Reference Viscosities of H2, CH4, Ar, and Xe at Low Densities

The zero-density viscosity η gas 0,T of hydrogen, methane, and argon was determined in the temperature range from 200 to 400 K, with standard uncertainties of 0.084% for hydrogen and argon and 0.096% for methane. These uncertainties are dominated by the uncertainty of helium’s viscosity ηHe 0,T , which we estimate to be 0.080% from the difference between ab initio and measured values at 298.15 K. For xenon, measurements ranged between 200 and 300K and the zero-density viscosity ηXe 0,T was determined with an uncertainty of 0.11%. The data imply that xenon’s viscosity virial coefficient is positive over this temperature range, in contrast with the predictions of corresponding-states models. Furthermore, the xenon data are inconsistent with Curtiss’ prediction that bound pairs cause an anomalous viscosity decrease at low reduced temperatures. At 298.15 K. the ratios ηAr 0,298/ηHe 0,298, η CH4 0,298/ηHe 0,298, η H2 0,298/ηHe 0,298, ηXe 0,298/ηHe 0,298, η N2 0,298/ηHe 0,298, and η C2H6 0,298 /ηHe 0,298 were determined with a relative uncertainty of less than 0.024% by measuring the flow rate of these gases through a quartz capillary while simultaneously measuring the pressures at the ends of the capillary. Between 200 and 400 K, a two-capillary viscometer was used to determine η gas 0,T /ηHe 0,T with an uncertainty of 0.024% for H2 and Ar, 0.053% for CH4, and 0.077% for Xe. From η gas 0,T /ηHe 0,T , η gas 0,T was computed using the values of ηHe 0,T calculated ab initio. Finally, the thermal conductivity of Xe and Ar was computed