Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 41041 times)

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Offline CW2274

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Re: Which sling psychrometer should I buy?
« Reply #75 on: July 07, 2018, 05:49:30 PM »
Not sure this is conclusive extra airflow knocks the humidity out of calibration
Granted, not conclusive, but if that were the case, I'd probably be having bigger problems than you, right, but I'm not. Three years on the big fan is conclusive to me that it's not degrading the sensor, at least not noticeably. All I can do is state what I see.
 

Offline openvista

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Re: Which sling psychrometer should I buy?
« Reply #76 on: July 07, 2018, 06:00:47 PM »
Interesting, Randy. Thanks for doing that.

At some point soon, I will swap the 31s in my two shields. Of course, the active shield humidity reading might start off better and then get wetter over time. I'd have to keep an eye on it.
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Offline ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #77 on: July 07, 2018, 06:17:50 PM »
Not sure this is conclusive extra airflow knocks the humidity out of calibration
Granted, not conclusive, but if that were the case, I'd probably be having bigger problems than you, right, but I'm not. Three years on the big fan is conclusive to me that it's not degrading the sensor, at least not noticeably. All I can do is state what I see.

From what I read in the study the solid state sensors do fine in dry conditions. Adding extra air with humidity may accelerate drifting with extra saturation.
I can't be conclusive without starting out with a couple untouched sensors. One left passive the other with a fan and see how fast they drift apart.
 
My disappointment with humidity side of sensor hasn't changed but at least we have some answers what may be going on.
It's a big thing for my website to be accurate, I owe it to my visitors who trust in it. 
Randy

Offline ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #78 on: July 07, 2018, 06:20:29 PM »
Not sure this is conclusive extra airflow knocks the humidity out of calibration
Granted, not conclusive, but if that were the case, I'd probably be having bigger problems than you, right, but I'm not. Three years on the big fan is conclusive to me that it's not degrading the sensor, at least not noticeably. All I can do is state what I see.
Quoting from page 2041 of that DOI article:

"For naturally ventilated screens, ... , errors may be larger in calm conditions. Relative humidity natural varibility and errors are somewhat larger during daylight hours than at night."

Thanks for pointing out.
Randy

Offline CW2274

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Re: Which sling psychrometer should I buy?
« Reply #79 on: July 07, 2018, 06:28:48 PM »
Not sure this is conclusive extra airflow knocks the humidity out of calibration
Granted, not conclusive, but if that were the case, I'd probably be having bigger problems than you, right, but I'm not. Three years on the big fan is conclusive to me that it's not degrading the sensor, at least not noticeably. All I can do is state what I see.
It's a big thing for my website to be accurate, I owe it to my visitors who trust in it.
Oh, completely get that, and I don't even have a web site (well, NWS). Being a perfectionist can be a curse, one that you, I, and more than a few others on here can relate to.
GL with everyone's experiments....hopefully something of true value can be gleaned. UU

Offline jgentry

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Re: Which sling psychrometer should I buy?
« Reply #80 on: July 07, 2018, 07:14:56 PM »
I noticed the Davis spec sheet says “Film capacitor element” in describing what kind of humidity sensor they use.

For this RW product, it says that they use a “thin film polymer capacitor.”

http://rainwise.com/products/attachments/6751/20060215113108.pdf

Is there any difference between the two sensors or are they the same?
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline openvista

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Re: Which sling psychrometer should I buy?
« Reply #81 on: July 07, 2018, 07:39:54 PM »
Here's the Sensirion datasheet on the humidity sensor. Sensirion claim drift of <0.25% per year.
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Offline jerryg

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Re: Which sling psychrometer should I buy?
« Reply #82 on: July 07, 2018, 10:00:16 PM »
 Here is an article about getting to 100% humidity that might hurt your brain to read it  :grin: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4431636/

Offline openvista

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Re: Which sling psychrometer should I buy?
« Reply #83 on: July 07, 2018, 10:47:43 PM »
OK, so I swapped out the two SHT31 sensors this evening. After giving about 20 minutes to acclimate, I'm seeing the reverse of what I was seeing previously. Now the newer sensor inside the active shield reads lower dew points (3-4F) than the passive station. I'll monitor it and report back.
« Last Edit: July 07, 2018, 10:57:06 PM by openvista »
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Offline jgentry

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Re: Which sling psychrometer should I buy?
« Reply #84 on: July 07, 2018, 11:04:21 PM »
What kind of sensors do they use in those electronic hand held devices?

