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[wxsig] T-238 Humidity Calibration

Will Beals will at beals5.com
Tue Sep 6 03:05:52 UTC 2005


One thing I can do is add some diagnostic data to occasional APRS packets.
I'm doing this now with the gust data to help isolate if I have a sensor
issue or a firmware issue.  Once that trial runs it's course, I can add a BP
diagnostic string next.  I like it as the data gets appended to an APRS
string that you can then go look at a few days worth of data on findu and
see what happened.

I have the raw humidity data displayed right now on the LCD diagnostic
screen starting with 1.15.? (forgot).  It shows the raw sensor data.  The
LCD is handier for issues that are either easily repeatable or happen for a
while (like saturated humidity sensors).

How confident are you the offset is a constant over both humidity and
temperature?  My concern with simply adding an offset is that it makes the
100% case look good, but in fact may make less obvious readings (ones harder
to verify) even more innacurate.


-----Original Message-----
From: Ken.A.Brown at noaa.gov [mailto:Ken.A.Brown at noaa.gov] 
Sent: Friday, September 02, 2005 6:14 AM
To: will at beals5.com; TAPR Weather Station SIG Mailing List
Subject: Re: RE: [wxsig] T-238 Humidity Calibration


I understand the problem with the BP sensor. You need to use a very accurate
and high resolution volt meter to read the voltages directly on the board
and even then it takes a lot of fooling around to get the span and offset
correct.  I had to calibrate mine several times. Each time I would determine
how many millibars in error it was, convert that error to meters of
elevation using the Smithsonian met tables, and compute new voltage values
for the settings.  I'm still a bit off but it is real close and tracks well.
I'm still troubled with out lying spikes in the indicated pressure that are
many millibars high, somewhat like the wind gust problem you described.
Most days I get none but other days there may be one or a few, hours apart.
I have never been sitting in front of the
T238 when it happens and only see it in the APRS output packets.  I'm sure
it isn't corrupted packets because I see it on my local APRS station and
also in the QC data on the web.

For humidity my sensors indicate saturation when the RH is only around 95%
and they seem to track about 5% high all the time.  I thought maybe scaling
the voltage from the sensor to the one-wire device might work but haven' 
tried it yet.

Ken Brown
----- Original Message -----
From: Will Beals <will at beals5.com>
Date: Friday, September 2, 2005 3:29 am
Subject: RE: [wxsig] T-238 Humidity Calibration

> Ken:
> I have been avoiding tackling this issue for BP sensors.  It isn't 
> justbeing lazy.  My concern is that it is already really hard to 
> calibrate the BP sensor with the two pots that interact with eachother 
> a lot.
> Adding two
> more calibration points makes the interactions eight times worse!
> If there was a thread on the humidity sensor, I may have misfiled it 
> under the BP issue, sorry.
> I am open to such an adjustment, but how would you calibrate it?  I 
> rememberwhen I first started fiddling with the humidity sensor and was 
> thinking of calibrating it, I had no practical way of doing so.  100% 
> wasn't too hard, but any other target value seemed much less accurate 
> than the sensors specs.
> Russ had given me the formula of salt to water such that at 
> equilibrium the air in a sealed container would be at different 
> humidity values, but there was no way I could be more accurate than 
> the sensors 5%.
> will
> -----Original Message-----
> From: wxsig-bounces at lists.tapr.org [mailto:wxsig- 
> bounces at lists.tapr.org] On Behalf Of Ken Brown
> Sent: Thursday, September 01, 2005 1:41 PM
> To: TAPR Weather Station SIG Mailing List
> Subject: [wxsig] T-238 Humidity Calibration
> Several weeks ago someone mentioned trying to put a pot on the 
> humiditysensor because they were reporting several percent too high.  
> I have the same problem and wondered if there was any news of success 
> in calibration of the sensor.
> Ken, APRS weather station N5KUK

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