The NIST Mise en Pratique since the Redefinition of the Kilogram

Summary

As part of a revised SI system of units adopted in 2019, the kilogram was redefined and realized in terms of Planck's constant. By experimental necessity, the new realization occurs at high vacuum pressures. This presents a unique challenge for the world's National Metrology Institutes (NMI’s); how do you transfer the new definition of the kilogram from high vacuum pressure to atmospheric pressure where nearly all mass metrology is performed? The solution to this challenge is a new Mise en Pratique for the revised SI kilogram. Mise en Pratique is a French phrase meaning "to put in practice" and the NIST Mass & Force Group has developed its own mise en pratique for transferring the vacuum realization of the kilogram to the world of mass metrology that operates at atmospheric pressure.

Description

Platinum and stainless-steel mass artifacts in one of NIST’s precision vacuum-mass comparators.

Realization

Since the redefinition of the kilogram, the NIST Mass and Force Group has been closely involved with the NIST-4 Kibble balance project and its realization of the electronic kilogram via the Kibble Principle. Traceability post-realization is maintained through what is referred to as the Consensus Value (CV). The CV is determined by the ensemble of realization experiments that participate in International Key Comparisons and represents the weighted average realization value referenced to Planck’s constant via the International Prototype Kilogram (IPK). By maintaining traceability through the 2019 calibration of NIST’s National Prototype K20 to IPK combined with the CV adjustment, the group has been able to accurately monitor K79/K4 in air and K85/K104 in vacuum, which were all used during the international Key Comparisons for determining the CV of the kilogram. On February 1, 2021, the world’s mass scale officially became traceable to the CV. To read more about this key comparison and dissemination process see the links at the bottom of this page.

A Cross-sectional view of the Magnetic Suspension Mass Comparator (MSMC).

Dissemination

The mise en pratique of the newly defined kilogram consists of the realization contribution from the NIST-4 Kibble Balance to the international Consensus Value, the transfer of that realization contribution to air, then the dissemination to the U.S. Measurement System, and ultimately storage of NIST-4 Kibble Balance Key Comparison value in a custom vacuum system through an ensemble of vacuum-mass artifacts. The realization/CV can be transferred to mass artifacts at atmospheric pressure using a vacuum-to-air process and one of the Mass and Force Group’s precision vacuum-mass comparators or using a specially designed instrument that enables direct comparison of a mass in vacuum to a mass in air using a Magnetic Suspension Mass Comparison (MSMC) technique. The MSMC project is ongoing but is currently in hiatus.

Storing a Mass Realization

The NIST-4 Kibble balance is used infrequently to realize mass due to its operational complexity. Therefore, storing a mass realization value and monitoring its mass evolution is critical for accurate dissemination. NIST has created an ensemble of kilogram artifacts, consisting of Platinum-Iridium and stainless-steel standards that act as a “flywheel” for the new kilogram in between Key Comparisons/Realizations. Half of the ensemble are stored in high vacuum and half are stored in laboratory air. The “flywheel” mass artifacts that are stored in vacuum are kept in a custom vacuum storage chamber. To transfer one of these mass artifacts from the NIST-4 Kibble Balance a Mass Transfer Vehicle (MTV) is used to transfer the artifact, under vacuum, from the NIST-4 Kibble balance to the Mass and Force Group’s custom vacuum storage chamber. The kilogram artifact used in the Key Comparison mass realizations from the NIST-4 Kibble Balance can subsequently be used to transfer the realization value to other artifacts by using one of the Mass and Force Group’s high precision vacuum-mass comparators. This process can be repeated periodically to monitor the evolution of the realization kilogram artifacts. A vacuum-to-air transfer process can then be used to transfer the realization value to the Mass and Force Group’s working standards in laboratory air.

• CCM detailed note on the dissemination process after the redefinition of the kilogram
• Report on the CCM key comparison of kilogram realizations (CCM.M-K8.2019)

The NIST Mass and Force Group’s custom vacuum storage chambers used for the storage of the ensemble of mass artifacts.