WXforum.net
Weather Station Hardware => Davis Instruments Weather Stations => Topic started by: ValentineWeather on July 08, 2017, 03:35:47 PM
-
Visited ASOS today for comparison with my portable 24 hr. VP2 with SHT31 temp/humidity sensor. I collected 40 minutes of data but temperature wasn't changing much and unfortunately the 5 minute data from ASOS skipped most sections ](*,) when there.... I took pictures of console every 5 minutes but will only provide the 2 times I received ASOS data.
The ASOS was reading +1F on one of the 5 minute intervals and the other time period both reported 91F. You can see the ASOS does read lower dewpoint /humidity as some that watch ASOS stations may have suspected. -4° DP which ends up being 6% humidity in 90-91F range.
Anyway thought I would give what I found. What I would really like to do is several full days of side by side comparison but this sample told me some of what I suspected.
-
You can see the ASOS does read lower dewpoint /humidity as some that watch ASOS stations may have suspected. -4° DP which ends up being 6% humidity in 90-91F range.
Somewhat surprised by this...granted a very small sampling though.
-
You can see the ASOS does read lower dewpoint /humidity as some that watch ASOS stations may have suspected. -4° DP which ends up being 6% humidity in 90-91F range.
Somewhat surprised by this...granted a very small sampling though.
Really? Almost forgot that's why you always end up in QC jail. :lol:
I wonder now if the issue you have isn't when DP/temp gets the big separation with sub 10% humidity. I'm always about 4° higher on DP. But then again each ASOS may be different but they aren't suppose to be. Remember the the debacle with the HO83 shield. I read somewhere recently they still haven't got all the bad HO83's out of field yet. https://wattsupwiththat.com/2008/01/10/inside-the-asos-ho83-tempdewpoint-sensor/
-
All I can say is, "very interesting" as used in the TV series, Laugh in, I believe! A portable VP2--must try that sometime! Maybe Davis is working on such a product--oops, sorry--I forgot that they are working on the VP3!
-
You can see the ASOS does read lower dewpoint /humidity as some that watch ASOS stations may have suspected. -4° DP which ends up being 6% humidity in 90-91F range.
Somewhat surprised by this...granted a very small sampling though.
Really? Almost forgot that's why you always end up in QC jail. :lol:
Only when the humidity gets below 2% does the 31's dew crash, otherwise it's completely comparable to my usual check points.
I think that the 31 has trouble way down low, just like about every sensor has problems way up high (asos's included).
-
I only took the shield, without anemometer or rain gauge but it could be easily attached. I would take a picture but it's all broke down and put away.
I clamp shield on 6' pipe that fits down in holder for dish. They call these non penetrating platform and looks just like this. You can weigh it down with cement blocks if needed.
-
8-)
-
Some may remember my theory about in town with grass and trees being cooler during summer.
I just got back from airport 2nd time and can confirm anywhere from +2-4° warmer at airport with same matching Davis SHT-31's instruments I'm using in town. Amazing what grass and trees can do. The airport may be +1° warmer than SHT31 at times but taking accuracy of instruments into consideration it's within specs if Airport was + 1/2 degree and SHT31 - 1/2 degree.
I guess I can't complain ASOS is reading hot.
-
Some may remember my theory about in town with grass and trees being cooler during summer.
I'm quite sure that Central Park in NYC (just as one example) is several degrees cooler than the surrounding area.
-
Some may remember my theory about in town with grass and trees being cooler during summer.
I'm quite sure that Central Park in NYC (just as one example) is several degrees cooler than the surrounding area.
I'm sure it is.
I guess the issue that pushed me into checking this for myself is my neighbor telling me why was the airport 97° and you are only 93°. My thermometer under house eve was also 97° today. :?: When I try to explain it's because your thermometer is against the house in a heat pocket the response is well it's the same as airport. ](*,)
-
Some may remember my theory about in town with grass and trees being cooler during summer.
I'm quite sure that Central Park in NYC (just as one example) is several degrees cooler than the surrounding area.
When I try to explain it's because your thermometer is against the house in a heat pocket the response is well it's the same as airport. ](*,)
Many just don't get it, nor care to.
-
Interesting! I also noticed that the pressure readings were very close. Did you adjust the elevation or did it just happen to work out? Since the VP2 console doesn't do altimeter and it was so warm, I would have expected there to be an appreciable difference between the two readings (MSLP vs QNH).
-
The altimeter I just used the zero elevation method and matched airport. I haven't touched pressure on the VP2 console for years but it's still very close. I use the VUE console as my primary unit because it does have altimeter and usually dead on.
-
The altimeter I just used the zero elevation method and matched airport. I haven't touched pressure on the VP2 console for years but it's still very close. I use the VUE console as my primary unit because it does have altimeter and usually dead on.
