Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 5208 times)

0 Members and 1 Guest are viewing this topic.

Offline jgentry

  • Senior Contributor
  • ****
  • Posts: 169
    • Gentry Field Station
What is Davis reply so far if any of you has contacted them?

...

As stated previously, I contacted the Davis support person (Brett Lane) and alerted him to the discussion in this thread and provided him a link hereto. He acknowledged receipt of my email but nothing more. If I were Davis I would be very careful posting anything in this Forum, rather I would post something on the Davis website, then post in this thread a link to the Davis website. My guess, Davis is talking internally whether or how to address these issues on its website. If I was a betting man, they will not publish anything; rather just consider our comments should they decide to replace the horribly aging VP-2!

Unrelated to the above, I have been reading this Forum for about 5 years and lately contributing, and there has been considerable fluff in many threads. Not this one. I just spent several hours re reading this thread and identified 11 useful takeaways. When I finish my list, I will seek editing review by contributors to this thread then post. Setting the record straight by a very knowledgeable poster, kcidwx, has been most refreshing.

Thanks Ron! This forum has been very educational for me. Hopefully Davis will take consideration to the information on this thread and make the necessary changes. Thanks gang for all your input!  Please keep them going.
Davis Vantage Pro2. SHT 31. 24hr FARS. WU: KXALJEMI2 & KALTHORS2 CWOP: C6353 & C6358

  

Offline ValentineWeather

  • Forecaster
  • *****
  • Posts: 4812
I should add just because they (Davis Inst) probably aren't going to admit any wrong doing something good may  still come from thread.
It brought to everyone's attention humidity has issues and proper handling of these sensitive sensors is important. Davis did fix the problem with Vue just didn't admit the problem existed.
Randy

Offline jgentry

  • Senior Contributor
  • ****
  • Posts: 169
    • Gentry Field Station
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.

How did those sensors perform?
Davis Vantage Pro2. SHT 31. 24hr FARS. WU: KXALJEMI2 & KALTHORS2 CWOP: C6353 & C6358

  

Offline jgentry

  • Senior Contributor
  • ****
  • Posts: 169
    • Gentry Field Station
Just reading the spec sheet, Davis would do well to get their transmitter to take I2c and switch over to IDT’s HS 3001. That’s IMO.

But aren't SHT35 and HS3001 pretty much on a par? Actually SHT35 looks better in the 90-100% region. Where would HS3001 score over SHT35?

Not saying there aren't differences - they're just not too obvious from the specs, other than details like 0.1% difference in nominal tolerance which may well get lost anyway in slightly different interpretation of the specs - eg what is the difference between tolerance and accuracy or nominal vs max - and differences in eg binning. And yes Davis would need to move away from Sensibus to modern I2C if these were to be relevant to use in VP2 units.

johnd- After carefully reading through the datasheet of the HS3001, I think you’re correct in your overall thinking. At first, I thought the HS3001 was a more “durable sensor” and it maybe is (when it comes to performance while the sensor is contaminated with dust or debris) but that’s about it. Other than that, it behaves basically the same as the Sensirion sensors. Here is what the HS 3001 datasheet says:

High humidity conditions:

“Important note: The HS300x series sensors are optimized to perform best in the more common temperature and humidity ranges of 10°C to 50°C and 20% RH to 80% RH, respectively. If operated outside of these conditions for extended periods, especially at high humidity levels, the sensors may exhibit an offset. In most cases, this offset is temporary and will gradually disappear once the sensor is returned to normal temperature and humidity conditions. The amount of the shift and the duration of the offset vary depending on the duration of exposure and the severity of the relative humidity and temperature conditions. The time needed for the offset to disappear can also be decreased by using the procedures described in sections 10 and 11.”


“Storage and Handling
Recommendation: Once the sensors are removed from their original packaging, store them in metal-in antistatic bags.
Avoid using polyethylene antistatic bags as they may affect sensor accuracy.
The nominal storage conditions are 10 to 50°C and humidity levels within 20% to 60%RH. If stored outside of these conditions for extended periods of time, the sensor readings may exhibit an offset. The sensor can be reconditioned and brought back to its calibration state by applying the following procedure:
1. Bake at a temperature of 100°C with a humidity < 10%RH for 10 to 12 hours.
2. Rehydrate the sensor at a humidity of 75%RH and a temperature between 20 to 30°C for 12 to 14 hours.”


