Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 5210 times)

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Offline Old Tele man

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Re: Which sling psychrometer should I buy?
« Reply #25 on: July 05, 2018, 02:22:16 PM »
Boy this question sure brought me back to the past. This is what I used some forty years ago to do fire weather forecasts. Still used today.

http://www.forestry-suppliers.com/product_pages/products.php?mi=85191&itemnum=89080
Did they issue a "slide-rule" back then or did you have a look-up table to use?


To "sanity check" your RH numbers see this earlier post: http://www.wxforum.net/index.php?topic=22767.msg219291#msg219291
« Last Edit: July 05, 2018, 03:38:47 PM by Old Tele man »
• SYS: Davis VP2 Vue/WL-IP & Envoy8X/WL-USB
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• WU - KAZTUCSO202, Countryside

Offline AF6GL

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Re: Which sling psychrometer should I buy?
« Reply #26 on: July 05, 2018, 04:06:53 PM »
We had look up tables. A slide rule was provided but we had multiple charts to flip through so it was easier, more consistent, to flip through the tables.

When electronic handheld calculators came out I used to sit in my grandfathers lap and race him while he was using his slide rule. Never did beat him.

Offline Old Tele man

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Re: Which sling psychrometer should I buy?
« Reply #27 on: July 05, 2018, 04:39:26 PM »
We had look up tables. A slide rule was provided but we had multiple charts to flip through so it was easier, more consistent, to flip through the tables.

When electronic handheld calculators came out I used to sit in my grandfathers lap and race him while he was using his slide rule. Never did beat him.

I've still got mine from USN and college, including a little 4-inch diameter circular slide rule (made in Japan).
• 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 #28 on: July 05, 2018, 07:36:57 PM »
This could be a completely new thread, but if issue is 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.  DP temperature run about 3°F high all the time winter summer doesn't matter but the damn thing still won't make 100% humidity on 4 different units all aspirated.
The SHT-15 I also recently put back in service for comparison reads closer to actual  psychrometer and nearby ASOS but also won't make 100% humidity.
Randy

Offline jgentry

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Re: Which sling psychrometer should I buy?
« Reply #29 on: July 05, 2018, 08:00:32 PM »
This could be a completely new thread, but if issue is 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.  DP temperature run about 3°F high all the time winter summer doesn't matter but the damn thing still won't make 100% humidity on 4 different units all aspirated.
The SHT-15 I also recently put back in service for comparison reads closer to actual  psychrometer and nearby ASOS but also won't make 100% humidity.

That has been my concern for a while. Either the SHT 31 having a wet bias or the ASOS has a dry bias during the day. Plus I also hate the fact that the SHT-31 humidity sensor doesn’t reach to 100% like it should. I guess it’s safe to say that the SHT-31 doesn’t operate within specs.

Now I wonder if the SHT-35 sensor is really all that better?

I wonder if Davis would be willing to use a different temp/hum sensor instead of Sensirion sensors?

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 #30 on: July 05, 2018, 08:04:10 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.

Offline ValentineWeather

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Re: Which sling psychrometer should I buy?
« Reply #31 on: July 05, 2018, 08:27:44 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°.
Randy

Offline jgentry

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Re: Which sling psychrometer should I buy?
« Reply #32 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 #33 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 #34 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 #35 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 #36 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 #37 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 Old Tele man

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Re: Which sling psychrometer should I buy?
« Reply #38 on: July 05, 2018, 09:28:34 PM »
Uh, 100% RH basically implies fog (ground cloud) formation at any moment.
• 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 #39 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 #40 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 #41 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 #42 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 #43 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 #44 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 #45 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 #46 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

  

Offline SLOweather

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Re: Which sling psychrometer should I buy?
« Reply #47 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 #48 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 #49 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