Absolute pressure

Absolute pressure is pressure measured relative to a perfect vacuum. It equals gauge pressure plus the local atmospheric pressure.

Vacuum: A region containing matter at a pressure below atmospheric pressure; a perfect vacuum contains no matter. Absolute pressure uses a perfect vacuum as its zero point.
Ideal gas law: An equation relating a gas's pressure, volume, amount, and absolute temperature. Its pressure variable must be absolute, not gauge pressure.
Pressure: The normal force exerted per unit area by a fluid, gas, or solid. Absolute pressure is one way of specifying the pressure of a system.
Gauge pressure: Pressure measured relative to the surrounding atmospheric pressure or another chosen reference pressure. Gauge pressure omits atmospheric pressure, so its numerical value differs from absolute pressure.
Pressure measurement: The determination of force per unit area exerted by a fluid or gas. Measurement systems must specify the reference used to report pressure.
Boyle's law: The inverse relationship between a fixed amount of gas's pressure and volume at constant temperature. Comparing gas states with Boyle's law requires absolute pressure values.
Vapor pressure: The pressure of a vapor in equilibrium with its liquid or solid at a given temperature. It provides a physical threshold often compared with absolute pressure.
Vacuum pressure: Pressure below atmospheric pressure, commonly expressed as the amount by which it falls short of atmospheric pressure. Vacuum-pressure scales may report a deficit from atmosphere rather than absolute pressure itself.
Pressure sensor: A device that detects pressure and converts it into a measurable signal. Absolute-pressure sensors reference a sealed vacuum, unlike vented gauge sensors.
Barometer: An instrument that measures atmospheric pressure. A barometer supplies the atmospheric-pressure value needed to convert gauge readings.

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