Results of new high-precision measurements of the viscosity of water vapor at low densities are reported. The measurements were performed using an all-quartz oscillating-disk viscometer with small gaps. Ten isochoric series were carried out at temperatures between (298 and 438) K and at densities from (0.001 to 0.013) mol‚L -1 . The uncertainty is estimated to be ( 0.2 % at ambient temperature increasing up to ( 0.3 % at higher temperatures. Isothermal values were analyzed with a density series for the viscosity in which only a linear contribution is considered. Values for the viscosity of the saturated vapor were also obtained at low temperatures. The new results and reliable data sets from literature are used to represent the viscosity of water vapor at moderately low densities. The second viscosity virial coefficient was modeled by means of the Rainwater-Friend theory. An individual correlation according to the extended theorem of corresponding states was necessary to describe the zero-density viscosity coefficient within its uncertainty and is particularly reliable at temperatures down to 280 K. The good agreement between the new values and older data from literature, not considered in the IAPWS formulation, requires the improvement of this correlation at low temperatures.

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The viscosity coefficient of methanol vapor was measured at low densities by means of an all-quartz oscillating-disk viscometer of high precision. The relative measurements were performed along 10 isochores at densities from (0.004 to 0.050) mol·dm-3 in the temperature range between (298 and 603) K. The uncertainty is estimated to be ± 0.2 % at room temperature, increasing up to ± 0.3 % at higher temperatures. Isothermal values recalculated from the original experimental data were evaluated with a first-order expansion for the viscosity in terms of density. A reasonable agreement with experimental values from the literature was found for those of Golubev and Likhachev (up to 1 % higher) measured with a capillary viscometer in an extended temperature range up to high pressures. The new results, some older ones obtained in our laboratory, and those of Golubev and Likhachev are used to model the viscosity of methanol vapor at moderately low densities. Whereas an individual correlation according to the extended theorem of corresponding states was necessary to represent the zero-density viscosity coefficient within its uncertainty, the Rainwater−Friend theory proved to be suitable for the description of the second viscosity virial coefficient. In addition, viscosity values of the saturated vapor were determined at low temperatures (299 to 339 K). They are in reasonable good consistency with values of Golubev determined at higher temperatures (373 to 513 K).

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