Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 48097 times)

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Offline jerryg

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Yes and no, i had the sf2 on the old and new for a few days and then put Davis filter on the new one to see if i could see any difference but could not detect any. Now today i just replaced both filters with the silk to test further. I am trying out several things right now out of curiosity if nothing else  :grin:

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Yes and no, i had the sf2 on the old and new for a few days and then put Davis filter on the new one to see if i could see any difference but could not detect any. Now today i just replaced both filters with the silk to test further. I am trying out several things right now out of curiosity if nothing else  :grin:
Thanks. No doubt SF2 provides better protection than the stock filter does, but as far as the silk...? I think the SF2 keeps the sensor itself from becoming saturated as it would unprotected or with the stock filter. I truly believe it helps with recovery.

Offline WheatonRon

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Yes and no, i had the sf2 on the old and new for a few days and then put Davis filter on the new one to see if i could see any difference but could not detect any. Now today i just replaced both filters with the silk to test further. I am trying out several things right now out of curiosity if nothing else  :grin:
Thanks. No doubt SF2 provides better protection than the stock filter does, but as far as the silk...? I think the SF2 keeps the sensor itself from becoming saturated as it would unprotected or with the stock filter. I truly believe it helps with recovery.

Isnít goofy that we have to put ďband-aidsĒ on Davis products to make them work correctly? Oh well, there isnít much (any?) competition in their product price range so I guess we have keep developing and using our band aids.
« Last Edit: November 17, 2019, 05:18:30 PM by WheatonRon »
Davis VP2 with SHT31 (3 complete VP2 systemsó2 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

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Yes and no, i had the sf2 on the old and new for a few days and then put Davis filter on the new one to see if i could see any difference but could not detect any. Now today i just replaced both filters with the silk to test further. I am trying out several things right now out of curiosity if nothing else  :grin:
Thanks. No doubt SF2 provides better protection than the stock filter does, but as far as the silk...? I think the SF2 keeps the sensor itself from becoming saturated as it would unprotected or with the stock filter. I truly believe it helps with recovery.

Isnít goofy that we have to put ďband-aidsĒ on Davis products to make them work correctly? Oh well, there isnít much (any?) competition in their product price range so I guess we have keep developing and using our band aids.
Not sure I'd call the SF2 a band aid, it's a Sensirion made option. Regardless, if it makes my PWS better without breaking the bank, I'm all for it. Suppose that's why I graduated from an SHT11 to a 15 to a 31, put in a case fan, use a Rainwise 111 as my hard wired tipper, and have my anny separate from my ISS.  All good. =D>

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Jerryg, have you seen a manufacture date on your sensor?

C2274, youíve probably mentioned this before, but what do you use to mount the SF2?

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Jerryg, have you seen a manufacture date on your sensor?

C2274, youíve probably mentioned this before, but what do you use to mount the SF2?
I know the question was not directed at me, but I know of no way of to determine the manufacture date other than go to Davis or Sensirion (I guess). There is labeling, but not with an apparent date.

As far as mounting, our board is not holed for the four feet of the cap (besides, I wanted nothing to do with specialized glue involved) so I trimmed the four feet closely and used very fine bare wire to cinch it down onto the "gummy" substance surrounding the sensor itself with the two holes provided in the board while leaving the membrane unrestricted. I can't guarantee a water tight seal, but I'll bet I have one.

Online CW7491

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The newest Davis 31 I have has a sticker on the back with the part number and Ē190610 DSĒ on it. I just assumed thatís either a manufacture or inspection date and initials. Total assumption obviously, but seemed reasonable.

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The newest Davis 31 I have has a sticker on the back with the part number and Ē190610 DSĒ on it. I just assumed thatís either a manufacture or inspection date and initials. Total assumption obviously, but seemed reasonable.
My old 31 from 3+ years ago has I60216SB. There is a "16" in it....

Offline jgentry

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Yes and no, i had the sf2 on the old and new for a few days and then put Davis filter on the new one to see if i could see any difference but could not detect any. Now today i just replaced both filters with the silk to test further. I am trying out several things right now out of curiosity if nothing else  :grin:

Iím curious to what your results are.
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

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Yes and no, i had the sf2 on the old and new for a few days and then put Davis filter on the new one to see if i could see any difference but could not detect any. Now today i just replaced both filters with the silk to test further. I am trying out several things right now out of curiosity if nothing else  :grin:

Iím curious to what your results are.
I think testing would have to be somewhat long term, not just a few days, but IMO a properly sealed SF2 cap is without a doubt the way to best protect the sensor. That's exactly what it's designed to do.

