Ab initio calculations for helium, acoustics, and metrology

Abstract

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: η=(19.800±0.010) μPa 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=(Cpη/λM)=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 ...