How is absolute pressure calculated?

Absolute pressure is calculated by adding atmospheric pressure to gauge pressure. Gauge pressure measures the pressure relative to the surrounding atmospheric pressure, which means it does not include the atmospheric pressure itself. To find absolute pressure, you need to account for the atmospheric pressure that is typically about 14.7 psi at sea level.

Therefore, the correct approach is to take the gauge pressure and add the atmospheric pressure value (approximately 14.7 psi). In this case, the choice specifying gauge pressure plus 15 likely approximates atmospheric pressure in an informal sense; while the exact value should be 14.7 psi, the rounding to 15 makes it more practical for quick calculations in some contexts.

Understanding how absolute pressure is determined is essential for applications in refrigeration and other scientific fields where accurate pressure measurements are crucial.