The Second Interaction (Cross) Virial Coefficient for Moist Air.

The results of calculations of the second interaction (cross) virial coefficient B aw for water vapor and air, based on enhancement data obtained at NBS at 30, 40, and 50 °C, are presented. Comparisons are made with the results of calculations based on the enhancement data of Politzer and Strebel, Webster, and Goff et al. and with the results of the theoretical calculations of Mason and Monchick and of Chaddock. An empirical equation is given for interpolation and extrapolation. The random (one standard deviation) uncertainty in the mean values of B aw , arising from the scatter of previously obtained NBS values of the enhancement factor, is estimated to range from 0.7 percent at 30 °C to 1.4 percent of 50 °C. The estimated systematic uncertainties range from 4 percent at 30 °C to 6 percent at 50 °C, respectively.