Author Topic: Is it possible to restore SHT31 humidity sensor  (Read 724 times)

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Offline JCA433

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Is it possible to restore SHT31 humidity sensor
« on: September 18, 2023, 12:29:05 PM »
Is it possible to restore the SHT 31 humidity sensor?   The dew point temperatures are 1 to 2 F higher than sling psychrometer reading when humidity is less than 75%.  The sling psychrometer I am using is the US Weather Bureau Type from Kessler Thermometer Corp.  This is a very good sling psychrometer and highly recommend it. It is advisable to use a good sling psychrometer to occasionally check sensor humidity and dew point readings.
I read that it may be possible to restore some humidity sensors by heating to 175 F for 24 hours.

Offline CW7491

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Re: Is it possible to restore SHT31 humidity sensor
« Reply #1 on: September 18, 2023, 01:09:45 PM »
Unfortunately I have been down this path for years and years. Baking the sensor in accordance with the manufacturer’s recommendation will bring the sensor back into calibration temporarily, but if you live in a high humidity environment, these Sensirion sensors simply have a wet dewpoint/humidity bias that I suppose is just a limitation of the technology. They will struggle to reach 100% at saturation and will read 5-7% high in humidity and 3-5°F high in dewpoint in the mid to low range. If you calibrate with software slope and offset as best you can, when the humidity dries out for an extended period, the high bias is reduced and your corrections will be artificially low. I’ve searched in vain for a better solution, but it appears one does not exist at the moment. I still have two analog Davis sensors that performed much better, but sadly I can’t find a 2 pin thin film capacitance humidity element that will work on such low power. The original elements are pretty tired. I might just have to move to the desert …

Offline JCA433

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Re: Is it possible to restore SHT31 humidity sensor
« Reply #2 on: September 18, 2023, 02:18:26 PM »
The temperature measurements seem very good and do not seem to drift much.
It is the humidity sensor especially in humid climates that have drift problems.  This problem is not limited to Davis humidity sensors. The other humidity sensors from weather stations on Weather Underground have very high dew point temperatures also some 2 to 3 F above mine.

Offline CW7491

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Re: Is it possible to restore SHT31 humidity sensor
« Reply #3 on: September 18, 2023, 03:01:33 PM »
Yes, temperature performance is very good. These Sensirion sensors have a heater like many high end sensors from Vaisala etc and Sensirion has begun to advertise their use to help prevent this “creep”. Unfortunately this brings in power requirements and obviously complicates the temperature measurement. I noticed this is one of the reasons the NWS uses two sensors.

Offline JCA433

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Re: Is it possible to restore SHT31 humidity sensor
« Reply #4 on: September 18, 2023, 03:42:37 PM »
I have a fan aspirated weather station and the higher air flow can increase the rate of humidity sensor drift.  Greater air flow benefits temperature measurement but not so good for humidity sensor.  Better to put the temperature sensor in aspirated shield and humidity sensor in passive shield.

I have tangle foot on mounting pole to prevent ant infestation problems and wonder if this might contaminate the humidity sensor.

Offline CW7491

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Re: Is it possible to restore SHT31 humidity sensor
« Reply #5 on: September 18, 2023, 09:27:59 PM »
I doubt it’s the tangle foot. I’ve tried every conceivable explanation over the years. It’s just the sensor. In a laboratory, it is probably right on spec. Practically though, in your backyard, good luck. I’m still using Davis’ old analog gauge (Part #7346.029) that was discontinued 15 years ago and getting better results.

As far as aspirated temp and passive humidity, that’s exactly what the NWS is doing. I posted about their new TRH sensor setup for ASOS that will be deployed next year in the NOAA/NWS section. Vaisala HMP 155 probe that measures temp and RH to calculate dewpoint in a passive shield. The HMP155 comes with an optional additional temp sensor that they are using in an aspirated RM Young shield for temperature readings
« Last Edit: September 19, 2023, 05:24:13 PM by CW7491 »

Offline jgentry

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Re: Is it possible to restore SHT31 humidity sensor
« Reply #6 on: December 10, 2023, 07:29:00 PM »
I just installed a brand new Davis VP2 station at Petals From The Past in Jemison, AL. So far the humidity sensor is performing well with no wet bias. 

You can see the comparison here with the nearby ASOS

Petals From The Past https://www.weatherlink.com/embeddablePage/show/66eab9495ce5496ba281d83872dcb2bf/summary



Shelby County Airport (ASOS) https://mesowest.utah.edu/cgi-bin/droman/meso_base_dyn.cgi?stn=KEET&unit=0&time=&hours=24&hour1=00&day1=11&month1=12&year1=2023&radius=25&past=0&order=1
Davis Vantage Pro2 & WeatherFlow Tempest. WU: KXALJEMI2, KALJEMIS7, KFLPANAM363 & KALTHORS2. CWOP/APRS: C6353 & E6358