Determining the Boltzmann constant and the temperature from sound speed measurements

Extraordinary speed-of-sound measurements in helium and argon gas determined Boltzmann’s constant k B with sub-part-per-million uncertainties. These measurements were conducted in preparation for the 2019 redefinition of the unit of temperature T, the Kelvin. We outline the motivation for these measurements, the principles that enabled them, and their limitations. Similar sound speed measurements revealed errors of order 50 × 10−6 T in the consensus International Temperature Scale of 1990 in the range 7 K < T < 550 K. Accurate acoustic thermometry stimulated improved, first-principles calculations of the virial coefficients and transport properties of helium gas. Now, the calculated properties are so accurate that they are often used to calibrate measuring apparatus.