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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).



Scientists from the Physikalisch-Technische Bundesanstalt (PTB) have implemented a novel pressure measurement method, quasi as a byproduct of the work on the "new" kelvin. In addition to being new, this procedure is a primary method, i.e. it only depends on natural constants. As an independent method, it can be used to check the most accurate pressure gauges, for which PTB is known as the world...

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SI-Grafik (BIPM)

This revolution has been a long time in the making. Behind it are metrologists working in their scientific laboratories, and scientific managers in their decision-making committees. Now the moment is approaching when many of the old quantities (specifically, the definitions behind them) will have to step down to make way for their successors, who are waiting in the wings. The quantities concerned...

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PTB scientist Christof Gaiser with the core of the dielectric-constant gas
thermometer. The different silver-colored pressure vessels have special
capacitors which are filled with helium to carry out the measurement that
takes place inside them.

PTB has succeeded in measuring the Boltzmann constant k independently. A fundamental condition laid down by the Consultative Committee for Thermometry (CCT) has thereby been met, so that all obstacles to the redefinition of the kelvin have been removed by fixing the value of the Boltzmann constant. The final measurements of k with a dielectric-constant gas thermometer were conducted with a...

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Die Boltzmann-Apparatur der PTB (Foto: PTB)

At present, the kelvin is literally based on no more than water – on the triple point of water, to be more precise. The base unit of temperature is therefore dependent on a material whose properties may vary. But this is about to change: In the fall of 2018, the kelvin, as well as all the other units of the International System of Units (SI), will be redefined to rest on a solid and invariable...

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Capacitor systems (cross capacitor with partial capacitances C<sub>XC1</sub> and C<sub>XC2</sub> and cylindrical capacitor with capacitance C<sub>Zylinder</sub>) with corresponding pressure vessels to measure the dielectric constant of helium.

New measurements performed with PTB's dielectric-constant gas thermometer have yielded a value of 1.3806509 ⋅ 10–23 J/K for the Boltzmann constant k. The measurement uncertainty was thereby reduced to 4 parts per million (4 ppm). The new value is within 1.5 ppm in agreement with the current CODATA value (CODATA – Committee on Data for Science and Technology). Reaching the final step towards the...

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