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T able 3. Weights of Saturated and Partly Saturated Air for Various Barometric and H ygrometric Conditions,b Pounds per Cubic Foot American Society of Heating and Ventilating Engineers Guide, 1935 6 Chapter 1--Fundamentals op Heating and Air Conditioning pressure and volume of a gas is a constant, or P V = C, when T is kept constant. Any change in the pressure and volume of a gas at constant temperature is called an isothermal change. Charles' Law refers to the relation among pressure, volume, and tem perature of a gas and may be stated as follows: The volume of a given weight of gas varies, directly as, the absolute temperature at constant pressure, and the pressure varies directly as the absolute temperature at constant volume. Hence, when heat is added at constant volume, Fc, the resulting equation is P=t- = Tt or, for the same temperature range at constant pres- JT1 l 1 y 'T ' ' ' " sure, Pc, the relation is -fy = =r. riii In general; for any weight of gas, Wy since volume is proportional to weight, the relation among P, V, and> T is where PV =>WRT (3) P = the absolute pressure of the gas, pounds per square foot. V = the volume of the weight W, cubic feet. W = the weight of the gas, pounds. R = a constant depending on the nature of the gas. The average value of R for air is 53.34. T -- the absolute temperature, degrees Fahrenheit. This is the characteristic equation for a perfect gas, and while no gases are perfect in this sense, they conform so nearly that Equation 3 will apply to most engineering computations. HUMIDITY Humidity is the water vapor mixed with dry air in the- atmosphere. Absolute humidity has a multiplicity of. meanings,; but usually the term refers to. the weight of water vapor per unit volume'of space occupied, expressed in grains or pounds per cubic foot. With'this meaning, absolute humidity is nothing but the actual density of the water vapor in , the mixture and might better be so called A study of Keenan's Steam Tables2 indicates that water vapor,'either saturated or super-heated, at partial pressures lower than 4 in.-.of mercury may be treated as a gas with a gas constant R of 1.21 in the characteristic equation of the gas pV. = * wR ft + 460). Within such limits, the, density (8) of water vapor is 8 = -y = 'f.21 (t + 460) (Punds Per cubic fot) (4a) = (grains per cubic foot) v~ , where e = actual partial pressure of vapor, inches of mercury. t = dry-bulb temperature, degrees Fahrenheit. ... "> (4b) - 'Published by American Society of McckanichI~Enginee7s, sec abstract'in Table 7. ' .V '