Measurements of the viscosity of sulfur hexaflouride up to 100 bar by a capillary-flow viscometer

A new capillary-flow viscometer has been developed for the precise determination of the viscosity of gases in the vicinity of the gas-liquid critical point. An important feature of the viscometer is the possibility to operate at pressure differences as low as 1 mbar. The apparatus, consisting mainly of a pressure vessel with the capillary, a differential pressure meter and a specially designed air thermostat has been described in detail together with the experimental procedure. The viscosity has been computed with a modified Poiseuille formula for compressible fluids. Measurements on sulfur hexaflouride are performed at 1 bar as a function of temperature between 298 and 333 K and at 333 K as a function of pressure up to 100 bar. Besides, for the purpose of calibration, some measurements are performed on nitrogen at several pressures and temperatures. The accuracy of the measurements is estimated to be about 0.1%. The agreement with previous results is found to be fairly satisfactory in several cases.