Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 25176 times)

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Offline jgentry

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Re: Which sling psychrometer should I buy?
« Reply #50 on: July 06, 2018, 07:14:25 PM »
But I bet the Davis transmitter couldn’t take it.
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Offline ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #51 on: July 06, 2018, 07:32:05 PM »
Randy

Offline ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #52 on: July 07, 2018, 08:00:37 AM »
Good news the Extech was dead on with the Davis this morning at 90% humidity range. Still doesn't change fact 97% humidity is max.



Randy

Offline openvista

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Re: Which sling psychrometer should I buy?
« Reply #53 on: July 07, 2018, 09:40:09 AM »
Randy, are any of your stations passive?

I say this because I've noticed that my passive station with the stock shield tends to run lower in humidity (~2-3%) than my  aspirated station (both VP2s with SHT31s). By the way, my FARS unit uses the same Mechatronics fan as yours I believe (UF80B12-BTH).

Could the sensor (and the firmware which lacks averaging capabilities) have been designed for passively ventilated enclosures? It seems forcing air onto it causes it to go a bit haywire with rapidly fluctuating dew points. I noticed this behavior previously with my solar powered stock fan too.
 
Take yesterday for example. Almost calm winds all day. The dew point from the FARS was a roller coaster varying 6F between the highs and lows (a complete cycle took as little as 5 minutes).

However, with a stronger, steadier wind (5-15 mph), as exists today, the DP smooths out considerably (<1F variations in same time period). This mimics the behavior of a passive enclosure. And the DPs between the two shields converge (1-2F diff with active still higher) under windier conditions.

Both mornings have been clear and sunny with temps in the upper 60s to lower 70s.

Perhaps humidity and DPs need to have a rolling average applied (last 10 readings?) before comparisons can be made on calmer days?
« Last Edit: July 07, 2018, 10:32:59 AM by openvista »
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Offline openvista

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Re: Which sling psychrometer should I buy?
« Reply #54 on: July 07, 2018, 09:50:27 AM »
Just thought about it and another explanation for the variations in DPs on days with little to no wind is that the air pockets the active shield is sampling are unmixed. Meaning, they are discrete parcels of air with distinct humidities and temps (which is also why temps fluctuate too (to a lesser degree than DPs). Normally, the wind mixes the air and performs an averaging function in a round about way.

Sampling calm air with an active shield would seem to require some averaging software to get a handle on the true DP. I've been thinking of maybe taking the last 5 DPs and doing a rolling average from that. Of course I'd have to do humidity and heat index too if I went that way.
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Offline ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #55 on: July 07, 2018, 10:44:13 AM »
The SHT's are all aspirated not the handheld extech I was checking against. Today we did have a stiff breeze when they all matched so maybe you are on to something but 100% without using Cumulus calibrator won't happen. 97% is max on all units even dense fog for 8 hours.

FYI you can slow and make less dewpoint/humidity swings by lowering the fan speed. This unit works but has a very narrow range as they all do. Aspiration speed even slowed is still 2x above solar fan.
 
I've tried another speed controllers that failed, would shut fan speed all the way off after few hours but found this unit to keep speed steady and never stops fan.

https://www.amazon.com/gp/product/B00BLOSG7I/ref=oh_aui_search_detailpage?ie=UTF8&psc=1
Randy

Offline ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #56 on: July 07, 2018, 10:49:53 AM »
I only turn fan speed down summer, DP swings aren't band until about 70° dewpoint temperature. Winter I keep it cranked full speed.
Randy

Offline jgentry

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Re: Which sling psychrometer should I buy?
« Reply #57 on: July 07, 2018, 11:05:26 AM »
The SHT's are all aspirated not the handheld extech I was checking against. Today we did have a stiff breeze when they all matched so maybe you are on to something but 100% without using Cumulus calibrator won't happen. 97% is max on all units even dense fog for 8 hours.

FYI you can slow and make less dewpoint/humidity swings by lowering the fan speed. This unit works but has a very narrow range as they all do. Aspiration speed even slowed is still 2x above solar fan.
 
