The Viscosity of Seven Gases Measured with a Greenspan Viscometer

The viscosity of seven gases (Ar, CH4, C3H8, N2, SF6, CF4, C2F6) was determined by interpreting frequency-response data from a Greenspan acoustic viscometer with a detailed model developed by Gillis, Mehl, and Moldover. The model contains a parameter ∈r that characterizes the viscous dissipation at the ends of the viscometer's duct. It was difficult to determine ∈r accurately from dimensional measurements; therefore, ∈r was adjusted to fit the viscosity of helium on the 298 K isotherm (0.6 MPa<p<3.4 MPa). This calibration was tested by additional viscosity measurements using four, well-studied, polyatomic gases (CH4, C2H6, N2, and SF6) near 300 K and by measurements using argon in the range 293 K<T<373 K. For these gases, all of the present results agree with reference values to within ±0.5% (±0.4% in the limit of zero density). The viscosities of CF4 and C2F6 were measured between 210 and 375 K and up to 3.3 MPa with average uncertainties of 0.42 and 0.55%, respectively. At the highest density studied for CF4 (2746 mol⋅m−3), the uncertainty increased to 1.9%; of this 1.9%, 0.63% resulted from the uncertainty of the thermal conductivity of CF4, which other researchers estimated to be 2% of its value at zero density. As an unexpected bonus, the present Greenspan viscometer yielded values of the speed of sound that agree, within ±0.04%, with reference values.