The METAS 1 kg vacuum mass comparator - adsorption layer measurements on gold-coated copper buoyancy artefacts

A new 1 kg mass comparator has been designed at the METAS to be used for research activities in comparing 1 kg mass and buoyancy artefacts between air and vacuum environments. The instrument is a single-pan mass comparator using electromagnetic force compensation. A novel feature is a wire suspension system for the balance beam. This suspension, together with the differential plane mirror interferometer used for the alignment, provides a very high resolution of 50 ng. The reproducibility of the comparator in vacuum is 0.8 µg. The vacuum mass comparator was first characterized and then used for the determination of the adsorption layers on special 1 kg gold-coated copper buoyancy artefacts with large variations in surface area. The adsorption layer information is used to correct the mass value (determined in air) for a watt balance experiment in which measurements on a gold test mass are performed under vacuum.