I've tried another speed controllers that failed, would shut fan speed all the way off after few hours but found this unit to keep speed steady and never stops fan.

https://www.amazon.com/gp/product/B00BLOSG7I/ref=oh_aui_search_detailpage?ie=UTF8&psc=1


Had a similar issue with one of my stations but after installing a new SHT-31 sensor, it will top at 99% fairly often but I’m sure the 100% reading would’ve been correct during some mornings.
Davis Vantage Pro2. SHT 31. 24hr FARS. WU: KXALJEMI2 & KALTHORS2 CWOP: C6353 & C6358

  

Offline jgentry

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Re: Which sling psychrometer should I buy?
« Reply #58 on: July 07, 2018, 11:12:15 AM »
To me, it seems like all SHT sensors act just about the same when it comes to humidity. The SHT 31 may do better on the low end of the scale but in the mid and upper range, it basically acts almost the same. In the mid range, it as a wet bias and on the high end, it has a dry bias. IMO.

It would be awesome if someone on here had the smarts and the capabilities of making a home brew sensor for the VP2 using something else besides Sensirion.
Davis Vantage Pro2. SHT 31. 24hr FARS. WU: KXALJEMI2 & KALTHORS2 CWOP: C6353 & C6358

  

Offline openvista

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Re: Which sling psychrometer should I buy?
« Reply #59 on: July 07, 2018, 11:23:28 AM »
Thank you for the suggestion on the variable fan speed controller. I'll consider that.

I went back and looked at data from the wee hours of this morning when winds kicked up (10-15mph) and the DP on the aspirated station smoothed out. I'm still seeing 4-6% discrepancies in humidity between passive and active shields. Active ALWAYS runs higher. So something is up.

The air temps were always within the margin of error (0.5F). So no problems there. The shields are only a few feet away from each other.

Something about forcing air against that sensor (SHT31) seems to be in play here. Not exactly sure what. I don't have a psychrometer so I can't test, but I have 2 VP2 stations within about a mile of me and they seem to agree with my active shield (dew points within about 1F). That would suggest my passive station has a sensor running low and that would be odd since that's about a 6 month old sensor.

OR... maybe it's a new revision of the sensor and it's more accurate? Might have to perform surgery and swap the two sensors to find out.


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Offline ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #60 on: July 07, 2018, 11:33:23 AM »
To me, it seems like all SHT sensors act just about the same when it comes to humidity. The SHT 31 may do better on the low end of the scale but in the mid and upper range, it basically acts almost the same. In the mid range, it as a wet bias and on the high end, it has a dry bias. IMO.

It would be awesome if someone on here had the smarts and the capabilities of making a home brew sensor for the VP2 using something else besides Sensirion.

Agree what I'm seeing too. 
The sensor's are basically really cheap just few $$ when purchased separately and may not have the ability to have linear accuracy everywhere.  Maybe we just expect too much. 

I wouldn't hesitate if there was option for high dollar unit but it won't happen. These Davis sensors already have special needs. Can't image getting a Vaisala to work unless it came with a compatible transmitter that could communicate with the console.   :idea:
Randy

Offline ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #61 on: July 07, 2018, 11:35:44 AM »

OR... maybe it's a new revision of the sensor and it's more accurate? Might have to perform surgery and swap the two sensors to find out.

Interesting thought where did you purchase the sensor :?:
Randy

Offline ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #62 on: July 07, 2018, 11:43:53 AM »
Thank you for the suggestion on the variable fan speed controller. I'll consider that.

I went back and looked at data from the wee hours of this morning when winds kicked up (10-15mph) and the DP on the aspirated station smoothed out. I'm still seeing 4-6% discrepancies in humidity between passive and active shields. Active ALWAYS runs higher. So something is up.



Hmm!  I've never tested passive shield and humidity. I'm wondering if this isn't how sensirion test and calibrates sensor in passive calibration chamber.  Somehow using aspiration knocks sensor calibration off. It is a thought...
Randy

Offline openvista

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Re: Which sling psychrometer should I buy?
« Reply #63 on: July 07, 2018, 11:46:51 AM »

OR... maybe it's a new revision of the sensor and it's more accurate? Might have to perform surgery and swap the two sensors to find out.

