– Change to come into force next May

TODAY May 20, Metrology (Measurement) Institutions around the world join in celebrating World Metrology Day under the theme: Constant Evolution of the International System of Units.

The Guyana National Bureau of Standards, as the National Metrology Institute, is proud to be a part of this celebration, as it offers a wide range of measurement services to the industry and commerce in Guyana.
The Bureau has also been at the forefront of promoting the use of the SI units (Metric System) in Guyana, so as to bring our national measurement system on par with the global measurement system.
The following extract was provided by the International Bureau of Weights and Measures (BIMP):

“The International System of Units, the SI, which is based on the second, the metre, the kilogram, the ampere, the kelvin, the mole and the candela (the base units), is being revised to update the definitions of four of these units.
“In November 2018, revised definitions of the kilogram, ampere, kelvin and mole are expected to be approved by the General Conference on Weights and Measures (CGPM), the international body responsible for the global comparability of measurements.
“The revised definitions are expected to come into force on May 20, 2019. The revised definitions will be based on seven physical constants, for example the speed of light, the Planck constant and the Avogadro constant, and are therefore inherently stable. 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 revision of the SI in this way was foreseen in Resolutions of the CGPM adopted in 2011 and 2014. Additional requirements contained in these Resolutions will ensure a smooth transition to the four revised definitions.

“Most users will not notice the change. Guidance on the practical realisation of the units will be available. Users will be able to obtain traceability to the SI from the same sources used at present (the BIPM, national metrology institutes and accredited laboratories). International comparisons will ensure their consistency.

The value of the Planck constant will be chosen to ensure that there will be no change in the SI kilogram at the time of redefinition. The uncertainties offered by NMIs to their calibration customers will also be broadly unaffected.

The ampere and other electrical units, as practically realised at the highest metrological level, will become fully consistent with the definitions of these units.

SMALL CHANGES
The transition from the 1990 convention to the revised SI will result in small changes to all disseminated electrical units. For the vast majority of measurement users, no action need be taken as the volt will change by about 0.1 parts per million and the ohm will change by even less. Practitioners working at the highest level of accuracy may need to adjust the values of their standards and review their measurement uncertainty budgets.

“The kelvin will be redefined with no immediate effect on temperature measurement practice or on the traceability of temperature measurements, and for most users, it will pass unnoticed. The redefinition lays the foundation for future improvements. A definition free of material and technological constraints enables the development of new and more accurate techniques for making temperature measurements traceable to the SI, especially at extremes of temperature. After the redefinition, the guidance on the practical realization of the kelvin will support its world-wide dissemination by describing primary methods for measurement of thermodynamic temperature and equally through the defined scales ITS-90 and PLTS-2000.

“The mole will be redefined with respect to a specified number of entities (typically atoms or molecules) and will no longer depend on the unit of mass, the kilogram. Traceability to the mole can still be established via all previously employed approaches including, but not limited to, the use of mass measurements along with tables of atomic weights and the molar mass constant. This uncertainty will be so small that the revised definition of the mole will not require any change to common practice.
The metre in the revised SI will continue to be defined in terms of the speed of light, one of the fundamental constants of physics. Dimensional metrology practice will not need to be modified in any way and will benefit from the improved long-term stability of the system.

THE CANDELA
“The candela will continue to be defined in terms of Kcd, a technical constant for photometry and will therefore continue to be linked to the watt. Traceability to the candela will still be established with the same measurement uncertainty via radiometric methods using absolutely-calibrated detectors.

The SI has been revised several times since its formal adoption by the CGPM in 1960. However, redefining four base units at one time is unprecedented, requiring simultaneous world-wide collaborations in diverse fields of metrology.

“As in the past, care has been taken to ensure that there will be no perceptible impact on daily life and that measurements made with previous definitions of the units remain valid within their measurement uncertainties. Few users outside national metrology laboratories will notice the changes. Reaching the experimental accuracies and fulfilling the conditions requested in the CGPM resolutions has been a remarkable accomplishment, which will ensure that the SI continues to meet the needs of even the most demanding users.”