What pressure scale must be used to calculate Compression Ratio?

The compression ratio is defined as the ratio of the volume of the gas in a cylinder when the piston is at the bottom of its stroke (the cylinder's total volume) to the volume of the gas when the piston is at the top of its stroke (the cylinder's clearance volume). To calculate this ratio accurately, it is essential to use the absolute pressure scale.

Using PSIA (pounds per square inch absolute) is the correct choice because absolute pressure includes atmospheric pressure along with the gauge pressure. This is crucial as the compression ratio is a measurement relative to the absolute vacuum state of the gas. When you express pressures in PSIA, you measure the total pressure on the gas including the atmospheric pressure. Thus, using PSIA provides an accurate basis for calculating the actual compression ratio.

On the other hand, using PSIG (pounds per square inch gauge) would not be appropriate as it measures pressure above atmospheric pressure and does not consider the atmospheric component, potentially leading to an inaccurate calculation of the compression ratio. Similarly, while BARa also refers to gauge pressure, it faces the same limitations. Hence, using PSIA ensures the calculation accounts for all relevant pressures in determining the compression ratio accurately.