[wxsig] A barometer for the T238+

[George]

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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