Redefined SI measurement standards come into effect

May 17th, 2019, Published in Articles: Energize, Articles: EngineerIT, Featured: EE Publishers, Featured: Energize

On World Metrology day, 20 May, four of the seven base units comprising the International System of Units (SI) changed. While this revision underlies all measurements in science, it will not affect our everyday lives. A metre will still be a metre, a litre of petrol will still be a litre of petrol, and one ampere will still be one ampere. Why then bother with the change? The redefinition of SI is done with the future in mind, for decades ahead when technologies we have yet to imagine may require more accurate measurements.

The 26th meeting of the Conférence Générale des Poids et Mesures or CGPM (known in English as the General Conference on Weights and Measures) adopted a set of far-reaching changes introducing a new approach to articulating the definitions of the units in general, and of the seven base units in particular, by fixing the numerical values of “defining” constants. Among them are fundamental constants of nature such as the Planck constant and the speed of light, so that the definitions are based on and represent the current understanding of the laws of physics. For the first time, a complete set of definitions is available that does not make reference to any artefact standards, material properties or measurement descriptions. Changes in the SI will ensure that the SI definitions remain robust for the future, ready for advancements in science and technology.

In preparation for the introduction of the redefined SI, the National Metrology Institute of South Africa (NMISA) hosted a three-day conference in Pretoria to introduce the revised SI. Dr Wynand Louw, the chairman of the International Committee for Weights and Measures (CIPM) explained that the SI has been used around the world as the preferred system of units. It is the basic measurement language for science, technology, industry and trade, since it was established in 1960 by a resolution at the 11th meeting of the CGPM.

Fig. 1: Delegates attending the NMISA Revised SI conference in Pretoria.

Definitions of the new units

The actual changes will impact four of the seven base units: the kilogram, ampere, kelvin and mole. The kilogram will be defined in terms of the Planck constant, a physical constant that is used extensively in quantum mechanics and fixes the scale of quantisation of many phenomena, such as the relation between the energy of a photon (a quantum of light) and its wavelength. Its value is approximately 6,626 x 10-34 joule-seconds (equivalent to units of angular momentum).

The ampere will be defined in terms of the elementary charge, the electrical charge carried by a single electron. The kelvin will be defined in terms of the Boltzmann constant. The Boltzmann constant relates temperature to energy. It is an indispensable tool in thermodynamics, the study of heat and its relationship to other types of energy. It’s named after Austrian physicist Ludwig Boltzmann (1844 – 1906), one of the pioneers of statistical mechanics.

The mole will be defined in terms of the Avogadro constant. The Avogadro constant relates to how many elementary entities (usually either molecules or atoms) are present in in one mole of a substance. It is named after Amedeo Carlo Avogadro, an Italian physicist (1776 – 1856).

Background to the SI

Historically, SI units have been presented in terms of a set of (most recently, seven) base units. All other units, described as derived units, are constructed as products of powers of the base units. Different types of definitions for the base units have been used such as:

  • The mass of the international prototype for the unit kilogram.
  • A specific physical state, such as the triple point of water for the unit kelvin.
  • Idealised experimental prescriptions, as in the case of the ampere and the candela.
  • Constants of nature, such as the speed of light for the definition of the unit metre.

In the case of an artefact, they involve the risk of loss, damage or change. For many years the kilogram was compared with the gravitational force on a reference piece of metal known as a “standard weight”. The standard weight is in turn compared with the International Prototype of the Kilogram (IPK). The concern is that the mass of the IPK may have changed since it was produced in 1884, and there is no way of knowing. Contamination, cleaning or just time may have increased or decreased the mass. The IPK has been conserved at the BIPM since 1889, when it was sanctioned by the First General Conference on Weights and Measures. It is of cylindrical form, with diameter and height of about 39 mm, and is made of an alloy of 90% platinum and 10% iridium. Initially, the IPK had two official copies; over the years, one official copy has been replaced and four others have been added, so that there are now six official copies.

Fig. 2: A replica Kibble Balance demonstrating the new method of defining the kg SI unit.

In the revised SI system, the standard weight will be compared with the gravitational force on an object with a magnetic force using a Kibble Balance (Fig. 2).

The other types of unit definitions are increasingly abstract. Here, the realisations are separated conceptually from the definitions so that the units can, as a matter of principle, be realised independently at any place and at any time. In addition, new and superior realisations may be introduced as science and technology develop, without the need to redefine the unit. These advantages can be clearly seen in the history of the definition of the metre from artefacts through an atomic reference transition to the fixed numerical value of the speed of light. This led to the decision to define all units by using defining constants. The choice of the base units was never unique, but grew historically and became familiar to users of the SI. This description in terms of base and derived units is maintained in the present definition of the SI, but has been reformulated as a consequence of adoption of the defining constants.

While in everyday life redefining of SI may have no effect, it has implications in the education sector. The curricula across the education spectrum will have to be adapted and new learning materials have to be developed. NMISA is working with the University of Cape Town on a project which includes educational posters creating awareness, as well as working with other entities to develop virtual reality training tools. A new brochure explaining the SI is available from the BIPM and can be viewed here.

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