Realisation of the awaited definition of the kilogram - resolving the discrepancies

Short Name: kNOW, Project Number: SIB03
Courtesy of PTB

Kilogram redefinition - resolving discrepancies

A redefined SI kilogram will have two possible practical realisations based on the Planck constant, one based on using the Kibble balance and another based on atom counting. The EMRP project Realisation of the awaited definition of the kilogram (kNOW) refined and improved the current approaches for measuring the Planck constant to ensure experimental results from different NMI and from different experiments are consistent and available for use in the realisation of the new kilogram. It also demonstrated a successful traceability route to enable industrial users to access the redefinition of the kilogram via silicon spheres opening up the possibility of more direct SI traceability for end users.


The Plank constant can be experimentally determined using a Kibble balance or by the Avogadro experiment. The Kibble balance essentially operates like a traditional balance, with a mass on one side, and a current-carrying coil of wire within a magnetic field on the other. By measuring the electromagnetic force needed to balance the mass, the Planck constant can be determined in terms of quantum electrical units. An alternative approach for experimentally determining the kilogram is to relate the Avogadro constant, the number of atoms in one mole of a substance, to the Planck constant by counting atoms that form a silicon sphere.


This project sought to address the unresolved inconsistencies between these two approaches and to bring practical realisations of the kilogram by both methods into more precise agreement, through:

  • Improving the European Kibble balances to optimise their performance for measuring the Planck constant.
  • Confirming the existing Avogadro number by measuring the number of atoms in two pure one kilogram silicon-28 spheres constructed by the International Avogadro Coordination Project.
  • Resolving experimental inconsistencies by comparing the Kibble balance and atom counting methods and confirming that both can generate an appropriate level of accuracy in realising the SI kilogram.
  • Participating in an international kilogram comparison exercise that demonstrated that the results of the Avogadro and Kibble balance experiments could be linked to the International Prototype Kilogram maintained by BIPM, in Paris.

Overcoming these technical challenges has enabled the European metrology community to make an important contribution to improving the determination of the Planck and Avogadro constants. The project partners also worked with NMIs in the USA (NIST) and Canada (NRC) to eliminate the previous differences in the value of the Planck constant realised by the Kibble balance and Avogadro approaches. This brings all measurements into agreement and is an essential step in the redefinition of the kilogram.

Research conducted in this project and its precursor projects under iMERA+ have made a significant contribution to the proposed redefinition of the kilogram in 2018. The redefinition will support future improvements in the accuracy of mass and related units which supports emerging nano- and biotechnologies that require measurements on ever-finer scales.

The Avogadro constant and a new definition of the kilogram

International Journal of Mass Spectrometry

A More Accurate Measurement of the28Si Lattice Parameter

Journal of Physical and Chemical Reference Data

Surface Layer Analysis of Si Sphere by XRF and XPS

IEEE Transaction on Instrumentation and Measurement

Homogeneity Characterization of Lattice Spacing of Silicon Single Crystals

IEEE Transaction on Instrumentation and Measurement

Etching of silicon surfaces using atmospheric plasma jets

Plasma Sources Science and Technology

Current State of Avogadro 28Si sphere S8


Instrumental neutron activation analysis of an enriched 28Si single-crystal

Journal of Radioanalytical and Nuclear Chemistry

Other Participants
Leibniz-Institut für Oberflächenmodifizierung (Germany)
National Institute of Advanced Industrial Science and Technology (Japan)