Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 24845 times)

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Offline Intheswamp

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1200 on: September 09, 2018, 10:09:10 AM »
Ok, I still haven't installed the new (bought a year ago?) temp/humidity sensor.  Looks like I need to.  I looked this morning and was surprised to see a report of 21% humidity.  This is after it being so foggy I couldn't even see the ISS this morning at 220' distant.  Uh,...no.  So, this week I hope to swap the sensors out.

09909_2018_HumidityChart by Intheswamp, on Flickr

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Offline Intheswamp

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1201 on: September 09, 2018, 10:14:48 AM »
Hmmm, and one more chart...weather trends.  It seems the humidity sensor is nose-diving as the fog clears out.   #-o
0909_2018_WxTrends by Intheswamp, on Flickr

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Offline jerryg

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1202 on: September 09, 2018, 11:44:47 AM »
For those of you who might want to try the 75 here is the link to the best price that i have found so far. They are in factory packaging and shipping packaged the proper way. http://www.futureelectronics.com/en/Technologies/Product.aspx?ProductID=SHT75SENSIRIONAG9049996&IM=0&utm_source=octopart.co

Offline ValentineWeather

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1203 on: September 09, 2018, 02:57:57 PM »
Good find Jerry.  =D>
Randy

Offline Intheswamp

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1204 on: September 09, 2018, 10:54:47 PM »
Interesting recovery for the plummeting humidity...
 09092018_temphumidity by Intheswamp, on Flickr

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Offline Intheswamp

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1205 on: September 10, 2018, 08:38:40 AM »
Well, it looks like we're getting another episode of the plummeting humidity chart this morning...another heavy fog morning.

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Offline galfert

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1206 on: September 10, 2018, 09:32:11 AM »
I've noticed a few Davis stations around me with plummeting humidity graphs. It always seems to drop to around that 30% range and then it recovers when the humidity drops around 75%. I've seen several graphs just like yours.

If I go back on those stations historic data I notice that they gradually loose the top end coming down from 96% down to about 88%. Then they hold that 88% top end for a while. Then it may get a bit random and unpredictable for a few weeks as it tried to keep things together. That's the big indicator that its about to blow. And then wham drops down to 30% when it should be above 75% to 100%. Never to ever recover to normal working condition unless the sensor is replaced. Seen it with VP2 and Vue. I've talked to a few of my neighbors with this issue.

Its like a drunk college kid. He is sober and working in 96% efficiency. Then he starts drinking and getting drunk loosing capacity down to 88%. Then they get unpredictable as they try and sober up. Someone gives them coffee or smacks them around. But they are a wreck and stumbling and dizzy. Then they feel like they are going to blow chunks and they try and hold it in for bit. Then wham they toss their cookies and pass out momentarily on the toilet down to barely breathing at 30% capacity. Then they feel better for a while and recover to 75%. But they are still drunk and think they are okay to drink more....only to pass out again. I don't know why I came up with this analogy. Just feeling silly this morning. Yes I'm sober.
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« Last Edit: September 10, 2018, 09:44:36 AM by galfert »
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Offline Intheswamp

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1207 on: September 10, 2018, 01:28:50 PM »
Words of experience....   :-k :-D

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Offline dendrite

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1208 on: September 11, 2018, 09:31:31 PM »
Rocking the 100% RH tonight. I haven't seen 100% this much since my analog sensor.

Offline CW2274

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1209 on: September 11, 2018, 10:06:13 PM »
Rocking the 100% RH tonight. I haven't seen 100% this much since my analog sensor.
Please forgive me, but after fifty pages of this thread, I've forgotten what you're "rocking" with, 75, 31, fan, no fan....

Offline dendrite

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1210 on: September 12, 2018, 07:51:56 AM »
Rocking the 100% RH tonight. I haven't seen 100% this much since my analog sensor.
Please forgive me, but after fifty pages of this thread, I've forgotten what you're "rocking" with, 75, 31, fan, no fan....
75 + 40CFM fan + no filter

Offline ValentineWeather

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1211 on: September 12, 2018, 09:11:44 AM »
Fine mist water droplets drawn onto sensor is making 100% much easier I'm sure. Sooner or later sensor will fail without any protection. This was what Jerry was experiencing in humid south Texas gulf region even using filter with FARS, killing sensors so decided to go with passive unit.

