Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 45686 times)

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Offline CW2274

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Re: Which sling psychrometer should I buy?
« Reply #125 on: July 09, 2018, 06:28:32 PM »
Thanks Ron, good job... =D> it helps clarify what we mostly had figured out. Until they do something different with the 31 either update or go with a new version I'll just swap sensors in spring provided they don't jack price up.

For those living in arid climates sub 60 dew points you should be good for several years.

but something is clearly wrong--unless you live in Arizona!
:-"

Offline CW2274

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Re: Which sling psychrometer should I buy?
« Reply #126 on: July 09, 2018, 06:36:29 PM »
Honestly, I'm half tempted to pull the ISS outta the desert (kinda a PITA) and put in the back yard so I can do instantaneous comparisons between my two 31's, which is exactly what I did when I picked up my second 31. At that time I estimated about 1/2% difference tween the two.

Offline ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #127 on: July 09, 2018, 06:54:58 PM »
Here is some information I received from the good folks at the NWS

Hi Justin! Here is the response below about your weather equipment question which we received from our electronics technicians.  Hope this helps. 🙂                                                                                                         -----------------------------------------------------------------------                                   Our Dew Point Sensor is the Vaisala DTS1.  This sensor uses a HUMICAP® capacitive thin film polymer sensor to determine RH. The measured capacitance of the DTS1 probe will vary depending on the amount of moisture in the air. The resistance of the PT 100  temperature sensor is used to determine the sensor air temperature. During periods of high humidity, the sensor electronics will turn on the sensor heater element to improve sensor humidity resolution. The algorithm then computes the dew point temperature using the measured sensor temperature and capacitance values.

Our calibration procedures include verifying accuracy every 90 days, there are no adjustments.  We also replace the sensor every 18 months as a time change requirement

We also use a NWS 1088 hygrothermometer to determine ambient air temperature.  This sensor uses a fan to draw air in and across the temperature probe, thus the temperature is aspirated.  Our calibrations procedures consist of cleaning, adjusting, and verifying accuracy every 90 days.                                                                                                              ----------------------------------------------------------------------------

Interesting those run around $800 retail.
And they go bad too, a couple years ago airport completely stopped reporting 100% humidity even after hours of dense fog, this went on for couple years at KVTN. I mentioned it to forecast office and they eventually replaced and now 100% all the time almost too often it seems. 
Randy

Offline ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #128 on: July 09, 2018, 07:05:16 PM »
If you think about it, (I) was expecting too much. 1 year on these sensors especially thoses aspirated isn't really that out of normal if NWS routinely changes there's every 18 months or when needed.
Randy

Offline CW2274

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Re: Which sling psychrometer should I buy?
« Reply #129 on: July 09, 2018, 07:06:59 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. :-({|=

Offline ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #130 on: July 09, 2018, 07:12:22 PM »
Anyone want to figure out what the shelf live is if never removed from bag?
Randy

Offline CW2274

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Re: Which sling psychrometer should I buy?
« Reply #131 on: July 09, 2018, 07:20:36 PM »
Anyone want to figure out what the shelf live is if never removed from bag?
Purely speculation, but with no current/humidity/dust/dirt/fan/big fan/moon phase......forever. ;)

Online jgentry

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Re: Which sling psychrometer should I buy?
« Reply #132 on: July 09, 2018, 08:02:12 PM »
This might have been posted on another thread but this is some interesting info

“There is a procedure used by the ASOS Technician to check the validity of the ASOS ambient/dew point temperatures by comparing the values with the Psychron Model 566-2 psychrometer.  A measurement is considered valid if the temperature difference is within ±5 F.  dew point validity is determined by values within tolerances based on the temperature/dew point depression and if the temperature falls above or below 32 F.”

https://training.weather.gov/nwstc/DATAACQ/d.ASOShuman/ASOSTemp.htm
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Online WheatonRon

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Re: Which sling psychrometer should I buy?
« Reply #133 on: July 09, 2018, 08:25:23 PM »
If you think about it, (I) was expecting too much. 1 year on these sensors especially thoses aspirated isn't really that out of normal if NWS routinely changes there's every 18 months or when needed.

