COMPRESSIBILITY FACTORS AND VIRIAL-COEFFICIENTS OF DIFLUOROMETHANE (HFC-32) DETERMINED BY BURNETT METHOD

The compressibility factors of difluoromethane (HFC-32) in the gaseous phase have been determined by means of the Burnett method, and the va por pressures have also been measured using the same apparatus. The ce ll constant of the Burnett apparatus has been calibrated by measuring the thermodynamic properties of gaseous helium. Using the cell constan t, we measured 95 compressibility factors at temperatures from 290 to 370 K, and pressures from 0.15 to 6.6 MPa. These experimental compress ibility factors for temperatures from 290 to 370 K have been discussed along each isotherm by applying the virial-type equation having secon d, third and fourth virial terms. We have also measured 9 vapor pressu res at temperatures from 280 to 350 K. On the basis of the present vap or-pressure measurements, the critical pressure of HFC-32 has been det ermined so as to correspond to the critical temperature directly measu red in our laboratory. The present data were also used to develop the vapor-pressure correlation which are effective for temperatures from t he normal-boiling-point temperature to the critical temperature. The p resent data are represented within a deviation of +/- 0.035% (+/- 0.6 kPa) by the derived vapor-pressure correlation. The uncertainty of tem perature measurements was estimated to be better than +/- 10 mK and th at of pressure measurements was estimated to be better than +/- 0.8 kP a. Experimental uncertainty for the compressibility factor was evaluat ed as not lower than +/- 0.2%. The purity of the HFC-32 sample used wa s higher than 99.98 wt%.