You can replace the 75 at $20 each futureelectronics link was posted above by Jerry. I ordered a few replacements myself.
Randy

Offline ValentineWeather

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1212 on: September 12, 2018, 10:47:31 AM »
I have some 100 mesh silk (not the real stuff), it comes much higher mesh also. Plan is cover all sensors eventually. 200-300 mesh may have been better choice but water still beads on top of the 100 mesh when tested and no air restriction I can tell. Its very sheer stuff and completely odorless.
 
I'm just going to wrap sensors, have single layer on sensor side of board. This is a cut down SHT31 I was using in Meteoshield Pro and simple wire tie you might find on loaf of bread. Nothing fancy but should work.
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Offline dendrite

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1213 on: September 12, 2018, 01:27:23 PM »
Fine mist water droplets drawn onto sensor is making 100% much easier I'm sure. Sooner or later sensor will fail without any protection. This was what Jerry was experiencing in humid south Texas gulf region even using filter with FARS, killing sensors so decided to go with passive unit.

You can replace the 75 at $20 each futureelectronics link was posted above by Jerry. I ordered a few replacements myself.
I had my old 75 installed this way for an entire Fall-Winter-Spring period and didn't notice any changes in readings to the naked eye. A lot of you guys have 31s failing or drifting and that's with filters and passive/stock FARS shields. It hasn't been easy to hit 100%. It was mostly 99% with occasional obs of 100% and this was after almost 2" of synoptic rain followed by 12hrs of drizzle. The local METAR reported 0F to 1F dewpoint depressions too so RH was definitely in that 98-100% range at times.

Offline ValentineWeather

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1214 on: September 12, 2018, 02:37:00 PM »
I had my old 75 installed this way for an entire Fall-Winter-Spring period and didn't notice any changes in readings to the naked eye. A lot of you guys have 31s failing or drifting and that's with filters and passive/stock FARS shields. It hasn't been easy to hit 100%. It was mostly 99% with occasional obs of 100% and this was after almost 2" of synoptic rain followed by 12hrs of drizzle. The local METAR reported 0F to 1F dewpoint depressions too so RH was definitely in that 98-100% range at times.

The 75 may be a more robust sensor being it has been used extensively in meteorological field.  Being last year of production I noticed many sellers like futureelectronics.com have on order (1900) so may be around awhile longer.
Randy

Offline jerryg

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1215 on: September 12, 2018, 07:23:28 PM »

Well this was an interesting day, we have had several days of cloudy rainy high humidity days in a row and today we finally got some sunshine and my sensors were both showing humidity around 84 to 86 percent when the one i put out for testing this am was showing 79 to 81 percent as well as a bunch of stations area wide. I put a new sensor in the standard shield and it showed the lower humidity but the main station was still high so i swapped it out and it was then showing the lower reading. Well i remember reading that the sensor could read high if exposed to high humidity for 60 or more hours and that's exactly what i had going on. Now staying in high humidity doesn't happen that often but it sure gives you a 3% higher reading just like the specs said and until the humidity lowers for some time your readings will have that offset.

Offline jerryg

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1216 on: September 12, 2018, 08:15:06 PM »
The more i think about it that really sucks if you live in an area where long periods of humidity above 80% exist, i guess you would have to change out your sensor every couple of days to keep from having wrong readings of humidity and dew point. Here is what it says, Sensor works stable within recommended normal range –
see Figure 4. Long term exposures to conditions outside
normal range, especially at humidity >80%RH, may
temporarily offset the RH signal (+3 %RH after 60h). After
return to normal range it will slowly return towards
calibration state by itself. See Section 1.4 “Reconditioning
Procedure” to accelerate eliminating the offset. Prolonged
exposure to extreme conditions may accelerate ageing.

