Author Topic: relative humidity calibration?  (Read 15323 times)

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Offline ncpilot

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relative humidity calibration?
« on: September 14, 2006, 10:47:03 AM »
I never seem to go above 95% RH, which probably is a bit low given some weather conditions in the past.

I haven't read elsewhere, and don't recall anything in the Davis literature about humidity calibration. Is that possible? Is it just an offset in the console. How would you go about doing this (put the unit in a bathroom that's full of steam?).

Then, if I don't want to record the data, is it as simple as pulling out the datalink module before any calibration procedure? Then making sure to clear the console data, and plugging the module back in?
Marc
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Offline mhweather

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relative humidity calibration?
« Reply #1 on: September 14, 2006, 11:39:18 AM »
I've never seen a 100% myself and I'm on my second SIM card.  I know I've been in situations with dense fog, etc, but I think I've only seen 97% at best.  Some people do experience (I think Gary has) the 100% reading in the obvious situations, so maybe a few SIM boards out there will show 100%.  Of course there is always a +/- percentage error in these readings anyway.
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Offline capeweather

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relative humidity calibration?
« Reply #2 on: September 14, 2006, 12:44:49 PM »
I've seen 100% during peak fog conditions. Last New Years Eve the fog was so thick I couldnt see the house directly across the street from me. I know mine works ok. Maybe the steam test will work.

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Offline ncpilot

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relative humidity calibration?
« Reply #3 on: September 14, 2006, 02:00:24 PM »
I guess the reason I was concerned was because I can see the value flatline for up to  an hour at a time at 95%... what are the chances that over a period of an hour that the humidity doesn't budge? From 9:18 to 11:08 AM today (data on Wunderground) it was pegged at 95%.

That's why I'm thinking it needs to be calibrated, it just seems so unlikely to have a steady value like that over such a period of time...

In fact, I don't think I've ever seen it go above 95%, yet, I'll have dew on surfaces in the morning. Isn't that by definition when the temp/dew pt are equal? Therefore 100% RH? (all things being equal, etc, I'm sure that's more theoretical, since other factors probably have influence).

I'll have to look at my detailed data at home...
Marc
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Offline Mark / Ohio

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relative humidity calibration?
« Reply #4 on: September 14, 2006, 08:12:52 PM »
Mine hits 100% frequently.



According to CWOP error checking it averages 1% high.  I'm not running any offsets.
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Offline racenet

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relative humidity calibration?
« Reply #5 on: September 14, 2006, 10:55:05 PM »
Same here, with getting 100% readings. Have them quite often. Have them now, since it is raining out. That's about as 100% as you can get. :)




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Offline Ravenstar

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relative humidity calibration?
« Reply #6 on: September 15, 2006, 02:39:42 AM »
To be honest, I didn't think 100% RH was possible unless you were swimming in a pool of water???


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Offline ncpilot

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relative humidity calibration?
« Reply #7 on: September 15, 2006, 10:46:47 AM »
Anyone make their own sling psychrometer? I may do that to have an independent measure of RH...
Marc
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Offline ocala

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relative humidity calibration?
« Reply #8 on: September 15, 2006, 01:01:23 PM »
I went back and looked and once in the past year I hit 100%. Usually it tops out at 95%.

Offline dalecoy

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Re: relative humidity calibration?
« Reply #9 on: September 16, 2006, 12:46:14 AM »
Quote from: "ncpilot"
I never seem to go above 95% RH, which probably is a bit low given some weather conditions in the past.

I haven't read elsewhere, and don't recall anything in the Davis literature about humidity calibration. Is that possible? Is it just an offset in the console. How would you go about doing this (put the unit in a bathroom that's full of steam?).

Then, if I don't want to record the data, is it as simple as pulling out the datalink module before any calibration procedure? Then making sure to clear the console data, and plugging the module back in?


If you are using Weatherlink with your data logger, there's an easy setting.  Click on setup, and then on "Temp and Hum Calibration".  It actually changes the calibration in the console (not simply an offset, I think).

If you don't have a more accurate humidity meter, then I would simply adjust the current humidity reading up by about 5% (in your situation), and watch it for several days/weeks.  That is, if the current humidity reading is (for example) 60%, adjust it to 63% - which is 5% higher.  

If you get peak humidity readings of 99% or 100% over several weeks, leave it alone.  If you get peaks over 100%, then reduce it a bit.

