A new apparatus for measurement of the critical p-ρ-T properties based on the variable-volume method

In the present study, a new variable-volume apparatus equipped with a metal-bellows volumeter is designed to measure the critical pressure-density-temperature (p-ρ-T) properties by charging the device only once. The expanded uncertainties of the temperature, pressure and density measurements are assessed to be less than 20 mK, 4.2 kPa, and 1.6% (k = 2, 95%), respectively. Moreover, the combined expanded uncertainty of the respective critical quantities is lower than 0.084 K, 10.2 kPa, and 2.4% (k = 2, 95%). The average relative absolute deviation (ARAD) between the experimental p-ρ-T data and those calculated by the Refprop for the nitrogen is evaluated to be 0.18%. The p-ρ-T data and the critical properties of the 1,1,1,2-tetrafluoroethane and difluoromethane are measured by the apparatus for verification. All the critical p-ρ-T data agree well with the accessible experimental data from the literature. The ARAD is evaluated to be 0.67% and 0.28% for the density of 1,1,1,2-tetrafluoroethane at the vapor and liquid phases, respectively.