Invited Paper VT-MoM1
The Important Role of Vacuum Technology in the Redefinition of the Kilogram

Monday, October 28, 2013, 8:20 am, Room 202 C

The kilogram is the sole remaining unit in the International System of Units (SI) that is still defined by a physical artifact, namely, the International Prototype Kilogram (IPK). There are two motivations for redefinition: To comply with a “New SI” that realizes units from invariant natural constants, and to address the discovery of a divergence in the mass values between the IPK and its official copies that was discovered during the third periodic verification of national prototypes of the kilogram (1988-1992). Because mass is such an important area of metrology for commerce, any redefinition of the kilogram must not hinder the metrological community in terms of unacceptably large uncertainties compared to what is currently available through traceability to the IPK. After careful consideration, in October of 2011 the General Conference on Weights and Measures (CGPM) adopted a resolution to redefine the kilogram in terms of the Planck constant, h. The Consultative Committee for Mass and Related Quantities (CCM) has recommended that the Planck constant be known to within a standard relative uncertainty of 2 parts in 10⁸ before redefinition occurs in order to address the metrological concerns mentioned above. World-wide, several efforts that measure the Planck constant are in progress, and more are expected to begin in the next few years. The two major apparatus used to measure the Planck constant are the Watt balance and the X-ray crystal diffraction (XRCD) experiments. Though these methods use very different physical principals, they have in common the necessity to operate in medium to high vacuum conditions in order to mitigate contamination and reduce uncertainties. Therefore, the new kilogram will be realized in a vacuum environment, which obviates the need for transferring this realization to air where it can easily be disseminated via conventional mass metrology. This transference from vacuum to air requires precision mass metrology under vacuum, an understanding of adsorption and desorption phenomena related to mass artifacts, transportation of mass artifacts under vacuum between experiments for inter-comparisons, and an understanding of the effects of long-term storage of mass artifacts in air, inert gases, and under vacuum. In short, application of many of the fundamentals of vacuum technology is absolutely essential in the realization and dissemination of a kilogram that is defined in terms of the Planck constant. This talk will provide details of the many experiments that are underway to address the vacuum technology challenges presented in the redefinition of the kilogram.