Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?

[jgentry]

From the side by sides I’ve done, the unmodified 75 runs consistently drier than the Davis-mounted 31 in the midrange as low as humidity in the 20s. (And I stress it is unmodified). That said, it’s not perfect and is still a Sensirion. It struggles to hit 100%, but gets there on occasion. It hits 98 and 99 regularly. It will still exhibit a wet bias if in high humidity for a prolonged period and I’d say it may have a slight wet bias generally, but it compares really closely to my psycho-dyne. I’d love to find a way to get a 31 or 35 that Davis hasn’t touched to work with my VP2, but until then, my experience is having the 75 is markedly superior to the Davis 31. Temp specs are not as good obviously and not exactly a rigorous study, but I’ve tested side by side with an ASOS at -8F and it was right on. Not trying to convince anyone, but just my experience ...

How much of the difference is there between the 75 & psych-dyne?

Usually pretty spot on. Within 1F in dew point. Sometimes 2F. The nice thing about the 75 is you can put it right in the intake of the psychro dyne to verify temp and dew point. I know some others are using the 75 so they may be able to provide more input in terms of temp/hum accuracy of the 75 across the extreme ranges

That’s good!  Have you ever done a comparison between the 75 & 31 in regards to air temp?

How did your 31 compared to the psychro-dyne?

The psychro dyne is dead on next to ASOS for temp and humidity. I’ve never found a difference in temp between the 75 and 31 when I’ve run them side by side. I could maybe do some more tests to get you some numeric answers. The Davis 31 is dead on for temp with the psychrodyne, but high on dewpoint usually by up to 4F, which is consistent with the comparison to ASOS. I have had the 75 next to an ASOS from -8F up to about 90F and it’s always been exactly on. I know the specs aren’t as good, but practically it seems to be a really good performer...

Thanks to some of these guys for going to Davis so they at least know the issue with their sensor. . I purchased the Acurite with the 31 after reading about it in this thread. It is really very good. I wish Davis’ 31 performed this way on the humidity side

How much of a variance in terms of RH did the psycho-dyne showed in comparison to the SHT-31?

[dendrite]

You have your new 75 up and running?

Not yet. I decided to give it another coat of conformal coating. I'll install it tomorrow evening once the shade hits the ISS in the evening.

[jgentry]

You have your new 75 up and running?

Not yet. I decided to give it another coat of conformal coating. I'll install it tomorrow evening once the shade hits the ISS in the evening.

Aight. Sounds good.

[WxLover16]

Oddly enough, after taking the filter off yesterday evening, I'm really not seeing that much better response time than with it on, even with a bit drier of air in place (48%). I remember accidentally leaving it off last summer and sometimes seeing .3-4 change in an update, but now it's almost as if I still have the filter on, and this with a 24 cfm fan so it's definitely bringing in some serious ambient air with it. Hmmm...

I should add that the humidity is changing alot more than usual, just not as much as the temp.

[CW7491]

How much of a variance in terms of RH did the psycho-dyne showed in comparison to the SHT-31?
[/quote]

I only compare the dewpoint as I found variances in the temperature can make comparing RH near impossible

[jgentry]

How much of a variance in terms of RH did the psycho-dyne showed in comparison to the SHT-31?

I only compare the dewpoint as I found variances in the temperature can make comparing RH near impossible
[/quote]

Gotcha.

[ValentineWeather]

Oddly enough, after taking the filter off yesterday evening, I'm really not seeing that much better response time than with it on, even with a bit drier of air in place (48%). I remember accidentally leaving it off last summer and sometimes seeing .3-4 change in an update, but now it's almost as if I still have the filter on, and this with a 24 cfm fan so it's definitely bringing in some serious ambient air with it. Hmmm...

I should add that the humidity is changing alot more than usual, just not as much as the temp.

