SM0486
Figure 21.
Manometer basic test connections
(6) It is not necessary to make adjustments prior to each reading, but they
should be made before each set of readings using the same scale.
If
absolute pressure readings are being made, the scan setting should be
checked against the actual barometric pressure hourly, or more often
during periods of rapidly changing weather conditions.
When checking
or calibrating altimeters, use the scale marked "Using Mercury, Read
Altitude in Thousands of Feet."
Position the scale so that the
millimeter graduation (on edge of scale) equal to the barometric
pressure at the test location is aligned with the fill mark on the
mercury-fill plate adjacent to the scale. Altitude must be related to
absolute pressure rather than gage pressure.
For the standard
atmosphere, the standard pressure at sea level (zero altitude) is 760
millimeters of mercury. Positioning the scale at the local barometric
pressure (in millimeters of mercury) automatically sets the altitude
scale at the correct pressure altitude for the test location.
It is
not necessarily the absolute altitude because the barometric pressure
at sea level, at the time of test, may not be 760 millimeters of
mercury.
When the altimeter (an instrument connected for a test) and
manometer are properly connected for a test, as shown in Figure 21,
application of vacuum (or pressure) simulates pressure altitude from
the test location.
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