International System of Units agreement being commemorated today

Intergovernmental organization BIPM outlines historic changes to the International System of Units, or the SI

Today, May 20, is World Metrology Day, commemorating the anniversary of the signing of the Metre Convention in 1875.

This treaty provides the basis for a coherent measurement system worldwide that underpins scientific discovery and innovation, industrial manufacturing and international trade, as well as the improvement of the quality of life and the protection of the global environment.

The theme for World Metrology Day 2019 is “The International System of Units – Fundamentally Better”.

This theme was chosen because on 16 November 2018, the General Conference on Weights and Measures agreed perhaps one of the most significant revisions to the International System of Units (the SI) since its inception.

In the revised SI, four of the SI base units – namely the kilogram, the ampere, the kelvin and the mole – will be redefined in terms of constants.

The new definitions will be based on fixed numerical values of the Planck constant (h), the elementary charge (e), the Boltzmann constant (kB), and the Avogadro constant (NA), respectively.

Further, the definitions of all seven base units of the SI will be uniformly expressed using the explicit-constant formulation, and specific mises en pratique will be drawn up to explain the realization of the definitions of each of the base units in a practical way.

This historic change towards using the rules of nature in the definitions will eliminate the final link between the SI and definitions based on physical artefacts, the last one being the International Prototype of the Kilogram (IPK), which was sanctioned by the 1st CGPM in 1889.

Great attention has been paid to ensure that the new definitions will be compatible with the old ones at the time the change is implemented, so that the changes will be unnoticeable to all but the most demanding users.

Research into new measurement methods, including those using quantum phenomena, underpin the change.

The SI is now based on a set of definitions each linked to the laws of physics and have the advantage of being able to embrace further improvements in measurement science and technology to meet the needs of future users for many years to come.

Across the world, national metrology institutes continually advance measurement science by developing and validating new measurement techniques at whatever level of sophistication is needed.

The national metrology institutes participate in comparisons coordinated by the Bureau International des Poids et Mesures (BIPM) to ensure the reliability of measurement results worldwide.

The BIPM also provides a forum for its Member States to address new measurement challenges. The International Organization of Legal Metrology (OIML) develops International Recommendations, the aim of which is to align and harmonize requirements worldwide in many fields.

“World Metrology Day recognizes and celebrates the contribution of all the people that work in intergovernmental and national organizations throughout the year on behalf of all,” says BIPM.