These are the sensor update rates filter or not.
ISS Weather Variable Update Intervals
(Transmitter ID Dependent)
Wind speed: . . . . . . . . . . . . . . . . . . . . . . 2.5 to 3 seconds
Wind direction: . . . . . . . . . . . . . . . . . . . . 2.5 to 3 seconds
Accumulated rainfall: . . . . . . . . . . . . . . . 20 to 24 seconds
Rain rate: . . . . . . . . . . . . . . . . . . . . . . . . 20 to 24 seconds
Outside temperature: . . . . . . . . . . . . . . . 10 to 12 seconds
Outside humidity: . . . . . . . . . . . . . . . . . . 50 seconds to 1 minute
Ultraviolet radiation: . . . . . . . . . . . . . . . . 50 seconds to 1 minute
Solar radiation: . . . . . . . . . . . . . . . . . . . 50 seconds to 1 minute

https://www.davisinstruments.com/product_documents/weather/manuals/07395-249_IM_06152.pdf

[WxLover16]

Oddly enough, after taking the filter off yesterday evening, I'm really not seeing that much better response time than with it on, even with a bit drier of air in place (48%). I remember accidentally leaving it off last summer and sometimes seeing .3-4 change in an update, but now it's almost as if I still have the filter on, and this with a 24 cfm fan so it's definitely bringing in some serious ambient air with it. Hmmm...

I should add that the humidity is changing alot more than usual, just not as much as the temp.

These are the sensor update rates filter or not.
ISS Weather Variable Update Intervals
(Transmitter ID Dependent)
Wind speed: . . . . . . . . . . . . . . . . . . . . . . 2.5 to 3 seconds
Wind direction: . . . . . . . . . . . . . . . . . . . . 2.5 to 3 seconds
Accumulated rainfall: . . . . . . . . . . . . . . . 20 to 24 seconds
Rain rate: . . . . . . . . . . . . . . . . . . . . . . . . 20 to 24 seconds
Outside temperature: . . . . . . . . . . . . . . . 10 to 12 seconds
Outside humidity: . . . . . . . . . . . . . . . . . . 50 seconds to 1 minute
Ultraviolet radiation: . . . . . . . . . . . . . . . . 50 seconds to 1 minute
Solar radiation: . . . . . . . . . . . . . . . . . . . 50 seconds to 1 minute

https://www.davisinstruments.com/product_documents/weather/manuals/07395-249_IM_06152.pdf

I know the update rates. All I'm saying is I'm seeing less change per update without the filter than when I saw the last time I had no filter. As opposed to .2-.5° or so change per update, I'm either seeing no change or mainly .1°, occasionally .2°. I suppose I just find it odd and perhaps not worth taking the filter off.

[CW2274]

Oddly enough, after taking the filter off yesterday evening, I'm really not seeing that much better response time than with it on, even with a bit drier of air in place (48%). I remember accidentally leaving it off last summer and sometimes seeing .3-4 change in an update, but now it's almost as if I still have the filter on, and this with a 24 cfm fan so it's definitely bringing in some serious ambient air with it. Hmmm...

I should add that the humidity is changing alot more than usual, just not as much as the temp.

These are the sensor update rates filter or not.
ISS Weather Variable Update Intervals
(Transmitter ID Dependent)
Wind speed: . . . . . . . . . . . . . . . . . . . . . . 2.5 to 3 seconds
Wind direction: . . . . . . . . . . . . . . . . . . . . 2.5 to 3 seconds
Accumulated rainfall: . . . . . . . . . . . . . . . 20 to 24 seconds
Rain rate: . . . . . . . . . . . . . . . . . . . . . . . . 20 to 24 seconds
Outside temperature: . . . . . . . . . . . . . . . 10 to 12 seconds
Outside humidity: . . . . . . . . . . . . . . . . . . 50 seconds to 1 minute
Ultraviolet radiation: . . . . . . . . . . . . . . . . 50 seconds to 1 minute
Solar radiation: . . . . . . . . . . . . . . . . . . . 50 seconds to 1 minute

https://www.davisinstruments.com/product_documents/weather/manuals/07395-249_IM_06152.pdf

perhaps not worth taking the filter off.

