PRESSURE
To accurately size control valves, we must fully understand the various Pressure terms used in the instrument
world. The pressure measurement identifications most frequently encountered in valve applications are: absolute
pressure, Gauge Pressure, vacuum, and differential pressure.
DEFINITIONS:
a) ABSOLUTE PRESSURE -expressed "pounds per square inch absolute," or psia.
b) GAUGE PRESSURE -expressed "pounds per square inch gauge," or psig.
c) VACUUM -is a special case of gauge pressure; i.e., vacuum is negative gauge pressure or any pressure
less than atmospheric pressure.
d) DIFFERENTIAL PRESSURE-is the difference between two pressure points in a system and is
expressed as ∆P .
Here are some basic relationships between gauge pressure, absolute pressure and vacuum.
a) absolute pressure = atmospheric pressure + gauge pressure.
b) absolute pressure = atmospheric pressure - vacuum.
c) gauge pressure= -vacuum.
EXAMPLES:
1. Convert 100 psig to absolute
a) at sea level 7 . 114 7 . 14 100 = + = abs P
b) at 4,000 feet 7 . 112 7 . 12 100 = + = abs P
2. Convert 20.36 in Hg Vacuum to psia
a) P in Hg
gauge =
−
= = 2 036
20 36
2 036 10 .
.
.
psig
Pgauge = -vacuum = -10 psig vacuum
b) Pabs = Patmos – vacuum
Pabs = 14.7 – 10 psig = 4.7 psia at sea level
Pabs = 12.7 – 10 psig = 2.7 psia at 4,000 feet
3. Convert 100 psia to psig
a) Pabs = Patmos + Pgauge
b) Pgauge = Pabs – Patmos
Pgauge = 100 psi – 14.7 psia = 85.3 psig at sea level