Offline jerryg

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1217 on: September 12, 2018, 10:36:05 PM »
Sure seems strange that above 80% is not normal range and can cause an error.
what kind of sensor is this? How can a manufacturer make a humidity sensor that can't take humidity over 80% for several days, sure seems like some kind of poor planning to me. Could this be what is causing the error in the 31?

Offline jgentry

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1218 on: September 13, 2018, 08:15:10 AM »
Sure seems strange that above 80% is not normal range and can cause an error.
what kind of sensor is this? How can a manufacturer make a humidity sensor that can't take humidity over 80% for several days, sure seems like some kind of poor planning to me. Could this be what is causing the error in the 31?

It could play a role, but I think it’s the way Davis mounts the 31 chip into the pcb. But I could be wrong.
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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 #1219 on: September 13, 2018, 08:51:54 AM »


It could play a role, but I think it’s the way Davis mounts the 31 chip into the pcb. But I could be wrong.

Yes, something is different between Davis 31's and Acurite 31's. Either the calibration or manufacturing process knocks Davis units off. The Acurite 31 calibration is very similar to the SHT75 running 2° DP lower mid to upper ranges.  I wonder if Davis didn't ask for a different calibration for desert regions so it wasn't going to 0% humidity? Making the mid to upper areas run wet. Just an  :idea:
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Offline dendrite

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1220 on: September 13, 2018, 09:38:47 AM »
It may be time for Davis to have a separate temperature and dewpoint option. I think that will be a project of mine come next spring as I'll be busy with my coop and getting the ultrasonic anemometer installed this fall. I'm thinking an SHT in a FARS for temp and an SHT in a passive with some form of light, supplemental heat for dewpoint.

Offline CW2274

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1221 on: September 13, 2018, 04:35:00 PM »


It could play a role, but I think it’s the way Davis mounts the 31 chip into the pcb. But I could be wrong.

I wonder if Davis didn't ask for a different calibration for desert regions so it wasn't going to 0% humidity?
I think the version of software on the console dictates whether it'll hit 0% or not.
Now that we're drying out here, the humidity/dews are really going down so I keep waiting for my -2% correction to catch up with me, but not so...yet. Myself and TUS have both hit 8% with dews of 28F and 29F respectively, so the shoe drop hasn't happened as I keep waiting to see when the -2% will start to show itself. In the mean time, the -2% has been money tween 8% and 91%.

Offline ValentineWeather

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1222 on: September 13, 2018, 06:03:54 PM »


 I think the version of software on the console dictates whether it'll hit 0% or not.

I've never heard that,  the 15's had dry bias issue below 10%, remember? Has a change been made to compensate, causing the wet bias we now see. The Davis versions are different from the acurite 31 is why I was saying it may be a special calibration request. These 31's with LS protocol are specifically made for Davis and not publically available as far as I know or we would be purchasing instead of the 75's.   
Randy

Offline CW2274

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1223 on: September 13, 2018, 06:21:27 PM »


 I think the version of software on the console dictates whether it'll hit 0% or not.

I've never heard that,  the 15's had dry bias issue below 10%, remember?
Hell Randy, I can't remember what was for dinner last night. Not saying the 15 didn't, I honestly don't recall.
As far as the 0%, this is for the Vue console, didn't look at the VP2. See version 2.14.

 

Version History

Firmware version 4.30

    Modified heat index table to better support high temp and high humidity environments.

Firmware version 4.18

    Supports new barometric pressure sensor. It will also support previous barometric sensors with a Mfg
    Code of MB or higher (e.g. MB, MC, etc.)

Firmware version 3.00

    Updated internal communication between console and data logger for improved performance.

Firmware version 2.14

    Solved problem of inadvertent low transmitter battery message. It was displaying as soon as a low voltage was detected, now it will not display unless a low battery has been detected for a full 2 minutes.
    Outside humidity no longer displays 0%.
    Modified heat index table to match the algorithm used by the WeatherLink software.


Offline jgentry

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1224 on: September 13, 2018, 10:34:21 PM »
Does anyone knows where you can get a crossover adapter for the RJ12?  If I can get a RJ 12 adapter, then I won’t have to worry about getting rid of the 45 plug that’s on the EH-4 cable.
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