I don't recall the predecessor to the SHT31 having this issue, but I may be wrong. That said, for a semi-professional station like the VP2,  having to replace this sensor every 12-18 months doesn't seem right to me. I think we should contact Davis support and get their views.
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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Re: Which sling psychrometer should I buy?
« Reply #134 on: July 09, 2018, 08:48:43 PM »
If you think about it, (I) was expecting too much. 1 year on these sensors especially thoses aspirated isn't really that out of normal if NWS routinely changes there's every 18 months or when needed.

I don't recall the predecessor to the SHT31 having this issue, but I may be wrong. That said, for a semi-professional station like the VP2,  having to replace this sensor every 12-18 months doesn't seem right to me.
Ron, the way I look at it, ya gotta pay to play (performance, performance, performance). Perhaps this is just the price of greater accuracy with this sensor, burn out.
My 2 cents.

Online WheatonRon

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Re: Which sling psychrometer should I buy?
« Reply #135 on: July 09, 2018, 08:57:51 PM »
If you think about it, (I) was expecting too much. 1 year on these sensors especially thoses aspirated isn't really that out of normal if NWS routinely changes there's every 18 months or when needed.

I don't recall the predecessor to the SHT31 having this issue, but I may be wrong. That said, for a semi-professional station like the VP2,  having to replace this sensor every 12-18 months doesn't seem right to me.
Ron, the way I look at it, ya gotta pay to play (performance, performance, performance). Perhaps this is just the price of greater accuracy with this sensor, burn out.
My 2 cents.


OR, Davis is testing us to see if we can thread three long screws through the rain base and the rest of the station, remove and install the sensor, and put the station successfully back together again and still love Davis in the morning!
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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Re: Which sling psychrometer should I buy?
« Reply #136 on: July 09, 2018, 09:07:55 PM »
If you think about it, (I) was expecting too much. 1 year on these sensors especially thoses aspirated isn't really that out of normal if NWS routinely changes there's every 18 months or when needed.

I don't recall the predecessor to the SHT31 having this issue, but I may be wrong. That said, for a semi-professional station like the VP2,  having to replace this sensor every 12-18 months doesn't seem right to me.
Ron, the way I look at it, ya gotta pay to play (performance, performance, performance). Perhaps this is just the price of greater accuracy with this sensor, burn out.
My 2 cents.


OR, Davis is testing us to see if we can thread three long screws through the rain base and the rest of the station, remove and install the sensor, and put the station successfully back together again and still love Davis in the morning!
Ha, easily, blindfolded to boot.

Offline Dj1225

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Re: Which sling psychrometer should I buy?
« Reply #137 on: July 10, 2018, 02:13:59 AM »
Yeah not interested in removing filter I think it would just cause another issue.

This is sunrise what I'm seeing this morning Extech outside for about 15 minutes, you can see it's dripping with sweat in short period. The SHT15 isn't a solution either for 100% or even close. Remember the SHT's are saturated all night in high humidity and should be reading at least 98%. Airport was 100% at same time.

I usually just lurk on this site have for years, and post very rarely. I came across this subject and had to chime in. I live on the Gulf Coast in Gulfport, MS. 8 miles north of the Gulf of Mexico. I've used Davis stations since 1996. The Weather Monitor II, the first VP and the VP2 all recorded 100% humidity. I upgraded my ISS two years ago with the SHT31. As humid as it is down here, the dense fog we see in fall and winter, and going through Hurricane Nate last October since I installed the new ISS with the SHT31 sensor the highest humidity I have recorded is 96% once! At first I thought it was really accurate, but then after awhile I started having my doubts, and now seeing this post it seems like it is more a problem than accurate. Also I agree with Valentine on the higher dp because down here if the sensors dp is high biased the heat index is out of control.

Offline ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #138 on: July 10, 2018, 08:47:05 AM »
FYI sent an email to Ryan at Scaled Inst. who I purchased all of mine through expressing concern and verifying they are genuine Davis sensors. If you read data sheet about sensor they have different qualities stamped on each one. Also emailed Davis Inst.
 
At least they will know now and maybe Davis will investigate on their own. 

Dj1225 thanks for chiming in. The topic heading probably should be changed since it turned into SHT31 reliability thread.
Randy

Online jgentry

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Re: Which sling psychrometer should I buy?
« Reply #139 on: July 10, 2018, 09:22:01 AM »
I’ve noticed the same thing DJ except my SHT-31s Typically top of at 99%...

