Dielectric-Constant Gas-Thermometry Measuring System for the Determination of the Boltzmann Constant at PTB

Dielectric-constant gas thermometry is being further developed at PTB to measure thermodynamic temperature and the Boltzmann constant k at the triple point of water. Due to the small electric susceptibility of gases, the targeted relative uncertainty of k of the order of 2 ppm can be achieved only if gas pressures up to 7 MPa and special 10 pF capacitors for the susceptibility measurements, including very large multi-ring toroidal cross capacitors, are used. This required development of a huge measuring system having a large heat capacity. Since the temperature measurement must be traceable to the triple point of water at a level of the order of 0.1 mK, a corresponding stability and homogeneity of the thermal conditions has to be realized. The design of the system and data characterizing its thermal parameters is described. The experimental results are compared with estimations based both on simple models and finite-element calculations.