Soo.... I think the best thing for Davis is to update their transmitter board to take pure I2c and switch to the SHT 35. But even more importantly, be sure that all sensors are packaged correctly.
Davis Vantage Pro2. SHT 31. 24hr FARS. WU: KXALJEMI2 & KALTHORS2 CWOP: C6353 & C6358

  

Offline ValentineWeather

  • Forecaster
  • *****
  • Posts: 4812
We really could use the special coating to protect from contamination.
As mentioned humidity sensors are not designed to be continually aspirated.
Not taking these special precautions will surely end sensor life sooner than later.
And yes handling and storage and not open air is very important. Open air starts the end of live for sensor prematurely who knows how old they are when we get them. 12 months 24?  Who knows. 
Randy

Offline jgentry

  • Senior Contributor
  • ****
  • Posts: 169
    • Gentry Field Station
I’m curious to know why Belfry Boy hasn’t make his version of the SHT-31 sensor like he did the 15?
Davis Vantage Pro2. SHT 31. 24hr FARS. WU: KXALJEMI2 & KALTHORS2 CWOP: C6353 & C6358

  

Offline kcidwx

  • Forecaster
  • *****
  • Posts: 314
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.

How did those sensors perform?

The LCD screen died on one of the eval boards so the testing is on hold until I get a replacement.
Meteorological Technician
NWS Certified Aviation Weather Observer
B.S. Meteorology

Offline jgentry

  • Senior Contributor
  • ****
  • Posts: 169
    • Gentry Field Station
We really could use the special coating to protect from contamination.
As mentioned humidity sensors are not designed to be continually aspirated.
Not taking these special precautions will surely end sensor life sooner than later.
And yes handling and storage and not open air is very important. Open air starts the end of live for sensor prematurely who knows how old they are when we get them. 12 months 24?  Who knows.

But man, it’s definitely frustrating to get a “Aged” sensor when you paid for a fresh new one
Davis Vantage Pro2. SHT 31. 24hr FARS. WU: KXALJEMI2 & KALTHORS2 CWOP: C6353 & C6358

  

Offline jgentry

  • Senior Contributor
  • ****
  • Posts: 169
    • Gentry Field Station
But just thinking about it, I bet this is the reason why RainWise hasn’t produced a FARS version of the MK III.

But I do believe that the RW stations have a similar problem in terms of humidity as the Davis except for in the 90 percentile humidities.

Just looking at some of RW’s data on the company’s website, their temperature readings have a warm bias (most likely due to the shield they use).

In response to the ASOS being warmer than the Davis: I think the ASOS stations may have a warm bias. There is a MMTS at the Extension office in my county and they also have the Davis VP2 locates inside the same fence. The Davis (24hr FARS might add) reads sometimes warmer or colder than the MMTS. Typically by 1°
Davis Vantage Pro2. SHT 31. 24hr FARS. WU: KXALJEMI2 & KALTHORS2 CWOP: C6353 & C6358

  

Offline ValentineWeather

  • Forecaster
  • *****
  • Posts: 4812
But just thinking about it, I bet this is the reason why RainWise hasn’t produced a FARS version of the MK III.

But I do believe that the RW stations have a similar problem in terms of humidity as the Davis except for in the 90 percentile humidities.

Just looking at some of RW’s data on the company’s website, their temperature readings have a warm bias (most likely due to the shield they use).

In response to the ASOS being warmer than the Davis: I think the ASOS stations may have a warm bias. There is a MMTS at the Extension office in my county and they also have the Davis VP2 locates inside the same fence. The Davis (24hr FARS might add) reads sometimes warmer or colder than the MMTS. Typically by 1°

The ASOS here for sure plus bias +2° on average. Don't see that with nearby AWOS or any of the Mesonet stations in area. All follow real close with aspirated Davis. 

You know who believes the ASOS are those with thermometers on the porches.   :lol:

I said this in jest my neighbor lady says the airport matches her porch thermometer...I told her airport thermometer is in middle of open field nothing but grass and hay no heat source at all and your porch thermometer is 2" off siding of house on southside of all places. What's wrong here? 
« Last Edit: July 13, 2018, 10:03:11 PM by ValentineWeather »
Randy

Offline CW2274

  • Forecaster
  • *****
  • Posts: 3535
    • Conditions @ CW2274
But just thinking about it, I bet this is the reason why RainWise hasn’t produced a FARS version of the MK III.
Even if that's true, which I highly doubt, I don't care if fans are bad for the humidity portion of the sensor, I'll never quit using one til something comes along that's superior.
The benefits a FARS to me far outweighs the potential negative.

