There are several types of pressure measurements. Absolute pressure is measured relative to a full vacuum and uses units like PSIA. Atmospheric pressure refers to pressure at sea level, about 14.7 PSIA. Barometric pressure measures atmospheric conditions using units like millibars. Compound pressure measures from full vacuum through atmospheric pressure to positive gauge pressure in PSIC. Differential pressure measures the difference between two pressures in PSID. Gauge pressure is measured relative to ambient atmospheric pressure in PSIG. Vacuum pressure refers to pressures below atmospheric measured in PSIV or inches of mercury.

1. TYPES OF
PRESSURE MEASUREMENTS

2. The relationships between gauge and absolute pressure are critical 2
Pressure Line
0 PSIA 14.7 PSIA
100 PSIA 114.7 PSIA
Vacuum Range
14.7 PSIV
-1 ATM
0 “ Hg
0 mB/hPa
0 Torr A
0 PSIV
ATM
29.92 "Hg
1013 mB / hPa
760 Torr A
Atmospheric Pressure
-14.7 PSIG 0 PSIG
85.3 PSIG 100 PSIG
85.3 PSIC

3. Absolute Pressure
Definition:
Pressure measured relative to full vacuum
(refers to a true zero reference)
Units of Measure:
Pounds per square inch absolute or (PSIA)
Applications:
best used in applications that require
higher accuracies over large temperature
range or for comparative studies at
different elevations

4. Atmospheric Pressure
Definition:
Pressure of the atmosphere at the earth’s surface.
At sea level this pressure is not constant (it varies
with the weather) but averages around 100
kilopascals (kP).
One atmosphere is also approximately the ambient
pressure at sea level and is equal to 14.7 PSIA
Units of Measure:
pounds per square inch (psi), atmospheres (atm),
mb, hPa can measure ambient pressure in other
circumstances
Applications:
remote weather stations, test stand reference
pressure

5. Barometric Pressure
Definition:
The measure of atmospheric
pressure
Units of Measure:
Often measured in millibars, in Hg
(inches of mercury), or hector
pascals
In other circumstances ambient
pressure may be measured in
pounds per square inch (PSIA)
Applications:
airports, AWOS systems, humidity
measurements, data buoys

6. Compound Pressure
Definition:
Pressure measured from full vacuum (-
14.7 PSIV) through atmospheric pressure
to a positive gauge pressure, being
referenced to current atmospheric
pressure
Units of Measure:
pounds per square inch compound (PSIC)
Applications:
autoclaves, instrument sterilization,
pumps

7. Differential Pressure
Definition:
Pressure measured relative from one
pressure to another
Units of Measure:
pounds per square inch differential
or (PSID)
Applications:
filter status, static pressure, chillers,
sealed tank

8. Gauge Pressure
Definition:
A form of atmospheric pressure
measured relative to ambient
atmospheric pressure
Units of Measure:
pounds per square inch (PSIG)
Applications:
measuring pressure in vented tank
(link to app note), pumps,
compressors, test stands

9. Vacuum Pressure
Definition:
Generally refers to pressures below
atmospheric pressure
Units of Measure:
pounds per square inch vacuum (PSIV) or 0-
30”Hg
Applications:
semiconductor fabs, plasma sterilizers,
vacuum packaging