Author Topic: Humidity Error at cold temperatures with WS-1400?  (Read 1141 times)

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Offline quailvalleywx

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Humidity Error at cold temperatures with WS-1400?
« on: January 19, 2018, 04:51:42 PM »
I have noticed my ~1.5 year old WS-1400 was recently reporting humidity often 20+% lower than most adjacent non-Ambient Weather PWSs and airports.  I occasionally compare readings when wind readings are > ~5MPH avg.  From another thread I found someone speculated the sensor in use is a HR202L.  If so, from that part's data sheet I see that it is an AC type that requires the driver software to temperature compensate to derive the correct humidity value.  From observing my and adjacent Ambient Weather stations during this recent cold spell it appears that on at least a couple other WS-1400 and WS-1001 model units I checked they also read low recently.  I've attached a comparison chart below of my station and an airport 30+ miles North of me - humidity averages Dec. 2017.  Using the same average humidity comparison for June 2017 my station is running about 7% low but it was significantly low for the coldest days in Dec 2016 and Dec 2017.

Is anyone else seeing significantly lower average humidity than airport / adjacent stations during cold temperatures?  While it's cold I'm now applying a fairly high + calibration but a constant calibration will not work well for a variable temperature bias.  The HR202L data sheet claims Stability: ≤ 1% RH / year - looks like a driver/temperature compensation issue to me.

https://www.wunderground.com/personal-weather-station/dashboard?ID=KKSSHAWN60

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Offline dupreezd

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Re: Humidity Error at cold temperatures with WS-1400?
« Reply #1 on: January 20, 2018, 08:45:32 PM »
@ quailvalleywx,
Quote
I've attached a comparison chart below of my station and an airport 30+ miles North of me
30 miles is a long way away to compare readings. You will have your own micro climate.
Quote
I have noticed my ~1.5 year old WS-1400
Up until end of 2015, Ambient used analog temp/hum sensors. That is a bead NTC for Temp and the HR202L for humidity. Some time in 2016 they switched to Digital sensors. So, it is difficult to say what you have. You might want to remove the radiation shield to see what is inside. Mine is analog  :-(
Quote
While it's cold I'm now applying a fairly high + calibration but a constant calibration will not work well for a variable temperature bias
It is not actually a calibration, but rather an offset. When you raise it by 10 you also raise the floor by 10. This means the humidity reading will not go lower than the offset. It might then read higher in high humidity.
« Last Edit: January 20, 2018, 08:48:40 PM by dupreezd »
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Offline quailvalleywx

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Re: Humidity Error at cold temperatures with WS-1400?
« Reply #2 on: January 21, 2018, 03:11:52 PM »
I only compare adjacent station temperature and humidity on windy days but yes, there is a lot of distance and low wind time in that MCI comparison graph data.  I bet I have the analog sensor also.  On closer look at the data sheet it does not even give conversion info for temperatures below 0 deg C.  This means Ambient's SW Eng would have had to guestimate out-of-range data for below 0C.  Bringing this problem to light for me was below 0C was majority temperature for this area the last 30 days.

HR202L data sheet:
https://akizukidenshi.com/download/ds/aosong/HR202L.pdf

I've got a good idea of the temperature induced error curve just from windy day adjacent station/airport observation.  For now I'll apply a temperature driven correction based on that curve.  I capture the data before it gets sent to WU, WeatherCloud, and PWS so will correct it there.

In the Spring I'm plan on taking the station down and moving the solar and UV sensors to the South (non shaded!) side of the sensor array (what were they thinking?).  At that time I'll confirm which type of humidity sensor I have.  Anybody know if the digital one is rated below 0C?

Offline quailvalleywx

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Re: Humidity Error at cold temperatures with WS-1400?
« Reply #3 on: January 23, 2018, 04:49:33 PM »
Via taking data from some very windy times from two airports and my station at various temperatures here is the correction graph I'm using.  From comparing some history on a couple close WS-1001 stations applying corrections from this graph would help there also.  Maybe this relative humidity drop at cold temperatures affects most/all Ambient Weather stations using the HR202L sensor?  Above 50F on my station the offset seemed constant and can be corrected on the calibration page.