This works well for the VP2 console in lieu of not having a Vue for the actual altimeter. As long as you're not in quite relatively high or low pressure, the VP2 is very close.
-
Well, you were a couple hundred feet away. Micro climate?
Just sayin.
-
Well, you were a couple hundred feet away. Micro climate?
Just sayin.
:oops:
-
Well, you were a couple hundred feet away. Micro climate?
Just sayin.
The only micro-climate (and think this term is pushing it here) that could apply at this short distance would be wind velocity/direction at any given moment and rainfall. Temp and humidity should be virtually identical.
-
I took it as a joke. We had a nice steady breeze so I'm pretty sure it was representative of conditions 100 yards away. 8-)
Its also flat as a pancake .....
-
I took it as a joke.
Oh....yeah. I get too wrapped up in this stuff sometimes. :oops:
-
I took it as a joke.
Oh....yeah. I get too wrapped up in this stuff sometimes. :oops:
I forgive you :-)
-
I took it as a joke.
Oh....yeah. I get too wrapped up in this stuff sometimes. :oops:
I forgive you :-)
You shouldn't forgive a Penguins fan!
-
Also bear in mind that if your station is set up in a locale with more vegetation (i.e. trees, shrubs, irrigated grass, etc.) such as a neighborhood/subdivision than at an open and exposed location like an airport/airfield, you will have a lot more transpiration of water vapor from the vegetation leading to locally higher vapor pressure, relative humidity, dewpoint, etc. With this considered, air with a higher water vapor content heats less efficiently than air with lower water vapor content. So there may very well be discrepancies this manner as well.
-
Also bear in mind that if your station is set up in a locale with more vegetation (i.e. trees, shrubs, irrigated grass, etc.) such as a neighborhood/subdivision than at an open and exposed location like an airport/airfield, you will have a lot more transpiration of water vapor from the vegetation leading to locally higher vapor pressure, relative humidity, dewpoint, etc. With this considered, air with a higher water vapor content heats less efficiently than air with lower water vapor content. So there may very well be discrepancies this manner as well.
Absolutely I'm located on a very densely vegetated side of town including a creek nearby.
-
Ran across tripod I had stored away. This is a better way of mounting VP2 shield on pvc pipe and using tent stakes for stability in wind.
-
Very hot day set a station record 106°. Took the portable back out to airport and matching 108° at 240pm. Didn't stay long it was too HOT!
-
heh...love this thread. Let's revisit this in a few weeks when the raw 1-min numbers come out for July. Then you can make better comparisons. Did you try this in June at all?
-
heh...love this thread. Let's revisit this in a few weeks when the raw 1-min numbers come out for July. Then you can make better comparisons. Did you try this in June at all?
Very nice...Hadn't heard that before.
-
heh...love this thread. Let's revisit this in a few weeks when the raw 1-min numbers come out for July. Then you can make better comparisons. Did you try this in June at all?
Very nice...Hadn't heard that before.
Here's VTN for June.
ftp://ftp.ncdc.noaa.gov/pub/data/asos-onemin/6406-2017/64060KVTN201706.dat
You have p-type there, precip, the 3 station pressure sensors, and then the raw temp and dew. Keep in mind that if you want the actual reported temp or dew for any particular minute that you have to take a 5-min average (that minute ob and the previous 4).
-
heh...love this thread. Let's revisit this in a few weeks when the raw 1-min numbers come out for July. Then you can make better comparisons. Did you try this in June at all?
Keep in mind that if you want the actual reported temp or dew for any particular minute that you have to take a 5-min average (that minute ob and the previous 4).
My WFO told me that to be "official" each 5 minute obs is used then the average algorithm must be met for 3 of those 5 minutes.
-
heh...love this thread. Let's revisit this in a few weeks when the raw 1-min numbers come out for July. Then you can make better comparisons. Did you try this in June at all?
Keep in mind that if you want the actual reported temp or dew for any particular minute that you have to take a 5-min average (that minute ob and the previous 4).
My WFO told me that to be "official" each 5 minute obs is used then the average algorithm must be met for 3 of those 5 minutes.
Some material was posted in another thread about humidity I started when the ASOS was reading low 90's humidity with soupy fog, they use a 5 minute running average. I'm pretty sure my nearby ASOS humidity is running low because of what it read in the fog and compared to the SHT31 I see 3-4° difference. I'm not sure I trust the new sensors as much as I did the chilled mirrors with DP.
-
heh...love this thread. Let's revisit this in a few weeks when the raw 1-min numbers come out for July. Then you can make better comparisons. Did you try this in June at all?
Keep in mind that if you want the actual reported temp or dew for any particular minute that you have to take a 5-min average (that minute ob and the previous 4).
My WFO told me that to be "official" each 5 minute obs is used then the average algorithm must be met for 3 of those 5 minutes.
