Absolute, Gauge, Bidirectional, Differential, Vacuum, Barometric Pressure - What’s the difference?

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Absolute, Gauge, Bidirectional, Differential, Vacuum, Barometric Pressure - What's the Difference?

The standard terminology used to describe the physical characteristic in a pressurized system can be a little confusing to someone new to pressure measurement. Knowing the standard terminology provides a common language that will ensure what you want is what you get when purchasing a pressure gauge, pressure controller or calibrator and pressure transmitter, transducer or sensor. It will also eliminate mismatch between the calibrated and the calibrator.

Pressure is a force applied perpendicular to the surface of an object per unit area. Mathematically it is P = F/A, where P is pressure, F is force, and A is area. Pressure is a scalar quantity, one that only has magnitude and no other characteristics. In a practical sense we can think of it as a force that acts equally on all surfaces it is exposed to, and results from the combined energy of the gas or liquid touching that surface.  There are two basic pressure types: absolute and gauge, distinguished by what pressure they are compared to, which is called the reference pressure.

Gauge pressure’s reference is ambient atmospheric pressure. Absolute pressure’s reference is an absolute vacuum. So both, in a sense, are reading the difference between the reference pressure and the pressure applied. However with gauge pressure, the reference pressure may vary depending on the current atmospheric pressure.

It is not intuitively apparent that we live in a world where every inch of our body is subjected to pressure (about 14.7 pounds per square inch at sea level). Vangelista Torricelli discovered this fact in the 17th century. He said, "We live submerged at the bottom of an ocean of the element air." The pressure exerted by the atmosphere in our gravitational field is also called barometric pressure, an absolute pressure that fluctuates with differing weather systems. The reason we don’t feel this tremendous amount of pressure is because there is an equal and opposite pressure exerted by the somewhat incompressible fluids of our body.

In situations where we have a vessel subjected to atmospheric pressure, we may want to ensure that the vessel does not explode or implode. In this case, we can measure the difference between the atmospheric pressure and the internal pressure of the tank using a gauge pressure sensor with the reference exposed to the atmospheric pressure.

Differential pressure is a special type of gauge pressure that is exposed to another arbitrary pressure  instead of the reference being ambient atmospheric pressure. This is used in level measurement applications and flow applications.

Vacuum pressure is also a special type of gauge pressure and is a measure of pressure below atmospheric, and is expressed as positive number.

Bidirectional pressure is also a type of gauge pressure but gives readings above atmospheric as a positive pressure, and readings below atmospheric as a negative pressure.

The video below gives a thorough explanation of all pressure types using visual aids.