Setra System Model 470 - Manual (Page 27)

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SECTION SIX
Altitude Measurement Functions
The Setra 470 Digital Pressure Transducer can report a variety of altitude measurements. It may be used
as an altimeter calibrator, a standard altimeter, a true corrected altimeter (self corrected or remote), or as
a relative altimeter. It may also be used to indicate small relative changes in altitude through the use of
the zero/tare function.
6.1 Standard Altitude Measurement
Standard altitude is defined as the altitude from the Standard Atmosphere Curve which corre-
sponds to the sensor’s measurement of absolute pressure.
The Standard Altitude does not necessarily coincide with the actual elevation or altitude the unit is
installed at. The only time it would is when the local sea level pressure happens to be 29.90 in.
Hg. See the next section, “Reporting True Altitude” in this manual to report True Altitude.
To report Standard Altitude with the Setra 470, follow this procedure:
1. Transmit U, for CONVERT (Section 4.2), repeatedly to select either feet or meters as the
engineering units of measure.
2. Transmit -Z, for -ZERO, to clear the zero/tare register.
3. The transducer now reports altitude as a function of the Standard Atmosphere Curve in
response to an INTERROGATION.
6.2 True Corrected Altitude Measurement
The Setra 470 can function as an altimeter reporting true altitude by correcting the Standard
Atmosphere conversion above for local barometric pressure. This correction is done in one of
two ways, depending on whether you know the station elevation (the altitude at which the trans-
ducer is installed) is known.
The station elevation and tare values used in these procedures are erased when power is turned
off. To report tare altitude this procedure must be executed each time the power is turned on.
A) If you 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. Put the transducer into DASI mode (Section 4.11) so that the Setra
470 correctly reports sea level pressure.
3. Transmit Z, for ZERO. (If interrogated now, the Setra 470 would
report the difference between the current pressure and the corrected
sea level pressure.)
4. Transmit U, for CONVERT (Section 4.2), repeatedly to select either feet or
meters as the engineering units of measure.
5. The transducer now functions as a corrected altimeter, and reports true