From ISA Handbook of Measurement Equations and Tables, 2nd Edition
Principles of Pressure
Pressure is the force per unit area a fluid or gas exerts on its surroundings. A container of gas contains numerous atoms and molecules constantly bouncing of its walls. The pressure they create is the average force those atoms and molecules produce on the walls.
Therefore, pressure, P, is a function of force, F, and area, A.
The SI unit for pressure is the Pascal (N/m2). Other frequently used units of pressure include atmospheres (atm), pounds per square inch (psi), bars, inches of mercury (in Hg), and millimeters of mercury (mm Hg).
Pressure measurements are typically described as either static or dynamic. Static pressure occurs where no motion is involved, such as air pressure inside a tire or balloon. When the motion of a fluid or gas changes the force applied to its surroundings, the pressure measurement is known as dynamic.
Head pressure (or pressure head) is the measurement of a static pressure in a tank or a pipe and is a function solely on the liquid's height and weight density.
Measurement Types and Sensors
There are three types of pressure measurements: absolute, differential, and gauge. Absolute pressure is measured relative to a vacuum. Differential pressure measurements are taken with respect to a specific reference pressure, while gauge pressure is measured relative to ambient atmospheric pressure. Pressure sensors come in many different types of designs. When pressure is converted to an intermediate form such as displacement, three universal types of pressure transducers...
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