Michael R. Moldover
NIST, Thermophys. Div., Gaithersburg, MD 20899
Lossy, gas-filled resonators are now being developed at NIST to measure the viscous and thermal diffusivities of gases. If these resonators become as well understood as the low-loss spherical resonators already developed at NIST, the uncertainty in the measured viscosity of a dilute gas will become less than 0.1%, possibly limited by the difficulty of measuring the dimensions of a double-Helmholtz resonator (i.e., a Greenspan viscometer). The accuracy of the data from the acoustic viscometer will be tested by comparing experimental results for helium with the recent ab initio result from quantum mechanical calculations: (eta)=(19.800(plus or minus)0.010) (mu)Pa s at 298.15 K. Similarly, the results for the Prandtl obtained from cylindrical acoustic resonators with inserts will be tested by comparison with the ab initio result: Pr=(C[sub p](eta)/(lambda)M)=0.66419. Helium-based gas thermometry (both traditional and acoustic) will benefit from the theoretical results for the second virial coefficient of helium. Perhaps pressure measurements in the 1- to 10-MPa region could be improved by combining the ab initio results with highly accurate measurements of the dielectric constant of helium.