Pressure measurement is an important process for many industrial control systems. Many techniques have been developed for the measurement of pressure and vacuum. Instruments used to measure pressure are called pressure gauges or vacuum gauges. Many instruments have been invented to measure pressure, with different advantages and disadvantages. Applications of pressure measurement are: Altimeter, Barometer, MAP sensor, Pitot tube, Sphygmomanometer, etc.

No pressure is an absolute quantity, everyday pressure measurements, such as for tire pressure, are usually made relative to ambient air pressure. In other cases measurements are made relative to a vacuum or to some other ad hoc reference. When distinguishing between these zero references, the following terms are used:

  • Absolute pressure is zero referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure.
  • Gauge pressure is zero referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Negative signs are usually omitted.
  • Differential pressure is the difference in pressure between two points.

Tire pressure and blood pressure are gauge pressures by convention, while atmospheric pressures, deep vacuum pressures, and altimeter pressures must be absolute. Differential pressures are commonly used in industrial process systems. Differential pressure gauges have two inlet ports, each connected to one of the volumes whose pressure is to be monitored. Atmospheric pressure is typically about 100 kPa at sea level, but is variable with altitude and weather. If the absolute pressure of a fluid stays constant, the gauge pressure of the same fluid will vary as atmospheric pressure changes.

Usually pressure is measured in the units of Pascal (Pa), Bar (bar), Technical atmosphere (at), Atmosphere (atm), Torr (Torr), and Pound-force per square inch (psi). An example pressure measurement units conversion is:
1 Pa = 1 N/m2 = 10−5 bar = 10.197×10−6 at = 9.8692×10−6 atm = 7.5006×10−3 torr = 145.04×10−6 psi.

Static pressure measurement and dynamic pressure measurements
are different because the Static Pressure is uniform in all directions, while Dynamic Pressure is the directional component of pressure in a moving (dynamic) fluid/air.