SM0486
and other pressure measuring devices.
We will also discuss
the
construction
principle applied in the design or the aneroid barometer.
a. Sighting the mercury level.
After you have completed the operational
adjustments, you should give the table which supports the barometer several raps.
Rapping the table before each reading helps you adjust the meniscus shape in the
tube and cistern.
The desired shape is shown in Figure 13.
Error incurred by
improper meniscus shape may be as large as 0.15 mm.
Figure 13.
Sighting the mercury level
b. Barometer application.
(1) When you use the barometer to calibrate aircraft altimeters, you
connect the equipment as shown in Figure 14. The altimeter operating principle has
already been discussed, but no attempt was made to establish its relation to the
altimeters.
Simply, atmospheric pressures change as distances from the earth's
center of gravity change.
The altimeter detects atmospheric changes and converts
atmospheric pressure values to altitude values.
The A-1 barometer performs the
same function, except that you can't install an A-1 barometer with its mercury in
an aircraft as easily as you can install an altimeter.
Let us return to our
equipment connection in Figure l4.
The connections should suggest the following
conclusions:
a. Because a vacuum valve and a relief valve are used, a vacuum pump is
necessary
b. Because the vacuum pump is connected to the test chamber by way of
the vacuum valve, a partial vacuum is created in the test chamber by
the pump.
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