Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 82833 times)

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Offline ValentineWeather

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Well I have psychro-dyne now which is pretty accurate and a new SHT-31 I always purchase the SI 25 foot 31's due to my custom station setup and I'm finding the dew point is either right on or reading low 1 degree not high. I only use dew point chasing humidity around is a waste of time.  Anyway, the highest I've seen in DP is 71 and the lowest is around 48 so limited where I've checked but I no longer see those crazy high heat indices of old. Yes if you heavy aspirate it moves around a couple of degrees once the ambient gets above about 84 or so but if you don't heavily aspirate with a higher-powered AC fan the spiking movement is less.

Now yes it reads higher than the ASOS but it's correct where the ASOS is actually reading low humidity and actually is a POS itself they are force-feeding this warming by cranking the rural ASOS temperature up so it's hard to trust anything about it. 
This crap started around March of 2017 the local official coop station happens to be run by the radio station owner in Valentine too so everyone pretty much knows what's up with the Valentine KVTN ASOS. Just a short example without getting into full detail about being above freezing during icing events, the ASOS inability to reach full saturation so now the max humidity is 94% the AP ASOS has had 9-100+ days while all the other local Wx stations including the COOP-3 this summer and it goes on and on. Really rock-solid case they are tampering with the temperatures on these FAA weather stations likely inserting a multiplier because the hotter it gets the wider the spread from other thermometers.

Totally agree with you regarding ASOS issue. The local site near me,KINT, has been reading higher temperatures than all surrounding ASOS and private stations going on 4 months now. Also their dew point is in my opinion way off, reading low. Max humidity I have seen over the past 4 months is 94%
I have contacted the NWS in Raleigh where their technicians are stationed and was told that they do a field spot check every so often and as long as it falls within 5 degrees itís considered good. Bullshit I say!! Trust my VP2, you bet.

Looks like to me they are trying to tie up some loose ends when they were called out by critics rural areas weren't warming anywhere close to the same rate as cities and some areas actually lost ground. So now rural areas next to the climate classified ASOS stations get to live with these fake temperatures destroying our historic records.
Randy

Offline CW7491

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For years I have been very critical of Davis' use of the Sensirion SHT31 in this thread. I have been frustrated that I have been able to get considerably more accurate humidity and dew point results with a home-brew, older style SHT75. I've been convinced that there was something Davis was doing in their mounting of the SHT31 on it's board that somehow was causing a high humidity bias in addition to the high humidity bias of the Sensirion sensor itself. I want to pass along that after testing a new Davis SHT31, I observed a notable improvement in the performance of the Davis SHT31 that matches the results I get from the SHT75. This is the first time I have ever seen this kind of performance and I've similarly tested 6+ Davis SHT31s over the years.

After hearing from a few people on this forum, I purchased a new Davis SHT31 with a manufacture date in 2021 because they noticed what they thought was an improvement in the humidity/dew point performance of the sensor. The Davis SHT31 I bought has 25ft of cable. Out of the box, it actually read slightly LOWER in humidity and dew point than an SHT75 and SHT35 I have. Never have I observed this. Every other new Davis SHT31 I have ever bought typically ready 3-5% higher than these sensors at room temperature.

I started to use the new Davis sensor outside for a few weeks to let it acclimate and experience prolonged high humidity. Then I co-located my home-brew SHT75, inside the same radiation shield, and compared the readings for a series of days. Humidity during these days ranged from saturation to around 30%. The Davis SHT31 followed the performance of the SHT75 in a way I have not observed before. Previously, as humidity dropped from saturation, the SHT75 would read consistently lower, but with this new SHT31, it stayed right with the 75.

The sensor still suffers from the limitations inherent to the Sensirion sensor. It still is reluctant to reach 100% at saturation (although it has jumped to 100%) and it still has a wet bias in the mid and lower ranges in my climate. I use a slope and offset to help correct this bias and to match it to my Belfort Psychron.

I do not know if I am just lucky with this sensor or if Davis has changed something in their processes that improve performance. There is no obvious or even less than obvious modification to the sensor board that I can tell. We'll never know, but I'm going to be an optimist here and give Davis credit for improving their product. I noticed Sensirion published a document for manufacturers specifically addressing the dos and don'ts of mounting their sensors with conformal coatings as Davis does: https://www.sensirion.com/fileadmin/user_upload/customers/sensirion/Dokumente/2_Humidity_Sensors/Application_Note/Sensirion_AppNotes_Humidity_Sensors_Handling_Instructions.pdf.
I like to think that combined with feedback, maybe even from this forum and the people on it, Davis took action to improve a deficiency in their product.
« Last Edit: November 16, 2021, 10:19:46 PM by CW7491 »

Offline CW2274

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I like to think that combined with feedback, maybe even from this forum and the people on it, Davis took action to improve a deficiency in their product.
I have no doubt. If ValentineWeather hadn't discovered this flaw in the then new 31's, who knows if anyone of us (including Davis) would have since. I feel confident that Davis heard our squeaky wheel and oiled in a fix.