Offline WheatonRon

  • Forecaster
  • *****
  • Posts: 849
    • WUnderground
But just thinking about it, I bet this is the reason why RainWise hasn’t produced a FARS version of the MK III.
Even if that's true, which I highly doubt, I don't care if fans are bad for the humidity portion of the sensor, I'll never quit using one til something comes along that's superior.
The benefits a FARS to me far outweighs the potential negative.

Having a FARS setup results in better temperature accuracy and not so accurate humidity readings—so pick your poison. Which is more important to you?
Davis VP2 with 24 hour FARS, SHT31 (3 complete systems-2 for uploading to the internet the other system for test and play); CWOP--CW5020 and FW3075; WU--KILWHEAT17 and KILWHEAT36; WeatherCloud.net; CoCoRaHS--IL-DP-132; Rainwise 111

Offline ValentineWeather

  • Forecaster
  • *****
  • Posts: 4812
But just thinking about it, I bet this is the reason why RainWise hasn’t produced a FARS version of the MK III.
Even if that's true, which I highly doubt, I don't care if fans are bad for the humidity portion of the sensor, I'll never quit using one til something comes along that's superior.
The benefits a FARS to me far outweighs the potential negative.

I was thinking about that also. But then I remembered the RM Young dual sensor temp/hum they run in aspirated shields. Maybe we took the fact because ASOS doesn't aspirate humidity it wasn't done.
Just look at sensor on the RMY temperature tracker and shields they offer both passive and aspirated for the dual sensor probe.
Randy

Offline CW2274

  • Forecaster
  • *****
  • Posts: 3535
    • Conditions @ CW2274
But just thinking about it, I bet this is the reason why RainWise hasn’t produced a FARS version of the MK III.
Even if that's true, which I highly doubt, I don't care if fans are bad for the humidity portion of the sensor, I'll never quit using one til something comes along that's superior.
The benefits a FARS to me far outweighs the potential negative.

Having a FARS setup results in better temperature accuracy and not so accurate humidity readings—so pick your poison. Which is more important to you?
Look at it this way, what happens when your passive shield is running 4-5F warmer than ambient? What's that do to your humidity.... #-o

Offline WheatonRon

  • Forecaster
  • *****
  • Posts: 849
    • WUnderground
But just thinking about it, I bet this is the reason why RainWise hasn’t produced a FARS version of the MK III.
Even if that's true, which I highly doubt, I don't care if fans are bad for the humidity portion of the sensor, I'll never quit using one til something comes along that's superior.
The benefits a FARS to me far outweighs the potential negative.

Having a FARS setup results in better temperature accuracy and not so accurate humidity readings—so pick your poison. Which is more important to you?
Look at it this way, what happens when your passive shield is running 4-5F warmer than ambient? What's that do to your humidity.... #-o

As many PWS owners would conclude, “you show me a happy moose, and I will find him an acorn!”

« Last Edit: July 13, 2018, 10:38:19 PM by WheatonRon »
Davis VP2 with 24 hour FARS, SHT31 (3 complete systems-2 for uploading to the internet the other system for test and play); CWOP--CW5020 and FW3075; WU--KILWHEAT17 and KILWHEAT36; WeatherCloud.net; CoCoRaHS--IL-DP-132; Rainwise 111

Offline jgentry

  • Senior Contributor
  • ****
  • Posts: 169
    • Gentry Field Station
But just thinking about it, I bet this is the reason why RainWise hasn’t produced a FARS version of the MK III.
Even if that's true, which I highly doubt, I don't care if fans are bad for the humidity portion of the sensor, I'll never quit using one til something comes along that's superior.
The benefits a FARS to me far outweighs the potential negative.

Having a FARS setup results in better temperature accuracy and not so accurate humidity readings—so pick your poison. Which is more important to you?

