What pressure scale is necessary to calculate a compression ratio?

To calculate a compression ratio, the absolute pressures of the gas are required. The compression ratio is defined as the ratio of the volume of gas before compression to the volume of gas after compression. In practical terms, this also means measuring the absolute pressures at these two states.

Using PSIA (pounds per square inch absolute) is ideal because this scale measures pressure relative to a perfect vacuum, providing an accurate portrayal of the true pressure of the gas in the system. Since the compression ratio involves comparing these true pressure values before and after compression, using absolute pressure ensures that the calculations reflect the actual conditions of the gas, eliminating any variations that might be caused by atmospheric pressure.

Considering other scales, PSIG (gauge pressure) measures pressure relative to atmospheric pressure, which can lead to inaccuracies in estimating the true compression occurring in the system. PSIV (velocity pressure) and simply stated absolute pressure do not provide the specific context needed for determining the compression ratio in this scenario, as they either lack the correct reference basis or do not directly relate to the necessary pressure context used in calculating compression ratios.