What is absolute pressure measured from?

Absolute pressure is defined as the pressure measured from a perfect vacuum. This means that absolute pressure provides a reference point that is free from the influence of atmospheric pressure. In a perfect vacuum, there is no matter present, so the pressure is effectively zero. When measuring absolute pressure, one considers the total pressure exerted by a gas or liquid above this zero-point reference.

In contrast, gauge pressure measures pressure relative to atmospheric pressure, which means it does not account for the atmospheric pressure at the measurement point. Hence, gauge pressure is always equal to absolute pressure minus atmospheric pressure. Other terms like standard atmospheric pressure and relative pressure also relate to the context of measuring pressure but do not serve as a baseline for absolute pressure as does a perfect vacuum. Absolute pressure is crucial in many scientific and engineering applications because it provides a consistent reference that ensures accurate calculations and assessments across varying atmospheric conditions.