Some material was posted in another thread about humidity I started when the ASOS was reading low 90's humidity with soupy fog, they use a 5 minute running average. I'm pretty sure my nearby ASOS humidity is running low because of what it read in the fog and compared to the SHT31 I see 3-4° difference. I'm not sure I trust the new sensors as much as I did the chilled mirrors with DP.
Frankly, I'd trust your 31. I don't know what kind are used here, but when the two ASOS's and one RAWS I always compare to "catch up" my humidity is "usually" no more than a percent off, plus or minus (usually plus) if not exact, and dew no more than two or three degrees, if not exact also. I never had this accuracy with my 11 or 15.
-
heh...love this thread. Let's revisit this in a few weeks when the raw 1-min numbers come out for July. Then you can make better comparisons. Did you try this in June at all?
Keep in mind that if you want the actual reported temp or dew for any particular minute that you have to take a 5-min average (that minute ob and the previous 4).
My WFO told me that to be "official" each 5 minute obs is used then the average algorithm must be met for 3 of those 5 minutes.
Can you clarify what you mean by this?
-
This section may explain the ASOS 5 minute running average.
3.1 Ambient and Dew Point
Temperature
Ambient and dew point temperature reports are among
the most widely disseminated of all the weather elements
in the surface observation. Because of keen public interest,
nearly all radio and most television stations report temperature
and humidity at least once an hour. Ambient and
dew point temperature are vital in determining aircraft performance
and loading characteristics and are critical for
accurate weather forecasts. To meet these needs, ASOS
provides a 5-minute average ambient air and dew point
temperature every minute
-
heh...love this thread. Let's revisit this in a few weeks when the raw 1-min numbers come out for July. Then you can make better comparisons. Did you try this in June at all?
Keep in mind that if you want the actual reported temp or dew for any particular minute that you have to take a 5-min average (that minute ob and the previous 4).
My WFO told me that to be "official" each 5 minute obs is used then the average algorithm must be met for 3 of those 5 minutes.
Can you clarify what you mean by this?
I just spoke to my WFO again and he stated that it's a running two minute average. Either way, ASOS's average things out, we obviously don't.
-
See 3.1.2 http://www.nws.noaa.gov/asos/aum-toc.pdf
Other good stuff included too.
-
What I find most interesting are the specs.
They aren't as tight as the sensirion SHT31. So much for comparing with ASOS. They should be calibrating with our instruments.
-
What I find most interesting are the specs.
They aren't as tight as the sensirion SHT31. So much for comparing with ASOS. They should be calibrating with our instruments.
Ha, true. Only thing is ASOS's are "supposedly" calibrated on a regular basis. The equipment I used was checked every six months.
-
What I find most interesting are the specs.
They aren't as tight as the sensirion SHT31. So much for comparing with ASOS. They should be calibrating with our instruments.
So the SHT31 is more accurate than ASOS climate sites?
-
What I find most interesting are the specs.
They aren't as tight as the sensirion SHT31. So much for comparing with ASOS. They should be calibrating with our instruments.
So the SHT31 is more accurate than ASOS climate sites?
Yes, here are the 31 specs showing typical and maximum tolerance. Specs are much tighter than what the ASOS publication claims.
-
What I find most interesting are the specs.
They aren't as tight as the sensirion SHT31. So much for comparing with ASOS. They should be calibrating with our instruments.
So the SHT31 is more accurate than ASOS climate sites?
Not necessarily, they just happen to have a fairly wide berth of acceptable range. A properly maintained/sited ASOS is about as good as it gets for public consumption. Remember, their basic function is for safety of the flying public so it's "suppose' to be accurate.
-
What I find most interesting are the specs.
They aren't as tight as the sensirion SHT31. So much for comparing with ASOS. They should be calibrating with our instruments.
So the SHT31 is more accurate than ASOS climate sites?
Not necessarily, they just happen to have a fairly wide berth of acceptable range. A properly maintained/sited ASOS is about as good as it gets for public consumption. Remember, their basic function is for safety of the flying public so it's "suppose' to be accurate.
Also a properly maintained/sited VP2 AC fan with 31 is about as good as it gets for us :grin:
BTW, off-topic but after a brief torrential downpour today, my AC fan beats on!!!
-
My conclusion for now unless I learn more after seeing ASOS specs in print, I'll just no longer look at ASOS as the absolute in accuracy as I once did.
Even the (.9F) which I assume is typical is more than SHT31 (.4C) (.72F) max tolerance on the SHT31. Typical on the 31 is just (.2-.3C) throughout.
-
What I find most interesting are the specs.
They aren't as tight as the sensirion SHT31. So much for comparing with ASOS. They should be calibrating with our instruments.
So the SHT31 is more accurate than ASOS climate sites?
