Absolute Pressure

Absolute Pressure is referenced to a complete vacuum.

 

Example

Therefore measuring a complete vacuum would give a pressure reading of 0 kPa. Atmospheric pressure at sea level when referenced to a complete vacuum measures approximately 101 kPa. A turbocharger producing 1 bar (approx.14.7 psi) of boost pressure would measure approximately 200 kPa.

 

Gauge Pressure

Gauge Pressure is referenced to barometric (atmospheric) pressure. Note that atmospheric pressure decreases with altitude.

 

Example

Measuring vacuum would give a negative gauge pressure reading (often measured in units such as mm Hg or In Hg). Measuring turbo boost pressure would give a positive gauge pressure.