Download Absolute Versus Gauge Pressure Units

Product Note, PN 438
Absolute Versus Gauge Pressure Units
March 17, 2017
Many are often confused by the difference between gauge pressure units of measurement
compared to absolute pressure units. The difference between them is very simple. Gauge units
are referenced from one atmosphere whereas absolute are referenced from perfect vacuum. One
atmosphere is 14.7 psia (1.013 bar) which is 0 in gauge units. Absolute units increase from 0 at
perfect vacuum whereas gauge units increase and decrease from the one atmosphere (0 psig)
reference. This is the source of the confusion because some measure using absolute reference
whereas others use gauge units. It gets even worse because industry convention (or laziness)
omits the minus sign from gauge units below 0 psig. Further, many simply state ‘psi’ for pounds
per square inch without adding a ‘g’ for gauge units or ‘a’ for absolute leaving one to guess as to
the 14.7 psi difference between the two. It is correct to simply state ‘psi’ or when referring to
pressure in a relative sense but incorrect if it is a measurement (more precise to include a or g).
Please note that atmospheric pressure is a standard atmosphere (atm), at mean sea level with
standard conditions.
The chart below is divided between absolute units on the left and gauge units on the right. One
can graphically see the difference – absolute increasing from absolute zero whereas gauge units
increase above one atmosphere, 0 psig, and decrease below. A typical convention in many parts
of the world is to use PSIG for above one atmosphere and inches of Hg for below.
Torr is roughly equivalent to mm Hg absolute, so most consider 100 Torr = 100 mm Hg . Many
incorrectly convert 100 mm Hg absolute to 4 in Hg gauge thinking it is -4 in Hg rather than -26
in Hg as it would show on the gauge.
687 TECHNOLOGY WAY, NAPA, CA 94558 ∙ PHONE (707) 259-0102 ∙ FAX (707) 259-0117
Absolute units
Gauge units
Referenced to absolute zero
Referenced to atmospheric pressure
MPa gauge
MPa
Bar
Bar
PSIG
PSIA
Atmospheric
pressure
Absolute zero
pressure
Perfect vacuum
0.1
0
1
0
14.7
0
Torr
mmHg
mbar
760
1013
0
0
in Hg
in W.C.
Vac
-29.9*
-408*
0
0
0
-1*
-0.1*
*Note: Gauge units shown as minus (-) values below atmospheric pressure but the minus
sign is omitted as a common practice
Units are roughly equivalent or rounded off for sake of clarity