absolute Pressure and Gauge Pressure
If the pressure inside a car tire is equal to atmospheric pressure, the tire is flat.
The pressure has to be greater than atmospheric to support the car, so the significant
quantity is the difference between the inside and outside pressures. When
we say that the pressure in a car tire is “32 pounds” (actually 32 lb/in.2
, equal to
220 kPa or 2.2 * 105
Pa), we mean that it is greater than atmospheric pressure
(14.7 lb/in.2
or 1.01 * 105
Pa) by this amount. The total pressure in the tire is
then 47 lb/in.2
or 320 kPa. The excess pressure above atmospheric pressure is
usually called gauge pressure, and the total pressure is called absolute pressure.
Engineers use the abbreviations psig and psia for “pounds per square inch gauge”
and “pounds per square inch absolute,” respectively. If the pressure is less than
atmospheric, as in a partial vacuum, the gauge pressure is negative.
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