Figure 11-1.-Comparison of Kelvin, Celsius, Fahrenheit, and Rankine temperature.convert Fahrenheit to absolute. For Fahrenheitreadings above zero, 460° is added. Thus, 72°Fequals 460° plus 72°, or 532° absolute (532°R).If the Fahrenheit reading is below zero, it issubtracted from 460°. Thus, -40°F equals 460°minus 40°, or 420° absolute (420°R).The Kelvin and Celsius scales are usedinternationally in scientific measurements; there-fore, some technical manuals may use these scalesin directions and operating instructions. TheFahrenheit scale is commonly used in the UnitedStates; therefore, it is used in most areas of thismanual.PRESSUREWe defined pressure in chapter 2 as force perunit area. Remember, liquids exert pressure onall surfaces with which they come in contact.Gases, because of their ability to completely fillcontainers, exert pressure on all sides of acontainer.In practice, we maybe interested in either oftwo pressure readings. We may desire either thegauge pressure or the absolute pressure.Absolute pressure is measured from absolutezero pressure rather than from normal oratmospheric pressure (approximately 14.7 psi).Gauge pressure is used on all ordinary gauges, andindicates pressure in excess of atmosphericpressure. Therefore, absolute pressure is equal toatmospheric pressure plus gauge pressure. Forexample, 100 psi gauge pressure (psig) equals 100psi plus 14.7 psi or 114.7 psi absolute pressure(psia). Whenever gas laws are applied, absolutepressuresGasesare required.COMPRESSIBILITY ANDEXPANSION OF GASEScan be readily compressed and areassumed to be perfectly elastic. This combinationof properties gives a gas the ability to yield to a11-3
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