The Air Density Equation and the Transfer of the Mass Unit

A new formulation of the equation for calculation of air density has been developed. The Cohen and Taylor value of the gas constant, currently accepted values of the atomic weights, and recent determinations of abundances of the various constituents of air have been used. The abundance of carbon dioxide has been treated as a variable and a factor enabling convenient adjustment of the apparent molecular weight of air for deviation of carbon dioxide abundance from a background value has been derived. A new table of the compressibility factor for the range of pressure and temperature of interest in standards laboratories has been calculated using recently determined values of virial coefficients. The enhancement factor, which has usually been ignored, has been explicitly included. A simple equation for the calculation of enhancement factor has been fitted to values in the range of pressure and temperature of interest. A simple equation for the calculation of saturation water vapor pressure has been fitted. Uncertainties, random and systematic, in the parameters and in the measurement of environmental variables and consequent uncertainties in calculated air density have been estimated. Application of the equation to air buoyancy determination and the transfer of the mass unit at the various national standards laboratories has been made.