Not necessarily, they just happen to have a fairly wide berth of acceptable range. A properly maintained/sited ASOS is about as good as it gets for public consumption. Remember, their basic function is for safety of the flying public so it's "suppose' to be accurate.
BTW, off-topic but after a brief torrential downpour today, my AC fan beats on!!!
I thought you gave up? Send me a PM so as not to gum up the thread.
-
What I find most interesting are the specs.
They aren't as tight as the sensirion SHT31. So much for comparing with ASOS. They should be calibrating with our instruments.
So the SHT31 is more accurate than ASOS climate sites?
Yes, here are the 31 specs showing typical and maximum tolerance. Specs are much tighter than what the ASOS publication claims.
Maybe the sensors changed since that publication? NWS in Birmingham, AL says that they believe the ASOS uses the HO-1088 for temperature & DTS-1 for DP
-
What I find most interesting are the specs.
They aren't as tight as the sensirion SHT31. So much for comparing with ASOS. They should be calibrating with our instruments.
So the SHT31 is more accurate than ASOS climate sites?
Yes, here are the 31 specs showing typical and maximum tolerance. Specs are much tighter than what the ASOS publication claims.
Maybe the sensors changed since that publication? NWS in Birmingham, AL says that they believe the ASOS uses the HO-1088 for temperature & DTS-1 for DP
I knew they changed the humidity sensor. I can't find much information on the new sensors even from the Allweatherinc who makes the ASOS. They seem to have the specs unavailable on the site.
-
ASOS is definitely still a 5-min running average...not 2-min. I've confirmed this just by comparing official high temps with the 1-min readings. I've seen times where a peak stretch of 94-94-95-95-94 resulted in a high of 94 rather than a high of 95.
And those specs are outdated...that was published in 1998. Anything ASOS is still superior (I guess we could argue the definition of superior) to the SHT3x.
-
I found this 2012 publication claiming +/- 2° also. https://www.weather.gov/media/lmk/pdf/educational_pages/ASOSandClimateObservations__What_Is_ASOS.pdf
WIKI has information on changeover from the :The current ASOS thermometer is designated the HO-1088, though some older systems still utilize the HO-83.
And new dew Point sensor: Due to problems with the chilled mirror sensor, NWS ASOS sites now use Vaisala's DTS1 sensor, which measures humidity only via capacitance. The sensor is based on a solid state capacitive relative humidity element that incorporates a small heater so that the sensing element is always above the ambient temperature, eliminating the formation of dew or frost. The sensor reports directly in dew point through a calculation based on measured relative humidity and the measured temperature of the heated capacitive element.[11]
https://en.wikipedia.org/wiki/Automated_airport_weather_station
allweatherinc has this sensor also: Accuracy .01C
http://www.allweatherinc.com/meteorological-sensors/temperature-sensor/#tabs=1151
-
+/- 0.01C sounds about right. Most of those higher end sensors run about +/- 0.01-0.05C
-
The ASOS humidity sensor is made by Vaisala. It's the DTS-1 which is based on the HMP243 probe utilizing the Humicap 180 sensor.
Relative humidity measurement: Measurement range: 0-100%RH; Accuracy at 68F: +-1.0% RH (0-90%RH), +-1.7%RH (90-100%RH); Sensor: Vaisala HUMICAP 180 Vaisala HUMICAP 180R
Also the HO-1088's chilled mirror was not decommissioned with the installation of the DTS-1. They are still in use as a backup should the DTS-1 fail or go out of tolerance. The HO-1088 was made by Technical Services Laboratory.
http://tslinc.com/systems/instruments-hygrothermometer/ (http://tslinc.com/systems/instruments-hygrothermometer/)
//edit: adding diagram of the DTS-1 sensor shield for reference.
(https://www.iprocessmart.com/images/Vaisala/vaisala_hmp243mika.gif)
-
The ASOS humidity sensor is made by Vaisala. It's the DTS-1 which is based on the HMP243 probe utilizing the Humicap 180 sensor.
Relative humidity measurement: Measurement range: 0-100%RH; Accuracy at 68F: +-1.0% RH (0-90%RH), +-1.7%RH (90-100%RH); Sensor: Vaisala HUMICAP 180 Vaisala HUMICAP 180R
Also the HO-1088's chilled mirror was not decommissioned with the installation of the DTS-1. They are still in use as a backup should the DTS-1 fail or go out of tolerance. The HO-1088 was made by Technical Services Laboratory.
http://tslinc.com/systems/instruments-hygrothermometer/ (http://tslinc.com/systems/instruments-hygrothermometer/)
Thanks for that information... =D>
-
ASOS 1088 specs finally...
-
ASOS 1088 specs finally...
