Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 41031 times)

0 Members and 1 Guest are viewing this topic.

Offline SLOweather

  • Administrator
  • Forecaster
  • *****
  • Posts: 3454
    • http://www.sloweather.com
Thread has went about as far as it can go unless anyone has something we didn't cover. Let Davis do their thing and see what they come up with.

Unless someone wants to tackle the reconditioning procedure and report back.
« Last Edit: July 20, 2018, 10:53:14 PM by SLOweather »

Offline ValentineWeather

  • Forecaster
  • *****
  • Posts: 5856
    • LiVE WEATHER
I would be interested in how to recondition but removing chip for baking I wouldn't know where to start.
Randy

Offline SLOweather

  • Administrator
  • Forecaster
  • *****
  • Posts: 3454
    • http://www.sloweather.com
I inferred from the docs that baking could be done with a mounted sensor. It's only at ~100c for 10 hours, so it shouldn't hurt anything, board or cable. The tough part would be holding that temp pretty close. Hmmm. Maybe in a crock pot on low with a thermometer to monitor the temp. The thermal mass should be great enough that you could manually control it.

Interesting... The reconditioning for the SHT1x series is slightly different:

Quote
Conditions outside the recommended range may
temporarily offset the RH signal up to 3%RH. After return
to normal conditions it will slowly return close to calibration
state by itself.
To accelerate this process we recommend the following
reconditioning procedure:
90C at <5%RH for 24h followed by
20-30C at >74%RH for 48h

 
« Last Edit: July 21, 2018, 08:36:40 AM by SLOweather »

Offline ValentineWeather

  • Forecaster
  • *****
  • Posts: 5856
    • LiVE WEATHER
Only 212F sounds like a plan. I have some of the correct bags non gaseous for Re-Hydration and wet rag.

Reconditioning Procedure
As stated above extreme conditions or exposure to solvent vapors may offset the sensor. The following reconditioning procedure may bring the sensor back to calibration state:
Baking: 100 105C at < 5%RH for 10h
Re-Hydration: 20 30C at ~ 75%RH for 12h 1.
Randy

Offline jgentry

  • Forecaster
  • *****
  • Posts: 440
Would it be ok to leave filter cap on or take it off before baking? Secondly, would it be ok to put the sensor on a metal baking pan?
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline ValentineWeather

  • Forecaster
  • *****
  • Posts: 5856
    • LiVE WEATHER
Take it off for sure. I'm doing one now I'll let you know how it comes out .  :?:  If this fixes sensor its something everyone can try. I'll be done around about 9pm tonight including hydration.
I'm using a small crockpot and it doesn't have temperature control and gets to about 120c if lid is on so have cracked open about 1/2" and sensor sitting on top of foil tent I built.
Randy

Offline Intheswamp

  • Early on one frosty morn'...
  • Forecaster
  • *****
  • Posts: 1418
  • Sure...I know enough to be dangerous, now.
    • Beeweather
Excellent, Randy!!! 

This sounds much like renewing silica desiccant, though at a higher temperature.  :-k  It will be interesting to see whether this improves performance.  If it does then it poses the question, for me, of whether I should do this before I install my new/old SHT31.

Thanks for taking the initiative!


Davis VP2, v3.12
Cumulus v1.9.4  Build 1079
Today's weather beats no weather at all!
www.beeweather.com
Tried to lift one of vreihen's gold stars but wasn't smart enough to do it....<grin>

Offline ValentineWeather

  • Forecaster
  • *****
  • Posts: 5856
    • LiVE WEATHER
Excellent, Randy!!! 

This sounds much like renewing silica desiccant, though at a higher temperature.  :-k  It will be interesting to see whether this improves performance.  If it does then it poses the question, for me, of whether I should do this before I install my new/old SHT31.

Thanks for taking the initiative!

If big IF  I'm just not sure how perfect baking temps must be. I've went over recommended and been under also being I don't have a controlled environment. I would hope Davis would do all sensors and ship fresh restored units so you don't have to do it.  My week old sensor has the high bias only difference from older sensors it will reach 98% vs 97 and 96%.
Randy

Offline openvista

  • Forecaster
  • *****
  • Posts: 453
    • marquetteweather.com
Yes, Randy, thank you for running this experiment.

