Setra System Model 470 - Manual (Page 28)

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altitude in response to an interrogation as it is moved from one location to
another.
CAUTION: In any true altimetry application, the accuracy of the measurement
depends on the quality of the correction made for local barometric pressure.
Barometric pressure changes often, and so the reading drifts over time as the
sea level corrected pressure drifts. To recalibrate the true altimeter, repeat the
above procedure at the known starting elevation often.
6. To exit the true altitude mode and report true barometric pressure, transmit -
U -Z -C, for -CONVERT -ZERO -CLEAR.
B) If you don’t know the station elevation, use this procedure to report true altitude.
1. Use the CONVERT function (see Section 4.2) so that the 470 reports data in
units of pressure in response to an interrogation. (i.e., the transducer should not
be reporting in units of altitude.)
2. Transmit B, for SEA LEVEL so that data is NOT reported in SEA LEVEL
mode in response to an interrogation. If the response to an interrogation
includes the symbol “SEA LEVEL”, transmit B, again to turn off the DASI mode.
3. Obtain an ASI (Altimeter Setting Indicator, or sea level corrected pressure)
from the local airport or weather station and convert it to the engineering units of
pressure currently in use by the transducer.
4. Transmit the converted ASI.
5. Transmit Z, for ZERO. (If interrogated now, the Setra 470 would report the
difference between the current pressure and the ASI.)
6. Transmit U, for CONVERT (Section 4.2), repeatedly to select either feet or
meters as the engineering units of measure.
7. The transducer now functions as a corrected altimeter, and reports true
altitude in response to an interrogation as it is moved from one location to
another.
CAUTION: In any true altimetry application, the accuracy of the measurement
depends on the quality of the correction made for local barometric pressure.
Distance from the reporting station and elapsed time from the measurement
adversely affects the accuracy of the altitude measurement as barometric
pressure changes. Repeat the above procedure as often as new ASI data is
available.
8. To exit the true altitude mode and report true barometric pressure, transmit -
U -Z -C, for -CONVERT -ZER0 -CLEAR.