Abstract
There is currently great interest in the international metrology community for new accurate determinations of the Boltzmann constant k, with a view to a new definition of the unit of thermodynamic temperature, the kelvin. Indeed, k is related to the quantum of energy kT, where T is the thermodynamic temperature. The value of the Boltzmann constant can be obtained from measurements of the velocity of the sound in a noble gas. In the method described here, the experiment is performed in a closed spherical cavity. To obtain an accurate value for k, all the parameters of the experiment (gas purity, static pressure, temperature of the device, exact shape of the cavity, etc.) have to be carefully controlled. As correction terms have to be applied to the acoustic signals, the validity of the theoretical models from which they are derived is crucial. The new determination carried out at the LNE-INM/CNAM is based on the same principles as in the acoustic experiment of Moldover et al. at NIST in 1988, which led to the most accurate determination of the Boltzmann constant up to now. However, several fundamental modifications and improvements have been made in this new experiment to measure and control the parameters that set the measured value of k. To cite this article: L. Pitre et al., C. R. Physique 10 (2009).
Highlights
Since the 10th (1954) and 11th (1960) General Conference of Weights and Measures (CGPM), the definition of the unit of thermodynamic temperature has been based on a temperature fixed point, the triple point of water assigned the exact value of 273.16 K [1,2].This definition links the magnitude of the unit of temperature, the kelvin, to a fundamental natural property of water, which is assumed to be uniform and sustainable
The uncertainty of the realisation of the triple point of water is unlikely to be reduced below the current value of 100 μK, which is higher than the differences in temperature that can be detected with the current sensors
The analysis of the results shows that one of the main constraints of the acoustic method used here is the realisation of a quasi-spherical resonator with a precise shape, adapted to the use of electromagnetic and acoustic models
Summary
Since the 10th (1954) and 11th (1960) General Conference of Weights and Measures (CGPM), the definition of the unit of thermodynamic temperature has been based on a temperature fixed point, the triple point of water assigned the exact value of 273.16 K [1,2]. The 20th and 21st CGPM strongly recommended the redefinition of the base units of the International System of Units (SI) in terms of fundamental constants of physics whose numerical values can be fixed [3,4], as has been done for the metre in 1983 [5] In this context, the kelvin could be redefined by fixing the value of the Boltzmann constant k, linking the thermodynamic temperature to the quantum of thermal energy and to statistical thermodynamics. – a sufficiently low relative uncertainty on the value of the Boltzmann constant, i.e. about 10−6; – an international consensus on the numerical value
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