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Aircraft
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Manifold Pressure Gauge
Description of Aircraft Manifold Pressure Gauge
Your MP gauge is a absolute vacuum pressure gauge that will read the
barometric pressure at altitude when the engine is turned off. Your altimeter setting is the barometric pressure of the station if the station
were at sea level. They are not the same as your altimeter setting is barometric
pressure with an altitude correction applied.
Absolute pressure is measurement relative to a perfect or zero vacuum, as in
outer space. A zero reading occurs when the pressure is reduced to near perfect
vacuum conditions. Therefore, when no vacuum is applied, the measured pressure
is the ambient atmospheric pressure, which at sea level is approximately 29.92
inches of mercury, or 14.7 pounds per square inch, or
760 mm of mercury. As you go up the ambient atmospheric pressure decreases and
your Absolute pressure gauge (MP) reading decreases.
If you're trying to draw a weather map of air pressure patterns, you need a way
to remove the effects of the station's elevation. That is, you want to see what
the pressure would be at the station if it were at sea level. Station pressure
shall be determined by adjusting the corrected barometric pressure to compensate
for the difference between the height of the
barometer and the designated station elevation. Your altimeter setting is not
the barometric pressure at that altitude but the barometric pressure if the
station were at sea level.
an excellent link on this subject is at
http://205.156.54.206/oso/oso1/oso12/fmh1/fmh1ch11.htm
 P39
Airacobra WWII Recognition Silhouette |