Note: here in dry New Mexico, it's not unusual for the humidity to be only 90% when it has been raining modestly for an hour or two.  Rain only requires that the RH be 100% at the altitude where the rain is being generated - not on the ground.  

I don't understand your last paragraph.

Offline Ravenstar

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relative humidity calibration?
« Reply #10 on: September 17, 2006, 08:13:30 AM »
This morning I've hit my highest RH yet in about 5 months, 98%. Temp is 57.3, dew point 56.7 just over a half deg less. The fog out is so bad I can barely see the outline of my neighbors house across the street.


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Offline ncpilot

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Re: relative humidity calibration?
« Reply #11 on: September 17, 2006, 11:11:33 PM »
Quote from: "dalecoy"
Quote from: "ncpilot"
Then, if I don't want to record the data, is it as simple as pulling out the datalink module before any calibration procedure? Then making sure to clear the console data, and plugging the module back in?


I don't understand your last paragraph.


If I were to physically take my ISS unit into the bathroom, with saturated air, in order to make sure it was in a 100% RH environment, I'd have to read the RH from the console. My console is recording the data and downloading to my computer.

What must I do to ensure that the calibration data doesn't end up in my data files on the computer?

Same question if you wanted to calibrate the rain gauge...

Did that explain it better?
Marc
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Offline dalecoy

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Re: relative humidity calibration?
« Reply #12 on: September 17, 2006, 11:53:28 PM »
Quote from: "ncpilot"
Quote from: "dalecoy"
Quote from: "ncpilot"
Then, if I don't want to record the data, is it as simple as pulling out the datalink module before any calibration procedure? Then making sure to clear the console data, and plugging the module back in?


I don't understand your last paragraph.


If I were to physically take my ISS unit into the bathroom, with saturated air, in order to make sure it was in a 100% RH environment, I'd have to read the RH from the console. My console is recording the data and downloading to my computer.

What must I do to ensure that the calibration data doesn't end up in my data files on the computer?

Same question if you wanted to calibrate the rain gauge...

Did that explain it better?


Oh.  Sorry I was being dense.

As you said, you COULD disconnect, measure, and then clear the data.  

However, my preferred approach would be to not clear the data - just let WeatherLink download the data, and then go into the Browse window and edit those values that you know to be wrong, to be "right".

With either method, you will have some concern with things like highs (daily, monthly, annual), lows, etc. that have different (or no) way to change/clear them.

There are two main reasons to not use the "steamy bathroom" approach.  The above is one reason, and the other reason is that you have no evidence that your "steamy bathroom" is 100% RH.   [If you have or can borrow an accurate humidity meter, then obviously you don't need a steamy bathroom].

Also, in my opinion, it's not advisable to try to calibrate the rain gauge.  Unless you want to make a career of doing that.  YMMV.

Offline ncpilot

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relative humidity calibration?
« Reply #13 on: September 18, 2006, 10:18:14 AM »
Sometimes I'll read what I've written and it won't make sense... :)

Oh hell, I need a project, maybe I'll build a psychrometer just for fun...

Maybe I could borrow one from one of our local news weather people...? If they'd even have one...

"There are two main reasons to not use the "steamy bathroom" approach. The above is one reason, and the other reason is that you have no evidence that your "steamy bathroom" is 100% RH. [If you have or can borrow an accurate humidity meter, then obviously you don't need a steamy bathroom]. "

Does that rule out the "steamy bedroom" scenario then?  :lol:  :wink:
Marc
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Offline dalecoy

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relative humidity calibration?
« Reply #14 on: September 18, 2006, 10:55:42 AM »
Quote from: "ncpilot"
Sometimes I'll read what I've written and it won't make sense... :)


Does that rule out the "steamy bedroom" scenario then?  :lol:  :wink:


That depends on which Relative the Humidity is about.....

Offline dalecoy

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relative humidity calibration?
« Reply #15 on: September 18, 2006, 12:45:35 PM »
Quote from: "ncpilot"


Oh hell, I need a project, maybe I'll build a psychrometer just for fun...



Building a psychrometer for short-term use is not difficult.

http://school.discovery.com/lessonplans/activities/weatherstation/airwaterboth.html

Offline ncpilot

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relative humidity calibration?
« Reply #16 on: September 18, 2006, 03:37:37 PM »
Quote from: "dalecoy"
That depends on which Relative the Humidity is about.....


