The new kilogram definition coming soon

2018 October 02d.

What is a kilogram?

The SI unit of mass, the kilogram (kg), is defined as being equal to the mass of the international prototype of the kilogram, the IPK. The origins of the kilogram date from 1799, when a cylinder of pure platinum was manufactured to have the same mass as one cubic decimetre (equivalent to a litre in today’s terms) of water at 4 ºC. The current kilogram replaced this original weight in September 1889 when the first General Conference on Weights and Measures (CGPM) sanctioned the IPK for use as the global standard for the mass. The IPK is a cylinder of 39 mm in height and diameter and is made from an alloy of platinum (90 %) and iridium (10 %).

The IPK is kept at the International Bureau of Weights and Measures (BIPM) in Sèvres just outside Paris. National Measurement Institutes, such as the National Physical Laboratory, possess copies that they use as their national standards. These national standard kilograms are tested against the IPK about every 40 years to see whether their mass has deviated from that of the IPK.

The “new” kilogram

For the last 30 years, a considerable amount of research has been undertaken to develop an alternative, more fundamental definition for the kilogram. In 2011, the General Conference on Weights and Measures (CGPM) and the International Committee for Weights and Measures (CIPM) decided that the kilogram should be redefined in terms of the Planck constant (h). The redefinition of the unit is scheduled for 2018. Such a definition would theoretically permit any apparatus that was capable of realising the kilogram in terms of the Planck constant to be used, as long as it possesses sufficient precision, accuracy and stability and had demonstrated equivalence with other realisations. In order for this to be feasible, the development of a practical means of traceability between a new definition of the kilogram and the (current) mass scale is necessary, both for initially fixing the value of the Planck constant to redefine the mass unit, with relation to current scale, and for the subsequent dissemination of the unit with reference to the fundamental realisation (in vacuum). The NewKILO JRP, a three year project of the European Metrology Research Programme (EMRP) involving 13 National Metrology Institutes (NMIs) and co-funded by the European Union (EU), was completed in May 2015. The future definition will require the unit of mass to be realised in a vacuum, instead being maintained and used in air, which presents some challenges to establish traceability to the mass scale. 

new old kilo
In parallel, METTLER TOLEDO has developed new apparatus to enable the transfer of artefacts to the M_one mass comparator under a controlled atmosphere (such as vacuum or inert gas), preventing air contamination. This apparatus consists of an Artefact Transfer Device (ATD) and an Artefact Storage and Transport Vessel (ASTV). The ASTV is made of high-grade stainless steel and can accommodate a range of 1 kg weight types including Pt-Ir cylinders, silicon spheres, and stainless steel OIML shaped or cylinder weights, without the need for adapter plates. Automated Gravimetric Centering (AGC) ensures correct positioning, minimizing eccentricity and ensuring the highest possible repeatability. 
The M_one mass comparator, with a resolution of 100 nanograms and a typical repeatability of 300 nanograms, enables NMIs to achieve the most accurate results for weights calibration, with the smallest possible measurement uncertainties, in the shortest time. External influences are reduced to a minimum using vacuum technology which enables measurements under a controlled environment from ambient pressure down to a vacuum of 10-6 mbar (10-4 Pa). As the future definition of the kilogram will be measured in a vacuum, the M_one mass comparator, along with the Artefact Transfer Device (ATD) and Artefact Storage and Transfer Vessel (ASTV), enable transfer of an artefact from one piece of apparatus to another whilst keeping it constantly under vacuum. These devices will play a vital role in current and future research to support the redefinition of the kilogram project, including dissemination of the new kilogram following the redefinition in 2018. 


METTLER TOLEDO is a leading global manufacturer of precision instruments. The Company is the world’s largest manufacturer and marketer of weighing instruments for use in laboratory, industrial and food retailing applications. The Company also holds top-three market positions for several related analytical instruments and is a leading provider of automated chemistry systems used in drug and chemical compound discovery and development. In addition, the Company is the world’s largest manufacturer and marketer of metal detection systems used in production and packaging. 


The M-one vacuum mass comparator from METTLER TOLEDO

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