Insert Rodney Dangerfield "No respect I tell ya, no respect". I posted this about 4-5 months ago having a "discussion" about whether ASOS's where aspirated or not. So I supplied this along with your pic... ;)
https://upload.wikimedia.org/wikipedia/commons/c/c3/2013-09-19_09_28_07_ASOS_HO-1088_thermometer_at_Eureka_Airport%2C_Nevada.JPG
-
ASOS 1088 specs finally...
Insert Rodney Dangerfield "No respect I tell ya, no respect". I posted this about 4-5 months ago having a "discussion" about whether ASOS's where aspirated or not. So I supplied this along with your pic... ;)
https://upload.wikimedia.org/wikipedia/commons/c/c3/2013-09-19_09_28_07_ASOS_HO-1088_thermometer_at_Eureka_Airport%2C_Nevada.JPG
You lost me :?:
-
ASOS 1088 specs finally...
Insert Rodney Dangerfield "No respect I tell ya, no respect". I posted this about 4-5 months ago having a "discussion" about whether ASOS's where aspirated or not. So I supplied this along with your pic... ;)
https://upload.wikimedia.org/wikipedia/commons/c/c3/2013-09-19_09_28_07_ASOS_HO-1088_thermometer_at_Eureka_Airport%2C_Nevada.JPG
You lost me :?:
It was my poor attempt at humor. I posted this pic months ago, I guess you didn't see it. http://tslinc.com/wp-content/uploads/2010/01/hygrothermometer-data-sheet-rev.pdf
-
Yes I either missed it or didn't sink in or just another senior moment. Was looking for the updated specs after what was linked to the publication from 1998. This is more what I expected.
So someone questioned if ASOS was aspirated :?: Now that's hilarious... :lol:
-
Yes I either missed it or didn't sink in or just another senior moment. Was looking for the updated specs after what was linked to the publication from 1998. This is more what I expected.
So someone questioned if ASOS was aspirated :?: Now that's hilarious... :lol:
Even after posting BOTH pics, he still wanted to argue. I eventually gave up. :roll: :roll:
-
As Bill would say...
-
:lol: Hadn't heard that one before... :lol:
-
Here's the brochure on the Vaisala DTS-1 sensor shield. Basically you can have your own DTS-1 if you purchase this shield (HMT330MIK) and the Vaisala HMT337 temperature/humidity transmitter.
http://www.vaisala.com/Vaisala%20Documents/Brochures%20and%20Datasheets/HMT330MIK-Datasheet-B210944EN-D-LoRes.pdf (http://www.vaisala.com/Vaisala%20Documents/Brochures%20and%20Datasheets/HMT330MIK-Datasheet-B210944EN-D-LoRes.pdf)
(https://upload.wikimedia.org/wikipedia/commons/thumb/d/d4/2013-09-19_09_27_41_ASOS_DTS-1_dewpoint_sensor_at_Eureka_Airport%2C_Nevada.JPG/640px-2013-09-19_09_27_41_ASOS_DTS-1_dewpoint_sensor_at_Eureka_Airport%2C_Nevada.JPG)
-
I like the brochure stating what the rest of us already know...
"In weather observations dew
formation makes reliable humidity
measurement difficult."
Indeed.
-
Back in December I did test the SHT31 in a true 100% RH test environment and achieved these results. You can see the condensation.
(https://farm5.staticflickr.com/4209/35719511122_0e637b6881_b.jpg)
-
Back in December I did test the SHT31 in a true 100% RH test environment and achieved these results. You can see the condensation.
Very nice. In the temp world that's 100, I assume in the humidity world that applies here too?
-
I should also state that I tested three SHT31's in the 100% RH environment at the same time and got mixed results. However, after looking at the specs, the maximal tolerance at 100% is 3.5%. So they would be considered in specification. The sensor in front in the photo below is the one in the photo I posted above. For that sensor I was testing repeatability. That is the sensors ability to indicate the same value when exposed to the same conditions.
(https://farm5.staticflickr.com/4262/35107558553_e9c915c7fb_b.jpg)
The sensors maximum allowable error is 2.5% and that escalates above 90% RH.
(https://farm5.staticflickr.com/4330/35107015133_17981536c2.jpg)
(https://farm5.staticflickr.com/4315/35107015143_ed304cae91.jpg)
While I was at it, I ran the daughters WMII sensor through the same test. It did get to 100% but it took 38 minutes.
(https://farm5.staticflickr.com/4279/35918497325_afa4ffb32a_z.jpg)
-
I've noticed a slight difference humidity also between my 3 VP2's aspirated. I run 3 stations #1&2 are over lawn, #4 is my portable located different area over less grass and more dirt. I switch over to #4 when I water the lawn every 3rd day.
-
I found out that the specs from the RAWS station quite interesting...
http://www.ncforestservice.gov/fire_control/pdf/RNTN01.pdf
-
Summary quote from page 3: "Ultimately, the station owner is responsible for ensuring the station is delivering
acceptable weather data."
