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 vapor pressures have also been measured using the same apparatus. The cell constant of the Burnett apparatus has been calibrated by measuring the thermodynamic properties of gaseous helium. Using the cell constant, we measured 95 compressibility factors at temperatures from 290 to 370 K, and pressures from 0.15 to 6.6 MPa. These experimental compressibility factors for temperatures from 290 to 370 K have been discussed along each isotherm by applying the virial-type equation having second, third and fourth virial terms. We have also measured 9 vapor pressures at temperatures from 280 to 350 K. On the basis of the present vapor-pressure measurements, the critical pressure of HFC-32 has been determined so as to correspond to the critical temperature directly measured in our laboratory. The present data were also used to develop the vapor-pressure correlation which are effective for temperatures from the normal-boiling-point temperature to the critical temperature. The present data are represented within a deviation of +/- 0.035% (+/- 0.6 kPa) by the derived vapor-pressure correlation. The uncertainty of temperature measurements was estimated to be better than +/- 10 mK and that of pressure measurements was estimated to be better than +/- 0.8 kPa. Experimental uncertainty for the compressibility factor was evaluated as not lower than +/- 0.2%. The purity of the HFC-32 sample used was higher than 99.98 wt%.