Setra System Model 470 - Manual (Page 29)

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6.3 Relative Altitude Measurement
The Setra 470 can measure relative altitude in either Standard or True Altimeter Mode. In this
mode, the 470 reports altitude relative to a reference altitude which you establish by setting the
reported altitude reading to zero, or by offsetting it to a higher or lower value by a specified
amount.
A) Follow this procedure to report relative altitude referenced to a zeroed starting point.
1. Put the transducer into an altitude reporting mode. Use either Standard or
True Altitude (Section 6.1 or Section 6.2).
2. Transmit Z, for ZERO.
3. The transducer saves the current reading as a tare value, and subtracts the
tare value from every subsequent reading before reporting it.
B) Follow this procedure to report relative altitude referenced to an offset starting point.
1. Transmit the amount of the offset (in the engineering units currently in use),
including the sign.
2. Transmit Z, for ZERO.
3. The transducer saves the offset as a tare value and subtracts that tare value
from every subsequent reading before reporting it.
CAUTION: In any 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 drifts with time as the
sea level corrected pressure drifts. To compensate for this effect, repeat the
above procedures often.