However, the handling and reconditioning instructions prompt me to ask two questions:

1) Why would 2 sensors less than a couple weeks old need "reconditioning"?

2) Why would Davis choose a sensor in which normal environmental conditions for most of its customers would knock it out of calibration not long after purchase (within a year) and require a complicated procedure to remain accurate?

Based on Sensirion's literature, it would appear these sensors are designed for indoor use.
« Last Edit: July 21, 2018, 09:41:37 AM by openvista »
Davis Vantage Pro2 AC FARS | https://marquetteweather.com | EW7933

Offline WheatonRon

  • Forecaster
  • *****
  • Posts: 1018
    • WUnderground
As discussed in several posts before, it is believed Davis has not done a very good job packaging the SHT31 when it leaves the Davis facility and secondly, the white cap that encases the sensor is not very effective in keeping dirt, etc. away from the sensor. Both reasons, if accurate, may give rise to a short life of the SHT31 requiring recalibration.
« Last Edit: July 21, 2018, 09:50:01 AM by WheatonRon »
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 ValentineWeather

  • Forecaster
  • *****
  • Posts: 5856
    • LiVE WEATHER
Yes, Randy, thank you for running this experiment.

However, the handling and reconditioning instructions prompt me to ask two questions:

1) Why would 2 sensors less than a couple weeks old need "reconditioning"?

2) Why would Davis choose a sensor in which normal environmental conditions for most of its customers would knock it out of calibration not long after purchase (within a year) and require a complicated procedure to remain accurate?

Based on Sensirion's literature, it would appear these sensors are designed for indoor use.



Well my thinking is because new sensors already have the high bias as stated but the Acurite units don't they are being contaminated in production process to board. By doing the baking we get rid of the production contaminates and sensor will work as advertised. This is wishful thinking I'm sure.  :lol:
 
And yes after some reading it does look like these aren't designed for continuous high humidity.
Randy

Offline openvista

  • Forecaster
  • *****
  • Posts: 453
    • marquetteweather.com
As discussed in several posts before, it is believed Davis has not done a very good job packaging the SHT31 when it leaves the Davis facility and secondly, the white cap that encases the sensor is not very effective in keeping dirt, etc. away from the sensor. Both reasons, if accurate, may give rise to a short life of the SHT31 requiring recalibration.

Not disputing that at all, but look again at the recommendations for the sensor.

Quote
The sensor shows best performance when operated within recommended normal temperature and humidity range of 5
60 C and 20 80 %RH
, respectively.

Quote
Prolonged exposure to extreme conditions may accelerate ageing.

So operating the sensor below 41F or in humid/foggy conditions will knock it out of calibration. Repeatedly exposing it to these temperatures and humidities will "age" the sensor (which I take to mean degrade).
« Last Edit: July 21, 2018, 10:34:27 AM by openvista »
Davis Vantage Pro2 AC FARS | https://marquetteweather.com | EW7933

Offline jgentry

  • Forecaster
  • *****
  • Posts: 440
Only 212F sounds like a plan. I have some of the correct bags non gaseous for Re-Hydration and wet rag.

Reconditioning Procedure
As stated above extreme conditions or exposure to solvent vapors may offset the sensor. The following reconditioning procedure may bring the sensor back to calibration state:
Baking: 100 105C at < 5%RH for 10h
Re-Hydration: 20 30C at ~ 75%RH for 12h 1.

Can you pick which option you want to do or do you have to do both for recalibration to take place (if it does)?
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline openvista

  • Forecaster
  • *****
  • Posts: 453
    • marquetteweather.com
Quote
After returning into the normal temperature and humidity range the sensor will slowly come back to calibration state by itself.

