The Future of the Mass Artifact in the New SI

Abstract

As the International System of Units continues the transformation from a system based on fundamental units to one based on fundamental constants the question is often asked: what will happen to the artifacts that once served as the cornerstone of the realization process? In the case of the kilogram, it is a common misconception that the International Prototype Kilogram (IPK) and its copies will have little more than historical value as the kilogram takes on its new definition based on the Planck Constant. The truth of the matter is, stable artifacts will be just as important in the new realization and dissemination of mass. At the National Institute of Standards and Technology (NIST), artifacts will play an integral role in the mise en pratique, or practical method, for the realization and dissemination of the new mass standards. The kilogram will be realized in the NIST-4 Watt Balance, where the power generated by moving a mass artifact through a gravitational field will be balanced by an electrical power involving quantum standards. From there, it is the artifact that will be transferred under vacuum, via a custom mass transport vehicle, to either a vacuum balance for sorption studies or the NIST Magnetic Suspension Mass Comparator for a direct calibration of another artifact in air. Following this, the artifacts will be housed in either vacuum or air environments, comprising the pool of standards from which mass is disseminated to customers. To prepare for this new system of mass, the Mass and Force Group has taken exquisite care to control the conditions that these artifacts will be exposed to. Identical chambers have been designed and fabricated to provide two separate environments for mass artifact storage. One chamber will be kept under vacuum and the other will contain filtered lab air at nominally atmospheric conditions. In this paper the design, fabrication and assembly of the chambers will be detailed. The mass transfer and handling processes employed by the Mass and Force groups will be explained. Finally, the overall impact of the redefinition on the artifact-based mass system will be discussed.