An absolute viscometer-densimeter and measurements of the viscosity of nitrogen, methane, helium, neon, argon, and krypton over a wide range of density and temperature

An apparatus for the simultaneous measurement of viscosity and density of fluids is presented. The viscometer-densimeter covers a viscosity range up to 150 muPa.s and a density range up to 2000 kg.m(-3) at temperatures from 233 to 523 K and pressures up to 30 MPa. Very accurate density measurements with uncertainties of +/- 0.02 to +/- 0.05% have always been carried out with this apparatus, although in its first version it was necessary to calibrate the viscosity measuring system on a reference fluid in order to achieve uncertainties of +/- 0.6 to +/- 1.0% in viscosity. After significant improvements, the apparatus now achieves uncertainties in viscosity of less than +/- 0.15% in the dilute gas region and less than +/- 0.4% for higher densities. Moreover, the viscosity measuring system can be described in an absolute way; calibration is no longer necessary. In order to test the advanced apparatus and to determine viscosity-density values of very high quality, comprehensive measurements on nitrogen, argon, and methane were carried out in the entire working range of the viscometer-densimeter. In addition, viscosity-density measurements on helium, neon, and krypton were made on two selected isotherms each. All measurements show that the estimated total uncertainty of +/- 0.15 to +/- 0.4% in viscosity and of +/- 0.02 to +/- 0.05% in density is clearly met. In order to verify the results of the combined viscometer-densimeter, a new apparatus for very accurate viscosity measurements was designed. While the working range of this apparatus is restricted to the dilute gas region, it yields uncertainties of less than +/- 0.07% in viscosity. Measurements carried out with this apparatus confirmed the previously measured values of the combined viscometer-densimeter within +/- 0.03%.