31 Field Instrumentation GEN Rev)
2.1 Definition of Pressure Pressure is defined as force divided by the area over which it is applied. Pressure is often defined in terms of Head. Pressure is a basic process variable in that it is utilized for measurement of flow (difference of two pressures, level (liquid pressure of back
pressure from a bubble tube, and temperature (fluid pressure in a filled thermal system. Pressure is measured as force per unit area. In English system force is measured in pounds and a common unit of pressure is pounds per square inch (psi. The pressure of atmosphere at sea level under standard conditions is 14.696 psi absolute.
UNITS OF PRESSURE Gravity dependent units Units such as Psi, Kg/cm2,
inches of water, and inches of mercury are all gravity dependent. The force at the bottom of each column is proportional to the height,
density, and gravitational acceleration.
Gravity independent units Units such as pounds-force per square inch and Kilogram-force per centimeter square are independent of gravity because a specific value of gravitational acceleration was selected in defining these units. Pascal is exactly
the same at every point, even on moon, despite changes in gravitational acceleration.
Pascal: The English or SI (systems international) unit of pressure is defined as the pressure or stress that arises when a force of one Newton (N) is applied uniformly over an area of one square meter (m. This pressure has been designated one Pascal (pa).
Pa=N/m2 (100Kpa=1 Kg/cm2)
Definitions Absolute pressure: Pressure above perfect vacuum or zero absolute.
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