Personally temperature, but being in a county that’s one of the top fruit crop producers in the state of Alabama (especially peaches) accurate temperature and dew point reports are a must for growers. But I have noticed that a Davis passive shield with a constant breeze performs very well IMO.
Davis Vantage Pro2. SHT 31. 24hr FARS. WU: KXALJEMI2 & KALTHORS2 CWOP: C6353 & C6358

  

Offline jgentry

  • Senior Contributor
  • ****
  • Posts: 169
    • Gentry Field Station
But just thinking about it, I bet this is the reason why RainWise hasn’t produced a FARS version of the MK III.
Even if that's true, which I highly doubt, I don't care if fans are bad for the humidity portion of the sensor, I'll never quit using one til something comes along that's superior.
The benefits a FARS to me far outweighs the potential negative.

I was thinking about that also. But then I remembered the RM Young dual sensor temp/hum they run in aspirated shields. Maybe we took the fact because ASOS doesn't aspirate humidity it wasn't done.
Just look at sensor on the RMY temperature tracker and shields they offer both passive and aspirated for the dual sensor probe.

If we are able to get a good SHT-31 (purely new one) with the Sensirion filter cap, I think we would be in good shape, even with FARS. The only thing is that we will need to switch them out around 9-12 months due to constant high humidities aging the sensor.
Davis Vantage Pro2. SHT 31. 24hr FARS. WU: KXALJEMI2 & KALTHORS2 CWOP: C6353 & C6358

  

Offline ValentineWeather

  • Forecaster
  • *****
  • Posts: 4812
RM YOUNG Temperature tracker they suggest the passive shield with the dual sensor.  ](*,)
Welp maybe I was thinking wrong.
Randy

Offline jgentry

  • Senior Contributor
  • ****
  • Posts: 169
    • Gentry Field Station
RM YOUNG Temperature tracker they suggest the passive shield with the dual sensor.  ](*,)
Welp maybe I was thinking wrong.

Probably the most idealistic set-up to keep contaminates out of the sensor, and slowing down the aging process of the sensor.

But according to jerryg, he doesn’t have any problems since he has Sensirion’s filter cap protecting the sensor.
Davis Vantage Pro2. SHT 31. 24hr FARS. WU: KXALJEMI2 & KALTHORS2 CWOP: C6353 & C6358

  

Offline kcidwx

  • Forecaster
  • *****
  • Posts: 314
So my daughter was on my computer reading this thread and saw the complaints about the SHT-31 not going to 100%. It started raining here a little while ago and she wanted me to share that her weather station goes to 100%. She said maybe you guys should buy one like hers.  :lol:

Hers is a Weather Monitor II.  :lol:  She thinks it's pretty neat she's the only one around here that gets to 100% on WU.  :lol:





My Vaisala HMT337 a few feet away is showing 99.5%. Typically, hers is running about +3% over my HMT337.
Meteorological Technician
NWS Certified Aviation Weather Observer
B.S. Meteorology

Offline jgentry

  • Senior Contributor
  • ****
  • Posts: 169
    • Gentry Field Station
So my daughter was on my computer reading this thread and saw the complaints about the SHT-31 not going to 100%. It started raining here a little while ago and she wanted me to share that her weather station goes to 100%. She said maybe you guys should buy one like hers.  :lol:

Hers is a Weather Monitor II.  :lol:  She thinks it's pretty neat she's the only one around here that gets to 100% on WU.  :lol:





My Vaisala HMT337 a few feet away is showing 99.5%. Typically, hers is running about +3% over my HMT337.


Hahaha.  Tell her that they don’t make em’ like they used too. Shoot, that’s not bad considering how old the WM II is.
Davis Vantage Pro2. SHT 31. 24hr FARS. WU: KXALJEMI2 & KALTHORS2 CWOP: C6353 & C6358

  

Offline jgentry

  • Senior Contributor
  • ****
  • Posts: 169
    • Gentry Field Station
So my daughter was on my computer reading this thread and saw the complaints about the SHT-31 not going to 100%. It started raining here a little while ago and she wanted me to share that her weather station goes to 100%. She said maybe you guys should buy one like hers.  :lol:

Hers is a Weather Monitor II.  :lol:  She thinks it's pretty neat she's the only one around here that gets to 100% on WU.  :lol:





My Vaisala HMT337 a few feet away is showing 99.5%. Typically, hers is running about +3% over my HMT337.

