Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 25177 times)

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Offline jgentry

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Re: Which sling psychrometer should I buy?
« Reply #25 on: July 05, 2018, 08:32:33 PM »
SHT31 looks high and you wanted to compare with another instrument I can confirm it does run high.

My findings on the SHT31 are disappointing with humidity. After long term not happy at all. 
Compared to my previous 11 and 15, the 31 is a rock star. I've used two 31's and they're virtually identical, between 1% and 99% this thing is absolute money. When I reach 1%, the dew does get a little cold, but a RAWS down the street does exactly the same thing. I also was W0X0F last winter and it wouldn't pull the 100% trigger, so I get that. I know you've seen some "competitors" PWS's, if you want disappointment, that's where to look. I don't know any sensor that's affordable that can mach the 31, not even close.

I typically don’t compare to other PWS systems except for RainWise units, Texas Instruments etc. I don’t compare it to cheap stations.  My main stations for comparisons are ASOS, AWOS, and RAWS. RAWS stations typically read closer to the Davis stations than the ASOS does. There is one AWOS station around 30-40 miles east of me and their DPs reads very close to mine station or even the same at times.

I’m curious to know how the RainWise and Texas Instruments stations DP readings compare to the ASOS units.

The Earth Networks (used to be known as WeatherBug) stations DPs read closely to ASOS.

I’m just waiting from Weather Metrics and see what the specs are for their units.
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 #26 on: July 05, 2018, 08:46:35 PM »
What I've done as work around using Cumulus calibrator I can work some of the high humidity out.

Not perfect but does work somewhat, usually dump 2% humidity without hurting scale but DP still runs high daytime unless I do like a straight -7% humidity cut for afternoon.  But then in the morning it's really low, example 90% vs actual 97%.
Randy

Offline CW2274

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Re: Which sling psychrometer should I buy?
« Reply #27 on: July 05, 2018, 08:48:44 PM »
It's anything but a rock star, I'm disappointed. Maybe it works better at extremely low humidity but for me they run high.
AP DP was 66 as was my psychrometer.

I can only give snapshot of 3 units but all 3 SHT31's - 69° and SHT15-67°.
Two degrees of dew point is nothing. What's that equate to in humidity, maybe 1/2 of a percent at your temp? If the 15 works for you, go for it. Mine went into the trash long ago.

Offline ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #28 on: July 05, 2018, 08:50:52 PM »
It's anything but a rock star, I'm disappointed. Maybe it works better at extremely low humidity but for me they run high.
AP DP was 66 as was my psychrometer.

I can only give snapshot of 3 units but all 3 SHT31's - 69° and SHT15-67°.
Two degrees of dew point is nothing. What's that equate to in humidity, maybe 1/2 of a percent at your temp? If the 15 works for you, go for it. Mine went into the trash long ago.

At 80° ambient it's 4%.
It's more than nothing and way out of specs.
Randy

Offline CW2274

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Re: Which sling psychrometer should I buy?
« Reply #29 on: July 05, 2018, 08:57:57 PM »
It's anything but a rock star, I'm disappointed. Maybe it works better at extremely low humidity but for me they run high.
AP DP was 66 as was my psychrometer.

I can only give snapshot of 3 units but all 3 SHT31's - 69° and SHT15-67°.
Two degrees of dew point is nothing. What's that equate to in humidity, maybe 1/2 of a percent at your temp? If the 15 works for you, go for it. Mine went into the trash long ago.

At 80° ambient it's 4%.
It's more than nothing and way out of specs.
Yes, I guess I'm used to my temps. What now?

Offline jgentry

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Re: Which sling psychrometer should I buy?
« Reply #30 on: July 05, 2018, 09:08:58 PM »
And believe it or not...My grandmother that lives besides me has an Acurite temp/hum/lightning sensor housed in an Acurite radiation shield and is underneath a tree; and it reports DPs that matches well with the ASOSs during the day. At night, it can have a little dry bias at times but it doesn’t take long for it to reach 99% (highest it can go) on a very humid night.
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 #31 on: July 05, 2018, 10:05:39 PM »

Yes, I guess I'm used to my temps. What now?

Punt I guess, just frustrated with the high humidity issue. When we get in advisory level on high heat index my website needs to be accurate and not just close.
Running around 6% high on humidity, 3° dew point range at 90° makes a big difference with heat index.
I would like to trust the humidity was accurate at least within claimed specs 2%.   ](*,)
« Last Edit: July 06, 2018, 01:19:00 AM by ValentineWeather »
Randy

Offline jgentry

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Re: Which sling psychrometer should I buy?
« Reply #32 on: July 05, 2018, 10:30:10 PM »
I’m in a similar issue here too. I upload data to WeatherLink Website, etc. and I need the DP to be accurate along with the temp. The county I’m in is one of the top produce producers in the state of Alabama. Growers here rely on the data coming from the Davis VP2s. Auburn University is switching to the Davis units for their agricultural Mesonet from the Campbell units.
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 #33 on: July 05, 2018, 10:43:32 PM »

Yes, I guess I'm used to my temps. What now?

