PSI vs PSIG vs PSIA

PSI vs PSIG vs PSIA: PSI is the raw unit. PSIG is pounds per square inch gauge, measured relative to local atmospheric pressure. PSIA is pounds per square inch absolute, measured relative to a perfect vacuum. At sea level, PSIA equals PSIG plus 14.7.

What it is

Pressure has to be referenced to something. Gauge pressure is what mechanical pressure gauges read by default; they sit at zero in open air. Absolute pressure is what thermodynamics requires, because the ideal-gas law and compression-ratio calculations only work when zero means “no molecules pushing back.” Vacuum systems quote PSIA because gauge values go negative.

Why it matters

A 100 PSIG rating is roughly 114.7 PSIA at sea level. The 14.7 PSI offset is the reason a compression-ratio calculation uses absolute units. A two-stage compressor with first-stage discharge of 50 PSIG and second-stage discharge of 175 PSIG operates at a stage ratio of (50+14.7)/14.7 = 4.4 then (175+14.7)/(50+14.7) = 2.9, not 50/14.7 and 175/50. Getting this wrong distorts every interstage temperature estimate.

How it is used in our catalog

All discharge pressures in our compressor listings are PSIG. CAGI data sheets quote both. When buyers ask “is 100 PSI enough?”, the answer is the same regardless of suffix, but anyone designing receivers, regulators, or interstage cooling needs to be precise. Browse the full compressor catalog by rated PSIG.

Common questions

Why does it matter at sea level if the offset is only 14.7?

For day-to-day plant operation it does not. For compression-ratio math, temperature rise, and dew point calculations, the offset is half the equation. Skipping it gives wrong answers, especially in vacuum and low-pressure work.

What about elevation?

Atmospheric pressure drops about 0.5 psi per 1,000 ft. A 100 PSIG compressor in Denver delivers air at a lower absolute pressure than the same gauge reading at sea level, which is one reason altitude affects effective capacity.