Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 24231 times)

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Offline mcrossley

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I haven't read this whole thread, but regarding contamination, Sensirion recommend that their sensors are placed behind a filter and supply chip enclosures that effectively use a Gortex like membrane. Davis chose not to incorporate such a filter.

I find that periodically giving my sht devices a wash in distilled water restores much of their humidity accuracy. I've not seen much drift in the temperature readings.
« Last Edit: July 11, 2018, 05:19:48 PM by mcrossley »
Mark

Offline ValentineWeather

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I haven't read this whole thread, but regarding contamination, Sensiron recommend that their sensors are placed behind a filter and supply chip enclosures that effectively use a Gortex membrane. Davis chose not to incorporate such a filter.

I find that periodically giving my sht devices a wash in distilled water restores much of their humidity accuracy. I've not seen much drift in the temperature readings.

So distilled water won't damage a unplugged chip. I have never heard this before but worth a try I've got several that I replaced already. I would like to find the bagging material they recommend also. Any other plastic bags will ruin chip in short order, even sealing bag don't use just any tape.

Found some bags and resealable  =D> https://www.amazon.com/gp/product/B07CX62R99/ref=ox_sc_act_title_1?smid=A3S6XKC1OQNPRR&psc=1
« Last Edit: July 11, 2018, 02:17:29 PM by ValentineWeather »
Randy

Offline kcidwx

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A lot of high humidity over the life of the sensor often accelerates aging of the sensor. Here's some info from an engineering doc I have on the SHT-31.

Quote
1.1
Recommended Operating Condition

The sensor shows best performance when operated within recommended normal temperature and humidity range of 5°C–60 °C and 20 %RH–80 %RH, respectively. Long-term exposure to conditions outside normal range, especially at high humidity, may temporarily offset the RH signal (e.g.+3%RH after 60h kept at >80%RH). After returning into the normal temperature and humidity range the sensor will slowly come back to calibration state by itself. Prolonged exposure to extreme conditions may accelerate ageing. To ensure stable operation of the humidity sensor, the conditions described in the document “SHTxx Assembly of SMD Packages”, section “Storage and Handling Instructions” regarding exposure to volatile organic compounds have to be met. Please note as well that this does apply not only to transportation and manufacturing, but also to operation of the SHT3x.

What this tells me is this sensor wasn't really designed for meteorological use. But most humidity sensors aren't.

Most people don't have the means but you can recondition the sensor to try and bring it back into spec.

Quote
Reconditioning Procedure SHT3x
The following reconditioning procedure may bring the sensor back to calibration state:
Baking: 100 – 105°C at < 5%RH for 10h
Re-Hydration: 20 – 30°C at ~ 75%RH for 12h
« Last Edit: July 11, 2018, 03:08:09 PM by kcidwx »
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Online openvista

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That image is from Stephen Burt's 2009 review of the VP2. This would be relative humidity as measured by the SHT11 (original OEM temp/humidity sensor package) compared against a Vaisala HMP45C capacitive humidity sensor (the dotted black line).

Keep in mind, that measurements on this graph start in January 2009 (red dots) at which point the VP2 is approximately 6 months old as the study commenced in July 2008. The May readings (in blue) are when it's nearly a year old.

Here are his comments on the graph:

Quote
5 minute sampled RH for both instruments for a winter and summer month (January 2009 data in red and May 2009 in blue): in all, 17 780 pairs of observations. The thick dashed line marks the trend line that would be expected with a ‘perfect’ agreement; points above the line indicate where the VP2 indicated higher than the Vaisala sensor, and vice versa below the line.

Look how often the sensor runs high in the middle of the range -- often by 5-10% or more. In fact, I don't see one instance of it running drier than the Vaisala until almost 80% humidity. He said over the whole range it was 2-4% higher on average, but that's balanced out by the dryness on the high end (above 80%).

Those results are why he rated the VP2's humidity measurement only as "adequate" (essentially a "C" letter grade) while giving the temperature portion an excellent rating (or "A"). The humidity sensor was acceptable, I suppose, in England, the testing location, where it's above 80% humidity a great deal of the time. Not so acceptable elsewhere where the middle ranges are more common.

In other words, the Sensirion humidity sensor has run wet in the middle for years at least within 6 months from installation. I'm sure Davis is fully aware of that review as it was widely publicized at the time. 

It doesn't appear that the issue was really "fixed" with the 31 model. Perhaps out of the gate, the readings are better, but not for long.
Davis Vantage Pro2 FARS | https://marquetteweather.com | EW7933

Offline CW2274

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I've stated this here before, but I use this on the sensor every time I service the ISS, which is approximately every six months. I've seen zero effect on my sensors except for the fact they still work. If you've got a bad sensor, whatta ya got to lose in case you're afraid to try it.
http://www.acehardware.com/product/index.jsp?productId=1451535
« Last Edit: July 11, 2018, 05:05:13 PM by CW2274 »

Offline WheatonRon

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I have prepared a possible response to this debacle from Davis. Maybe this might help Davis and its legal team get started with an actual Davis response.

