Absolute pressure is measured relative to which reference?

• A. Relative to ambient atmospheric pressure
• B. Relative to sea level
• C. Vacuum
• D. Relative to atmospheric pressure

Answer: (C)

Absolute pressure uses a zero reference of a perfect vacuum. That means the measured value represents how much pressure exists above empty space, so the pressure cannot be negative and is always above vacuum. In practice, you convert gauge pressure to absolute pressure by adding the atmospheric pressure: P_abs = P_gauge + P_atm. For example, with an atmospheric pressure around 101 kPa, a gauge reading of 50 kPa yields an absolute pressure of about 151 kPa. The other references describe pressure relative to the surrounding environment or at a given altitude, but the standard reference for absolute pressure is vacuum.