Absolute pressure is measured from what reference?

• A. Zero pressure or vacuum
• B. Sea level pressure
• C. Atmospheric pressure at altitude
• D. Inside the cabin pressure

Answer: (A)

Absolute pressure uses a complete vacuum as the reference point. In other words, it is the total pressure exerted on a surface relative to zero pressure (a perfect vacuum). This means any surrounding atmospheric pressure is already included in the reading, so the value represents all pressures present, not just what lies above ambient.

This is different from gauge pressure, which is measured relative to the surrounding air, so it would read zero at ambient pressure. For example, a tire’s gauge reading tells how much pressure is above atmospheric, and to get the absolute pressure you would add the current ambient atmospheric pressure to that gauge reading. Atmospheric pressure at sea level or at altitude, or pressure inside the cabin, are not the reference for absolute pressure; they are characteristics of the environment or a separate measurement, not the baseline for the absolute value.