-
And here is the specs of the AWOS stations
http://www.allweatherinc.com/wp-content/uploads/AWOS-III-PTHU-Specification-1505220-16-Latest-Edition1.pdf
-
Summary quote from page 3: "Ultimately, the station owner is responsible for ensuring the station is delivering
acceptable weather data."
That is definitely true!
-
And here is the specs of the AWOS stations
http://www.allweatherinc.com/wp-content/uploads/AWOS-III-PTHU-Specification-1505220-16-Latest-Edition1.pdf
Never mind I was gong to say something about it but this is the AWOS.. ](*,) I had this one.
-
Noticed now that the digital SHT 3x has different specs than the analog versions
Analog: https://www.sensirion.com/fileadmin/user_upload/customers/sensirion/Dokumente/2_Humidity_Sensors/Sensirion_Humidity_Sensors_SHT3x_Datasheet_analog.pdf
Digital: https://www.sensirion.com/fileadmin/user_upload/customers/sensirion/Dokumente/2_Humidity_Sensors/Sensirion_Humidity_Sensors_SHT3x_Datasheet_digital.pdf
If only the Davis transmitter can handle the digital sensors and use the SHT35....
-
Humm..Interesting they keep changing the longer sensor has been out. I do remember they mentioned these are initial specs when the sensor came out new.
-
Humm..Interesting they keep changing the longer sensor has been out. I do remember they mentioned these are initial specs when the sensor came out new.
And I noticed that in the analog sheet, there is no SHT 35, just the SHT 30 & 31.
-
One thing that seriously concerned me with these SHT31 evaluation boards was the long term shift in accuracy on the humidity side. I only test a particular type of sensor in minimum groups of three. When a sensor company is wanting to bid on a government contract it's usually pretty easy for me to secure at least three for testing. Although that's not the case with these evaluation boards, it was more for my own curiosity. The issue is approximately 3 weeks after receiving all three of these evaluation boards, I ran a test at 50% RH. All three were reading low by -1.2%, -1.7% and -1.9%. Almost a year later they were reading +0.5% to +0.8%. I'm at a loss to explain why the large shift over time. I'm not quite ready to call it "drift" because that's an insane amount of drift. I was so concerned, I sent my NIST reference instrument back to Washington D.C. to get it certified again after it was just certified 6 months before. It came back having tested just fine. In the hundreds of sensors I've tested over 20+ years, I never seen this much shift before over the course of a year. I was thinking of getting a fourth one and seeing how it compares right out of the box against these three units that have been running for almost a year. Some sensors do require a conditioning period when first used or what I refer to as exercising the sensor. I know that with some sensor manufacturers when they send you a sample for evaluation, sometimes these are not their best spec'ed sensors. Often they are sensors that are usable but don't quite pass QC 100% for some reason. I wonder if that's what these evaluation boards are. I never did ask Sensirion. The only thing I can think of is since these SHT31 sensors don't have a filter over them, they have become contaminated with dust over time.
-
One thing that seriously concerned me with these SHT31 evaluation boards was the long term shift in accuracy on the humidity side. I only test a particular type of sensor in minimum groups of three. When a sensor company is wanting to bid on a government contract it's usually pretty easy for me to secure at least three for testing. Although that's not the case with these evaluation boards, it was more for my own curiosity. The issue is approximately 3 weeks after receiving all three of these evaluation boards, I ran a test at 50% RH. All three were reading low by -1.2%, -1.7% and -1.9%. Almost a year later they were reading +0.5% to +0.8%. I'm at a loss to explain why the large shift over time. I'm not quite ready to call it "drift" because that's an insane amount of drift. I was so concerned, I sent my NIST reference instrument back to Washington D.C. to get it certified again after it was just certified 6 months before. It came back having tested just fine. In the hundreds of sensors I've tested over 20+ years, I never seen this much shift before over the course of a year. I was thinking of getting a fourth one and seeing how it compares right out of the box against these three units that have been running for almost a year. Some sensors do require a conditioning period when first used or what I refer to as exercising the sensor. I know that with some sensor manufacturers when they send you a sample for evaluation, sometimes these are not their best spec'ed sensors. Often they are sensors that are usable but don't quite pass QC 100% for some reason. I wonder if that's what these evaluation boards are. I never did ask Sensirion. The only thing I can think of is since these SHT31 sensors don't have a filter over them, they have become contaminated with dust over time.
Yes, well over the claimed drift on spec sheet.
Maybe it's time to ask Sensirion for a response.
-
I got in contact with them. I'm waiting to hear back from the engineer however I was asked if I was testing using the EK-H4 system. I told them I have one of those but didn't realize it worked with the SHT31's. I used it with the SHT2x and SHT7x sensors. So now I have a box of board mounted SHT31's coming.