Offline jerryg

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What i was looking at is if there is any difference in response time between the types of filters. The silk has the best time of the three with the sf2 being next and the Davis last which my be due to the large size of the filter rather than the material. The factory filters fit down tight on the sensor so have very little air space to moisten up and better response. Now the silk filter will not let water pass just vapor and where i live out in the country the only thing i really need to worry about is dust which is pretty heavy when the farmers are plowing for planting and harvesting the crops. No real gas vapors to worry about. We had very dense fog this am and the 31 did not show any bias once again, so looking good still.

Offline jgentry

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What i was looking at is if there is any difference in response time between the types of filters. The silk has the best time of the three with the sf2 being next and the Davis last which my be due to the large size of the filter rather than the material. The factory filters fit down tight on the sensor so have very little air space to moisten up and better response. Now the silk filter will not let water pass just vapor and where i live out in the country the only thing i really need to worry about is dust which is pretty heavy when the farmers are plowing for planting and harvesting the crops. No real gas vapors to worry about. We had very dense fog this am and the 31 did not show any bias once again, so looking good still.

Nice!
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

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Fingers crossed on some pea soup tonight! Phfttt, I'll even take some chicken broth...

Offline jerryg

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 :grin: well i have had a bunch of upper 90's on the humidity side and the sensor has worked just fine so far no sign of any bias at all. I am still waiting for one of the long drawn out periods where the humidity is in the high 90's for several days.

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:grin: well i have had a bunch of upper 90's on the humidity side and the sensor has worked just fine so far no sign of any bias at all. I am still waiting for one of the long drawn out periods where the humidity is in the high 90's for several days.
Well to be sure I'll never see that, just looking for some true saturation for several hours.  Thanks for checking in and bringing more hopeful news!

Offline tweatherman

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Yes and no, i had the sf2 on the old and new for a few days and then put Davis filter on the new one to see if i could see any difference but could not detect any. Now today i just replaced both filters with the silk to test further. I am trying out several things right now out of curiosity if nothing else  :grin:
Thanks. No doubt SF2 provides better protection than the stock filter does, but as far as the silk...? I think the SF2 keeps the sensor itself from becoming saturated as it would unprotected or with the stock filter. I truly believe it helps with recovery.

What case fan are you using and is it solar powered?

Thanks,
tweatherman

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Yes and no, i had the sf2 on the old and new for a few days and then put Davis filter on the new one to see if i could see any difference but could not detect any. Now today i just replaced both filters with the silk to test further. I am trying out several things right now out of curiosity if nothing else  :grin:
Thanks. No doubt SF2 provides better protection than the stock filter does, but as far as the silk...? I think the SF2 keeps the sensor itself from becoming saturated as it would unprotected or with the stock filter. I truly believe it helps with recovery.

What case fan are you using and is it solar powered?

Thanks,
tweatherman
No solar. It's 120vac from the house then has a transformer included that converts it to 12vdc for the fan. The size is 80x80x25mm and there's a ton of choices. I suggest no less than 20CFM.

Online fkapp

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I just read the whole thread to understand this whole SHT 31 situation.
Seems like Jerry has had better luck with this newest sensor.
How is it going for others out there with recent new SHT 31 sensors.
I know time will tell, but trying to understand if the quality of these has changed recently, or maybe Davis is doing something differently as part of build process


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I just read the whole thread to understand this whole SHT 31 situation.
Seems like Jerry has had better luck with this newest sensor.
How is it going for others out there with recent new SHT 31 sensors.
Mine's fine too... :roll:

Offline weather34

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Mine is fine too installed January 2019  :roll:
"The internet a place where people who you never met or have known make it their mission to disagree with everything you say or do , call it socially inept noise"..

https://weather34.com/homeweatherstation/meteobridge-mb-smart.html

Online fkapp

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ok. am on the fence about purchasing VP2 after reading this thread.
I mean sounds like a known defect that will be triggered by our damp winters here in Buffalo Ny.
In effect need to change sensors like tires i guess if you sign up for that by going VP2. ](*,)

Offline DaleReid

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I'm not an engineer, but I would think from the reading that there are a limited number of ways for a humidity sensor,  of the types that electronic stations use and excluding blonde hair or horse hair gizmos of the 40s, can work.

That being said, why would a sensor be substantially different from others, if the design were nearly the same.  Why would one be more sensitive to high humidity damage than others?

I know that Young and Vaisala have some very long life, extreme conditions, types of humidity sensors.  Are their construction that much more expensive than the glut of low cost things that are like the SHT stuff or the little almost pennies, ones for Arduino etc?

