Setra System Model 370 - Manual (Page 28)

5.3 MEASURING RELATIVE ALTITUDE
To measure relative altitude in either Standard or True Altimeter Mode, a
reference altitude may be established by setting the displayed reading to
zero, or by offsetting it by a specified amount entered as a tare value.
1. Use one of the above procedures to display either Standard or True
Altitude.
2. To set the displayed reading to zero, press ZERO.
3. To subtract an offset from the display, enter the amount of the offset and
press ZERO.
4. To add an offset to the display, press "-", then enter the amount of the
offset and press ZERO.
5. The gauge is now displaying altitude relative to the reference point, and will
indicate changes in altitude as it is moved from one location to another.
CAUTION: In any true altimetry application, the accuracy of the measurement
is dependent on the quality of the correction made for local barometric pressure.
Barometric pressure changes often, and so the reading will drift with time as
the sea level corrected pressure drifts. To compensate for this effect, repeat
the above procedures often.
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