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Redefinition of the Kilogram


This project will conduct the research and development required to support the international efforts to prepare for a future redefinition of the unit of mass - the kilogram - the last remaining SI basic unit defined by an artifact. We use magnetic levitation techniques combined with precision mass measurements to provide a direct link between the future definition realized in vacuum and the current kilogram defined in ambient air.


Experiments to redefine the Kilogram with an invariant of nature (such as the Watt balance or the Avogadro project) are performed in vacuum, while current realization of the mass unit and all associated methods of dissemination of the unit are done in air. Transfer of the unit to vacuum requires an unbroken traceability chain to the International Prototype Kilogram, as well as characterization of the stability of the artifacts and their surfaces during transfers from air to vacuum and vice versa. This requirement remains a major challenge and obstacle for further advances in redefining the unit of mass. This project will establish the first complete stable realization and dissemination system that directly ties the current air-based Kilogram definition to the vacuum-based alternative definition using stable artifacts with wear-resistant and chemical inertness characteristics that are maintained in controlled environments.

Major Accomplishments:

  • Completed the design and in-house construction of the non-magnetic aluminum vessel accommodating separate air and vacuum chambers. Incorporated appropriate vacuum technology (pumps, gauges, and components), vacuum and leak testing, and design and construction of vibration isolated support structure.
  • Completed the design and construction of a new, non-magnetic aluminum magnetic suspension system to interface with sensitive (20 µg resolution) mass balance in air-to-vacuum vessel.
  • Achieved stable (less than 1 mg) levitation with 200 g – 1 kg mass artifacts using new suspension outside of the vacuum chamber.
  • Completed long-term (over 18 months) study of several diamond-like carbon (DLC) coated stainless steel 1 kg mass artifacts for mass stability. Several coating processes are being considered to achieve chemical inertness, wear resistance, and overall stability in the dissemination of the unit of mass.
U.S. National Prototype Kilogram. It is made of 90% platinum, 10% iridium.
U.S. National Prototype Kilogram. It is made of 90% platinum, 10% iridium.

Start Date:

February 1, 2008

Lead Organizational Unit:




  • U.S. States Weights & Measures Laboratories and other Calibration Laboratories
  • BIPM, National Metrology Institutes
  • SI Metrology and Fundamental Constants scientific communities


  • Argonne National Laboratory
  • BIPM
  • National Metrology Institutes


Dr. Zeina J. Kubarych, Program Manager

Related Programs and Projects:


Physical Measurement Laboratory (PML)
Mechanical Metrology Division (681)

General Information:
301-975-6624 Telephone
301-417-0514 Facsimile

100 Bureau Drive, M/S 8221
Gaithersburg, Maryland 20899-8221