Many of us, including Ron, have noted that, at some point, these sensors don't return to calibration "slowly" as Sensirion claims. Perhaps that's why they include the caveat about sensor aging. If a year and a half old sensor is "aged" due only to the fact that it wasn't kept above 40F and under 80% humidity, the most logical conclusion would be it shouldn't be installed outdoors in most climates.

Now, perhaps the sensor is damaged by Davis' handling. If so, then Randy's reconditioning procedure should not help.
« Last Edit: July 21, 2018, 10:15:20 AM by openvista »
Davis Vantage Pro2 AC FARS | https://marquetteweather.com | EW7933

Offline SLOweather

  • Administrator
  • Forecaster
  • *****
  • Posts: 3454
    • http://www.sloweather.com
Can you pick which option you want to do or do you have to do both for recalibration to take place (if it does)?

In closely reading the procedure, it seems to me that the 2 steps are required, and that they must be done in that order.  Baking at low humidity would dry out the sensor, and the the rehydrating part would recondition it to a "normal" operating condition.

Offline ValentineWeather

  • Forecaster
  • *****
  • Posts: 5856
    • LiVE WEATHER
Ugh  ](*,)  So if these aren't even the correct sensor for outdoors applications as suggested this process may only work until over saturated with moisture again.  I'm in the rehydration phase now maybe I should skip, it may not be necessary because once outside it will get all the moisture it needs plus.
 [ You are not allowed to view attachments ]
 [ You are not allowed to view attachments ]
« Last Edit: July 21, 2018, 10:38:57 AM by ValentineWeather »
Randy

Offline openvista

  • Forecaster
  • *****
  • Posts: 453
    • marquetteweather.com
I'm in the rehydration phase now maybe I should skip, it may not be necessary because once outside it will get all the moisture it needs plus.

No, please complete the procedure. It's helpful information to have either way!
Davis Vantage Pro2 AC FARS | https://marquetteweather.com | EW7933

Offline openvista

  • Forecaster
  • *****
  • Posts: 453
    • marquetteweather.com
Question: does Acurite have any outdoor weather stations with the SHT31 installed?
« Last Edit: July 21, 2018, 11:04:36 AM by openvista »
Davis Vantage Pro2 AC FARS | https://marquetteweather.com | EW7933

Offline ValentineWeather

  • Forecaster
  • *****
  • Posts: 5856
    • LiVE WEATHER
Question: does Acurite have any outdoor weather stations with the SHT31 installed?

Don't think so.. The unit that comes with multiple stations specs different.

Temperature Range   -40 to 158 degrees Fahrenheit; -40 to 70 degrees Celsius
Temperature Accuracy   +/- 2 degrees Fahrenheit; Learn more
Humidity Range   1% to 99% Relative Humidity
Humidity Accuracy   +/- 5% from 0% to 10% Relative Humidity
+/- 4% from 10% to 20% Relative Humidity
+/- 3% from 20% to 80% Relative Humidity
+/- 4% from 80% to 90% Relative Humidity
+/- 5% from 90% to 100% Relative Humidity
Randy

Offline openvista

  • Forecaster
  • *****
  • Posts: 453
    • marquetteweather.com
Indeed, according to a forum posting 7 months ago by an Acurite Rep (https://support.acurite.com/acurite/topics/are-you-planning-to-release-a-wireless-external-measuring-instrument-or-sensor-using-the-sht31-sensor), the answer is no.

Wonder why they would put the sensor in their new indoor unit (the one Randy's been testing), but not an outdoor unit like the Atlas?
Davis Vantage Pro2 AC FARS | https://marquetteweather.com | EW7933

Offline openvista

  • Forecaster
  • *****
  • Posts: 453
    • marquetteweather.com
Now why would Acurite recently release the Atlas to compete against the VP2 and have a less accurate temperature sensor? It's billed as a "professional" station. Specs I've seen claim +/- 1F. Davis, of course, is +/- 0.5F. Humidity specs are, supposedly, the same. See: https://www.acurite.com/media/documents/AcuRite-Atlas-Elite-Specifications.pdf

They obviously know about the Sensirion sensor. They put it in their much cheaper indoor-only product. So awareness or cost cannot be the issue.