Running around 6-7% high on humidity
:???: If I were to average out my humidity compared to the two ASOS's and one RAWS around me over a decent span (say a week) I'd bet it'd be within 0 to 1% with the 31. I compare at least a dozen times every single day (night and day), and have done so since the local WFO picked up my PWS on their website about six years ago.
Me thinks there's more afoot....

Offline jgentry

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Re: Which sling psychrometer should I buy?
« Reply #34 on: July 05, 2018, 10:52:58 PM »

Yes, I guess I'm used to my temps. What now?

Running around 6-7% high on humidity
:???: If I were to average out my humidity compared to the two ASOS's and one RAWS around me over a decent span (say a week) I'd bet it'd be within 0 to 1% with the 31. I compare at least a dozen times every single day (night and day), and have done so since the local WFO picked up my PWS on their website about six years ago.
Me thinks there's more afoot....

My only guess then (since I have the Davis 24hr FARS and no boost in airflow) that Davis’ bulky filter cap may have a role in this but then again, it may not be the case.

Maybe Randy can take the filter cap off of one of his units and see if it makes any difference.
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 #35 on: July 05, 2018, 11:04:14 PM »

Yes, I guess I'm used to my temps. What now?

Running around 6-7% high on humidity
:???: If I were to average out my humidity compared to the two ASOS's and one RAWS around me over a decent span (say a week) I'd bet it'd be within 0 to 1% with the 31. I compare at least a dozen times every single day (night and day), and have done so since the local WFO picked up my PWS on their website about six years ago.
Me thinks there's more afoot....

My only guess then (since I have the Davis 24hr FARS and no boost in airflow) that Davis’ bulky filter cap may have a role in this but then again, it may not be the case.

Maybe Randy can take the filter cap off of one of his units and see if it makes any difference.
Honestly, I think all that would do is make the response time a touch quicker, it'll do nothing for accuracy. However, be sure it'll certainly dirty up the sensor quicker.

Offline jgentry

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Re: Which sling psychrometer should I buy?
« Reply #36 on: July 06, 2018, 05:54:19 AM »

Yes, I guess I'm used to my temps. What now?

Running around 6-7% high on humidity
:???: If I were to average out my humidity compared to the two ASOS's and one RAWS around me over a decent span (say a week) I'd bet it'd be within 0 to 1% with the 31. I compare at least a dozen times every single day (night and day), and have done so since the local WFO picked up my PWS on their website about six years ago.
Me thinks there's more afoot....

My only guess then (since I have the Davis 24hr FARS and no boost in airflow) that Davis’ bulky filter cap may have a role in this but then again, it may not be the case.

Maybe Randy can take the filter cap off of one of his units and see if it makes any difference.
Honestly, I think all that would do is make the response time a touch quicker, it'll do nothing for accuracy. However, be sure it'll certainly dirty up the sensor quicker.

You’re probably right. Just a wild guess on my part. Just trying to figure out what’s the cause.
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 #37 on: July 06, 2018, 08:04:21 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.


Randy

Offline jgentry

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Re: Which sling psychrometer should I buy?
« Reply #38 on: July 06, 2018, 09:54:51 AM »
I wish Davis would ditch the Sensirion sensor and use Vaisala.
Davis Vantage Pro2. SHT 31. 24hr FARS. WU: KXALJEMI2 & KALTHORS2 CWOP: C6353 & C6358

  

Online SLOweather

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Re: Which sling psychrometer should I buy?
« Reply #39 on: July 06, 2018, 10:30:16 AM »
Do you have a model number?

Offline ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #40 on: July 06, 2018, 10:43:42 AM »
I wish Davis would ditch the Sensirion sensor and use Vaisala.
I'll second that. I would be willing to spend some $$$ for reliable humidity.
 
For sure the claimed 2% accuracy across the entire range is false at least with the units I've received. It's more like +/- 6% from what I see.
At this point I would be willing to try Rainwise but it's not compatible with my software and website so I'll just live with it and hope they upgrade soon. I give grade on sensirion humidity (D+) but the temperature gets solid (A).     
Randy

Offline jgentry

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Re: Which sling psychrometer should I buy?
« Reply #41 on: July 06, 2018, 12:13:17 PM »
Do you have a model number?

For the SHT-31?
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 #42 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 CW2274

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Re: Which sling psychrometer should I buy?
« Reply #43 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 #44 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 #45 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 #46 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 #47 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 #48 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 #49 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