World’s standard definition of kilogram, Second&Metre

26th General Conference on Weights & Measures in a historic decision unanimously redefine World’s standard definition of kilogram, Second&Metre 

A landmark moment in scientific measurement, New International System of Units based on fundamental constants of nature to come into force from 20th May, 2019


The 26th meeting of the General Conference on Weights and Measures (CGPM) was held during November 13-16 2018 at Palais des Congrés, Versailles, France. CGPM is the highest international body of the world for accurate and precise measurements. The 26th CGPM meeting was very special and historic as the members have voted for the redefinition of 130 years old “Le grand K – the SI unit of kg” in terms of the fundamental Planck’s constant(h). The new definitions will come into force on 20 May 2019.

The CGPM comprises 60 countries including India and 42 Associate Members. In this meeting India was represented by Shri Avinash K. Srivastava, Secretary, Department of Consumer Affairs, Government of India, Dr. D.K. Aswal, Director, National Physical Laboratory(NPL), New Delhi and Dr. T.D. Senguttuvan, Head Planning, Monitoring & Evaluation, NPL.

The International Bureau of Weights and Measures (BIPM), the main executive body of CGPM has the responsibility of defining the International System of Units (SI). This revision of the SI is the culmination of many years of intensive scientific cooperation between the National Metrology Institutes (The national Physical Laboratory for India) and the BIPM. The dissemination of SI units for the welfare of society and industries in the country is the responsibility of Legal Metrology, Department of Consumer Affairs, Government of India.

Out of five draft resolutions the revision of the International System of Units and the definition of timescales are the important. The most important is the resolution on the revision of International System of Units. The definition of the seven base units namely, second, metre, kilogram, ampere, Kelvin, mole and candela has been changed from being linked to artefacts to being based on the fundamental constants on nature. Notably the definition of kilogram has been changed from being the mass of prototype sanctioned by the 1st CGPM held in Paris in 1889 and deposited at the BIPM to the Planck constant which is a physical constant. Likewise, the definition of metre has been changed to link it to the speed of light. There is a change in definition of time also. The change in the definition will result in uniform and worldwide accessible SI system for international trade, high- technology manufacturing, human health and safety, protection of environment, global climate studies and the basic science under-pinning these. The units are expected to be stable in the long term, internally self-consistent and practically realisable being based on the present theoretical description of nature at the highest level.

The International prototype of kilogram (IPK) is kept at the BIPM, Paris and serves as the international standard of kilogram. It is made of 90% platinum and 10% iridium and is a cylinder of 39 mm diameter and 39 mm height. Replicas of the IPK are made of the same material and used at BIPM as reference or working standards and national prototype of kilogram (NPK), kept at different National Metrology Institutes (NMIs). NPK-57, kept at CSIR- National Physical Laboratory, is sent periodically to BIPM for calibration. NPK further is being utilised through transfer standards of mass to provide unbroken chain of traceability for dissemination of mass through Legal Metrology to the user industries, calibration laboratories etc. The precise and accurate measurements help country in the production of international quality products and help commerce through elimination of the technical barrier to trade.

Kibble balance is a self-calibrating electromechanical balance and provides the measurements of mass, traceable in terms of electrical parameters and provides linkage of macroscopic mass to the Planck constant(h). NPL-UK, NIST-USA,NRC- Canada, PTB-Germany etc. Have successfully developed Kibble balance for 1 kg with an uncertainty of measurement in order of 10-8. NPL-India, in association with Department of Consumer Affairs, Government of India is looking forward for the development of 1 kg Kibble balance.

The advantages of Kibble balance would be that the NPK need not to be sent to BIPM for calibrations and the accuracy and stability of Kibble balance is very high which is very important where low weights with high accuracies are essential, for example in pharmaceuticals and biotechnologies.

After the kilogram’s definition is changed officially- on 20th May, 2019, also known as World Metrology Day- most people will never notice the difference. It would not change baking ingredients on a kitchen scale, or even have an effect on the tons of goods shipped globally every day.

For astronomers calculating the movements of stars and galaxies or for pharmacologists trying to define doses of medications sown to the molecule, the new standard of measurement could change the way they work.But for many metrologists, thatday to day work is not necessarily what inspired this change. The metric system was intended to be rational, universal set of units “for all people, for all time”.The SI unit will finally be truly universal system, free of any human artefacts.

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