgentry

  • Forecaster
  • *****
  • Posts: 529
I'm wondering if some software can't fix this issue.
I don't know enough about how they put code in sensor but I've got 2-SHT31's by Acurite that doesn't display the high bias just the opposite almost right with the airport all the time. 

What did they do different with these $15 dollar units vs their new Atlas running high also?  Maybe they didn't realize there was an issue. Looks like they are following Davis even having aspiration by fan, maybe they thought everything was good.

Maybe thats what RW did. They probably pregammed the console to correct the inaccuracies of the SHT-15 in certain humidity ranges.
Davis Vantage Pro2 & WeatherFlow Tempest. WU: KXALJEMI2, KALJEMIS7, KFLPANAM363 & KALTHORS2. CWOP/APRS: C6353 & E6358

Offline drew1021

  • Senior Contributor
  • ****
  • Posts: 211
Yes, I was thinking the same thing. Maybe a software update would correct the wet bias.
Another thought. I wonder how the Visalia probe would perform in the 24 hour FARS?
VP2 with 24 hour FARS. WU: KNCLEWIS2. CWOP/APRS: DW4712, COCORAHS: NC-FR-7

Offline jgentry

  • Forecaster
  • *****
  • Posts: 529
Yes, I was thinking the same thing. Maybe a software update would correct the wet bias.
Another thought. I wonder how the Visalia probe would perform in the 24 hour FARS?

Im no expert on Vaisala but I would think the same. It might age the sensor quicker in high humidity environments. Might have to switch out every 11-12 months instead of 18 months.
Davis Vantage Pro2 & WeatherFlow Tempest. WU: KXALJEMI2, KALJEMIS7, KFLPANAM363 & KALTHORS2. CWOP/APRS: C6353 & E6358

Offline ValentineWeather

  • Forecaster
  • *****
  • Posts: 6230
    • LiVE WEATHER
I'm wondering if some software can't fix this issue.
I don't know enough about how they put code in sensor but I've got 2-SHT31's by Acurite that doesn't display the high bias just the opposite almost right with the airport all the time. 

What did they do different with these $15 dollar units vs their new Atlas running high also?  Maybe they didn't realize there was an issue. Looks like they are following Davis even having aspiration by fan, maybe they thought everything was good.

Maybe thats what RW did. They probably pregammed the console to correct the inaccuracies of the SHT-15 in certain humidity ranges.

 I'll look at WD software and see what the calibrator can do. Cumulus can only really trim 1 dp and still reach 100%. That's about 3-4% humidity in area of issue but isn't a full fix.
Randy

Offline openvista

  • Forecaster
  • *****
  • Posts: 470
    • marquetteweather.com
Maybe thats what RW did. They probably pregammed the console to correct the inaccuracies of the SHT-15 in certain humidity ranges.

I don't know how RW would do that, as the errors don't seem consistent. Since everyone's siting these in different environments, each sensor will degrade differently. Some may stay within spec for months or years. Others arrive out of spec. Seems likely most go out within a year, but we don't have enough data yet to say for sure what the timeline and exact triggering mechanism (how many times must it encounter saturated air and for how long?).

For instance, currently, my 15 is reporting 66/56 and my 31 is reporting 65/56. That's around 70% humidity for both. Both are within spec according to my Kestrel and nearby reference stations.

See WheatonRon's report about his three different 31s all having different humidity readings (5% apart).

I have two 31s, one of them is right on and the other is way off. So if Davis tried to perform a calibration curve on both, it wouldn't result in two accurate sensors. I'd still have 1 accurate and 1 inaccurate sensor and, quite possibly, two inaccurate sensors.
« Last Edit: July 22, 2018, 02:52:25 PM by openvista »
Davis Vantage Pro2 AC FARS | https://marquetteweather.com | EW7933