Vacuum mass comparator supports the redefined kilogram

Jon Lawson

The METTLER TOLEDO M_one has long established itself as the reference instrument of choice for mass comparison under a controlled environment. Today, improved artefact storage and transfer capabilities are helping to minimize uncertainty even better than past models, putting the newly-designed M_one at the centre of the race to define a new kilogram based on physical constants.

The race to redefine the kilogram involves two experiments: one using a Watt balance and the other using the Avogadro principle. To share and disseminate the investigated artefacts, each artefact must be transferred in a contamination-free atmosphere. Eliminating human error through automation, the M_one vacuum pumping system enables the necessary controlled environment starting at ambient pressure and going down to a vacuum level of 10-6 mbar.

Ergonomic design allows users to place weights directly onto the M_one turntable through a large, quick-loading door. The system is sealed and allowed to reach desired atmospheric conditions. Once this pressurized environment is reached, the artefact is never exposed to air again. A newly designed Artefact Storage and Transport Vessel (ASTV), Artefact Transfer Device (ATD), and automatic Load Lock arm facilitate easy transfer of the artefact and preserve the pressure required to deliver necessary stability.

Once the artefact reaches the measurement chamber during comparisons, it is placed on a star-shaped weighing pan designed to accept various artefact shapes, such as cylindrical weights and silicon spheres from 100g – 1kg without adapter plates. Automated Gravimetric Centreing(AGC) ensures correct positioning, minimizing eccentricity and ensuring the highest possible repeatability.

The M_one is famous for its reproducibility; in fact, 28 NMIs have staked their reputation on the modular METTLER TOLEDO system. Extensive M_one adoption is further enhancing redefinition efforts by allowing artefact intercomparisons at minimised uncertainty.

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