PSI—this term is short for “pound-force per square inch,” typically referring to gas or liquid. Although it’s a basic label, there are infinite ways to complicate it. For the purposes of this blog post, we’ll stick with the basics and continue to refer to the example of air in a tire. When you pump air into a tire, the molecules bounce around exerting measurable pressure against the inside of it.
PSIA—this designation applies to PSI Absolute. It refers to pressure in a perfect vacuum. In a vacuum, if the tire in our example were completely empty of air, 0 PSIA would be the measurement.
PSIG—this is the term used for PSI in relation to atmospheric pressure. PSIG is also known as Gauge Pressure. The ambient pressure at sea level is about 14.7 PSIA, but ambient PSIG is always 0. Taking that into account, our completely empty tire’s PSIG reading would be -14.7. That’s because the gauge measures the pressure inside the tire compared to the atmospheric pressure outside of it.
Psig = Psia - 14.7