26th General Conference on Weights and Measures redefines standard definition of Kg

The 26th CGPM was special and historic, as the members 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 May 20, 2019.

Nov 21, 2018 09:54 IST
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The 26th meeting of the General Conference on Weights and Measures (CGPM) was held from November 13-16, 2018 at Palais des Congrés, Versailles, France.  The CGPM is the highest international body of the world for accurate and precise measurements.


The 26th CGPM was special and historic, as the members 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 May 20, 2019.

CGPM Members

The CGPM comprises 60 countries including India and 42 Associate Members. India was represented by Consumer Affairs Secretary Avinash K Srivastava, National Physical Laboratory (NPL) Director D K Aswal and Head of NPL’s Planning, Monitoring & Evaluation TD Senguttuvan.

Key Highlights

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).

The 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 important.

The most important is the resolution on the revision of the 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.

New Definitions

Kilogram: The definition of the 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.

Metre: The definition of the metre has been changed to link it to the speed of light. Similarly, there is a change in the definition of time also.


The change in the definitions will result in uniform and worldwide accessible SI system for international trade, high- technology manufacturing, human health and safety, protection of the environment, global climate studies and the basic science underpinning 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.

Old Definition of Kilogram

  New International System of Units to come into force

The International prototype of kilogram (IPK), which serves as the international standard of kilogram,  is kept at the BIPM, Paris.

It is made of 90 per cent platinum and 10 per cent iridium and is a cylinder of 39 mm diameter and 39 mm height.

The 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).

The NPK-57, kept at CSIR- National Physical Laboratory, is sent periodically to BIPM for calibration.

What was the problem with the system?

Having an artifact as the world standard for a measurement unit has two fundamental problems.

1.  The first is that it can change value over time but, because it’s the world standard, there is no way of knowing how it changes, as it’s the standard. If the world standard changes, all measurements traced to it will also change.

2. The second problem is that it is difficult to bring every mass-measurement device to France every time it needs calibration. Thus, the national standards labs such as NIST in the US and NPL in the UK have their own kilogram standards that they periodically bring to France for comparison against the Prototype.

New Definition of Kilogram

New Definition: The kilogram is defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 × 10−34 when expressed in the unit J s, which is equal to kg m² s−1, where the meter and the second are defined in terms of c and Δv.

When it comes into force on May 20, 2019, the new kilogram definition will do away with the comparisons to the International Prototype.

It will enable the national labs to create their own standard for the kilogram because it will be reproducible.

However, it is important to note that Kilogram won’t be changing, just the way it is defined.

Thereon, the process for realising a kilogram will rely on a tool called Kibble balance.

What is Kibble Balance?

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).

  New International System of Units to come into force

The national labs of UK, US, Canada and Germany have successfully developed Kibble balance for 1 kg with an uncertainty of measurement in order of 10-8.

The NPL-India, in association with the Department of Consumer Affairs, Government of India is looking forward for the development of 1 kg Kibble balance.

Advantages of Kibble Balance

The main advantage of the tool would be that the national prototype of kilogram would not have to be sent to BIPM for calibrations.

Further, the accuracy and stability of the Kibble balance would be very high, which is very important where low weights with high accuracies are essential, for example in pharmaceuticals and biotechnologies.

How does it work?

The tool uses coils and magnets wherein the current in the coil exerts a known force and that current can be measured to sufficient accuracy.

As the definition of current has already been established in terms of magnetic-field force, it can be used to derive an upward force from which the mass of an unknown object can be compared. When the forces are equal, the mass of the unknown object can be measured.

The voltage and current measurements are based on Planck’s Constant.

Significance: As Kibble balances already exist in national metrology labs, the kilogram can now be recreated across the planet.

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