The Design and Construction of the In-Vacuum Mass Exchange System for the Realization and Dissemination of the New SI Unit of Mass

The international system of units will complete a transition in 2018 from a system based on seven fundamental units to a system of seven fundamental constants. More specifically, regarding the SI unit of mass, the kilogram (kg) will be realized in terms of a fixed value of the Planck constant. At the National Institute of Standards and Technology, a watt balance will be used to relate the kilogram to the Planck Constant. One major challenge introduced with this new definition involves the environment where the realization occurs. In traditional mass metrology, all comparisons are completed in air and a chain of traceability can be completed back to the International Prototype Kilogram using conventional mass balances. In the new SI, the watt balance will be operated under vacuum. The vacuum environment is important for multiple reasons, including the elimination of the buoyancy correction and the introduction of sorption effects if the artifacts are transferred from vacuum to air. In order for the mass community to utilize the new vacuum-based definition in air, the scientists in the Mass and Force Group are constructing a magnetic suspension balance. This experiment will allow comparison of a mass in vacuum from the watt balance to an artifact in air that will be used for dissemination. This vacuum-to-air transfer is only worthwhile if the artifacts in the watt balance can be transferred to other instruments without breaking vacuum. To solve this problem, a complete, custom vacuum transfer system was developed for mass artifacts. The system is comprised of a series of load locks, mass exchange points and vacuum transfer arms to connect the mass transport vehicle to each experiment. Each stage of mass transfer introduces new challenges including material selection of artifact handlers, strict component design constraints and the logistics of maintaining a clean, reproducible vacuum environment during transfers that can span 100 meters of labs and hallways. This paper will explain the growing importance of vacuum technology in mass metrology and discuss how each of the aforementioned design problems were solved to create the vacuum transfer system in place today.