I'll test them with the EK-H4. I have all this free time now and still can't get away from this stuff. :lol:
https://www.sensirion.com/en/environmental-sensors/humidity-sensors/evaluation-kit-ek-h4/ (https://www.sensirion.com/en/environmental-sensors/humidity-sensors/evaluation-kit-ek-h4/)
-
So now I have a box of board mounted SHT31's coming.
I'll test them with the EK-H4. I have all this free time now and still can't get away from this stuff. :lol:
Nice =D>
-
Here's what I use to test humidity sensors across the range.
(https://farm5.staticflickr.com/4324/35955359842_cc20dc5cdc_b.jpg)
(https://farm5.staticflickr.com/4305/35955304352_4ca39e313b_b.jpg)
(https://farm5.staticflickr.com/4305/35990528311_045736b162_b.jpg)
-
Nice toy. If you ever get bored of them you can donate them here. :-P
-
So, you don't use the "salted-water vapor" in plastic bag/container method? Just straight water vapor?
-
So, you don't use the "salted-water vapor" in plastic bag/container method? Just straight water vapor?
Correct. The saturated salt solution/plastic bag method is not accurate. When using a salt solution it has to be contained to a very tiny area and the sensor has to be a couple millimeters above the solution to be accurate. For example if you are doing the 75% RH test, the humidity is only 75% within a few millimeters of the salt solution. That's why they make tiny calibrating chambers like this.
http://www.vaisala.com/en/products/humidity/Pages/HMK15.aspx (http://www.vaisala.com/en/products/humidity/Pages/HMK15.aspx)
-
Here's what I use to test humidity sensors across the range.
Wow, is that from the 70's? (certainly nothing wrong with that) How do you know that it's properly calibrated itself?
-
Once the heating or cooling bulbs go off, it's calibrated and you can read the sensor you are testing. It will maintain the humidity level until you turn off the unit. It's fairly old but there's no date on it. This one was retired from the weather office and once it was decommissioned I offered $20 for it. Deal was accepted. :lol:
-
I just ran the patent #, it was filed in 1959! Here's the link https://www.google.com/patents/US3171473
-
That's almost as old as me ;)
-
I looked it up via serial number with the government NSN and it was manufactured in 03-77. Probably says that on the metal tag but the numbers have faded to unreadable. Still works great. No parts available though. These were built like a tank. I always throw an NIST reference device in when testing just to make sure it's working right. My only concern is I believe it uses R12 refrigerant so I hope it doesn't leak out.
-
Work at the CWO at Charlotte Douglas Int'l Airport. We have a VP2 located at the office and just from experience (no official comparison) the VP2 dewpoint runs an average of about 1-2F above the asos but there are times when the VP dew point is lower. Our office is located about 1.5 miles away from the ASOS sensor group so you have to take this with a grain of salt.
Even across an airport as large as CLT you can have localized microclimates. The only way you could truly know is to set the sensors side by side.
-
What ever happened to the days of using a highly accurate mercury psychrometer to verify humidity? Certainly cheaper. I recently switched over to the sht31 and am still waiting for a foggy morning when I can do a comparison. Davis claims accuracy of +/-2% from 1-100% Sensiron is claiming +-3% for their analog version.
-
I should also state that I tested three SHT31's in the 100% RH environment at the same time and got mixed results. However, after looking at the specs, the maximal tolerance at 100% is 3.5%. So they would be considered in specification. The sensor in front in the photo below is the one in the photo I posted above. For that sensor I was testing repeatability. That is the sensors ability to indicate the same value when exposed to the same conditions.
(https://farm5.staticflickr.com/4262/35107558553_e9c915c7fb_b.jpg)
The sensors maximum allowable error is 2.5% and that escalates above 90% RH.
(https://farm5.staticflickr.com/4330/35107015133_17981536c2.jpg)
(https://farm5.staticflickr.com/4315/35107015143_ed304cae91.jpg)
While I was at it, I ran the daughters WMII sensor through the same test. It did get to 100% but it took 38 minutes.
(https://farm5.staticflickr.com/4279/35918497325_afa4ffb32a_z.jpg)
That tells me that you’re lucky if you hit 100%. Overall, your test tells us that the SHT-31 isn’t reliable when it comes to humidity readings
-
Overall, your test tells us that the SHT-31 isn’t reliable when it comes to humidity readings
Compared to what? Seriously folks, what else is better that we can realistically use?
-
Overall, your test tells us that the SHT-31 isn’t reliable when it comes to humidity readings
Compared to what? Seriously folks, what else is better that we can realistically use?
After reading that UK paper, it seems by switching to capacitive-RH sensors, they traded ONE problem (dry bias) for TWO (albeit cheaper) problems (wet bias & variability).
Should we go 'back' to sling psychrometers maybe?
-
Should we go 'back' to sling psychrometers maybe?
