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[wxsig] A barometer for the T238+

Will Beals will at beals5.com
Mon Mar 13 05:24:04 UTC 2006


George:

Thanks for noting this for other folk!  

will 

-----Original Message-----
From: wxsig-bounces at lists.tapr.org [mailto:wxsig-bounces at lists.tapr.org] On
Behalf Of George
Sent: Sunday, March 12, 2006 7:51 AM
To: wxsig
Subject: [wxsig] A barometer for the T238+

Folks,
Finally, I got a barometer working with the T238+ It is a modified B1-R1-K
from Hobby-Boards.
see
http://www.hobby-boards.com/catalog/product_info.php?cPath=25_27&products_id
=37
It needs two mods.

1. Adjust potentiometer R4 for Vref=3.789V (Sensor's Vout at 931mBar @5.1V
P.S.) Maybe need readjustment due to slight differences in voltages. Mine
adjusted at 3.73V.
2. Solder in parallel of R1 (40.2k) - a resistor that sets the gain of the
INA122 instrumentation amplifier - a SMT resistor of 39k. This sets the gain
to 5+(200k/[40.2k//39k]) = 5+200/19.7955 = 15,10333 That's all

ANY DS2438 based Barometer can work with the T238+ if it is calibrated for
Intercept point = 931mBars and Slope = 14.42mBars per volt.
This means that Vref must be equal to sensor's Vout at 931mBar and the gain
is calculated as follows:
Sensor has an output of 4.59mV/mBar and we need 14.42mBar/V=0.01442mBar/mV
or 1/0.01442mV/mBar = 69.34812mV/mBar Total gain = 69.34812/4.59 = 15.10852

If you know the current atm. pressure you can fine adjust the Vref in order
to read the same value. I have a Garmin GPSMap76S that has an internal
barometric sensor and adjusted mine according to GPSMap 76S' s reading.

73 de George, SV2DOF


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