A new apparatus for measurement of the critical p-?-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-p-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-p-T data and those calculated by the Refprop for the nitrogen is evaluated to be 0.18%. The p-p-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-p-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.

Automated summary

In this study, a new variable-volume apparatus equipped with a metal-bellows volumeter is designed to measure the critical pressure-density-temperature (p-p-T) properties by charging the device only once. The uncertainties of temperature, pressure, and density measurements are evaluated, and the ARAD between experimental and calculated data is also assessed. The critical properties of 1,1,1,2-tetrafluoroethane and difluoromethane are measured by the apparatus for validation.