Anyone know the science behind it?
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Online fkapp

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I think part of it comes down to manufacturing and assembly process quality.
The weather monitor II I had for over 15 years was great, but had the different type of sensor pin installed vs this on board install.
Davis has their own custom sensor which again raises quality questions.
They seem to not being in a hurry to evolve and more troublesome is seems quality focus is not what used to be.

All the same the NWS changes sensors every 18 months, but my guess this is driven by precaution vs necessity beacause sensor is shot from humidity exposure
Not the science answer you were looking for, but seems to be cost driven IMHO.
What is the life expectancy and cost of these other stations?
Go there because total cost of ownership equation needs to include a new $50 SHT 10 times over ten years, all of a sudden Davis is not the bargain it used to be and further complicates and maybe changes decision process.
No news from Davis is confusing. Did they change something to address the issue or is it a lost cause no one knows.

And one more thing is peace of mind, is my sensor shot or is this the accurate reading. in the end that is why we are so passionate re this topic. 
Yes i dont own a VP2 yet, but this is all going through my mind as a prospective buyer.  Sharing so the group and Davis sees what this quality issue is doing and creates real question if VP2 is correct investment based on current state of things.


« Last Edit: December 08, 2019, 01:52:09 PM by fkapp »

Online CW7491

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I'm not an engineer, but I would think from the reading that there are a limited number of ways for a humidity sensor,  of the types that electronic stations use and excluding blonde hair or horse hair gizmos of the 40s, can work.

That being said, why would a sensor be substantially different from others, if the design were nearly the same.  Why would one be more sensitive to high humidity damage than others?

I know that Young and Vaisala have some very long life, extreme conditions, types of humidity sensors.  Are their construction that much more expensive than the glut of low cost things that are like the SHT stuff or the little almost pennies, ones for Arduino etc?

Anyone know the science behind it?

I am no engineer and am no expert on the science, but I am familiar with the history for Davis. Johnd and Pro Data used to have this very well documented, but I am not sure they still do. I also know that the old analog film capacitor element Davis used to use was superior to the current capacitor/digital technology, despite what the specs say. Why that is is a great question.

My first VP2 was circa 2005 and had the analog temp/hum sensor. On paper, specs weren't as good as they promote with the current SHT31. The specs for the temp sensor (a platinum wire thermistor) weren't as good and at the extremes, resolution in readings degraded. But in practice, I found these sensors to be superior in the humidity realm. The specs are still available online as +/-3% in humidity and +/-4% above 90%. In real world conditions, these sensors did the 2 things most people complain about with the current Sensirion lineup: they routinely hit 100% humidity and did not exhibit a wet bias as they "dried out" better when humidity dropped.

In 2006 I bought a second VP2 and noticed almost immediately that the humidity and dew point readings weren't as accurate (it would never hit 100% humidity and at lower humidity levels it had a considerable wet bias).  This new station had the Sensirion SHT11 and Davis has been using Sensirion ever since. As far as I can tell, the weaknesses of these sensors continue now nearly 14 years later. As I also understand it, the reason for the switch was the obsolescence of the humidity sensing element for the analog sensor. Initially, the SHT11 did not have any coating protecting the soldering joints of the digital sensor and they had a rash of failures. Davis then began protecting the solder joints with a coating. Davis then briefly offered the improved-spec SHT15 shortly before going to the SHT31. By all accounts, the temperature performance of this sensor is outstanding, which is not insignificant. On the humidity side however, there is no way it lives up to the +/-2% humidity spec. The sensors are not designed for high humidity environments for prolonged periods of time creating an offset in the reading. Each chip is supposed to have a temperature and linear calibration correction for humidity loaded onto the chip. Maybe these are extremely accurate in a dry, laboratory setting, but these corrections are seemingly insufficient in a weather station. Plus many of us have experienced a further wet bias offset present specifically in Davis mounted sensors compared to homemade Sensirion sensors we've begun using.

As johnd has pointed out numerous times, aside from a few of us who are frustrated by the humidity performance, most users are probably either oblivious or indifferent to the humidity performance, so if it hasn't been improved in 14 years, I can't imagine many resources are being put into improving accuracy. Plus the other issue is compared to what? The VP2 is still a great weather station for the price and even the Rainwise brand has gone to the Sensirion sensor, so it does seem that the options for accurate, low cost humidity sensors designed for meteorological monitoring is quite limited.

Online CW2274

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I may be completely in left field, but my 31 is about 6 months old and I still think using the SF2 cap from the get go (which I did, never have before) may aid in not letting the sensor get too "wet" and delaying proper drying. Regardless, I'm still happy with it....that's my story and I'm sticking to it.