Atmospheric, gauge, absolute pressure

• Atmospheric pressure (P_atm): the pressure exerted by the weight of the air column above the earth's surface. At sea level it is 101.325 kPa = 760 mm Hg = 1 atm. It is measured by a barometer.
• Gauge pressure (P_gauge): the pressure of a fluid measured relative to the local atmospheric pressure, i.e. the reading of a pressure gauge or manometer. It is positive when the pressure is above atmospheric.
• Absolute pressure (P_abs): the pressure measured relative to a perfect vacuum (zero pressure). It is the true pressure used in thermodynamic equations.

Relationship

For pressure above atmosphere:
P_abs = P_atm + P_gauge

For pressure below atmosphere (vacuum):
P_abs = P_atm − P_vacuum

 P_abs
  ^
  |   ___ pressure above atm: P_gauge
  | ----------------- P_atm (local)
  |   ___ pressure below atm: P_vacuum
  | ----------------- absolute zero (vacuum)

Example: a tyre gauge reads 200 kPa where P_atm = 101 kPa, so P_abs = 301 kPa.