ABSTRACT
In various fluid flow systems the fluid of interest is moist air, that is, a real-gas mixture of dry air and water vapor. Of these, the water vapor mixing ratio, r, the ratio of the mass of water in the mixture to the mass of dry air in the mixture, will be discussed and a simple equation for the calculation of the saturation mixing ratio, rs, will be developed. In the absence of sources and sinks of water vapor, r is conserved. Therefore, under these conditions, r can be inferred from a measurement of relative humidity in a convenient region in a system and used to characterize air in other regions of the system. The contribution to the uncertainty in the calculation of γ for moist air also is of the order of 0.001% and is therefore considered negligible for engineering calculations. It is concluded that the relationships developed can be useful in flow measurement or calibration calculations.
