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The kelvin: The Boltzmann Project

The Boltzmann constant is the key factor when it comes to redefining the base unit "kelvin" by relating it to a fundamental constant. At the international level, the aim is therefore to determine the numerical value of the Boltzmann constant and to replace the current definition via the triple point of water. Then, also the base unit "kelvin" would no longer depend on a special material property. This, however, requires the Boltzmann constant to be known with a very low uncertainty – so low that it equals the uncertainty of the realization of the triple point of water. To achieve this, scientists worldwide are working on determining the Boltzmann constant in different ways. At PTB, a Opens internal link in current windowdielectric-constant gas thermometer is used for this purpose in Department 7.4: Temperature (Berlin site).


The national metrology institutes prepare intensively for the redefinition of the base units which is scheduled - from 2015 on – to be traced back to fundamental constants. For that purpose, recommendations for implementation - so called Mises en Pratiques - are elaborated and published. According to the planned redefinition of the Kelvin - the base unit for the temperature - temperature...

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Initial measurements performed with PTB's dielectric-constant gas thermometer have yielded, a value of 1.380655 • 10–23 J/K with an uncertainty of approx. 8 ppm for the Boltzmann constant. The ultimate step towards an uncertainty of 2 ppm is expected within the next two years, which will pave the way for the redefinition of the unit “kelvin” via the definition of k.

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It is to generate an exactly defined helium absolute pressure within the scope of the Boltzmann project: a pressure balance which mainly consists of a piston-cylinder system. PTB now succeeded in measuring the form and the diameter of these pistons and cylinders exact to a few nanometres and, thus, in remaining below the required accuracies.

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