Page:Journal of the American Society of Mechanical Engineers, Volume 33.pdf/648

specific volume is increased, in accordance with the law of gases, in direct proportion to the increase of absolute temperature, while its density is changed in an inverse proportion, as shown in Fig. 3; that is, ${\frac {D_{2}}{D_{1}}}={\frac {T_{1}}{T_{2}}}$, where $D_{1}$ and $D_{2}$ are the densities corresponding to the absolute temperatures $T_{1}$ and $T_{2}$, respectively, and $\left(T_{2}-T_{1}\right)$ is the degree of superheat. If $D_{2}$ is the density of saturated vapor at temperature $T_{2}$, then the ratio ${\frac {D_{2}}{D_{1}}}$ is said to be the per cent of saturation, or more exactly, the per cent of isothermal saturation. When these relationships are considered with respect to water vapor in

Fig. 3.—Temperature-Density Diagram.

air, this ratio is termed the per cent of relative humidity, while the densities $D_{1}$, $D_{2}$, $D'_{2}$, etc., customarily expressed in grains of moisture per cubic foot, are termed absolute humidities.

DEW POINT

8⁠It should be noted that although the total weight of the water vapor remains the same, the absolute humidity $D_{2}$, is less than the absolute humidity $D_{1}$,. However, if water vapor, or air containing water vapor, having a temperature $T_{2}$, and an absolute humidity of $D_{2}$, be cooled to $T_{1}$, it will become saturated, and if cooled further, moisture will be precipitated. Therefore $T_{1}$ is termed the dew point of air having a temperature $T_{2}$ and an absolute humidity, $D_{2}$, or a corresponding relative humidity, ${\frac {D_{2}}{D'_{2}}}$. Therefore, the dew point