Pressure
Pressure, in mechanics, the force per unit area exerted by a liquid or gas on a body or surface, with the force acting at right angles to the surface uniformly in all directions. In the British system, pressure is usually measured in pounds per square inch (PSI); in international usage, in kilograms per square centimeters, or in atmospheres; and in the international metric system (SI), in newtons per square meter. The unit atmosphere (atm) is defined as a pressure of 1.03323 kg/sq cm (14.696 lb/sq in), which, in terms of the conventional mercury barometer, corresponds to 760 mm (29.921 in) of mercury. The unit kilopascal (kPa) is defined as a pressure of 0.0102 kg/sq cm (0.145 lb/sq in). PRESSURE GAUGES Most gauges record the difference between the fluid pressure and local atmospheric pressure. For small pressure differences, a U-tube manometer is used. It consists of a U-shaped tube with one end connected to the container and the other open to the atmosphere. Filled with a liquid, suc