Figure 6-4. - Atmospheric pressure.

Scale Relationships

A relationship exists between the readings of a gauge calibrated in psig and calibrated in psia. As shown in figure 6-5, when the psig gauge reads 0, the

Figure 6-5. - Pressure relationship.

psia gauge reads the atmospheric pressure (14.7 psia at sea level). In other words, the psia reading equals the psig reading plus the atmospheric pressure (7.7 psia at 16,400 feet), or, a psig reading equals the psia reading minus the atmospheric pressure.

For pressure less than the atmospheric pressure (partial vacuums), a measuring device with a scale reading in inches of mercury (Hg) or in inches of water (H₂ O) is used. A perfect vacuum is equal to -30 inches of mercury or -408 inches of water (fig. 6-5). In refrigeration work, pressures above atmospheric are measured in pounds per square inch, and pressures below atmospheric are measured in inches of mercury.

Effects of Pressure on Gases

The exertion of pressure on a substance with a constant temperature decreases its volume in proportion to the increase of pressure. For example, suppose that a given amount of gas is placed in a cylinder that is sealed on one end and has a movable piston on the other end. When 60 psi of absolute pressure is exerted on the piston, as shown in view A of figure 6-6, the volume of the gas is compressed to 3 cubic feet. When 90 psi of absolute pressure is exerted on the piston, as shown in view B, the volume of the gas is compressed to 1.5 cubic feet. Finally, when 180 psi of absolute pressure is exerted on the piston, as shown in view C, the volume of the gas is compressed to 1 cubic foot. Thus, if a given amount of gas is confined in a container and subject to changes of pressure, its volume changes, so the product of volume multiplied by absolute pressure is always the same.

Pressure has a relationship to the boiling point of a substance There is a definite temperature at which a liquid boils for everv definite pressure exerted upon it.

Figure 6-6. - Pressure-volume relationship.