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On the ground, set your altimeter for a reading of zero feet and read
the pressure from the window. Turn on power to your instrumentation and
compare their readings to the altimeter pressure setting. You may be
very surprised at the results!
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-might be difficult for the folks not near sea level.
Another option is to get a calibrated test gauge and compare MP readings
across the full operating range.  I have sent brand new MP instruments
back to the factory after checking their calibration.

Chris Zavatson
N91CZ
360std
www.N91CZ.com