In this short podcast episode, Bryan covers the differences between absolute and gauge pressure, as well as measuring pressure with a micron gauge or a manometer.

Compression ratio deals with absolute suction and absolute discharge pressures. Absolute pressure requires us to add atmospheric pressure to the gauge pressure. We usually measure gauge pressure in pounds per square inch (PSI). PSIG is the gauge pressure (zeroed to atmospheric pressure), and PSIA is the gauge pressure plus the atmospheric pressure (usually around 14.7 PSI).

When we measure vacuum pressure, we have "negative pressure" with respect to the atmosphere. We're not measuring less than zero pressure; we are in a positively pressurized environment, but the pressure is negative relative to the atmosphere (not absolutely). We use microns to measure deep vacuums; they are tiny pressure units equivalent to a millionth of a meter of mercury column. Since microns measure absolute pressure, they always dip below atmospheric pressure and approach 0 (but never reach it because we're not in a perfect vacuum). We don't have to zero the micron gauge because it automatically measures with respect to zero.

Manometers measure pressure differentials; they measure pressure in reference to something else, whether that's the room around you (whatever you've zeroed it to) or another point on the system (measured at the other port). In any case, we're not referencing 0 PSIA. Many manometers may pick up readings in the inches of water column scale, and some even measure very small scales like Pascals.

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