Pressure is one of the primary measurements used in instrumentation and control Its meaning, units, and measuring devices are important to a proper understanding of not only what is occurring in a process but also how instruments and controls serve to monitor the process. The atmosphere consists of gases and liquids. These gases and liquids have weight and exert a pressure on the surface of the earth. Pressure is force exerted over a unit area. Mathematically, pressure is expressed as P = F/A where P = pressure, F = force, A = area, Therefore, the amount of force exerted by a substance directly affects the amount of pressure. Note that pressure is not a fundamental quantity but is derived from force and area, which in turn are derived from mass, length, and time, the latter three being fundamental quantities.
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