We spoke of this quite a while back, leaving the filter off has no real benefit, but then, maybe leaving it on doesn't either. 

[WxLover16]

Oddly enough, after taking the filter off yesterday evening, I'm really not seeing that much better response time than with it on, even with a bit drier of air in place (48%). I remember accidentally leaving it off last summer and sometimes seeing .3-4 change in an update, but now it's almost as if I still have the filter on, and this with a 24 cfm fan so it's definitely bringing in some serious ambient air with it. Hmmm...

I should add that the humidity is changing alot more than usual, just not as much as the temp.

These are the sensor update rates filter or not.
ISS Weather Variable Update Intervals
(Transmitter ID Dependent)
Wind speed: . . . . . . . . . . . . . . . . . . . . . . 2.5 to 3 seconds
Wind direction: . . . . . . . . . . . . . . . . . . . . 2.5 to 3 seconds
Accumulated rainfall: . . . . . . . . . . . . . . . 20 to 24 seconds
Rain rate: . . . . . . . . . . . . . . . . . . . . . . . . 20 to 24 seconds
Outside temperature: . . . . . . . . . . . . . . . 10 to 12 seconds
Outside humidity: . . . . . . . . . . . . . . . . . . 50 seconds to 1 minute
Ultraviolet radiation: . . . . . . . . . . . . . . . . 50 seconds to 1 minute
Solar radiation: . . . . . . . . . . . . . . . . . . . 50 seconds to 1 minute

https://www.davisinstruments.com/product_documents/weather/manuals/07395-249_IM_06152.pdf

perhaps not worth taking the filter off.

We spoke of this quite a while back, leaving the filter off has no real benefit, but then, maybe leaving it on doesn't either. 

There is sooo much discussion in here (but perhaps the MOST informative and exciting read I've ever seen here!) of filters and good and bad humidity errors in high humidity with a FARS and the pros and cons to everything that I am confused and my head is spinning. 

[jgentry]

Oddly enough, after taking the filter off yesterday evening, I'm really not seeing that much better response time than with it on, even with a bit drier of air in place (48%). I remember accidentally leaving it off last summer and sometimes seeing .3-4 change in an update, but now it's almost as if I still have the filter on, and this with a 24 cfm fan so it's definitely bringing in some serious ambient air with it. Hmmm...

I should add that the humidity is changing alot more than usual, just not as much as the temp.

These are the sensor update rates filter or not.
ISS Weather Variable Update Intervals
(Transmitter ID Dependent)
Wind speed: . . . . . . . . . . . . . . . . . . . . . . 2.5 to 3 seconds
Wind direction: . . . . . . . . . . . . . . . . . . . . 2.5 to 3 seconds
Accumulated rainfall: . . . . . . . . . . . . . . . 20 to 24 seconds
Rain rate: . . . . . . . . . . . . . . . . . . . . . . . . 20 to 24 seconds
Outside temperature: . . . . . . . . . . . . . . . 10 to 12 seconds
Outside humidity: . . . . . . . . . . . . . . . . . . 50 seconds to 1 minute
Ultraviolet radiation: . . . . . . . . . . . . . . . . 50 seconds to 1 minute
Solar radiation: . . . . . . . . . . . . . . . . . . . 50 seconds to 1 minute

https://www.davisinstruments.com/product_documents/weather/manuals/07395-249_IM_06152.pdf

I know the update rates. All I'm saying is I'm seeing less change per update without the filter than when I saw the last time I had no filter. As opposed to .2-.5° or so change per update, I'm either seeing no change or mainly .1°, occasionally .2°. I suppose I just find it odd and perhaps not worth taking the filter off.

Were you using the Davis filter or the Sensirion filter?

[WxLover16]

Oddly enough, after taking the filter off yesterday evening, I'm really not seeing that much better response time than with it on, even with a bit drier of air in place (48%). I remember accidentally leaving it off last summer and sometimes seeing .3-4 change in an update, but now it's almost as if I still have the filter on, and this with a 24 cfm fan so it's definitely bringing in some serious ambient air with it. Hmmm...

