In calculating compression ratio, how is PSIG converted to PSIA?

To understand the conversion from PSIG (pounds per square inch gauge) to PSIA (pounds per square inch absolute), it's important to consider the definitions of these pressure measurements. PSIG measures pressure relative to atmospheric pressure, meaning it does not account for atmospheric pressure itself. In contrast, PSIA represents pressure measured relative to a perfect vacuum.

Atmospheric pressure at sea level is approximately 14.7 psi, which is a common approximation used for these calculations. Therefore, to convert PSIG to PSIA, you must add atmospheric pressure to the gauge pressure. The typical conversion factor used is about 14.7 psi, but 14.66 psi can also be utilized as a rounded approximation for simplicity in various contexts.

Thus, the correct approach to convert PSIG to PSIA is to add 14.66 psi, indicating the total absolute pressure including atmospheric pressure. This understanding is crucial in compression ratio calculations where absolute pressure values are needed for accurate assessments.