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News article20 February 2019

Measurements matter in our daily life: International System of Units revised in a historic decision

International System of Units (SI)

Citizens, consumers, patients, doctors, control authorities, travellers and industry need to be able to rely on sound and comparable measurements to take the right decisions.

As a significant part of measurements are done in laboratories, they need to be able to measure correctly and provide reliable results independent of national borders. Very recently, in a landmark decision, representatives from 54 of the BIPM's (Bureau International des Poids et Mesures) Member States voted on 16 November 2018 to revise the International System of Units (SI), changing the world's definition of the kilogram, the ampere, the kelvin and the mole.

The decision, made at the 26th meeting of the General Conference on Weights and Measures (CGPM) in Versailles, France, means that all SI units will now be defined in terms of constants that describe the natural world. This will assure the future stability of the SI and open the opportunity for the use of new technologies, including quantum technologies, to implement the definitions.

The new SI (System of Units)

The International system of units, SI, has been transformed from a system of manmade objects, towards the application of devices using certain selected natural constants as a base. This is the largest revision of the SI system since 1890.

Already in 1983 (at the 18th General Conference on Weights and Measures, CGPM) the unit of time (the second) and the unit of length (the metre) were defined in terms of natural constants.

At the recent, 26th, CGPM it was decided, among other things, to replace the kilogram artefact, which over time has shown to be the hardest change to make.

How it is done

Within the SI, the values of the natural constants are now defined, and thus become fundamental. This is in comparison to earlier times, when the natural constants were measured and the manmade objects were fundamental.

- Time -> hyperfine transitions in caesium (1983)

- Length -> speed of light (1983)

- Mass -> Planck's constant (2018)

- Temperature -> Boltzmann's constant (2018)

- Electric current -> electron charge (2018)

- Amount of substance -> Avogadro's number (2018)

- Light intensity -> KCd 1/683 watt / steradian (1967) (The power unit watt is derived from natural constants above)

What the implications are

Anybody with enough resources and expertise can build or purchase a device producing the well- known units based on fundamental natural constants. The quantities have been chosen so that the revised definitions will not need to be modified to accommodate future improvements in the technologies used to realise them.

The BIPM will no longer be the sole provider of the kilogram. The future role, as already is the case with time and length, is to maintain the highest quality of measurements, and act as coordinator for the national measurement institutes.

How it affects the common European citizen

The common citizen will not notice anything in daily life. Buying a kilogram of bananas or potatoes will be the same as before the redefinitions.

The main impact is where accurate measurements are a basic prerequisite, like in science, engineering, industry, health and commerce. In addition, accurate and traceable measurements contribute to better management of resources, sustainable growth, a clean environment, and to a better world to live in.

JRC's involvement

JRC has contributed with so-called mass spectrometry measurements to determine the isotopic purity of special materials used in the realization of the new SI.

Further information on the website.


Publication date
20 February 2019