I should add that the humidity is changing alot more than usual, just not as much as the temp.

These are the sensor update rates filter or not.
ISS Weather Variable Update Intervals
(Transmitter ID Dependent)
Wind speed: . . . . . . . . . . . . . . . . . . . . . . 2.5 to 3 seconds
Wind direction: . . . . . . . . . . . . . . . . . . . . 2.5 to 3 seconds
Accumulated rainfall: . . . . . . . . . . . . . . . 20 to 24 seconds
Rain rate: . . . . . . . . . . . . . . . . . . . . . . . . 20 to 24 seconds
Outside temperature: . . . . . . . . . . . . . . . 10 to 12 seconds
Outside humidity: . . . . . . . . . . . . . . . . . . 50 seconds to 1 minute
Ultraviolet radiation: . . . . . . . . . . . . . . . . 50 seconds to 1 minute
Solar radiation: . . . . . . . . . . . . . . . . . . . 50 seconds to 1 minute

https://www.davisinstruments.com/product_documents/weather/manuals/07395-249_IM_06152.pdf

I know the update rates. All I'm saying is I'm seeing less change per update without the filter than when I saw the last time I had no filter. As opposed to .2-.5° or so change per update, I'm either seeing no change or mainly .1°, occasionally .2°. I suppose I just find it odd and perhaps not worth taking the filter off.

Were you using the Davis filter or the Sensirion filter?

Davis, never have used sensirion.

[jgentry]

Interesting that Campbell Scientific sells their version of the SHT-75...

https://www.campbellsci.com/cs215-l

[Old Tele man]

Interesting that Campbell Scientific sells their version of the SHT-75...

https://www.campbellsci.com/cs215-l

Quoted from the data sheet:

"The CS215 uses the Sensirion SHT75, a combined relative humidity and temperature element, to provide accurate, stable measurements. The Sensirion SHT75 element is field-replaceable, eliminating the downtime typically required for the recalibration process. The CS215 outputs an SDI-12 signal that's measurable by many Campbell Scientific dataloggers."

What's the SDI-12 signal? Compatible with Sensibus?

[jgentry]

Interesting that Campbell Scientific sells their version of the SHT-75...

https://www.campbellsci.com/cs215-l

Quoted from the data sheet:

"The CS215 uses the Sensirion SHT75, a combined relative humidity and temperature element, to provide accurate, stable measurements. The Sensirion SHT75 element is field-replaceable, eliminating the downtime typically required for the recalibration process. The CS215 outputs an SDI-12 signal that's measurable by many Campbell Scientific dataloggers."

What's the SDI-12 signal? Compatible with Sensibus?

That I do not know...I’m sure someone who is smarter than I can answer that question for you.

[johnd]

What's the SDI-12 signal? Compatible with Sensibus?

No - something fairly different. SDI-12 is another serial protocol that's quite widely used for instrumentation. It's one of the options on EM. There's a load more information online as ever, but one starting point would be:

en.wikipedia.org/wiki/SDI-12

[miraculon]

For what it's worth, I just took my Belfort 566 psychrometer outside. We are showing 97% RH on my Davis ISS and 95% on the Davis Temp/Hum station. I believe that these are both SHT-11 (or -15) and definitely not SHT-31.

Using distilled water I got 65°F dry bulb and 64°F wet bulb.

The slide rule shows about 94% RH a web calculator gives 94.88% RH. http://linricsoftw.web701.discountasp.net/webpsycalc.aspx

I ordered one of those Sensirion SHT-31 eval boards from Mouser.

Greg H.

Hmm. Interesting. Let us know the results using the 31

This morning the humidity was higher and I had the chance to check the Sensirion eval board. I have it placed in my NovaLynx thermometer housing and the shelter is in the shade this time of day.

Here is what I saw:

ISS Humidity: 92%
T/H Station: 88%
SHT-31 eval: 88.86% (Temp was 65.78°F)
Belfort Psychrometer Wet/Dry: 66°F Dry, 63½°F Wet giving 87.58%. Another reading was 67°F and 64°F for 85.38%

Conditions were changing, especially on the Sensirion board and Belfort. I tried to stay close to the therm shelter and the Sensirion while using the Belfort.

