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NIST Mise en Pratique of the New Kilogram Definition


As part of a New SI system of units to be adopted in 2018, the kilogram will be redefined so that it may be realized in terms of Planck's constant. By experimental necessity, this new realization will occur in a high vacuum environment, which presents a challenge for the world's National Metrology Institutes (NMI), namely, how do we transfer the new definition of the kilogram, realized in high vacuum, to atmospheric pressure air where mass metrology is performed? Mise en Pratique is a French phrase meaning "to put in practice", and by way of application a Mise en Pratique is a sequence of steps that are enacted to accomplish a desired outcome. The NIST Mass & Force Group is developing its own Mise en Pratique for transferring a vacuum realization of the kilogram to the world of measurement that operates in atmospheric pressure air.


The Mise en Pratique consists of the realization of the new kilogram in the NIST Watt Balance, and then transferring the realization to air and the U.S. Measurement System. The artifact is transferred from the Watt Balance under vacuum to a specially designed instrument that enables direct comparison of a mass in vacuum to a mass in air using a magnetic suspension technique.

Figure 1. Realization and Dissemination of the Redefined Kilogram. Left: Watt Balance realization; Center: Masses are transported back and forth by the Mass Transport Vehicle (MTV); Right: Dissemination of vacuum realization by magnetic suspension technique.


magnetic suspension technique
Figure 2. Magnetic suspension technique for direct comparison of the vacuum-realized kilogram to an artifact in atmospheric pressure air. Left: Suspension of empty mass pan in lower (air) chamber; Center: Suspension of artifact in lower (air) chamber; Right: Weighing in upper chamber (vacuum) of standard mass artifact calibrated by NIST Watt Balance.

Created July 1, 2015, Updated January 23, 2018