When any pressure is detected above the absolute zero of pressure, it is labelled as absolute pressure. It is measured using a barometer, and it is equal to measuring pressure plus the atmospheric pressure.
Diagram showing absolute pressure, vacuum and gauge
Absolute pressure formula (pabs) is given by,
where pgauge is the gauge pressure
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.
Let’s see some examples of absolute pressure:
Problem 1: A pressure gauge measures the pgauge reading as 31 psi. If the atmospheric pressure is 14.2 psi. Compute the absolute pressure that corresponds to pgauge reading.
patm (Atmospheric pressure) = 14.2 psi
pgauge (Gauge pressure) = 31 psi
Absolute pressure (pabs) = patm + pgauge
= 14.2 psi + 31 psi
pgauge= 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.
Atmospheric pressure patm = 15 psi
Absolute pressure pabs = 35.8 psi
The Gauge pressure is
pgauge = 35.8 psi – 15 psi
gauge = 20.8 psi.