ADM-880C 07/20/09
39
7.1.4 PITOT TUBE "TOTAL PRESSURES"
Total pressure measurements may be obtained using the pitot tube and the differential pressure mode by connecting the
positive (+) port on the meter to the total pressure connection (in line with the main shaft) of the pitot tube and leaving the
negative port of the meter exposed to the ambient pressure. Insert the pitot tube into the airstream as discussed under
Section 7.1.2 PITOT TUBE VELOCITY PRESSURES above. The resulting differential pressure reading represents total
pressure.
7.2 ABSOLUTE PRESSURE
The absolute pressure function is intended mainly to provide automatic air density correction for the velocity and flow
measurements. Absolute barometric pressure measurements are obtained when the negative (-) port is open to the
atmosphere and the meter is in the absolute pressure mode. The measurement range is 10-40 in Hg.
Specific absolute pressure source measurements may be obtained by connecting the pressure source directly to both the
positive (+) and the negative (-) ports (
in
common
) of the meter. Press the MODE key until [ABS PRES] is displayed. The
meter will then display [Hg READ] or [Bar READ]. Press the READ key to take a reading.
CAUTION
If an absolute pressure source
is likely to be greater than 60 in Hg (30 psia
), the pressure source
must
be connected in common to both the positive (+) and negative (-) meter ports
simultaneously
. This
precaution avoids excessive differential pressure input which will damage the pressure transducer.
Maximum safe pressure is 60 psia common mode and 20 psi differential pressure.
The absolute pressure measuring accuracy should be checked periodically. Readings taken with the negative port (-) open
to the atmosphere should be within ± 2% of the actual barometric pressure to assure rated accuracy of density corrected
flow and velocity readings.
NOTE: Weather service or airport reports of barometric pressure have usually been adjusted for altitude, so the pressure
can be used for altimeter adjustment. The barometric pressure announced on television or radio stations is generally
obtained from weather service reports. This altitude corrected barometric pressure must
not
be used in density correction
equations for comparison with a FlowHood. An estimation of the actual barometric pressure may be obtained by deducting
approximately 1.0 in Hg for each 1000 feet above sea level.
Summary of Contents for AIRDATA ADM-880C
Page 42: ...ADM 880C 07 20 09 37 FIGURE 6 3 VELGRID ASSEMBLY...
Page 50: ...ADM 880C 07 20 09 45 FIGURE 10 1 FRAME STORAGE FIGURE 10 2 FLOWHOOD IN CASE...
Page 51: ...ADM 880C 07 20 09 46 FIGURE 10 3 FLOWHOOD ASSEMBLY...
Page 53: ...ADM 880C 07 20 09 48 FIGURE 10 7 1X5 FRAME ASSEMBLY FIGURE 10 8 3X3 FRAME ASSEMBLY...