I broke too many... :lol:
-
Overall, your test tells us that the SHT-31 isn’t reliable when it comes to humidity readings
Compared to what? Seriously folks, what else is better that we can realistically use?
Should we go 'back' to sling psychrometers maybe?
Dunno about you, but uploading that data every five minutes 24/7 365 might.. just might get a little tiresome.... :mad:
-
Overall, your test tells us that the SHT-31 isn’t reliable when it comes to humidity readings
Compared to what? Seriously folks, what else is better that we can realistically use?
Should we go 'back' to sling psychrometers maybe?
Dunno about you, but uploading that data every five minutes 24/7 365 might.. just might get a little tiresome.... :mad:
Actually, I was thinking more about a dual-temp & wet-sock (wetbulb depression) unit; fitted into an AC-fan aspirated shield it should work OK, but getting clean/fresh distilled water to it might get interesting during either a dust storm or dust-devil.
-
Overall, your test tells us that the SHT-31 isn’t reliable when it comes to humidity readings
Compared to what? Seriously folks, what else is better that we can realistically use?
Should we go 'back' to sling psychrometers maybe?
Dunno about you, but uploading that data every five minutes 24/7 365 might.. just might get a little tiresome.... :mad:
Actually, I was thinking more about a dual-temp & wet-sock temperature unit; fitted into an AC-fan aspirated shield it should work OK, but getting clean/fresh distilled water to it might get interesting during either a dust storm or dust-devil.
Thanks, but I'll be sticking to my auto-loading, crappy 31. :oops:
-
Overall, your test tells us that the SHT-31 isn’t reliable when it comes to humidity readings
Compared to what? Seriously folks, what else is better that we can realistically use?
Should we go 'back' to sling psychrometers maybe?
Dunno about you, but uploading that data every five minutes 24/7 365 might.. just might get a little tiresome.... :mad:
Actually, I was thinking more about a dual-temp & wet-sock temperature unit; fitted into an AC-fan aspirated shield it should work OK, but getting clean/fresh distilled water to it might get interesting during either a dust storm or dust-devil.
Thanks, but I'll be sticking to my auto-loading, crappy 31. :oops:
Remember, your climate is more ideal for the SHT-31
-
Slow day in weatherland to dig up this dated topic. Oh well, WU continues to suck and kill great radar apps they once had; Davis ignores the need for a VP-3; CWOP as a website is so dated it is embarrassing but no one at NOAA cares since Russ Chadwick retired, despite volunteers like me who have offered to help bring CWOP somewhat up to date by at least identifying the hundred or so deadlinks in those webpages; —we must discuss something. The new WiFiLogger recently developed by Wojtek is a plus and the SHT31 is still a great tool despite words to the contrary in this thread.
All this said, life will continue more or less as we know it today!
-
Slow day in weatherland to dig up this dated topic. Oh well, WU continues to suck and kill great radar apps they once had; Davis ignores the need for a VP-3; CWOP as a website is so dated it is embarrassing but no one at NOAA cares since Russ Chadwick retired, despite volunteers like me who have offered to help bring CWOP somewhat up to date by at least identifying the hundred or so deadlinks in those webpages; —we must discuss something. The new WiFiLogger recently developed by Wojtek is a plus and the SHT31 is still a great tool despite words to the contrary in this thread.
All this said, life will continue more or less as we know it today!
ANALOGY: "...and the can continues to be kicked down the road."
-
the SHT31 is still a great tool despite words to the contrary in this thread.
Sounds like you got lucky. Just dropped another $100 for two more Sensirion sensors. Gets old adjusting for bad humidity readings all day for website accuracy easier to just replace every few months.
On the bright side at about $50 pop with shipping you can buy 16 or so for the price of 1 Vaisala.
-
the SHT31 is still a great tool despite words to the contrary in this thread.
Sounds like you got lucky. Just dropped another $100 for two more Sensirion sensors. Gets old adjusting for bad humidity readings all day for website accuracy easier to just replace every few months.
On the bright side at about $50 pop with shipping you can buy 16 or so for the price of 1 Vaisala.
Why do you think you keep getting bad humidity readings?
-
the SHT31 is still a great tool despite words to the contrary in this thread.
Sounds like you got lucky. Just dropped another $100 for two more Sensirion sensors. Gets old adjusting for bad humidity readings all day for website accuracy easier to just replace every few months.
On the bright side at about $50 pop with shipping you can buy 16 or so for the price of 1 Vaisala.
Why do you think you keep getting bad humidity readings?
Read this thread, starting with those posted about July 4, 2018 to the present:
https://www.wxforum.net/index.php?topic=34658.0
I was in the camp of liking the SHT31, but when carefully reviewing the data from my 3 VP2s, I now have a different view. Apparently, these sensors “wear out” after 12 to 18 months or so, a fact I find troubling, if true.