Absolute Pressure - is one which is measured relative to a perfect vacuum.
Barometric Pressure or Atmospheric Pressure - is the total outside air pressure measured with reference to absolute vacuum. The pressure varies depending on geographical location, altitude and local weather conditions. For weather reporting purposes the barometric pressure is normally adjusted to a sea level value so that all locations can be compared independent of the altitude at each location
Differential Pressure (DP) - is the difference in pressure between two separate points.
Gauge Reference Pressure or G - is a pressure measured relative to atmospheric or barometric pressure.
Hydrostatic Pressure - is what is exerted by a liquid when it is at rest. The height of a liquid column of uniform density is directly proportional to the hydrostatic pressure.
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