Primary Dielectric-Constant Gas Thermometry in the Range from 2.4 K to 26 K at PTB

Abstract

New dielectric-constant gas-thermometry (DCGT) measurements were performed at PTB from 2.4 K to 26 K in order to establish a temperature scale with reduced uncertainty. The progress concerning the measurement of capacitance changes, temperature, and pressure compared with the results published in 1996 is described. This is the first step on the way to determine the Boltzmann constant at the triple point of water. At more than 20 temperatures, isotherms were measured and evaluated performing both single- and multi-isotherm fits. Based on this evaluation, a more accurate DCGT scale has been established that is compared with the constant-volume gas-thermometry scale NPL-75, being one basis of the International Temperature Scale of 1990. Coincidence has been found within only a few tenths of a millikelvin above 3.3 K. This gives, together with literature data, confidence with respect to thermodynamic temperature at this level. Emphasis is also given to the results obtained for the virial coefficients, especially below 3.3 K, where the 1996 DCGT results show strong deviations from the expected behavior. The new experimental data for the second virial coefficient are compared with ab initio calculations.