Page:Journal of the American Society of Mechanical Engineers, Volume 33.pdf/663

1330 :$$t + 459.64$$ = absolute temperature
 * $$D_s$$= density in lb. per cu. ft. of saturated water vapor at temperature $$t$$
 * = reciprocal of speciﬁc volume of steam
 * $$P$$ = 29.92 = assumed standard of barometric pressure in in. of mercury
 * $$e$$ = vapor pressure of saturated water vapor
 * $$5284$$ = constant of the equation

The derivation of equation [5] is given in Appendix No. 3.

47The speciﬁc weight of saturated water vapor is not constant, but varies with the temperature of saturation and may be calculated from equation [1]. The theoretical value, computed from its molecular weight, assuming it to be a perfect gas, is 0.6221. This is the assumption made in the computation of the psychrometric tables published by the United States Weather Bureau, which are inaccurate, therefore, in proportion to the variation of $$S$$ from this theoretical value. The actual values for the speciﬁc weight of water vapor at various saturation temperatures, computed from equation [35] are given in Table 1.