18.05.2015
Fig. 1: The most accurate pressure balances worldwide in the pressure range up to 7 MPa have been compared in PTB's Boltzmann booth.
Fig. 2: Capacitor systems with their pressure vessels for the measurement of the dielectric constant of helium.
To determine the Boltzmann constant k which is necessary for the redefinition of the SI base unit of thermodynamic temperature, the kelvin, numerous research groups are using acoustic gas thermometry (speed of sound measurements) which has provided the most exact values so far. PTB has followed an alternative, completely independent path to rule out systematic error sources and, thus, to place the redefinition on a solid basis by using Dielectric-Constant Gas Thermometry (DCGT). This method is based on measuring the density of the measuring gas helium in-situ via the dielectric constant. In practice, the researchers measure to which extent the gas changes the capacitance of a special, highly stable measuring capacitor. From experiments at the triple point of water with different gas pressures (isotherm measurements), k can be determined applying fundamental relations. The researchers have now succeeded in reducing the relative uncertainty for k to 4 × 10-6.
This experiment is an extremely demanding task with regard to capacitance, pressure and temperature measurements; it could only be successfully completed by means of close cooperation with several other PTB working groups and with industry. Pressure measurements up to 7 MPa must, for example, be carried out with pressure balances that are accurate to one part per million (ppm), and in the case of capacitance changes, measurements have to be performed with a relative uncertainty even in the order of one part per billion. The required temperature stability is provided by a large bath thermostat which was manufactured and optimized in cooperation with the national metrology institute of Italy.
Exhaustive comparisons between pressure balances of different effective cross-sectional areas, combined with dimensional data, have now contributed to PTB's ability to measure gas pressures up to 7 MPa with an uncertainty as low as 1 ppm, which is unique worldwide. This improvement has led to a relative uncertainty of 4 ppm for the value of the Boltzmann constant obtained with DCGT. DCGT and noise thermometers are – next to acoustic gas thermometers – the most accurate methods of determining this fundamental constant. Fixing its value, hopefully in 2018, will represent the basis for the redefinition of the unit of temperature, the kelvin.
Contact
C. Gaiser, 7.43, e-mail: Christof.Gaiser(at)ptb.de