Researchers at the Physikalisch-Technische Bundesanstalt (PTB), the national metrology institute of Germany, have used a dielectric-constant gas thermometer to accurately determine the Boltzmann constant, named after Ludwig Boltzmann and describes the relationship between the thermal energy of particles in a gas and the gas’ temperature, and remove any obstacles to the redefining of the kelvin as a unit of temperature by the International System of Units (SI).
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.
An article on the PTB website explained, “Currently, the definition of the SI base unit kelvin is still based on a material property of water: the triple point. At a well-defined temperature, water can simultaneously be a solid, a liquid and a gas. However, because no two waters are alike, the triple point of water depends on the isotopic composition of the water used. Although physicists from all around the world have agreed upon a ‘standard water’, this situation is far from ideal.”
The SI is expected to be redefined soon (possibly as early as the fall of 2018) with a set of fundamental constants.
The Boltzmann constant (represented by kb) has already been measured by researchers around the world with acoustic gas thermometers to within 1.0 ppm, which is the first condition for the redefinition.
The researchers at PTB began using a dielectric-constant gas thermometer project to fulfill the second piece of the redefinition requirements and they have now gotten with 1.9 ppm and met the required accuracy.
According to the article, “This special thermometer exploits the fact that helium, as a rare gas and a dielectric, changes the capacitance of a capacitor. It is therefore possible to measure the density of helium at a given pressure by means of an electric measurement – and via density, to measure temperature as well. Moreover, electric capacitance measurements can be carried out with great accuracy. In this case, the measurement uncertainty is only a few parts per billion.”
In order to reach that level of accuracy, scientists needed to determine the material properties of the capacitor at high pressures and ensure the purity of the gas was better than 99.99999 percent.
Now that the two methods for measuring the Boltzmann constant have been concluded, its final value will be computed by CODATA (The Committee on Data for Science and Technology)Task Group on Fundamental Constants in the fall.
The research was recently published in Metrologia. Read the full report at http://iopscience.iop.org/article/10.1088/1681-7575/aa62e3/meta;jsessionid=22C307C13AB8C74527DFE35B3E1C1ABF.