I don't live in one of those states where it's ok to have a relative in the bedroom!  :lol:
Marc
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Offline talbert1952

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relative humidity calibration?
« Reply #17 on: September 18, 2006, 06:42:19 PM »
My readings maxed out at 98% last winter so I dialed a +2% offset into the console.

The Davis sensor is is only rated 4% accuracy above 90% RH. If it bugs you not seeing 100% RH wait for a cool to cold day with fog and tweak the calibration. The colder it is the more likely the air will be saturated.

It is very difficult to accurately measure humidity above 90% with a psychrometer. The temperature difference between the dry bulb and wet bulb  is very small. At 70 degrees F and 95% humidity the temperature difference is around 1 degree. At 40 degrees F and 95% humidity the difference is only 0.6 degrees.
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Offline tinplate

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relative humidity calibration?
« Reply #18 on: September 19, 2006, 12:49:18 PM »
I went through an experience with my VP2 humidity. We had a spell of very foggy/misty weather, and I was a bit disappointed that my RH refused to hit 100% even though it was dripping outside. I then looked through my history and found that for the year I had the station, the RH had never gotten above 98%. I was on the verge of calibrating it up two notches when the inversion strengthened even further and it got incredibly foggy. And the VP humidity hit 100% and stayed there for about 10 hours until a front swept through.

Steve

Offline ncpilot

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relative humidity calibration?
« Reply #19 on: September 20, 2006, 01:51:47 PM »
I figured I'd just calibrate using a psychrometer at whatever the RH was at the time, but then realized that it wouldn't really help at the extreme end due to the reduced accuracy above 95%...

Thanks all for the feedback...
Marc
Wilmington, NC
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Offline dalecoy

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relative humidity calibration?
« Reply #20 on: September 20, 2006, 02:19:24 PM »
Quote from: "ncpilot"
I figured I'd just calibrate using a psychrometer at whatever the RH was at the time, but then realized that it wouldn't really help at the extreme end due to the reduced accuracy above 95%...

Thanks all for the feedback...


Well, it MIGHT help at the top end.  And it SHOULD help (or confirm the present accuracy) at the midrange.  

It's an interesting exercise.  [And particularly with the indoor humidity reading, which can be affected by illumination and other factors]

I happen to have 3 other "electronic" temp/humidity meters (Radio Shack, etc.), and an old non-electronic humidity meter.  I have in the past gone to the effort to compare the set of four meters with each other and with the VP2 (inside and outside).  The non-electronic meter was "far afield" - so I adjusted it.  The VP2 was essentially on the average of the other 3 meters (which differed about 4 or 5% between meters in the set).  

At least, it's a fun hobby.

Offline ncpilot

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relative humidity calibration?
« Reply #21 on: September 20, 2006, 04:14:01 PM »
That reminds me of when I had my console outdoors while (yes...) checking the calibration of the rain gauge...

I noted a temp difference between the 2 sensors--can't remember if there was a humidity diff...

I'll probably build a sling blade psychrometer, then I'll weed whack with it after calibration.......... :roll: (sure hope Billy Bob don't show up!!)
Marc
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Offline Andy Overton

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relative humidity calibration?
« Reply #22 on: October 10, 2006, 03:08:30 PM »
I bought a good quality Cassella whirling hygrometer so that I can keep regular calibration checks on both the air temperature readings and the humidity.  They are not so expensive when you think you'll have a good reference instrument for a lifetime.  You have to make sure this itself is right by putting both thermometers in a distilled water/ice mix to make sure they show 0C.  If not you need to allow that discrepancy in your reading of the thermometers.  You can then use this to check your air temperature readings and humidity-remember if you're thinking about constructing your own, are the thermometers high quality, do you have the psychrometric tables to work out humidity?  The VP is a good quality instrument, there is no point comparing a high precision instrument against something which is poor.

Remember also that all instruments suffer from lag and hysteresis, so doing comparisons when values are changing rapidly may give false 'errors'.  Wait until values are reasonably steady.  Also, an instrument may work well at one value and poorly at another, so you need to do a range of calibration checks over time with temperature and humidities of different values.  Allow for the stated accuracy of the sensor (if it is within this it is ok, you won't get an exact match and this isn't an error) and the stated accuracy of the thermometers (good quality ones will have this stated, with cheap ones you don't know where you are).

Once you've done a range of checks you'll be able to identify any consistent error bias and value and apply an offset.  Never do this on the basis of one or two comparisons, you need to do a comprehensive calibration assessment as described.  Have fun!