Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 27225 times)

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Offline CW2274

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1250 on: September 19, 2018, 08:25:20 PM »
Has anyone else had spikes in temps and dew point in such a short amount of time like that?
You definitely had some kind of boundary go through there based on the temp drop and consistent rise in dewpoint.
See the loop at 20Z.
https://weather.cod.edu/satrad/nexrad/index.php?type=LIX-N0Q-1-200

Offline Dj1225

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1251 on: September 19, 2018, 11:13:29 PM »
Has anyone else had spikes in temps and dew point in such a short amount of time like that?
You definitely had some kind of boundary go through there based on the temp drop and consistent rise in dewpoint.
See the loop at 20Z.
https://weather.cod.edu/satrad/nexrad/index.php?type=LIX-N0Q-1-200

I have had a some type of Davis weather station for over 20 years and I've never seen the sea breeze make the humidity jump that high in such a short amount of time. I wonder if it would have shown such a spike in that short of time if I had the 31 installed instead of the 75? CW2274 you live out in the desert have you ever seen your humidity spike that much in such a short time with the monsoon or the influx of Pacific moisture?

Offline CW2274

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1252 on: September 20, 2018, 12:21:50 AM »
Has anyone else had spikes in temps and dew point in such a short amount of time like that?
You definitely had some kind of boundary go through there based on the temp drop and consistent rise in dewpoint.
See the loop at 20Z.
https://weather.cod.edu/satrad/nexrad/index.php?type=LIX-N0Q-1-200

 CW2274 you live out in the desert have you ever seen your humidity spike that much in such a short time with the monsoon or the influx of Pacific moisture?
Honestly, I'm not sure, but if so, I'd be infrequent. It's certainly possible your sensor is acting up, obviously study it closely, you'll be the best judge as whether it's going out the door or not. I posted the radar loop because you could see a 'boundary' or 'outflow' at the time of your spike move from south to north, but the Gulfport airport did not show the spike, but that's only one observation per hour compared to yours every 5 minutes.

Online dendrite

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1253 on: September 20, 2018, 10:22:14 AM »
I saw similar jumps as that with my 75...mostly with quick moving t'storms where the sun quickly comes back out and there's some evaporative cooling. The type of situation where you see the sun blazing and the steam rising off the roof.

Offline ValentineWeather

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1254 on: September 20, 2018, 12:13:21 PM »
Been above 85% now solid 30 hours most of the time near 100%.
I've actually had 2-SHT31's reach 100% on their own and this SHT31 graph is inside Davis FARS has reached 99%. I finally had 1- SHT75 out of 3 different ones reach 98% but took almost 20 hours of actual humidity near 100% saturation. I'm disappointed in the 75's ability to go high. Still have more to test but so far the 3 I've tested all duds on high end.

I find the SHT31's are better on top end near 100% but as we know have the high bias also.
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Randy

Offline ValentineWeather

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1255 on: September 20, 2018, 12:38:02 PM »
At same time here is under porch where I keep the acurite 31 dry. Here you can see it's also reached 99% overnight but as you can see has dropped down to 96% currently with rain falling and the Davis 31's are all near 99% currently. This is what I mean when I say the 31's don't all act the same. The Acurite version doesn't display the same wet bias but still reaches 99% under porch cover.
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Randy

Offline jerryg

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1256 on: September 20, 2018, 01:04:56 PM »
All of mine reach 99% outside in the shields  :grin: i don't see why yours don't unless you are just cursed  :lol:.

Online dendrite

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1257 on: September 20, 2018, 01:14:20 PM »
heh...my 75s have always trumped the 31s on reaching the high end too. I'm not sure I've ever had a 31 over 98%.

Offline ValentineWeather

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1258 on: September 20, 2018, 01:42:32 PM »
All of mine reach 99% outside in the shields  :grin: i don't see why yours don't unless you are just cursed  :lol:.

I've had horrible luck so far.  :-(
Randy

Offline Dj1225

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1259 on: September 20, 2018, 01:55:43 PM »
All of mine reach 99% outside in the shields  :grin: i don't see why yours don't unless you are just cursed  :lol:.
Mine also has no trouble reaching 99%

Offline ValentineWeather

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1260 on: September 20, 2018, 06:55:43 PM »
Well trying another 75. Did the damp rag test for about 2.5 hours it finally hit 98% went another hour and didn't go beyond.  3.5 hours total..

So I heated some water in big pot and gave sensor some hot steam humidity just a few minutes around 138° and saw 97% once. Took out cooled down to room temperature, 40% humidity.
Did the wet rag test again and it quickly reached 98% this time followed by 99% within 10 minutes total time where its still holding now for 15 minutes.

So my question do these sensors need broke-in?

As I was getting ready to post just noticed 100%....Hee haw  \:D/

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« Last Edit: September 20, 2018, 07:37:30 PM by ValentineWeather »
Randy

Offline jgentry

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1261 on: September 20, 2018, 06:59:56 PM »
Well trying another 75. Did the damp rag test for about 2.5 hours it finally hit 98% went another hour and didn't go beyond.  4.5 hours total..

So I heated some water in big pot and gave sensor so hot steam humidity just a few minutes around 138° and saw 97% once. Took out cooled down to room temperature, 40% humidity.
Did the wet rag test again and it quickly reached 98% this time followed by 99% within 10 minutes total time where its still holding now for 15 minutes.

So my question do these sensors need broke-in?

