Fellmuth et al (2011 Metrologia 48 382-90) published the first value of the Boltzmann constant k determined by dielectric-constant gas thermometry at the triple point of water (k = 1.380 654 × 10~(-23) J K~(-1), standard uncertainty 9.2 parts per million (9.2 ppm)). Since that time, essential progress of this primary thermometry method has been achieved concerning the design and the assembly of the measuring capacitor, the determination of its effective compressibility, the sensitivity of the capacitance bridge, the influence of stray capacitances, the purity of the measuring gas, the pressure measurement, and the scattering and the evaluation of the data. The resulting new k value amounts to k = 1.380 6509 × 10~(-23) J K~(-1) with a standard uncertainty of 4.3 ppm. This value is about 1.5 ppm larger than the CODATA 2010 one, which has a relative uncertainty of 0.9 ppm.
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