16.08.2011
Figure 1: Measuring system of the dielectric-constant gas thermometer with two cylindrical capacitors within stainless steel pressure vessels on a central copper plate.
In view of the redefinition of the base units of the International System of Units (SI) via fundamental constants, research groups from all over the world have been dealing with determining also the value of the Boltzmann constant with uncertainties of a few ppm. If they are successful, the unit "kelvin" could be redefined. Many groups apply the so-called acoustic gas thermometry, a method which has provided the first reliable values with an uncertainty of order 2 ppm. PTB has chosen a different and independent path to detect systematic sources of errors and, thus, put the redefinition on solid ground. PTB's "Temperature" Department hereby uses dielectric-constant gas thermometry (DCGT). This method is based on the in-situ determination of the density of the measuring gas (helium) which changes the capacitance of a special capacitor. After first DCGT measurements, a value of 1.380655 10–23 J/K with a relative uncertainty of about 8 ppm has been obtained for the Boltzmann constant. The ultimate step towards an uncertainty of 2 ppm is expected within the next two years in order to smooth the way for the redefinition of the unit "kelvin".
Contact:
C. Gaiser, 7.43, e-mail: 
Christof.Gaiser(at)ptb.de