Let us know what Ryan and Davis has to say...

I’ve noticed that many good stations use a thin film polymer capacitive sensor. According to Davis’ spec sheet, their VP2s use a thin film capacitor element. Is there a difference between the thin film polymer sensor and the thin film capacitive sensor that Davis uses?

Why I ask is that there has to be a reason why the SHT-31 humidity sensor goes off calibration quicker (or least it seems) and for the sensor not reaching 100% most of the time (even in actual 109% humidity).
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Online jgentry

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Re: Which sling psychrometer should I buy?
« Reply #140 on: July 10, 2018, 09:26:14 AM »
FYI sent an email to Ryan at Scaled Inst. who I purchased all of mine through expressing concern and verifying they are genuine Davis sensors. If you read data sheet about sensor they have different qualities stamped on each one. Also emailed Davis Inst.
 
At least they will know now and maybe Davis will investigate on their own. 

Dj1225 thanks for chiming in. The topic heading probably should be changed since it turned into SHT31 reliability thread.


I’ve changed the thread title.
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Offline Aardvark

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I’m thinking about getting a sling psychrometer to make sure that my RH sensor is within specs. I would like to know which psychrometers are accurate and reliable.  Thanks!

So rather than me reading the 163 posts, have you bought your sling yet?  solve your problem?
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Online WheatonRon

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I too contacted Davis support (Brett Lane, supervisor) and alerted him to this thread. I also alerted Johnd, a frequent contributor in this Forum as well as a Davis reseller in the UK to this thread as well.
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

Online jgentry

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I’m thinking about getting a sling psychrometer to make sure that my RH sensor is within specs. I would like to know which psychrometers are accurate and reliable.  Thanks!

So rather than me reading the 163 posts, have you bought your sling yet?  solve your problem?

Not planning on getting a sling...Thought about getting a digital psychrometer but I decided to get this fan-aspirated one whenever I have some extra spending money.

http://www.iisusa.com/mobile/Psychro-Dyne-Battery-Operated-Psychrometer-Fahrenheit-BLACK-ORGANIC/productinfo/22014/
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Online jgentry

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Did noticed that the 1088 hygrothermometer has a very similar specs in terms of ambient temperature as the SHT-31


Looking at the RainWise MK III data from RainWise’s factory location and comparing it to the nearby airport; it appears to me that their humidity sensor has a similar wet bias as the Davis does...

The only difference I’ve noticed is that RW humidity sensor reaches 100% more easily and often than the SHT-31.

I would love to hear what Brett Lane from Davis and others’s take on this issue.
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline ValentineWeather

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Heard back from Ryan he doesn't think there is an issue with Davis covering for replacements.
If you purchased through him send defective units back.
As far as solution which I would rather have than keep replacing nothing yet but hopefully it will now get addressed as they say the ball is rolling with appropriate people notified.


« Last Edit: July 10, 2018, 09:17:29 PM by ValentineWeather »
Randy

Offline openvista

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The question now becomes: how long is the warranty period?

My out-of-spec sensor was purchased April 2017. Does that qualify?
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Offline openvista

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The question now becomes: how long is the warranty period?

My out-of-spec sensor was purchased April 2017. Does that qualify?

Ryan will take any 31 back if purchased through others it will be a Davis decision but being an known issue my guess all will be covered.

That's good news! For us at least. I hope it's not bad news for Ryan and that he can convince Davis to step up and Davis can convince Sensirion to stabilize drift or else risk losing a big contract.

Davis might not have the leverage it needs since its sensor platform, Sensibus (?), is so old. What other modern sensor could it run without a major overhaul of the SIM and its power section (solar, battery)? 10 years of hinting at a VP3 with nothing to show for it would now seem to be haunting them.
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Offline CW2274

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Wow, I'm happily shocked about returns. I figured it would be a "pound sand " approach, nature of the beast thing. Nicely done Randy. =D>

Offline CW2274

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The question now becomes: how long is the warranty period?

My out-of-spec sensor was purchased April 2017. Does that qualify?

Ryan will take any 31 back if purchased through others it will be a Davis decision but being an known issue my guess all will be covered.

 convince Davis to step up and Davis can convince Sensirion to stabilize drift or else risk losing a big contract.

Now that I'll believe when I see it.