Not sure...

But I think I saw where you stated that you had a RW MK III station? If so, how did the DPs compared to the ASOS? Does RW stations have the “wet bias” like the VP2s?
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Offline jgentry

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Re: Which sling psychrometer should I buy?
« Reply #85 on: July 07, 2018, 11:07:13 PM »
OK, so I swapped out the two SHT31 sensors this evening. After giving about 20 minutes to acclimate, I'm seeing the reverse of what I was seeing previously. Now the newer sensor inside the active shield reads lower dew points (3-4F) than the passive station. I'll monitor it and report back.

Please do!

I’m thinking it’s all about the  capacitive sensor element that Sensirion uses to measure humidity.
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Offline ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #86 on: July 07, 2018, 11:12:18 PM »



I ordered a fresh sensor also, I'm going to use in passive shield exclusive and do comparisons before I screw it up in FARS shield.
Randy

Offline CW2274

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Re: Which sling psychrometer should I buy?
« Reply #87 on: July 08, 2018, 12:24:39 AM »
do comparisons before I screw it up in FARS shield.
Please ....if anything, loses it's virginity..

Offline ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #88 on: July 08, 2018, 12:48:22 PM »
I know the frustration some are having with the humidity.
My little not so secret secret and why I use Cumulus not the MX version which also uploads to all the favorite weather outlets including CWOP and website.

You can't just remove 5% humidity without hurting the already low upper end. My sensors only max at 97%.

Cumulus has a calibration function that can reduce those mid range humidities but still reach 100%.

The value that works best on all my SHT31's and still reach 100% humidity is this.

-11% humidity (89%) and use multiplier  x 1.156 = 102.79%   Some sensors may require a multiplier of 1.157 for 100%.
The reason my multiplier number goes above 100% is because my sensors stop at 97% so need it for 100%. 


Edit: I should add this works above 35% humidity. If you get below you may need to change multiplier something less -5% (95) multiplier x 1.083 or even remove completely for humidity in the teens. So yes you end up chasing your tail when sensor doesn't work as advertised.
« Last Edit: July 08, 2018, 04:08:07 PM by ValentineWeather »
Randy

Offline jerryg

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Re: Which sling psychrometer should I buy?
« Reply #89 on: July 08, 2018, 04:06:56 PM »
I use weather display and it has a very good humidity calibration setup. I have finally got over the must have 100% reading or go nuts lol. For the sake of this  thread i broke out my wr-25 station from texas weather inst. which is a high dollar set up that was given to me by a friend who worked at a cable vision that went fiber and quit using it, the only thing i had to do was replace the bearings in the wind cup and direction and it works fine. I don't use it because it is wired and you have to use the cables that came with it or the calibrations are way off. I believe it uses analog voltage readings thus the cable can not be messed with. Anyway i was going to compare humidity readings with my two 31's i have up and running but a problem with that is the Davis only senses the humidity about once a minute and the wr-25 is basically real time, reporting every second, so i get almost 60 readings to 1 for the Davis and yes the humidity does go up and down quite a bit in a minute. But just by eye and looking at it my readings are near enough to each other and my dp according to cwop error is 0.0F so it looks like my 31's are doing fine. I sure do like the near real time updates of the wr-25 and it came with the computer interface wps-10 so i can use wd to see the info. I found out along time ago that trying to compare my readings with other stations or metars was a waste of time. I have a metar 10 miles nw of me and one 12 miles se and sometimes they have 20 to 30 percent difference between them. I sometimes wonder about the metar readings, i report one of the metars was showing -40 degrees every time the temp got down in the 30's and they said they would report it to the repair crews and it went all winter without being fixed. I say trust your own gear and no one elses.

