Evolution of SF6 pressure at constant volume versus temperature between liquefaction point and +20°C experiment by Thuries

Switchgear must be able to operate within a temperature range anywhere from -50°C to +55°C. The practical problem of how to perform significant tests at +20°C then presents itself. It becomes mandatory to know the equivalent of the density at the lowest temperature to which the switchgear will be subjected. A perfect mathematical knowledge of SF₆ evolution at constant volume versus temperature between the liquefaction point and +20°C is then very important. There are three main state equations used by SF₆ switchgear users and manufacturers and they do not truly agree between themselves. The authors evaluate them and also describe a simple experiment as defined by E. Thuries to check these state equations. The last part of the paper describes various experimental data that lead to the establishment of a state equation using the new notion of fugacity. The last part of the paper compares this state equation based on fugacity and the other equations as used today against the results of the new SF₆ Thuries experiment