Relative Humidity (RH)=p/ps, where p is the vapour pressure at 37 deg C and ps is the saturated vapour pressure at 37 deg C. From steam tables we can calculate ps at 37 deg C. Therefore vapour pressure, p = RH x ps.
Specific Humidity (SH) = 18/MW x p/(P-p), where P is the pressure of the gas and MW is the mole weight of the dry gas. 18 is assumed to be the mole weight of water vapour. Hence we can calculate SH. But SH is the ratio of mass of water vapour to dry gas. Thus, given the mass of gas we can calculate the mass of water vapour.
