Progress towards an acoustic determination of the Boltzmann constant at CEM-UVa

An acoustic gas thermometer was used to achieve a determination of the Boltzmann constant, kB, using a misaligned stainless steel (316L) spherical cavity with an internal volume of approximately 268 cm3. Measurements of the speed of sound while the cavity is filled with argon at the temperature of the triple point of water, 273.16 K, and at different pressures between 78.2 kPa and 0.9 MPa, were used to extrapolate the value of the speed of sound in argon at zero pressure. The internal volume of the resonator was accurately determined by measuring microwave resonance frequencies at the same temperature and pressure conditions as for the acoustic measurements. The measurements were taken at pressures from 78.2 kPa up to 901.3 kPa, and at 273.16 K. As the results of the measurements, we determined kB = (1.380 644 1   ±   0.000 022 1)  ×  10−23 J K−1 which means a relative standard uncertainty of 16 parts in 106.