Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 40942 times)

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Offline openvista

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Really the readings are in spec, temp is +- .5f and humidity is typically +-2% but can run up higher to 3% or so.

No, Sensirion's specs are tighter than Davis'. He's testing OEM sensors, not sensors run through Davis electronics (which introduce their own uncertainties, apparently).

The key question here: is the chamber drifting upwards in temperature or the sensors? We need to know.
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Offline jerryg

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Those specs are from the sht 31 data sheet not from Davis.

Offline ValentineWeather

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Chamber I'm guessing would be dead on since its used for calibration. Looks like 31's ran about -.3 low new and +.5 high after two years.

But after further review I'm comparing a brand new sensor 1 day old against 26 month old and see no difference at night. The two sensors swing from  0-.3 delta T with neither sensor biased one direction so I'm thinking temperature is solid.
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Offline openvista

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Those specs are from the sht 31 data sheet not from Davis.

Correct. And because he's testing Sensirion equipment with no Davis connection, +/- 0.36F is the correct specification.
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Offline Jim's Weather

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The chamber is accurate. The problem is repeatability with the sensors. Repeatability means subjecting the sensor to the exact same conditions multiple times to see if it produces the same reading. If I were to do this test ten times with these, I'm thinking I would end up with a maximum repeatability window of 0.7°F and a standard window of 0.5°F. The average temperature between all tests would probably come out around 83.1°F.

I did the test one more time and got different but better results on the temperature side.

Chamber Conditions
Temperature: 83.0°F
Humidity: 100.0%



« Last Edit: July 14, 2018, 08:42:05 PM by kcidwx »
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Offline openvista

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Whew. Yes, of course, repeatability. Forgot about that bugaboo.

Thanks for repeating the test!
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Offline Jim's Weather

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Hitting 100% on the SHT-31 is not really part of the spec. At 100% humidity it's considered in calibration at >96.5%. If you really want to see 100% when it's 100% humidity then you will have to get into the pro level sensors and they start around $400 each.

« Last Edit: July 15, 2018, 12:22:56 AM by kcidwx »
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Offline Dj1225

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Yesterday I hit 99% on my new sensor today max 98%...Tomorrow?
I did just pull it out and double filtered with the Davis filters.


My new sensor maxed out at 97% First time I've seen 97% in a year and a half. The old sensor was barely making it over 94% I was reading in this thread about the sensor being shipped in bubble wrap and it causing issues to the sensor. Mine was wrapped up in bubble wrap, should that be a cause for concern?

Offline Jim's Weather

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They do make ESD bubble wrap bags but because of what they're typically made out of, it usually violates the sensor handling/packaging requirements.

My 31's came in this.

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Offline Dj1225

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They do make ESD bubble wrap bags but because of what they're typically made out of, it usually violates the sensor handling/packaging requirements.

My 31's came in this.




Are you talking about the actual sensor from Sensirion, or the temp/hum sensor for the VP2?

Offline jgentry

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Hitting 100% on the SHT-31 is not really part of the spec. At 100% humidity it's considered in calibration at >96.5%. If you really want to see 100% when it's 100% humidity then you will have to get into the pro level sensors and they start around $400 each.

Hmm... That hurts my feelings. Lol

The one I have at my friend’s farm has reported 100% for the second time in a row. This is after he received 2” of rain yesterday. My main station with FARS is currently reporting 97%
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Offline Jim's Weather

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They do make ESD bubble wrap bags but because of what they're typically made out of, it usually violates the sensor handling/packaging requirements.

My 31's came in this.



Are you talking about the actual sensor from Sensirion, or the temp/hum sensor for the VP2?

My sensors came PCB mounted from Sensirion. The VP2 sensor should be shipped in similar qualifying packaging according to Sensirion's guidelines.
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Offline Jim's Weather

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Hitting 100% on the SHT-31 is not really part of the spec. At 100% humidity it's considered in calibration at >96.5%. If you really want to see 100% when it's 100% humidity then you will have to get into the pro level sensors and they start around $400 each.

