Watt Balance METAS II

• Research unit: METAS
• Project number: F-5010.30017

Key words (English)

Watt balance, kilogram, unit of mass, International System of Units SI, Planck constant

Short description (English)

During the last years, a considerable international metrological effort has been made in the projects related to the new definition of the kilogram. Within the framework of this activity, METAS was strongly implied by initiating three projects: Watt Balance, Vacuum Comparator M-One, Surface Analysis by Coupled Methods. These activities are strongly dependent to each other and allow METAS to participate actively in a metrological effort of world range.

The essential condition for a new definition of the kilogram is an agreement between the results obtained by three different watt balances and the Avogadro experiment with a relative uncertainty of about 10^-8.

In agreement with the resolutions of the "23ème Conférence Générale des Poids et Mesures", and to ensure the future realization of the kilogram at METAS, a new watt balance will be developed in collaboration with universities, industry and technical schools.

Project aims (English)

The goal of BWM II project is to ensure the continuity of the efforts made at METAS during the last ten years for the new definition of the kilogram and its realization. In order to maintain METAS in the leading group of the institutes implied in the new definition of the kilogram, BWM II proposes to develop, on the basis of the knowledge and results acquired over the years, a competitive watt balance, satisfying the essential requirements for a new definition of the kilogram.

Abstract (English)

The watt balance BWM II was developed and built according to the project plan and is fully operational. Comprehensive test measurements were carried to assess the uncertainty of the instrument. A combined standard uncertainty of 0.3 ppm was reached which is considerably higher than the target uncertainty. The main reasons for the limited accuracy were identified and improvement measures have been worked out. The implementation of these measures is the subject of a follow-up project.

Transfer and application (English)

On the basis of the project results, an improved version of the experimental set-up will be built. The target uncertainty of a few parts in 10⁸ should be reached shortly after the date of implementation of the revised SI in May 2019. The plan is to contribute to the realization of the newly defined kg and to disseminate the unit to customers on a regular basis.

Publications / Results (English)

[A. Eichenberger, G. Genevès, and P. Gournay, “Determination of the Planck constant by means of a watt balance,” Eur. Phys. J. B, vol. 172, pp. 363–383, 2009.
Baumann H, Klingel´e E E, Eichenberger A, Richard P and Jeckelmann B 2009 Eval-uation of the local value of the earth gravity field in the context of the new definition of the kilogram Metrologia 46 178–86
A. Eichenberger, H. Baumann, B. Jeanneret, B. Jeckelmann, P. Richard, and W. Beer, “Determination of the Planck constant with the METAS watt balance,” Metrolo-gia, vol. 48, pp. 133–141, 2011.
Eichenberger A, Baumann H, Jeckelmann B, Tommasini D, Cosandier F, Clavel R, Beguin C, and Reber D 2012 The METAS watt balance Mark II experiment Confer-ence on Precision Electromagnetic Measurements (CPEM) pp. 426-427
Baumann H et al 2013 Design of the new METAS watt balance experiment Mark II Metrologia 50 235
F. Cosandier, Phd Thesis, ‘Conception d’Axes Motorisés Rectilignes d’Ultra-Haute Précision’, EPFL 2013
F. Cosandier, A. Eichenberger, H. Baumann, B. Jeckelmann, M. Bonny, V. Chatagny, and R. Clavel, "Development and integration of high straightness flexure guiding mechanisms dedicated to the METAS watt balance Mark II", Metrologia 51, S88-S95 (2014)
Baumann H et al 2015 Experimental assessment of the speed of light perturbation in free-fall absolute gravimeters Metrologia 52 635