Offline jgentry

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Re: Which sling psychrometer should I buy?
« Reply #90 on: July 08, 2018, 04:24:04 PM »
I use weather display and it has a very good humidity calibration setup. I have finally got over the must have 100% reading or go nuts lol. For the sake of this  thread i broke out my wr-25 station from texas weather inst. which is a high dollar set up that was given to me by a friend who worked at a cable vision that went fiber and quit using it, the only thing i had to do was replace the bearings in the wind cup and direction and it works fine. I don't use it because it is wired and you have to use the cables that came with it or the calibrations are way off. I believe it uses analog voltage readings thus the cable can not be messed with. Anyway i was going to compare humidity readings with my two 31's i have up and running but a problem with that is the Davis only senses the humidity about once a minute and the wr-25 is basically real time, reporting every second, so i get almost 60 readings to 1 for the Davis and yes the humidity does go up and down quite a bit in a minute. But just by eye and looking at it my readings are near enough to each other and my dp according to cwop error is 0.0F so it looks like my 31's are doing fine. I sure do like the near real time updates of the wr-25 and it came with the computer interface wps-10 so i can use wd to see the info. I found out along time ago that trying to compare my readings with other stations or metars was a waste of time. I have a metar 10 miles nw of me and one 12 miles se and sometimes they have 20 to 30 percent difference between them. I sometimes wonder about the metar readings, i report one of the metars was showing -40 degrees every time the temp got down in the 30's and they said they would report it to the repair crews and it went all winter without being fixed. I say trust your own gear and no one elses.

The NWS in Birmingham, AL does a good job in maintaining the ASOS stations and they do not get wacky readings very often. All the ASOS stations DP reports are sensible and congruent. Sensirion, on the other hand, has a pretty much of a "wet bias" until it reaches in the 90 percentile and then, it starts to have a "dry bias" of sorts.

Shoot, the RW stations performs better when the humidity hits near or at saturation.
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Offline jerryg

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Re: Which sling psychrometer should I buy?
« Reply #91 on: July 08, 2018, 04:28:42 PM »
Well the primary problem i have is unless you are co-located with the station you are comparing to it is all a guessing game. If your station is properly sited then the readings you get from a quality station like Davis should be pretty good. I am lucky that i live in the country and have my station out in the clear over grass and my anny at 33 feet so i can be as accurate as i can be, i am glad i don't have to compromise on my station like a lot of the guys do because of living in town.

Offline jerryg

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Re: Which sling psychrometer should I buy?
« Reply #92 on: July 08, 2018, 04:36:54 PM »
When you look at the specs of the really high dollar sensors nearly all of them are speced between 20 to 40 percent up to 90 percent and above 90 percent they go up in error percentage. The 31 specs at 2% or called nominal and above 90% they show it can go up higher on the error scale. I read a study about sling thermos and how they did a study at a military school where they taught how to use them and found a 5 to 10 percent error in the readings, so many things to consider when using one, how many spings good water supply to wet bulb and reading the chart right to get the humidity. Sure is a lot of places to goof up.

Offline jgentry

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Re: Which sling psychrometer should I buy?
« Reply #93 on: July 08, 2018, 04:49:55 PM »
When you look at the specs of the really high dollar sensors nearly all of them are speced between 20 to 40 percent up to 90 percent and above 90 percent they go up in error percentage. The 31 specs at 2% or called nominal and above 90% they show it can go up higher on the error scale. I read a study about sling thermos and how they did a study at a military school where they taught how to use them and found a 5 to 10 percent error in the readings, so many things to consider when using one, how many spings good water supply to wet bulb and reading the chart right to get the humidity. Sure is a lot of places to goof up.

True. But I’m believing the problem is with Sensirion’s humidity sensing element is that it is a capacitive sensing element and it seems to go off calibration in high humidity environments. If Sensirion used something different, we might not have these “wet bias” issues. Then again, who knows...
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Offline openvista

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Re: Which sling psychrometer should I buy?
« Reply #94 on: July 08, 2018, 04:53:47 PM »
Here are some interesting quotes from The Weather Observer's Handbook by Christopher Burt (2012 Kindle Version; locations 4569-4631):

"Aspirated screens are ideal for accurate temperature measurements, and when fitted with RH sensors will generally give more representative RH values too, but the greater volume of air movement over humidity sensors tends to exacerbate dust and salt ingress problems and probably ultimately shortens their working life." [emphasis mine]

"Calibration drift is a problem with humidity sensors, particularly in less expensive systems where it can exceed 5% per year. Whatever type of equipment is used, regular checking over a range in humidities is essential if reasonably accurate long-period humidity measurements are sought. Calibration checking is best carried out annually, or more frequently if spot checks indicate the sensor is regularly more than about 5% different from independent instruments."