Hmm... That hurts my feelings. Lol

The one I have at my friend’s farm has reported 100% for the second time in a row. This is after he received 2” of rain yesterday. My main station with FARS is currently reporting 97%

Sure. You will find some of the SHT31's will go to 100% humidity, however most of them will not. That doesn't mean there is something wrong with the sensors that don't reach 100%. Of course they should reach at least 96.5% to be in spec. What you're seeing with the different maximum values is normal behavior from a budget sensor. You are pushing the operational limits of the sensor once you get above 95% and you will get varying results. That's why they relax the spec above 90%. If you want an SHT31 that goes to 100% then you have to play the lottery. Buy a pack of 10 sensors, test and hope you win by getting one sensor that will reach 100%. Actually, I've seen really picky guys do this with budget sensors. They will buy ten, test them all at 100% and keep the one that shows the highest humidity and send the rest back.  :lol:

« Last Edit: July 15, 2018, 11:09:30 AM by kcidwx »
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Offline DaleReid

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Weather Display, for example, has a tweak that says IF the humidity is 95% or greater, display as 100%

Clearly this has been a situation that has been around for awhile.

I have read this discussion and yet still am wondering what the technology is that the high priced, more-likely-to-read-100%, type of sensors use?  I assume we're talking RM Young, Vaisala, Campbell Scientific and perhaps a few others. 

Is it an entirely different technology?  do they actually measure dew point (doubt it from looking at most of the sensors on-line) and then do a conversion?

Perhaps we're all arguing how many angels can dance on the head of a pin.  It just "feels" uncomfortable when the humidity is that high.  I wonder what my built in sensor is that tells me that, other than sweating in Florida this time of year? 

I know the local talking heads on the weather portion of the news always talk about 'how it will feel' especially with the heat waves and incredible tropical humidity we have been having in the MidWest.  How does my sense of oppressive environment get sensed by my body?  And the wonderful feeling of low humidity and clear air and you can actually breath more easily after a front goes through?  Or the 'fall like' feeling for those of us up north here in October.  I even remember  doing a ride at Disney where they were showing how energy came to be with a trip through dino land with noises, calling birds, sort of a wet smell piped over the little cars carrying us along, but also a high humidity mist being sprayed along side to keep the feel of being in the tropics along that whole segment.

I'm just wondering about sensing things, just with our bodies, as a side question about experiencing humidity.

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Offline Jim's Weather

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I have read this discussion and yet still am wondering what the technology is that the high priced, more-likely-to-read-100%, type of sensors use?  I assume we're talking RM Young, Vaisala, Campbell Scientific and perhaps a few others. 

Is it an entirely different technology?  do they actually measure dew point (doubt it from looking at most of the sensors on-line) and then do a conversion?


It's the same technology just different manufacturing process with tighter tolerances. Of course the tighter the manufacturing tolerance the higher the manufacturing cost but you will get a better spec'ed sensor and better performance above 90%. Apples to apples, the SHT31 chip itself runs about $5. My Vaisala Humicap chip runs $80. SHT31 spec is 2%, Vaisala Humicap is 1%. An engineer once told me that once you get down around 2% spec, it costs a lot more to shave off another 0.5% in manufacturing.

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Offline jerryg

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I ran across this cheap indoor sensor that i just had to order, it is spec out as an sht31 sensor. The sensor being used is the sht 3x on the picture but says .5 +/- temp and 2% humidity. It would make a nice cheap unit to use for testing purposes. Here is the link https://www.amazon.com/gp/product/B01HDW58GS/ref=od_aui_detailpages00?ie=UTF8&psc=1

Offline jerryg

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It's due here Tuesday and i will find out keep in mind the sensor determines the accuracy and it is  an sht sensor so i got my fingers crossed.

Offline dalecoy

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Measures indoor temperature and humidity from -4 to 158°F (-20 to 70°C) and 1-99% RH

Professional accuracy: +/- 0.5°F, +/- 2% RH with manual calibration option

Offline ValentineWeather

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I ran across this cheap indoor sensor that i just had to order, it is spec out as an sht31 sensor. The sensor being used is the sht 3x on the picture but says .5 +/- temp and 2% humidity. It would make a nice cheap unit to use for testing purposes. Here is the link https://www.amazon.com/gp/product/B01HDW58GS/ref=od_aui_detailpages00?ie=UTF8&psc=1

If it's the same as our 31's nice little unit. Easy to read.

Weather Display, for example, has a tweak that says IF the humidity is 95% or greater, display as 100%



Cumulus has a similar but it's 98% I didn't have SHT31 that would make 98% until this new one and yes I have 98% = 100% on Cumulus for website. I can watch actual humidity on my Envoy8x display or console.


