Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 5211 times)

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Offline Old Tele man

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Re: Which sling psychrometer should I buy?
« Reply #50 on: July 06, 2018, 02:49:39 PM »
Do you have a model number?

For the SHT-31?
I believe SLOweather was asking about the model number for that Vaisala sensor you wish Davis would use.
• SYS: Davis VP2 Vue/WL-IP & Envoy8X/WL-USB
• CWOP: DW6988 - 2 miles NNE of Cortaro, AZ
• WU - KAZTUCSO202, Countryside

Offline ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #51 on: July 06, 2018, 04:53:20 PM »
Auburn U. should look at the Dyacon stations. They have web portal capabilities.  https://dyacon.com/
Randy

Offline Old Tele man

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Re: Which sling psychrometer should I buy?
« Reply #52 on: July 06, 2018, 05:30:24 PM »
Auburn U. should look at the Dyacon stations. They have web portal capabilities.  https://dyacon.com/
They use a capacitive RH sensor...but, I DO like their RAIN gauge leveling and drain tubes approach:

« Last Edit: July 06, 2018, 06:20:28 PM by Old Tele man »
• SYS: Davis VP2 Vue/WL-IP & Envoy8X/WL-USB
• CWOP: DW6988 - 2 miles NNE of Cortaro, AZ
• WU - KAZTUCSO202, Countryside

Offline CW2274

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Re: Which sling psychrometer should I buy?
« Reply #53 on: July 06, 2018, 05:53:56 PM »
Auburn U. should look at the Dyacon stations. They have web portal capabilities.  https://dyacon.com/
Merely food for thought reference your complaints about the SHT31, the Daycon specs out as the same for temp accuracy, and the humidity only 0.2% (1.8% vs 2%) more accurate, but that's only between 10 and 80%. They don't even state what it is outside those parameters. I know that is no satisfaction for you, but just sayin'....

Offline ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #54 on: July 06, 2018, 06:01:53 PM »

They use a capacitive RH sensor...but, I DO like their RAIN gauge leveling and drain tubes approach:


The capacitor type no good I suppose or no better than Davis I assume.
Randy

Offline ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #55 on: July 06, 2018, 06:02:39 PM »
Auburn U. should look at the Dyacon stations. They have web portal capabilities.  https://dyacon.com/
Merely food for thought reference your complaints about the SHT31, the Daycon specs out as the same for temp accuracy, and the humidity only 0.2% (1.8% vs 2%) more accurate, but that's only between 10 and 80%. They don't even state what it is outside those parameters. I know that is no satisfaction for you, but just sayin'....

I saw that.
Randy

Offline ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #56 on: July 06, 2018, 06:37:25 PM »
One day, this is about as close to professional equipment I can afford.  ....http://www.youngusa.com/products/2/29.html
I know it's just a big expensive thermometer but I've always wanted something I know is accurate and will have a good comparison for the Davis SHT series.
 
Wonder if the probe could be retrofitted inside a Davis FARS? 7.25 ".

Randy

Offline jgentry

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Re: Which sling psychrometer should I buy?
« Reply #57 on: July 06, 2018, 06:39:37 PM »
Auburn U. should look at the Dyacon stations. They have web portal capabilities.  https://dyacon.com/

Looks like a cool system but Auburn has already made the decision to go with the Davis. I think they would’ve stuck with the Campbell units if they were not so expensive to get and to maintain.
Davis Vantage Pro2. SHT 31. 24hr FARS. WU: KXALJEMI2 & KALTHORS2 CWOP: C6353 & C6358

  

Offline CW2274

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Re: Which sling psychrometer should I buy?
« Reply #58 on: July 06, 2018, 07:04:01 PM »
One day, this is about as close to professional equipment I can afford.  ....http://www.youngusa.com/products/2/29.html
I know it's just a big expensive thermometer but I've always wanted something I know is accurate and will have a good comparison for the Davis SHT series.
 
Wonder if the probe could be retrofitted inside a Davis FARS? 7.25 ".
I love RMY. That being said, the only thing that disappoints me about the SHT31 is that it wouldn't pull the 100% trigger, stayed on 99 forever, but that was my old 31, haven't had the opportunity with the new one. Remember, the only difference between 99 and 100% is 0.4F spread between temp and dew point, that's pretty razor thin. Both 31's certainly have gotten to 1%, and have done so a few times this year, hell even the TUS ASOS and a RAWS station hit 1%, I've never seen that before.
Perhaps you have other issues, but maybe not.

Offline jgentry

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Re: Which sling psychrometer should I buy?
« Reply #59 on: July 06, 2018, 07:13:24 PM »
Do you have a model number?

In terms of the Vaisala sensor, the HMP 155 would be a good one...

https://www.vaisala.com/sites/default/files/documents/HMP155-Datasheet-B210752EN-F.pdf
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 #60 on: July 06, 2018, 07:14:25 PM »
But I bet the Davis transmitter couldn’t take it.
Davis Vantage Pro2. SHT 31. 24hr FARS. WU: KXALJEMI2 & KALTHORS2 CWOP: C6353 & C6358

  

Offline Old Tele man

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Re: Which sling psychrometer should I buy?
« Reply #61 on: July 06, 2018, 07:18:18 PM »
Do you have a model number?

In terms of the Vaisala sensor, the HMP 155 would be a good one...

https://www.vaisala.com/sites/default/files/documents/HMP155-Datasheet-B210752EN-F.pdf
Curiously, the RS-485 output is tighter toleranced than the voltage (0-5V?) output?!?

Can't the Davis ISS accept 0-5Vdc inputs?

However the real problem is getting adequate power (7-28Vdc) to the Vaisala which the wireless ISS can't provide.

Also, they spec the Platinum Temp sensor at <35 seconds to 90% of full value...BUT...that's at 7 mph airflow! Which ain't gonna happen inside that static radiation shield!
« Last Edit: July 06, 2018, 07:53:05 PM by Old Tele man »
• SYS: Davis VP2 Vue/WL-IP & Envoy8X/WL-USB
• CWOP: DW6988 - 2 miles NNE of Cortaro, AZ
• WU - KAZTUCSO202, Countryside

Offline ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #62 on: July 06, 2018, 07:32:05 PM »
Randy

Offline ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #63 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 #64 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 »
Davis Vantage Pro2 | https://marquetteweather.com

Offline openvista

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Re: Which sling psychrometer should I buy?
« Reply #65 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.
Davis Vantage Pro2 | https://marquetteweather.com

Offline ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #66 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 #67 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 #68 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 #69 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 #70 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 #71 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 #72 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 #73 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 #74 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.
Davis Vantage Pro2 | https://marquetteweather.com

 

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