### ASA 129th Meeting - Washington, DC - 1995 May 30 .. Jun 06

## 4pPA9. Ab initio calculations for helium, acoustics, and metrology.

**Michael R. Moldover
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*NIST, Thermophys. Div., Gaithersburg, MD 20899
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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.