psi vs psig vs psia: Gauge and Absolute Pressure Explained
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psi, psig, psia — what is the difference? This one-minute video shows that every pressure reading has a reference: absolute pressure (psia) starts at a perfect vacuum, gauge pressure (psig) starts at the surrounding air, and at sea level the atmosphere adds about 14.7 psi — so 34.7 psia is 20 psig.
Chapters
Key points
- Absolute zero is a perfect vacuum; at sea level the atmosphere adds about 14.7 psi.
- psia is measured from vacuum; psig is measured from the air around the sensor.
- The same process reads 34.7 psia or 20 psig; below atmosphere, −10 psig is 4.7 psia.
- Use gauge for most tanks and hydraulic systems, absolute for vacuum or sealed processes.
- 1 bar = 14.5 psi.
Transcript
psi, psig, psia: what is the difference?
Pressure is always measured from a reference. Absolute zero is a perfect vacuum. At sea level, the atmosphere adds about 14.7 psi.
Absolute pressure, psia, is measured from the vacuum. This process sits at 34.7 psia.
Gauge pressure, psig, is measured from the air around the sensor. The same process reads 20 psig.
Below atmosphere, gauge pressure turns negative. −10 psig is 4.7 psia.
Use gauge for most tanks and hydraulic systems, and absolute for vacuum or sealed processes. 1 bar = 14.5 psi.
SEGMENsensor pressure transmitters: gauge, absolute and differential. Try our free unit converter at segmensensor.com.