Mass Calibration at NIST in the Revised SI

Abstract

The unit of mass has been realized by the International Prototype Kilogram (IPK) for over 130 years. This will change very soon. The revision of the International System of Units (SI) that will take effect on May 20, 2019 will fundamentally change the way the United States mass scale is realized by NIST at the one kilogram level and below. For example, below 50 mg, very precise measurements of capacitance gradient by the NIST Electrostatic Force Balance (EFB) will extend the lower end of the NIST mass scale to 100 micrograms or less and improve uncertainties by a factor of ten over what they are now, all while eliminating laborious work-downs from one kilogram standards. Between 100 g and one kilogram, the NIST-4 Kibble balance will realize mass from quantum-based electrical and mechanical power measurements. Mass transfer between the vacuum environment of the Kibble balance and the mass metrology performed in laboratory air pressure will be accomplished by a unique-to-NIST magnetic suspension-based mass comparator that will allow a test mass to be directly calibrated against an artifact whose mass has been determined by the Kibble balance. When considered in its entirety, the NIST mass scale under the revised SI will be easier to realize, easier to maintain, and have equal or smaller uncertainties that the mass scale that is traceable to the IPK. This presentation will illustrate how the NIST mass scale at one kilogram and below is constructed using the new instruments described above. An uncertainty budget covering the range from 1 kilogram to 100 micrograms will be given and the techniques that are used for mass dissemination in this range will be described.