As I was getting ready to post just noticed 100%....Hee haw  \:D/

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I think I’ve read somewhere (maybe on this forum) that Davis does say that the sensor needs to be “broke in.”
Davis Vantage Pro2. SHT 31. 24hr FARS. WU: KXALJEMI2 & KALTHORS2 CWOP: C6353 & C6358

  

Offline ValentineWeather

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1262 on: September 20, 2018, 07:07:18 PM »
Now I wonder if the key was the few minutes ( about 10) in the hot steam, at least to reduce the time... Next time I may try the steam first.
Randy

Offline CW2274

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1263 on: September 20, 2018, 07:09:55 PM »
Could we trouble you to try a 31 also? :-)

Offline CW2274

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1264 on: September 20, 2018, 07:23:48 PM »
As I was getting ready to post just noticed 100%....Hee haw  \:D/
Not to be Debbie Downer, but I just noticed your dew is higher than your temp. So, I guess the choice is 99%, or 100% with a 'technically' too high of a dew.

Offline ValentineWeather

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1265 on: September 20, 2018, 07:35:38 PM »
As I was getting ready to post just noticed 100%....Hee haw  \:D/
Not to be Debbie Downer, but I just noticed your dew is higher than your temp. So, I guess the choice is 99%, or 100% with a 'technically' too high of a dew.
\

It came together, dewpoint is calculated so it was slower posting. I've got 8 stations so the envoy8x is behind occasionally and wasn't there why I captured the image.
Randy

Offline CW2274

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1266 on: September 20, 2018, 07:37:07 PM »
As I was getting ready to post just noticed 100%....Hee haw  \:D/
Not to be Debbie Downer, but I just noticed your dew is higher than your temp. So, I guess the choice is 99%, or 100% with a 'technically' too high of a dew.
\

It came together, dewpoint is calculated so it was slower posting. I've got 8 stations so the envoy8x is behind occasionally and wasn't there why I captured the image.
=D>

Offline ValentineWeather

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1267 on: September 20, 2018, 07:42:46 PM »
Could we trouble you to try a 31 also? :-)

Currently fresh out of 31's but worth a try. The way these measure humidity is resistance across a little bridge so maybe a little heat and steam loosens it up. Just a guess but seemed to work that time.
Randy

Offline CW2274

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1268 on: September 20, 2018, 08:03:53 PM »
Could we trouble you to try a 31 also? :-)

Currently fresh out of 31's but worth a try. The way these measure humidity is resistance across a little bridge so maybe a little heat and steam loosens it up. Just a guess but seemed to work that time.
IIRC, back when I picked up my 15, I called Sensirion and asked about all the 'conditioning' in their technical literature and if that was necessary, I basically got a "no", perhaps that was bad info. At this point, who knows.  :roll:


Offline ValentineWeather

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1269 on: September 20, 2018, 08:20:30 PM »
Could we trouble you to try a 31 also? :-)

Currently fresh out of 31's but worth a try. The way these measure humidity is resistance across a little bridge so maybe a little heat and steam loosens it up. Just a guess but seemed to work that time.
IIRC, back when I picked up my 15, I called Sensirion and asked about all the 'conditioning' in their technical literature and if that was necessary, I basically got a "no", perhaps that was bad info. At this point, who knows.  :roll:

Don't know either. Jerry has built a chamber with fan to speed up saturation so all the sensors he puts in the field he knows what the humidity will do.

Guess I'll find out if it helped or not next time we have high humidity.
Randy

Offline WxLover16

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1270 on: September 20, 2018, 09:06:22 PM »
heh...my 75s have always trumped the 31s on reaching the high end too. I'm not sure I've ever had a 31 over 98%.

Very odd because the 31 I just bought almost a month ago (installed 8/24) has hit 100% a whole weeks worth of data (if not more, I'd have to look), something I've never seen before. I was almost thinking the 100 was erroneous as I'd never seen it that much before. I'll just chalk it up as my best sensor yet?  \:D/

Also, even though I don't have a filter on it, I also didn't on the previous and it rarely hit 99%.

Edit: Forgot to add I have a 24 cfm fan going in it, dunno if that matters or not...
« Last Edit: September 20, 2018, 09:09:01 PM by WxLover16 »
Davis Wireless VP2 SHT31 24hr 24CFM FARS

Offline ValentineWeather

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1271 on: September 22, 2018, 06:48:11 AM »
Well as temperatures are dropping into the low 40's the trend I'm seeing on the sht75 humidity is now running slightly higher than the 31's.  Not by much just 1-2%. I was wondering if the Davis sht31 wet bias was only active at higher temperatures and yes that appears to be.

The 31 for sure is the sensor of choice at colder climates come winter with it's better low temperature accuracy and no difference in humidity it's a no brainer for me which sensor to use.
 
Graph shows Dp's from yesterday with ambient temperatures 40's-50's. Both sensors in same stock Davis FARS.
It's clear the 75 is running slightly higher at these lower dp temperatures. 
Randy

Offline ValentineWeather

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1272 on: September 22, 2018, 07:29:32 AM »
This is what I like about the 75 notice response to change is really apparent. Sensors are in same fars shield.
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Randy

Online dendrite

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1273 on: September 22, 2018, 08:50:23 AM »
My new transmitter (2016+), aerocone, and new SHT31 came in from SI today. My old transmitter (late 2006) was running an older 31 so I had a -0.9F offset on it. With the new transmitter it is back to no calibration. I assume I did everything correctly on that front?

Anyway, here's the readings before and after the switch. Conditions were similar throughout the day...overcast with a S wind and no precip. It looks like my dewpoint dropped a solid 1F with the new 31 and the temperature response seems to have improved (appears to be more "swings" in the readings). My dewpoint is right in line with the METARs this morning.

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Offline ValentineWeather

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1274 on: September 22, 2018, 09:02:53 AM »
Transmitter manufacture date is the key. Look at the first 2 numbers after the letters, as long as they are 16,17,18 etc. no offset is needed with 31.
Yes you can see the humidity is lower on new sensor for sure.
Randy