Characterization of the NIST Magnetic Suspension Mass Comparator Apparatus and Facility

With the upcoming redefinition of the kilogram set for 2018, National Metrology Institutes are working to identify and reduce uncertainties related to the realization and dissemination of the kilogram. In the current system the kilogram is equal to the mass of the International Prototype Kilogram (IPK) with zero uncertainty. In the "New SI", the Watt Balance and Avogadro Experiment will use a fixed value of Planck's constant (zero uncertainty) and realize the kilogram based on that fixed value. The previous uncertainty associated with Planck's constant will essentially be transferred to the kilogram. In addition, the new realization will occur under vacuum and new sources of uncertainty in the dissemination to air will have to be accounted for. At the National Institute of Standards and Technology, the Mass and Force group is developing a unique system for disseminating the kilogram realized in vacuum to air where customers can continue their usage of calibrated masses without affect. Currently, the widely accepted method for vacuum-to-air dissemination involves making measurements in both environments and then building an empirical model to account for the sorption of particles when transferring between vacuum and air. At NIST, a magnetic suspension mass comparator (MSMC) is utilized in order to directly compare a mass in vacuum to a mass in air. In order to meet customers needs it is vital that the uncertainty in our MSMC measurement is well understood so that it can be accounted for. Contributing to the overall uncertainty are uncertainties in the measurement environment, in the suspension apparatus and in the measurement facility itself. To accurately characterize this process, the Mass and Force group has measured the gravitational gradient along the measurement axis, the vibrations of the lab floor and the ambient temperature and humidity stability of the room. Additionally, we have worked to model the stray magnetic fields emanating from the MSMC. This talk will detail the methodology and results of these characterization efforts and explain how each factor influences the final uncertainty budget.