I sent an email to Ambient Weather support but no response yet.  This is pretty easy to verify.  Find a very cold day when a cold front blew strong winds in from the North and compare humidity with an airport or official NOAA site.

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Offline rods55555

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Re: Humidity Error at cold temperatures with WS-1400?
« Reply #4 on: January 24, 2018, 05:57:00 AM »
My station humidity sensor is dead (outdoor sensor), it show 99% for almost the entire day, and when the humidity is actually high, it just show - - % on the screen.

My station is a Misol, an Ambient Weather clone (HP2000). Does anyone know if it's possible to replace just the humidity sensor?
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Offline Pooley

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Re: Humidity Error at cold temperatures with WS-1400?
« Reply #5 on: January 24, 2018, 11:11:45 AM »
Up until end of 2015, Ambient used analog temp/hum sensors. That is a bead NTC for Temp and the HR202L for humidity. Some time in 2016 they switched to Digital sensors. So, it is difficult to say what you have. You might want to remove the radiation shield to see what is inside. Mine is analog  :-(

Hi Dupreezd,

You seem to be quite an expert on these weatherstations as you have helped me once before with the correct specifications for the NTD bead thermistor (see http://www.wxforum.net/index.php?topic=30689.msg328158#msg328158). So as it would have it, about the middle of December my humidity sensor stopped working on the outdoor unit. Humidity readings were only measuring in a range between about 30% and 50%. I determined (from your other posts) that it is of the HR202L type  :-( so I went ahead and ordered these from Ebay ( https://www.ebay.com.au/itm/5pcs-HR202L-Humidity-Resistance-HR202L-Humidity-Sensor-for-Arduino-with-Case/191736128218?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649. Have soldered it in but my humidity readings are still way too "centred" as in always reading in the middle ranges - Too high when it is hot and dry (which we will be having - 41C/106F for the next 3 days) and reads too low when there is moisture in the air which does not match up even closely with any of my surrounding stations - including the official government stations.

Would you have any recommendations for an appropriate HR202L sensor that may be more accurate than the ones that I purchased and installed?

Offline dupreezd

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Re: Humidity Error at cold temperatures with WS-1400?
« Reply #6 on: January 25, 2018, 05:36:30 AM »
@Pooley, I had a look at your graphs and yes, your humidity readings are off as you say.
 
Quote
Humidity readings were only measuring in a range between about 30% and 50%.
before you replaced the sensor is about the same with the new sensor.
I would say the problem is not with the sensor itself, but rather with the controller board inside the unit. Open the unit (carefully) there is a bunch of wires going from the bottom to the top half. I found it was easier to leave the short mounting pipe on the unit, clamp it in a wise then lay the top half on a box the same height as the bottom.
This will allow you to get to the controller board.
All the best.
« Last Edit: January 25, 2018, 05:38:40 AM by dupreezd »
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Offline wxmanmhd

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Re: Humidity Error at cold temperatures with WS-1400?
« Reply #7 on: February 01, 2018, 01:14:07 PM »
Just to add my 2 cents.  I am having the same problem with my Ambient WS1001. Like this morning the WS1001 is showing -1.5 with Humidity of 34%. My BloomSky2 -0.8 humidity of 64%:  Ag weather network -1 /63%  NWS KFAR 0 /65%.  This is the 2 ND winter that I have had the Ambient Ws1001. Once the temperatures get around 40 and warmer it will match a closer. Wish I would of bought the Davis Vantage Vue now.
« Last Edit: February 01, 2018, 06:12:02 PM by wxmanmhd »

Offline Jjm457

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Re: Humidity Error at cold temperatures with WS-1400?
« Reply #8 on: February 01, 2018, 02:19:15 PM »
+1 my ws-1001 died (same issue) after 15 months or so, I too opted for the Vantage Vue.

