Page 17 - ebook fluid mechanics_finalize
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FLUID MECHANICS
1.2.2 Gauge pressure, PG
The pressure of a system measured by a gauge, above or below atmospheric
pressure. It is the pressure, measured with the help of a pressure measuring
instrument, in which the atmospheric pressure is taken as datum; in other words the
atmospheric pressure at the gauge scale is marked zero.
1.2.3 Absolute pressure, PAbs
It is the pressure equal to the algebraic sum of the atmospheric and gauge pressures.
Absolute pressure = Gauge pressure + Atmospheri c pressure
p = p + p
G
Abs
atm
1.2.4 Vacuum pressure, Po
In a perfect vacuum which is a completely empty space, the pressure is zero. There is
no pressure in a vacuum, a vacuum is the absence of anything, there is nothing there
to push. Air from a pressurized space that becomes open to a vacuum may push you
into the vacuum, but the vacuum itself does nothing as it is literally nothing.
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