From the earliest civilizations through the discovery of the Higgs boson, measurement of the quantity now known as "mass" has always been important; contemporary mass measurements are required for commerce, technology, and even tests of fundamental physics. Reliable measurements of mass and its "related quantities" which include force, pressure, hardness and density of materials, viscosity of liquids, the flow of liquids and gases through pipelines and other conduits, and the gravitational acceleration on the free-fall Earth's surface underpin measurement results in many diverse areas. At the highest level of accuracy, these activities require the collaboration of National Metrology Institutes whose experts participate in the specialized working groups of the Consultative Committee for Mass and Related Quantities (CCM). Much of their work involves "key comparisons" designed to test the principal techniques of each field and to help assess participants' measurement capabilities.
At present, a significant CCM activity focuses on the unit of mass, the kilogram. Since 20 May 2019 the kilogram is defined in terms of the Planck constant, a fundamental constant of quantum physics, which by its nature is invariant and universally accessible. It has replaced the last artefact definition of the SI, the mass of a unique object known as the "international prototype of the kilogram" which had served to define the kilogram since 1889. The new definition of the kilogram can be realized by any suitable method, for example using a Kibble balance, based on the equivalence of electrical and mechanical power, or by a specially-prepared sphere of silicon, based on the perfect crystal structure. The methods which can be used to realize the kilogram in practice are described in the "Mise en pratique for the definition of the kilogram in the SI".
An important task of the CCM is to ensure the consistency of independent realizations in the revised SI, and to provide international recognition of the NMIs' measurement capabilities within the framework of the CIPM MRA, by organizing key comparisons at the highest metrological level.