Offline dupreezd

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Re: Humidity Error at cold temperatures with WS-1400?
« Reply #9 on: February 02, 2018, 09:57:29 AM »
How important is indoor humidity to you? If you want, you can remove the humidity sensor from the indoor unit and transplant it to the outdoor unit. Place the indoor unit outside for a few days to make sure it gives the right readings before doing the swap.

I moved my indoor unit outside under the porch just so I can compare the two readings. It is usually within +- 2%. I have a 24 hour fan on my outside unit.
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Offline Rychu

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Re: Humidity Error at cold temperatures with WS-1400?
« Reply #10 on: February 09, 2018, 02:46:02 PM »
I have noticed my ~1.5 year old WS-1400 was recently reporting humidity often 20+% lower than most adjacent non-Ambient Weather PWSs and airports. (...)

The problem you write about is described together with my friend here_click

The manufacturer's instructions tell you about such conditions of the sensor - termohigrometr - read the attached link.

The SHT1x information is provided by the manufacturer "Values higher than 99% RH, which must be processed and displayed as 100% RH (13).", I.e. values higher than 99% RH must be processed and displayed as 100% RH.

  A reference (13) contains "If wetted excessively (strong condensation of water on sensor surface), 100% RH output sensor (even below 0% RH in some cases), but the sensor will completely disable the water droplets. . ", i.e. what you wrote, with strong water condensation on the surface of the sensor, the output signal may drop below 100% and even below 0% in some cases.

It is also important that it is not allowed to expose the sensor of the internal sensor to the outside. Also, do not interchange sensors between external and internal sensors - they are completely different sensors and will not work properly with the station's software.


« Last Edit: February 09, 2018, 02:55:03 PM by Rychu »

Offline dupreezd

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Re: Humidity Error at cold temperatures with WS-1400?
« Reply #11 on: February 09, 2018, 04:11:31 PM »
Quote
It is also important that it is not allowed to expose the sensor of the internal sensor to the outside. Also, do not interchange sensors between external and internal sensors - they are completely different sensors and will not work properly with the station's software.

On my unit they are exactly the same part and the internal unit can be moved outside. Have to mention that mine does not have the SHT type sensors.
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Offline quailvalleywx

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Re: Humidity Error at cold temperatures with WS-1400?
« Reply #12 on: February 09, 2018, 05:17:51 PM »
I have noticed my ~1.5 year old WS-1400 was recently reporting humidity often 20+% lower than most adjacent non-Ambient Weather PWSs and airports. (...)

The problem you write about is described together with my friend here_click


Thanks for the link but that appears to be a different failure mode with a different sensor.  I believe I have the HR202L.  I did review my station's data and found no discontinuity in humidity around rain events.  This implies no water intrusion/condensation to the HR202L surface.  Most suspect is the undefined operation as given in the datasheet below 0 deg C and Ambient Weather's assumptions in that operating range.  I've been running the following PHP code to apply temperature driven correction and am now seeing closer humidity values to adjacent stations with the original sensor.

   // - Correct Relative Humidity which drifts low at low temperatures
   $humidity = $_GET['humidity'];
   $tempf = $_GET['tempf'];
   $dewpointf = $_GET['dewptf'];
   if ( $tempf > 50.0 ) {
      $humidity_adj = $humidity + 7;
   } else {
      $correction = round( $tempf*$tempf*0.005 + $tempf*-0.707 + $tempf/54.7 + 28.8 );
      $humidity_adj = $humidity + $correction;
   }
   if ( $humidity_adj > 99 ) {
      $humidity_adj = 99;
   }
   $tempc = ($tempf - 32) * 5 / 9;
   $H = (log10($humidity_adj)-2)/0.4343 + (17.62*$tempc)/(243.12+$tempc);
   $dewpointc_adj = 243.12*$H/(17.62-$H);
   $dewpointf_adj = round((($dewpointc_adj * 9 / 5) + 32), 1);