[wxsig] T-238 Humidity Calibration

[Ken.A.Brown at noaa.gov]

Will,

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
>