Interesting thought where did you purchase the sensor :?:

Both were purchased from Ryan at SI about 1 year apart.
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Offline DaleReid

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Re: Which sling psychrometer should I buy?
« Reply #64 on: July 07, 2018, 12:53:16 PM »
What kind of sensors do they use in those electronic hand held devices?

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Offline openvista

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Re: Which sling psychrometer should I buy?
« Reply #65 on: July 07, 2018, 01:07:15 PM »
Both were purchased from Ryan at SI about 1 year apart.

Strike that. The first sensor was purchased Apr 2017 from Ryan and it's currently in my aspirated shield. The newer sensor was purchased Nov 2017 and it lives in my passive shield. Could Sensirion have pushed out a minor (unpublished?) revision in those 7 months (and whatever lag Ryan has in stock turnover)?

Also in reviewing data from the winter it looks like this was an issue too but much less so. Active station always runs higher but only by a couple percent in cold temps (20F).

Could have a bad sensor that's getting worse over time. The only wrinkle here is that the "bad" sensor might actually be more accurate. And this might be the rare case where others are wrong.

I'd have to get a reliable device to verify that claim and that's a chore and not cheap. See upthread. A margin of error +/- 3% is not what I'd call a reference device. And how do we know that device is accurate without paying hundreds to get it certified? I have an NIST calibrated temp sensor (PRT) that's supposed to be within 0.1C at all times and is 0.5 - 0.7C off at all times. How do I know? Well I tested it against 4 Davis sensors all on the same bench. Either all 4 of those sensors (SHT11, SHT15, SHT31 sensors purchased years apart) were up to 1.2F off all in the same direction (warm) or the certified unit is bad (within 3 months of certification). So... just be aware that anyone can print off a piece of paper or state some specifications. Extech is a mass manufacturer. If you're serious, you need a specialty manufacturer like, say, this: https://www.alphaomega-electronics.com/en/calibration-instruments/2552-cr-4-higrometro-de-espejo-frio.html. I doubt any of us want to drop the briefcase of dough to get our hands on that kind of unit.
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Offline ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #66 on: July 07, 2018, 01:36:55 PM »
I just set up a passive shield with SHT31 using a 24 hr shield core. I've got plenty of spare parts so drilled some ventilation holes on core for added ventilation and extra louver and 2 extra shields on top.  So far with stiff wind the passive is doing ok +.2 warmer than AC fan. Dewpoint 2° lower which is a sign maybe.

I'll test a few days may just leave out as is a see how warm it gets on calm days.  :idea:
Randy

Offline jgentry

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Re: Which sling psychrometer should I buy?
« Reply #67 on: July 07, 2018, 02:33:09 PM »
I just set up a passive shield with SHT31 using a 24 hr shield core. I've got plenty of spare parts so drilled some ventilation holes on core for added ventilation and extra louver and 2 extra shields on top.  So far with stiff wind the passive is doing ok +.2 warmer than AC fan. Dewpoint 2° lower which is a sign maybe.

I'll test a few days may just leave out as is a see how warm it gets on calm days.  :idea:

Please let us know what your results are. You would think the SHTs  would be designed to take constant airflow from the FARS. There has to be a good temp/hum sensor that Davis could use besides Sensirion.
Davis Vantage Pro2. SHT 31. 24hr FARS. WU: KXALJEMI2 & KALTHORS2 CWOP: C6353 & C6358

  

Offline jgentry

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Re: Which sling psychrometer should I buy?
« Reply #68 on: July 07, 2018, 02:34:28 PM »
Here is an interesting journal article on humidity measurements if anyone hasn’t seen it....

https://journals.ametsoc.org/doi/full/10.1175/JTECH-D-12-00232.1
Davis Vantage Pro2. SHT 31. 24hr FARS. WU: KXALJEMI2 & KALTHORS2 CWOP: C6353 & C6358

  

Offline jgentry

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Re: Which sling psychrometer should I buy?
« Reply #69 on: July 07, 2018, 02:45:55 PM »
Here is an interesting journal article on humidity measurements if anyone hasn’t seen it....

https://journals.ametsoc.org/doi/full/10.1175/JTECH-D-12-00232.1

Here is an interesting excerpt...