Offline drew1021

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So with this new information would it be best to order the 31 with 25í cable? And did you order it from Davis directly?
VP2 with 24 hour FARS. WU: KNCLEWIS2. CWOP/APRS: DW4712, COCORAHS: NC-FR-7

Offline CW7491

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I got mine from Scaled Instruments. I wanted the 25' of cable and I thought I'd have a better chance at getting a more recently manufactured sensor since the 25' option is a relatively new option. I've heard great things about Mr. Wilhour at SI, so he may be able to provide information about the recency of his supply if the shorter cable is desired. If you do order the 25', I don't imagine you'd be getting a sensor older than mine ...

Offline johnd

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FWIW I'm not aware of any differences whatsoever between the sensor elements of the standard and the 25' parts. It's just a question of what cable length is fitted to the sensor PCBA.
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Sorry, but I have no time to help with individual issues by email. Please post your issue in the relevant forum section here & I will comment there if I have anything useful to add.

Offline drew1021

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Well I was curious to see if there was any change so I purchased a 31 with the short cable from Ryan and checked the performance against my Belfort psychrometer and the results were disappointing. At room temperature I got the usual 5 to 6 percent difference to the high side. Temperature was .5 to the low side. I put in a offset to the humidity of -2% and a +.5 offset for the temperature. Iíd rather have the readings to be accurate in the mid range than at saturation. I give up!
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Offline jgentry

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Well I was curious to see if there was any change so I purchased a 31 with the short cable from Ryan and checked the performance against my Belfort psychrometer and the results were disappointing. At room temperature I got the usual 5 to 6 percent difference to the high side. Temperature was .5 to the low side. I put in a offset to the humidity of -2% and a +.5 offset for the temperature. Iíd rather have the readings to be accurate in the mid range than at saturation. I give up!

Probably means some of us got a good batch. Right now, all of mine have a wet bias.
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline CW7491

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Well I was curious to see if there was any change so I purchased a 31 with the short cable from Ryan and checked the performance against my Belfort psychrometer and the results were disappointing. At room temperature I got the usual 5 to 6 percent difference to the high side. Temperature was .5 to the low side. I put in a offset to the humidity of -2% and a +.5 offset for the temperature. Iíd rather have the readings to be accurate in the mid range than at saturation. I give up!

Iím sorry to hear that. Iíve been hopeful. If itís accessible, can you provide the info or take a picture of the sticker on the back side of the sensor? Davis puts a manufacture/inspection sticker on it.

Offline drew1021

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Iím sorry to hear that. Iíve been hopeful. If itís accessible, can you provide the info or take a picture of the sticker on the back side of the sensor? Davis puts a manufacture/inspection sticker on it.

Sorry to say Iíve already mounted it outside in the radiation shield
VP2 with 24 hour FARS. WU: KNCLEWIS2. CWOP/APRS: DW4712, COCORAHS: NC-FR-7

Offline drew1021

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Original 31 purchased was returned. After watching the newest 31 I purchased for about 10 weeks now I have to say Iím impressed by the accuracy Iíve seen so far. Not only has this sensor on multiple occasions hit 100%, it also is performing much better in low dew points. Will have to see what happens when our Carolina summer weather takes hold. Also, Iím using the fs2 filter cap.
« Last Edit: January 30, 2022, 08:04:33 AM by drew1021 »
VP2 with 24 hour FARS. WU: KNCLEWIS2. CWOP/APRS: DW4712, COCORAHS: NC-FR-7

Offline CW2274

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Also, Iím using the fs2 filter cap.
=D>

Offline drew1021

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In full disclosure, Iíve been using the fs2 for a few years now
VP2 with 24 hour FARS. WU: KNCLEWIS2. CWOP/APRS: DW4712, COCORAHS: NC-FR-7

Offline CW2274

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Oh, didn't realize that. I have as well. I figured all my preaching about it had fallen on deaf ears. I'm also using the stock mesh filter now (didn't before) as I think it helps with the over sensitivity from the strong fan and obviously keeps the SF2 much cleaner. The last SF2 was coated in dirt. Either way, the SF2 will always be in use for me.

Offline drew1021

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I'm also using the stock mesh filter now (didn't before) as I think it helps with the over sensitivity

So your using both sf2 and the stock filter cap?
VP2 with 24 hour FARS. WU: KNCLEWIS2. CWOP/APRS: DW4712, COCORAHS: NC-FR-7

Offline CW2274

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Yep.

Offline Bashy

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It looks like my UV doesnt go high enough or low enough when tracking with the 3  RAF bases nearby

https://weather.gladstonefamily.net/qchart/E9093?date=20220205&addnl=EGUL&addnl=EGUN&addnl=EGYM&Add+to+charts=Add+to+charts&.cgifields=addnl
Kind regards
Bashy

Offline William BAZ

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Good morning guys, I'm reading the topic, and I had problems with the davis pro 2 and davis vue sensors, could you tell me if it's the same sensor in both? I just wanted to change the sensor chip without changing the whole part

  Sorry my english I'm from Brazil

 

anything