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 η He 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 300 K and the zero-density viscosity η Xe 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 η Ar 0,298 /η He 0,298 , η CH 4 0,298 /η He 0,298 , η H 2 0,298 /η He 0,298 , η Xe 0,298 /η He 0,298 , η N 2 0,298 /η He 0,298 , and η C 2 H 60,298 /η He 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 /η He 0,T with an uncertainty of 0.024% for H 2 and Ar, 0.053% for CH 4 , and 0.077% for Xe. From η gas 0,T /η He 0,T , η gas 0,T was computed using the values of η He 0,T calculated ab initio. Finally, the thermal conductivity of Xe and Ar was computed 1086 May, Berg, and Moldover from η gas 0,T 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 twoand three-body intermolecular potentials for these gases. Reference viscosities for seven gases at 100 kPa are provided for gas metering applications.

We determined the zero-density viscosity η Ar 0,T and thermal conductivity λ Ar 0,T of argon with a standard uncertainty of 0.084% in the temperature range 200 K to 400 K. This uncertainty is dominated by the uncertainty of helium's viscosity η He 0,T , which we estimate to be 0.080% based upon the difference between ab initio and experimental values at 298.15 K. Our results may improve (1) the argon-argon interatomic potential, (2) calculated boundary-layer corrections for primary acoustic thermometry, and (3) calibrations of laminar flow meters as well as instruments for measuring transport properties. At 298.15 K, we determined the ratio η Ar 0,298 /η He 0,298 = 1.138 00 ± 0.000 13 from measurements of the flow rate of these gases through a quartz capillary while simultaneously measuring the pressures at the ends of the capillary. Between 200 K and 400 K, we used a two-capillary viscometer to determine η Ar 0,T /η He 0,T = 1.211 67 − 0.820 34 exp(−T /123.78 K) with an uncertainty of 0.024%. From η Ar 0,T /η He 0,T , we computed η Ar 0,T using the values of η He 0,T calculated ab initio. Finally, we computed the thermal conductivity of argon from η Ar 0,T and values of the Prandtl number that we computed from argon-argon interatomic potentials.

Commercially manufactured meters that measure the flow of a process gas are often calibrated with a known flow of a surrogate gas. This requires an accurate model of the flow meter and accurate values of the relevant thermophysical properties for both gases. In particular, calibrating a "laminar" flow meter near ambient temperature and pressure requires that the ratio (process gas viscosity)/(surrogate gas viscosity) be known to approximately 0.1%. With this motivation, we critically reviewed measurements of viscosity conducted with 18 instruments near 25°C and zero density for 11 gases: He,