The zero-density viscosity \documentclass[12pt]{minimal}
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\begin{document}$${\eta_{0,T}^{\rm gas}}$$\end{document} 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 \documentclass[12pt]{minimal}
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\begin{document}$${\eta_{0,T}^{\rm He}}$$\end{document} , 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 300 K and the zero-density viscosity \documentclass[12pt]{minimal}
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\begin{document}$$\eta _{0,T}^{\rm Xe} $$\end{document} 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 \documentclass[12pt]{minimal}
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\begin{document}$$\eta _{0,298}^{\rm Ar}\!/\eta_{0,298}^{\rm He} ,{\eta _{0,298}^{{\rm CH}_{4}} }\!/\eta_{0,298}^{\rm He },{\eta _{0,298}^{{\rm H}_2} }\!/{\eta_{0,298}^{\rm He}},{\eta_{0,298}^{\rm Xe} }\!/{\eta _{0,298}^{\rm He} }, {\eta _{0,298}^{{\rm N}_2} }\!/{\eta _{0,298}^{\rm He}}$$\end{document} , and \documentclass[12pt]{minimal}
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\begin{document}$${\eta _{0,298}^{{\rm C}_2{\rm H}_6} }/{\eta _{0,298}^{\rm He} }$$\end{document} 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 \documentclass[12pt]{minimal}
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\begin{document}$${\eta_{0,T}^{\rm gas} }/{\eta_{0,T}^{\rm He} }$$\end{document} with an uncertainty of 0.024% for H2 and Ar, 0.053% for CH4, and 0.077% for Xe. From \documentclass[12pt]{minimal}
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\begin{document}$${\eta_{0,T}^{\rm gas} }/{\eta_{0,T}^{\rm He} }, \eta_{0,T}^{\rm gas} $$\end{document} was computed using the values of \documentclass[12pt]{minimal}
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\begin{document}$$\eta_{0,T}^{\rm He}$$\end{document} calculated ab initio. Finally, the thermal conductivity of Xe and Ar was computed from \documentclass[12pt]{minimal}
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\begin{document}$$\eta_{0,T}^{\rm gas} $$\end{document} and values of the Prandtl number that were computed from interatomic potentials. These results may help to improve correlations for the transport properties of these gases and assist efforts to develop ab initio two- and three-body intermolecular potentials for these gases. Reference viscosities for seven gases at 100 kPa are provided for gas metering applications.