Dew Point per the Belfort was 62°F to 63°F and 63°F on the Davis ISS.

I have a feeling that this is one of those "measure it with a micrometer, mark it with chalk and cut it with an axe" kind of things...

Greg H.

[jerryg]

 

[Bobvelle]

Just checked my history tables on WU and noticed, to my surprise, that I was getting 100% RH readings as recently as Sept 2017 and in June and July 2017. I had no Humidity compensation added and no software formulas converting or interpreting data, just console to Weatherlink IP to WU.  This is on a SHT-31 that was at least 2 years old (or more) at that time and in a 24hr FARS.
The rest of the year was had RH highs of 98% & 99%. I guess it's just a crap shoot. This new one hasn't had the conditions right for RH that high yet... but I'm showing 98% this morning.

[openvista]

This morning the humidity was higher and I had the chance to check the Sensirion eval board. I have it placed in my NovaLynx thermometer housing and the shelter is in the shade this time of day.

Here is what I saw:

ISS Humidity: 92%
T/H Station: 88%
SHT-31 eval: 88.86% (Temp was 65.78°F)
Belfort Psychrometer Wet/Dry: 66°F Dry, 63½°F Wet giving 87.58%. Another reading was 67°F and 64°F for 85.38%

Conditions were changing, especially on the Sensirion board and Belfort. I tried to stay close to the therm shelter and the Sensirion while using the Belfort.

Dew Point per the Belfort was 62°F to 63°F and 63°F on the Davis ISS.

I have a feeling that this is one of those "measure it with a micrometer, mark it with chalk and cut it with an axe" kind of things...

Greg H.

Even so, that's a 6% average humidity difference (ISS vs psychrometer) in a temp/humidity range where the Davis sensor is more accurate (above 70% in relatively cool air). However, let us know if you can test when the temp rises above 77 (25C) with a dp above 60 (15C) as that's when even larger discrepancies begin to appear.

[Old Tele man]

Devil's Advocate Humor: Sorta makes you wonder about the accuracy of all the PAST weather data, don't it?

[ValentineWeather]

Sent email to metoshield owner about the issue with the SHT31 humidity and asked him to consider production of the SHT75 which is a meteorological grade sensor unlike the SHT31.

They tried to develop the SHT31 just for their shields and failed with the protocol issue but the 75 is already working in homemade units would be great addition.  Something for sure many with the wet bias would jump on.
Say a combo deal shield and sensor providing the shield works as advertised.

[Bobvelle]

So I put on the Sensirion SF2 filter on my SHT31. I decided to use Rocksett, an engeneering ceramic adhesive. Its clear, no oder, no fumes, non toxic and most importantly easy to remove. Its a mild bond on plastics. Working good so far.
http://www.flexbar.com/shop/pc/ROCKSETT-2-OZ-p4019.htm
 [ You are not allowed to view attachments ]

Takes about 15 min to hold and 12hr to full cure.

I did not re-install the big Davis filter. So it sits in the FARS with just the SF2 filter. I was also able to get my 2nd Temp/Hum station installed today. It is a Passive shield SHT11 that I baked an re-hydrated yesterday. It's been going a few hours now and yes, the SHT31 is avg 4% higher RH but the temp difference is hovering around .5 deg F.

[ValentineWeather]

Glad you posted that if good stuff I need something. I ordered some of those filters also primarily for the metoshield I have coming in couple weeks. Less restriction inside temperature chamber the better I would think.

[ValentineWeather]

I found this also. http://ecobondit.com/ecobond-products/

http://ecobondit.com/what-we-stick-too/

and this https://www.amazon.com/Supertite-Universal-Industrial-Strength-0-63oz/dp/B077KGT3K7/ref=sr_1_8?ie=UTF8&qid=1532993103&sr=8-8&keywords=Eco-Bond+Adhesives