For anyone wondering about using double filters I started and don't see noticeable response time differences, accuracy was unchanged comparing with another sht31 side by side so recommend on new sensors to double up filter on new units. I just washed old and installed with the new. This at least doubles the filtration I'm thinking.
Randy

Offline Jim's Weather

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I ordered this one since it reads out in tenths. I just need that extra resolution for my satisfaction.  :lol: These will be great for monitoring humidity in garage, basement etc.

https://www.amazon.com/gp/product/B000XTJRRA
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Offline ValentineWeather

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I ordered this one since it reads out in tenths. I just need that extra resolution for my satisfaction.  :lol: These will be great for monitoring humidity in garage, basement etc.

https://www.amazon.com/gp/product/B000XTJRRA

X2  :-)
Randy

Offline WheatonRon

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Re: SHT31
« Reply #321 on: July 15, 2018, 02:45:57 PM »
SHT 31 drifts with age! I have two VP2 stations both have the SHT31 sensor--one station is about 2 months old and it has a 24 hour fan (CWOP FW3075) and the other station is about 2 years old and has a daytime fan (CWOP CW5020). Right now, in a western suburb of Chicago (where I live), the new station reports 91, humidity of 44 and dewpoint of 66 whereas the older station reports 91, humidity of 50 and dewpoint of 70. The stations are about 10 feet apart on the same fence and the sun is making both fans rumble today! The newer station always gets lower humidity and dewpoint readings as demonstrated by the attached QC graphs from CWOP.  I am not a fan (pun intended) of the QC checks on CWOP but this does raise some eyebrows!

My post above, a few days old, is no longer accurate. My new VP2 with the 24 hour fan is now showing higher humidity than my older 2 year old VP2 with a daytime fan. In short, the accuracy of the SHT31 is any person’s guess. One day it looks correct, the next day, not so correct. If I could be assured I could get a replacement SHT31 (properly packaged) from Davis for my new VP2 with the 24 hour fan, I would do that but I guess I will accept the inaccuracies in my humidity readings (all three VP2s have the SHT31 and report consistent temperatures) knowing at least my temperatures are solid. What a shame—we spend good money on what we believe is “semi-professional” weather gear only to get something less.
« Last Edit: July 15, 2018, 04:14:52 PM by WheatonRon »
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Offline WheatonRon

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I ordered this one since it reads out in tenths. I just need that extra resolution for my satisfaction.  :lol: These will be great for monitoring humidity in garage, basement etc.

https://www.amazon.com/gp/product/B000XTJRRA

X2  :-)

X3  👌
Davis VP2 with SHT31 (3 complete VP2 systems—2 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 CW2274

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Hitting 100% on the SHT-31 is not really part of the spec. At 100% humidity it's considered in calibration at >96.5%. If you really want to see 100% when it's 100% humidity then you will have to get into the pro level sensors and they start around $400 each.

Hmm... That hurts my feelings. Lol

The one I have at my friend’s farm has reported 100% for the second time in a row. This is after he received 2” of rain yesterday. My main station with FARS is currently reporting 97%
If you want an SHT31 that goes to 100% then you have to play the lottery. Buy a pack of 10 sensors, test and hope you win by getting one sensor that will reach 100%.
That's fine and dandy if you're just buying the sensor, but then you've got to put it on the PCB. Don't know about you guys, but mounting that tiny, tiny sensor is waaay above my pay grade. And buying 10 from Ryan ready to go, as much as he'd like it, I ain't shelling out that kind of money for a crap shoot.

Offline CW2274

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Re: Which sling psychrometer should I buy?
« Reply #324 on: July 15, 2018, 06:27:30 PM »
Well I've decided to pull the ISS and see what I see. The monsoon is in town so I'm not sure when I'll get out to it, I would MUCH prefer cloud cover, possibly tomorrow. At least we can see if the desert hypothesis holds any water with a two year old 31 vs five months.
Think of me when I'm pulling cacti hypo's outta my legs in the interest of science. :-({|=
Weather cooperated with overcast skies so I pulled the ISS and did my comparisons, which are at ideal conditions now. Simply put, I found absolutely no noticeable difference between the 31 that has 2 years of service vs the 5 month old one. Temp, dew, humidity were basically exactly the same. Now, one could say they've drifted the exact same amount, perhaps.
This certainly lends credence to the "dry climate syndrome" making the sensors last longer, but I think hurts the notion that aspiration degrades the sensor considering the very dusty climate here, and that one 31 has almost 500% more service time than the other.
I"m sure not the most scientific method, but I got some results, take it for what it's worth.