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

摘要

A paradigm shift will occur in 2018 when the International System of Units (SI) is redefined, providing an opportunity to realize a mass artifact upon the precise determination of a value of the Planck constant, h. Thus, the new primary mass standard for the US will no longer be an object but rather realized through a fundamental constant of nature determined by a self-calibrating instrument, the NIST-4 watt balance. In order to disseminate this new definition to the public, the National Institute of Standards and Technology (NIST) has also developed a novel Magnetic Suspension Mass Comparator (MSMC) capable of transferring a vacuum-based definition into air. This vacuum-to-air transfer is only worthwhile if the artifacts measured by the watt balance can be physically transferred to the MSMC without exposing the masses to atmospheric air. To achieve this goal, a custom vacuum transfer system was developed to transport mass artifacts from NIST-4 to the MSMC, which are separated by roughly 100 meters of laboratories and hallways. A Mass Transport Vehicle (MTV), or a mini mobile vacuum chamber housing a mass, is capable of docking to a series of load locks, mass exchange points and transfer arms built into both instruments. Each stage of the mass transfer process introduces challenges including material selection of artifact handlers, strict dimensional design constraints and logistics of maintaining a clean, reproducible vacuum transfer environment.