Davis has learned that the humidity-temperature sensor in our Vantage 2 product line (enter model numbers and date of manufacturing here) is not achieving the advertised accuracy we promised. Specifically, over time the humidity being reported by your sensor will likely be reading too high. We apologize for this deficiency and as a result, are offering to send you, post paid, a replacement every two years since you bought your unit up to a maximum of 4 replacements—the expected life-span (10 years) of your unit. For example, if your unit is two years old now, we will send you a replacement now and another one in two years, and so forth so you will have ten years of service from your Davis unit.

Replacing the sensor is not difficult, but requires you to take down your integrated sensor suite, remove the rain collector and base, remove a few screws, remove your temperature-humidity sensor and replace it with a new sensor and reassemble. Directions for this effort are documented in an instruction manual enclosed with the sensor. If you are not concerned that your humidity readings are likely reading too high, you do not need to do anything. We apologize for any inconvenience we may have caused you.
« Last Edit: July 11, 2018, 05:53:45 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

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I’m curious if you can change out the Davis’ filter cap with Sensirion’s

https://www.sensirion.com/en/environmental-sensors/humidity-sensors/filter-cap-sf2/
Davis Vantage Pro2. SHT 31. 24hr FARS. WU: KXALJEMI2 & KALTHORS2 CWOP: C6353 & C6358

  

Online openvista

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Just found an interesting document from Davis: https://www.davisinstruments.com/product_documents/weather/Doc_Sensor-Maintenance.pdf.

On page 2 it says it expects that the temperature/humidity sensor will need recalibration every 5 years.

Also, why do they not put such information in product manuals?!
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Online openvista

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Apparently DAVIS is only cautious about "dust" and not actual water condensing upon the RH sensor and thus physically "wetting" the mylar capacitive sensor.

I seem to recall discussion awhile back about the choice of that cap over the sensor. There's some reason they chose it and decided against the manufacturer's recommended cap. But I can't recall it exactly. Not enough air movement, perhaps.

In any event, they should have known that by choosing that cap design (which lets in just about anything), they were sentencing the sensor to a short life.

Now reconsider their doc where they claim you can go 5 years without the sensor needing "recalibration" (whatever that means; replacement probably). That seems like a bad joke.
Davis Vantage Pro2 FARS | https://marquetteweather.com | EW7933

Offline jerryg

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Using the factory filter is not hard to do, i use them on the 31 and the 15. They are different types with the 31 being smaller than the fl1 which fits the 15. I just cut the little mounting tabs off and hold the filter in place with a little sealing putty i have on hand, takes very little. The small filter gives better response time compared to the large one from Davis. I have used them for years without any problems.

Online openvista

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Using the factory filter is not hard to do, i use them on the 31 and the 15. They are different types with the 31 being smaller than the fl1 which fits the 15. I just cut the little mounting tabs off and hold the filter in place with a little sealing putty i have on hand, takes very little. The small filter gives better response time compared to the large one from Davis. I have used them for years without any problems.

I see them on Amazon: https://www.amazon.com/SENSIRION-SF2-FILTER-CAP-SHT2X/dp/B018CQ2USS. But you say there are different sizes. Where do you obtain the filter for the 31?
Davis Vantage Pro2 FARS | https://marquetteweather.com | EW7933

Offline jgentry

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Apparently DAVIS is only cautious about "dust" and not actual water condensing upon the RH sensor and thus physically "wetting" the mylar capacitive sensor.

I seem to recall discussion awhile back about the choice of that cap over the sensor. There's some reason they chose it and decided against the manufacturer's recommended cap. But I can't recall it exactly. Not enough air movement, perhaps.

In any event, they should have known that by choosing that cap design (which lets in just about anything), they were sentencing the sensor to a short life.

Now reconsider their doc where they claim you can go 5 years without the sensor needing "recalibration" (whatever that means; replacement probably). That seems like a bad joke.

Sensirion’s filter cap worked well with Bellfry Boy’s version of the SHT 15 sensor. I wonder if he can make his version of the SHT-31 that would work with our VP2s?

But I do wish Davis would update their transmitter to where it can take a quality sensor from a different manufacturer. More and more colleges and other professionals are switching to Davis (i.e. WeatherSTEM). It would be wise for them to take in the just criticism and use it for their advantage. Otherwise, some other manufacturers may take advantage of Davis’s lack of action.
Davis Vantage Pro2. SHT 31. 24hr FARS. WU: KXALJEMI2 & KALTHORS2 CWOP: C6353 & C6358

  

Offline jgentry

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Using the factory filter is not hard to do, i use them on the 31 and the 15. They are different types with the 31 being smaller than the fl1 which fits the 15. I just cut the little mounting tabs off and hold the filter in place with a little sealing putty i have on hand, takes very little. The small filter gives better response time compared to the large one from Davis. I have used them for years without any problems.

Awesome!
Davis Vantage Pro2. SHT 31. 24hr FARS. WU: KXALJEMI2 & KALTHORS2 CWOP: C6353 & C6358

  

Offline jgentry

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Using the factory filter is not hard to do, i use them on the 31 and the 15. They are different types with the 31 being smaller than the fl1 which fits the 15. I just cut the little mounting tabs off and hold the filter in place with a little sealing putty i have on hand, takes very little. The small filter gives better response time compared to the large one from Davis. I have used them for years without any problems.

