When we talk about pressure, the terms atmospheric, absolute, relative and differential are used. What is the exact meaning of these terms?
Pressure is defined as the force acting perpendicularly and uniformly on a defined area; the formula representing it is therefore the ratio between the intensity of the force acting on a surface and the area of that surface:
Consequently, for the same force, pressure is inversely proportional to the area to which it is applied while, for the same surface area, pressure is directly proportional to force. In the International System, the unit of pressure is the Pascal (Pa) corresponding to the force of 1 N acting on an area of 1 m2.
Atmospheric or barometric pressure
The atmosphere consists of a mixture of gases, called air, which, held by the force of gravity, surrounds the Earth.
Air, like all fluids, exerts a pressure on the surface of bodies immersed in it, this pressure is the atmospheric pressure, which is greatest at ground level and decreases progressively with altitude until it is zero. At ground level, atmospheric pressure has an order of magnitude of about one hundred thousand pascals (+/- 1000 mbar).
Atmospheric pressure is a variable parameter that can fluctuate by up to ± 5% in different meteorological zones.
Gauge pressure or relative pressure
On the gauge scale, zero is assigned to atmospheric pressure, so the gauge pressure measurement is given by:
This means that the relative pressure varies depending on the height above sea level and atmospheric conditions; however, since most industrial processes are under the same environmental conditions, the relative pressure measurement is sufficient for most applications.
Absolute pressure
In the absolute scale, zero value is attributed to the absolute vacuum. The relative pressure is called absolute pressure and is given by:
This measurement is independent of fluctuations in atmospheric pressure, using absolute vacuum as a reference.
On an industrial level, some specific cases require the measurement of absolute pressure; absolute pressure measuring instruments are, for example, used in aviation altimeters and in the monitoring of liquid vapour pressure, distillation processes and hazardous gases.
Differential pressure
Differential pressure, often also called ‘Delta p’, is the difference between two independent pressures.
The measuring instruments most suitable for monitoring the process of interest will therefore be chosen according to the type of pressure to be measured.
