Thermodynamics TEMPERATURE AND PRESSURE MEASUREMENTSPressurePressure is a measure of the force exerted per unit area on the boundaries of a substance (orsystem). It is caused by the collisions of the molecules of the substance with the boundaries ofthe system. As molecules hit the walls, they exert forces that try to push the walls outward. Theforces resulting from all of these collisions cause the pressure exerted by a system on itssurroundings. Pressure is frequently measured in units of lbf/in2 (psi).PressureScalesWhen pressure is measured relative to a perfect vacuum, it is called absolute pressure (psia);when measured relative to atmospheric pressure (14.7 psi), it is called gauge pressure (psig). Thelatter pressure scale was developed because almost all pressure gauges register zero when opento the atmosphere. Therefore, pressure gauges measure the difference between the pressure ofthe fluid to which they are connected and that of the surrounding air.If the pressure is below that of the atmosphere, it is designated as a vacuum. A perfect vacuumwould correspond to absolute zero pressure. All values of absolute pressure are positive, becausea negative value would indicate tension, which is considered impossible in any fluid. Gaugepressures are positive if they are above atmospheric pressure and negative if they are belowatmospheric pressure. Figure 2 shows the relationships between absolute, gauge, vacuum, andatmospheric pressures, as do Equations 1-9 and 1-10.Figure 2 Pressure RelationshipsRev. 0 Page 9 HT-01
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