“New capacitive sensors perform well but they usually drift to higher values during deployment (except in arid climates); they perform best at lower humidities and need regular adjustment and recalibration...”

That would explain why CW is having different results with the SHT-31 than Randy & I.


It would also explain why there is a “wet bias” with the SHT-31 for those living in higher humidity environments.
« Last Edit: July 07, 2018, 02:48:23 PM by jgentry »
Davis Vantage Pro2. SHT 31. 24hr FARS. WU: KXALJEMI2 & KALTHORS2 CWOP: C6353 & C6358

  

Offline ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #70 on: July 07, 2018, 03:02:44 PM »
[

Here is an interesting excerpt...

“New capacitive sensors perform well but they usually drift to higher values during deployment (except in arid climates); they perform best at lower humidities and need regular adjustment and recalibration...”

That would explain why CW is having different results with the SHT-31 than Randy & I.


It would also explain why there is a “wet bias” with the SHT-31 for those living in higher humidity environments.

Bingo you just won the lottery.  =D&gt;
Randy

Offline openvista

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Re: Which sling psychrometer should I buy?
« Reply #71 on: July 07, 2018, 03:11:40 PM »
It would also explain why there is a “wet bias” with the SHT-31 for those living in higher humidity environments.

Which may be worse in actively ventilated enclosures? We'll see what Randy's test produces.

Another takeaway from that paper is how little data exists on measuring humidity on solid state devices. Also, as of 2010, the WMO was saying not to expect better than +/- 3% from ANY solid state humidity sensor.

The study dates back to 2013 using 2012 data. That's a lifetime ago in terms of technology. Perhaps some mass manufacturer has made some improvements? Sensirion, of course, claims that the SHT31 was a breakthrough. But we're finding "discrepancies", let's say.  #-o
« Last Edit: July 07, 2018, 03:15:07 PM by openvista »
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Offline DaleReid

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Re: Which sling psychrometer should I buy?
« Reply #72 on: July 07, 2018, 03:19:31 PM »
Just like the calculation of humidity with look up tables when you know the two temperatures, if we only knew accurate humidity levels we could construct a look up chart (table) to take the reading and use the correction to make adjustments to know what the real temperature and humidity were.

Old programs for the Campbell Scientific Dataloggers sometimes did this. Your sensor says 4.234 volts?  Hmm, no linear forumula that is good enough, so I'll consult my built in matrix.  Ah yes, it is  93%, based on calibration with a standard.

The Hmmm comes in by having the ability to place a gizmo into an enclosure and really know what the humidity is.  The saturated salts method works for three readings that I know of but it is a royal pain.  Plus it says nothing about the linearity of the readings, which as noted may really start to deviate in certain ranges of the instrument's output.

But then again one might wonder if 94% humidity is too much different from 97?
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Offline CW2274

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Re: Which sling psychrometer should I buy?
« Reply #73 on: July 07, 2018, 04:15:26 PM »
I've said this before, I think you guys over do the fan speed thing. Mine runs at 67CFM 24/7 365 and has so for three years now and I don't have y'alls problems, on the contrary for that matter.
I skimmed the article about the dry climate effect on the sensor, maybe that's "helping" mine be more accurate than you's guys, I have no idea.
Good luck with your experiments, my fan will continue to suck at full power. ;)

Offline ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #74 on: July 07, 2018, 05:01:41 PM »
 I switched the passive to #1 transmitter for now..... Actual dewpoint per ASOS and my Extech is 63°.
All sensors are SHT31 with highest #3 (+5°dp) and the sensor with most airflow with AC fan.
#5 sensor is solar fan (+4°dp).
#1 sensor passive (+3°dp).

The #1 sensor was a spare but had also been used previously with solar fan but seems to be least off +3 dp. 

Not sure this is conclusive extra airflow knocks the humidity out of calibration but it does show for sure humidity drift is apparent. I noticed last year they were running high but over time they are getting worse. 

« Last Edit: July 07, 2018, 05:04:19 PM by ValentineWeather »
Randy

 

anything