# 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.

**Frequently Asked Questions**

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