Speed of sound measurements and derived third and fourth acoustic virial coefficients of supercritical neon

We report comprehensive and accurate measurements of the speed of sound in neon. These measurements were carried out by a double-path-length pulse-echo technique and cover the temperature range between 200 K and 420 K with pressures up to 100 MPa. The standard uncertainties are 1.9 mK in temperature, 22 parts in 10 6 in pressure and 35 parts in 10 6 in speed of sound. The third and fourth acoustic virial coefficients of neon were derived from the speed of sound data in the temperature range of the measurements by fitting a fourth-order acoustic virial expansion in pressure with the second acoustic virial coefficient constrained from first-principles calculations. To support our claimed uncertainty, we determined the ratio M / γ 0 between the molar mass M and the ideal-gas heat capacity ratio γ 0 of the neon sample with a relative standard uncertainty of 7.7 parts in 10 6 by additional speed of sound measurements using a spherical resonator at 273.16 K.