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When any pressure is detected above the absolute zero of pressure, it is labeled as absolute pressure. It is measured using barometer, and it is equal to measuring pressure plus the atmospheric pressure.
Absolute Pressure

Diagram showing absolute pressure, vacuum and gauge

Absolute pressure formula (pabs) is given by,


where pgauge is gauge pressure and patm is atmospheric pressure.

The vacuum pressure is articulated as,

Vacuum Pressure=Atmospheric Pressure-Absolute Pressure

At sea level it is around 14.7 pounds per square inch.

Solved Examples

Let’s see some examples of absolute pressure:

Problem 1: A pressure gauge measures the pgauge reading as 31 psi. If the atmosphere pressure is 14.2 psi. Compute the absolute pressure that corresponds to pgauge reading.

Given: patm (Atmospheric pressure) = 31 psi
pgauge (Gauge pressure) = 14.2 psi
Absolute pressure (pabs) =  patm + pgauge
= 31 psi + 14.2 psi
= 45.2 psi
Problem 2: The psia pressure instrument gives the reading as 35.8 psi. If the atmospheric pressure is 15 psi, calculate the corresponding guage pressure.

Given: Atmospheric pressure patm = 15 psi
Absolute pressure pabs       = 35.8 psi

The Gauge pressure is
pgauge = 35.8 psi – 15 psi
= 20.8 psi.