One of my VP2s (SHT-31 in passive shield) reached 100% for the first time this morning. Will note that it’s a new sensor (only around a month old).
« Last Edit: July 14, 2018, 08:22:44 AM by jgentry »
Davis Vantage Pro2. SHT 31. 24hr FARS. WU: KXALJEMI2 & KALTHORS2 CWOP: C6353 & C6358

  

Offline jerryg

  • Forecaster
  • *****
  • Posts: 552
  • weather is never boring
    • victoria texas weather
Update on sf2 and Davis filter at same time. When i put new sensor in use on my main station i used the sf2 filter, the same has been in use on the old sensor for a year, i removed the sf2 from the old and put the Davis in place of it for comparison with the new sensor. Well the sf2 filter specs say the sensor performance is the same with or without the filter so i put the sf2 back on the old sensor with the Davis filter on also. I can not see any difference in performance of the old sensor readings with both filters in use. It looks like it could give added protection in high dust and humidity areas. I figured the manufacturer says the sf2 does not have any effect on the sensor readings that maybe it would be worth a try even though the bigger filter does add some lag time on the readings. It is hard to measure lag time using the Davis set up because of the high sampling rates of the temp and humidity readings. Since i started using the sf2 i have not had any humidity related problems like i did when using the big Davis filter. I will never run a sensor without the sf2 in place. I am going to let the test sensor run for some time with both filters in place to see if anything shows up down the road.

Offline kcidwx

  • Forecaster
  • *****
  • Posts: 314
So my daughter was on my computer reading this thread and saw the complaints about the SHT-31 not going to 100%. It started raining here a little while ago and she wanted me to share that her weather station goes to 100%. She said maybe you guys should buy one like hers.  :lol:

Hers is a Weather Monitor II.  :lol:  She thinks it's pretty neat she's the only one around here that gets to 100% on WU.  :lol:





My Vaisala HMT337 a few feet away is showing 99.5%. Typically, hers is running about +3% over my HMT337.


Hahaha.  Tell her that they don’t make em’ like they used too. Shoot, that’s not bad considering how old the WM II is.

I picked it up on eBay a couple years ago new in the box with three extra temp/humidity sensors still sealed in the package for $125. I thought it would be a good starter weather station for her. I'm very impressed with it. She wants a VP2 6163 for Xmas because she wants (as she calls it) the "sun" sensor. I told her she needs to have a better reason than that. So I told her she needs to justify getting a VP2 in a technical paper presented to me on how the VP2 6163 will enhance her weather watching. She also needs to explain the pros and cons of switching from the WM II to the VP2 including outlining the sensor specs. She also needs to identify any common problems people are having with the VP2 and if there is a solution to those problems. If no solution is available, then how does she plan to live with that problem. I know the SHT-31 not going to 100% will drive her crazy  :lol: So she's been doing a lot of research on here. I gave her until October 1st to finish the paper. Then we'll see if she still wants a VP2. Yes, I expect a lot out of my 4th grader.  :lol:

« Last Edit: July 14, 2018, 11:00:56 AM by kcidwx »
Meteorological Technician
NWS Certified Aviation Weather Observer
B.S. Meteorology

Offline ValentineWeather

  • Forecaster
  • *****
  • Posts: 4812
Update on sf2 and Davis filter at same time. When i put new sensor in use on my main station i used the sf2 filter, the same has been in use on the old sensor for a year, i removed the sf2 from the old and put the Davis in place of it for comparison with the new sensor. Well the sf2 filter specs say the sensor performance is the same with or without the filter so i put the sf2 back on the old sensor with the Davis filter on also. I can not see any difference in performance of the old sensor readings with both filters in use. It looks like it could give added protection in high dust and humidity areas. I figured the manufacturer says the sf2 does not have any effect on the sensor readings that maybe it would be worth a try even though the bigger filter does add some lag time on the readings. It is hard to measure lag time using the Davis set up because of the high sampling rates of the temp and humidity readings. Since i started using the sf2 i have not had any humidity related problems like i did when using the big Davis filter. I will never run a sensor without the sf2 in place. I am going to let the test sensor run for some time with both filters in place to see if anything shows up down the road.

Might be on to something Jerry. Aspiration helps sensor go out fast so the dual filter makes sense.

Maybe doubling up on the Davis fiter material would also be a huge benefit. Instead of 1 have 2 or 3 of the filters inside the filter housing.  :idea: Much easier than gluing Sf2 on I would think and safer not knowing what toxic fumes could be produced with whatever glue or sealer you use. I'm not worried about a little lag time it's still going to be much faster than any passive setup.
Randy