"The errors inherent in the measurement of humidity - whether by dry- and wet-bulb or by electronic sensor - mean that the RH is at best accurate only to about 2-3% in its mid-range.... At high humidities response will be slow, while at low humidities and low temperatures, errors increase and the accuracy falls off further. The same goes for dew point - although often quoted to a precision of 0.1 degC, in reality the measurement is probably no better than +/- 0.5-1 degC when derived from humidity measurements, with still wider error ranges at low temperatures and humidities." [emphasis mine]

"Combined temperature/RH sensors are popular, but can become expensive and inconvenient if the relatively short working lifetime of the humidity component mandates replacement (and recalibration) of the temperature sensor too." [emphasis mine]

"The calculation varies somewhat depending upon the airflow over the sensors, and for accurate work this needs to be taken into account. For this reason there are different psychometric formulae and tables for sensor exposed in a passively ventilated shelter such as a Stevenson screen and for those in a forced airflow, such as an aspirated or whirling psychrometer. There are also slight differences in the method of calculation for temperatures below 0 degC, owing to differences in the saturation vapour pressure over liquid water and ice surfaces."

*************

Essentially what he's been saying, and it's been known for years even before he wrote it, is that, at least in non-arid climates, our sensor packages are disposable since they cannot really be calibrated at multiple points (which is required because they aren't linear). I would add, if you have a fan-aspirated Davis unit, expect to order replacements well before the sensors fail (yearly? every other year?) if you want to be within the ballpark on humidity. Also, the manufacturers specs are marketing, not reality, as we've been discovering.  This isn't a problem limited to Davis. It's just that Davis is one of very few manufacturers selling aspirated stations. The extra air movement exacerbates the wear and tear on the humidity sensor. Over time, they all drift, though.

Humidity checks should only be made against known reference-quality weather stations. One-off comparisons are not advised because even very expensive AWOS systems at airports can sometimes be out of calibration (which is currently the case at my local airport). But if you look at enough NWS/RAWS stations (https://www.wrh.noaa.gov/map/) within about 50 - 100 miles (assuming synoptic weather patterns and relatively flat terrain, no mesoscale influences, etc) a consensus dew point range begins to emerge. Plus you can check the software models for projected dew points in your area (weathernerds.org) and, likewise, perform an average (i.e. HRRR + NAM + Euro divided by 3). Do NOT use other neighborhood weather stations (unless you know them to be accurate) because, at least in my area, all of them run well above the reference stations. Very few station owners properly maintain their stations due to the expense and complexity. So the stations humidities and DPs almost always drift up over time and run wet.

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Offline openvista

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Re: Which sling psychrometer should I buy?
« Reply #95 on: July 08, 2018, 05:08:04 PM »
Key wording above was the repeated use of "calibration" which is something VERY few CWOP owners do.

Well, when the sensor isn't linear and the consoles and the major software packages only offer one-point calibration, how do you calibrate it? You don't really. You play whack-a-mole. The best you can do is a work-around like Randy advises using a specific piece of software.
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Offline jgentry

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Re: Which sling psychrometer should I buy?
« Reply #96 on: July 08, 2018, 05:24:59 PM »
I know that Sensirion claims that their humidity sensor drifts around .25% a year but I think that only applies to more arrid climates and not humid climates like Alabama.
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Offline ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #97 on: July 08, 2018, 05:52:42 PM »
Someone here one of the weather techs only occasionally visits warned about year ago the SHT31 chip was losing humidity calibration fast after just a few months of use he retested and it was off substantially enough he posted results.
It was one of the Sensirion stand alone temp/hum gadget with the SHT31 chip.
He was using his own humidity chamber in testing.   
Randy

Offline jgentry

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Re: Which sling psychrometer should I buy?
« Reply #98 on: July 08, 2018, 06:02:09 PM »
Someone here one of the weather techs only occasionally visits warned about year ago the SHT31 chip was losing humidity calibration fast after just a few months of use he retested and it was off substantially enough he posted results.
It was one of the Sensirion stand alone temp/hum gadget with the SHT31 chip.
He was using his own humidity chamber in testing.

I’ll try to find the thread but if you do, would you post it on here?

I think I’ve seen on the SHT-31 datasheet that if the humidity is really high (mainly in the 90 percentile range) for a while, it will knock the sensor out of calibration.
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Offline jgentry

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Re: Which sling psychrometer should I buy?
« Reply #99 on: July 08, 2018, 06:36:01 PM »
Would resistive humidity sensors perform better than capacitive sensors?

https://www.sensorsmag.com/components/choosing-a-humidity-sensor-a-review-three-technologies
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