I would think they'd want to be at least as accurate as their competitor(s) in a certain product segment. They are not. But maybe they decided they didn't want all the warranty headaches that come from putting a sensor best suited for indoor conditions in an outdoor unit. I'm sure there are other considerations as well.
« Last Edit: July 21, 2018, 11:15:56 AM by openvista »
Davis Vantage Pro2 AC FARS | https://marquetteweather.com | EW7933

Offline DoctorKnow

  • Forecaster
  • *****
  • Posts: 1581
I have the atlas. I also have the 5n1. The humidity is spot on with the Atlas. The temperature is within 1 degree for sure. I hope you guys will try it when it starts being sold. I think you will be very happy with the hardware. I hope the software will continue to offer more, like CWOP uploads because everything is much improved, compared to the 5in1.


Offline jerryg

  • Forecaster
  • *****
  • Posts: 778
  • weather is never boring
    • victoria texas weather
Ok here is some test info mainly for people in high humidity areas. I have been running 3 vp2 shields with Davis fans on ac power, two test shields running 2.3vots on fan all the time and my main station runs 2.3 volts during the day and with the use of a photocell and relay drops down to 1 volt at night. I put in sht11 sensors in all three that match pretty well in performance. What i found is the amount of airflow at night when the humidity rises is important. When the sun came up the main station humidity reading started to drop pretty soon after the sun was up and the temp began to rise. The two test sensors did not show a drop until almost 30 minutes later and the main reading dropped steadily while the two test lagged behind around 4% higher and did not get even with the main reading until the sun had dried off the grass shields. The higher fan speed at night is causing the sensor chamber to get a lot more moisture in the shield and takes longer to dry out. Now is put the new 31 in one test shield and a two year old 31 in the other and the results were the same except it took longer for the 31 to start dropping than it did the sht11 and at one point the main station was running 7% drier than the 31's. It looks like the fans at night are putting a lot of moisture inside the shields at night and the 31's don't handle it as well as the 11's. I ordered two of the sht15 from
Ryan and they will be here Monday and i will repeat the test using the new 31, the new 15 and the two year old 31, should be interesting to find out what shows up.

Offline dport

  • Senior Contributor
  • ****
  • Posts: 196
Now why would Acurite recently release the Atlas to compete against the VP2 and have a less accurate temperature sensor? It's billed as a "professional" station. Specs I've seen claim +/- 1F. Davis, of course, is +/- 0.5F. Humidity specs are, supposedly, the same. See: https://www.acurite.com/media/documents/AcuRite-Atlas-Elite-Specifications.pdf

They obviously know about the Sensirion sensor. They put it in their much cheaper indoor-only product. So awareness or cost cannot be the issue.

I would think they'd want to be at least as accurate as their competitor(s) in a certain product segment. They are not. But maybe they decided they didn't want all the warranty headaches that come from putting a sensor best suited for indoor conditions in an outdoor unit. I'm sure there are other considerations as well.

Keep in mind acurite plans to introduce two new stations.  The atlas and the atlas elite.  The atlas, which some folks are testing now, is your all-in-one unit.  Atlas Elite looks more like a Davis with the separate of the anemometer.  Not even sure that one exists in the field today though, at least I haven't seen any real pictures yet.  Guessing it could be quite some time before we see it.  This is why I bought a VP2.  I'm very happy with the unit overall.  Doubtful acurite will offer the replaceable hardware the way Davis does.

Let's hope we find some solutions to the SHT31 though.  Not sure baking is in my future :)

Offline jerryg

  • Forecaster
  • *****
  • Posts: 778
  • weather is never boring
    • victoria texas weather
Just to add to my testing results i am now using the sht11 instead of the new 31 because my humidity and dp readings are so much better with the 11, in every why the basic readings are better with the sht11 and i am glad that i have a few to spare because the sht1x line is no longer being sold. That is why i ordered to 15's from Ryan before they are gone for good. In my opinion the 31's have good on paper performance but poor field performance, I wonder if they even test these sensors in the home weather station settings.

 

anything