So the mounting tabs won’t clip into Davis pcb board like their filter caps does?
Davis Vantage Pro2. SHT 31. 24hr FARS. WU: KXALJEMI2 & KALTHORS2 CWOP: C6353 & C6358

  

Offline jgentry

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Using the factory filter is not hard to do, i use them on the 31 and the 15. They are different types with the 31 being smaller than the fl1 which fits the 15. I just cut the little mounting tabs off and hold the filter in place with a little sealing putty i have on hand, takes very little. The small filter gives better response time compared to the large one from Davis. I have used them for years without any problems.

I see them on Amazon: https://www.amazon.com/SENSIRION-SF2-FILTER-CAP-SHT2X/dp/B018CQ2USS. But you say there are different sizes. Where do you obtain the filter for the 31?

Get the SF2. It’s for both the SHT 2x and 3x series of sensors.
Davis Vantage Pro2. SHT 31. 24hr FARS. WU: KXALJEMI2 & KALTHORS2 CWOP: C6353 & C6358

  

Offline jerryg

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No it won't fit in the holes, the filter just barely covers the sensor itself.

Offline jerryg

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Here is link to Mouser the filter is 1.06 plus shipping.https://www.mouser.com/Sensirion/Sensors/Sensor-Hardware-Accessories/_/N-11ei3?P=1ytt2dn  It is the sf2 filter.
« Last Edit: July 11, 2018, 06:33:29 PM by jerryg »

Offline CW2274

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For those that read my last post, I deleted it, I had the wrong cover. :oops:

Offline ValentineWeather

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For those that read my last post, I deleted it, I had the wrong cover. :oops:

Too late  :grin: But you brought up a good point even if by mistake.  Davis made the decision not to include the polyimide foil dedicated to protect the sensor opening from pollution.  Cheaper but may have solved the problem of contamination if it's the problem.
Randy

Offline CW2274

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For those that read my last post, I deleted it, I had the wrong cover. :oops:

Too late  :grin: But you brought up a good point even if by mistake.  Davis made the decision not to include the polyimide foil dedicated to protect the sensor opening from pollution.  Cheaper but may have solved the problem of contamination if it's the problem.
Poo... :-) Ya know what? If we go in on a group buy of 10, we can save one cent per filter!! It's our lucky day guys!!! \:D/

Offline jerryg

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I bought 10 of them because of paying shipping only once and when i change one out i put it up and put a new one on and don't have to clean any until i get them all used. I had one on for a year and it never showed any signs of clogging up.

Offline ValentineWeather

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I bought 10 of them because of paying shipping only once and when i change one out i put it up and put a new one on and don't have to clean any until i get them all used. I had one on for a year and it never showed any signs of clogging up.

Nice Jerry thanks for info.. I can always use a little quicker reaction time.
Randy

Online openvista

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Davis made the decision not to include the polyimide foil dedicated to protect the sensor opening from pollution.  Cheaper but may have solved the problem of contamination if it's the problem.

Not just pollution but WATER. As many have pointed out, it would seem water ingress is the main culprit here.

For those ordering the cap, please post pics and procedures so we know what works. I'm not sure a putty like substance is going to be enough to keep this cap in place in a FARS where it is installed perpendicular to the ground (rather than parallel like in the passive shield) and will have air blowing on it constantly. Super glue would make it impossible to remove when it gets dirty. Not sure if there's a substance in between these two that will secure it but not permanently. Ideas?
Davis Vantage Pro2 FARS | https://marquetteweather.com | EW7933

Offline CW2274

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The way I look at it, since it's basically an "impenetrable" shield when sealed, you might as well super glue it into place. When said sensor gets unreliable, chuck it. That pretty much the way electronics are anymore, use and lose.

Offline ValentineWeather

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I was thinking about what kcidwx said about how abnormal it is to aspirate the humidity sensor. There must be reason for it...Possibly they wear out much faster...Anyway 4 days ago I started testing the Davis 24 hr shield in passive mode. 
We've only had 1 day with light enough winds I considered a good test day. It's the best passive shield I've ever tested. The sensor is well protected inside from thermal heat and painted inside flat black, radiation and reflective heat is reduced also.
Peak temperature was only 1 degree warmer than the AC fan and 1.5° warmer than the dc solar fan. The DC fan actually runs cooler than the AC on average .5F.

What I did to modify was removed fan, painted interior where sensor mounts all the way to bottom flat black added a couple more top shields, but those with rain bucket not necessary and drilled some holes on outer shield core...This may not be necessary also.  Final step I used same 5/8" bit and drilled a bottom center hole through plastic for a little more upward airflow and removed the small pyramid shaped black shield on bottom.

This is graph of the one day with wind speed light enough to consider a valid test under clear cloudless sky with high summer sun angle 43° longitude.
#3 orange is passive, #1 AC fan #5 Dc fan

Added wind speed and dewpoint
 
« Last Edit: July 12, 2018, 06:21:22 AM by ValentineWeather »
Randy