I recently had a conversation with Dr Wynand Louw, president of the International Committee for Weights and Measures (CIPM) and director for research and international liaison at the National Metrology Institute of South Africa (NMISA), with the aim of creating a better understanding of the metrology and its importance.

Metrology comes from the Greek words “metron” (measure) and “logos” (study). The study of measurement covers both the experimental and theoretical aspects of measurement and the determination of the levels of uncertainty of these aspects. The term “metrology” is often confused with “meteorology” which is the branch of science concerned with the processes and phenomena of the atmosphere, especially as a means of forecasting the weather.

For thousands of years, nations developed their own measurement standards based on body dimensions and their immediate surroundings. During the 19th century the French led a process to define measurement standards based on geodetic dimensions that could be adopted globally. This led to the Metre Convention of 1875 that introduced the “metric” system with as foundation the circumference and rotation of the earth and the mass of water, to define units for length (metre), mass (kilogram) and time (second). Over the next 85 years, units were added for temperature (kelvin), luminous intensity (candela) and current (ampere) to be named the International System of Units (SI) in 1960. In 1974, the quantity mol with unit mole was added for amount of substance, defining the SI as we know it today with its seven base units, derived units and other units approved for use with the SI.

In the previous century, the measurement standards as defined in the SI were mostly vested in various science laboratories and science councils. It was primarily the world wars that changed the focus and saw the formation of metrology institutes.

“During the World War II it became clear that standardisation with accurate measurement as a cornerstone had become absolutely essential for global ‘component’ manufacture, as ammunition made in one country was in many cases not compatible with weaponry manufactured in another country,” Dr Louw explained.

“In South Africa the National Physical Laboratory was established at the CSIR in 1947 and was tasked to keep and maintain physical measurement standards comparable to standards internationally, as ‘national measurement standards’. In parallel with measurement infrastructure, the other legs of a quality infrastructure were developed (written standards, accreditation and conformity assessment) and in the 2000’s this was brought together under one umbrella called the “Technical Infrastructure” under the auspices of the Department of Trade and Industry (the DTI). Along with Australia, we were very much the pioneers in establishing TI in this way. These days this system is called the ‘Quality Infrastructure’ in most parts of the world, but we retained the name TI.”

One would have thought that the Department of Science and Technology (DST) would have been a more logical home for the TI, but Dr Louw explained that as the TI is indispensable for production and export of locally manufactured goods (thus trade), there is a good case to be made for the DTI to govern the TI. Metrology, due to its scientific nature, maintains close links to DST.

Besides the NMISA, the other three TI partners are:

South African Bureau of Standards (SABS), responsible for the South African National Standards (SANS) which also forms the basis for compulsory specifications. NMISA gives input to measurement issues incorporated into the SANS.

National Regulator for Compulsory Specifications (NRCS), responsible for the administration of technical regulations including compulsory specifications based on standards that protect human health and safety, and the environment. The NRCS administers the Legal Metrology Act. The Act and technical regulations set the requirements for measurements of quantity for trade purposes and for environmental health and safety. The outputs of scientific metrology are critical to the effective legislation and prosecution of regulations.

South African National Accreditation System (SANAS), responsible for giving formal recognition that Laboratories, Certification Bodies, Inspection Bodies, Proficiency Testing Scheme Providers and Good Laboratory Practice (GLP) test facilities are competent to carry out specific tasks.

The 2019 revised SI

On 20 May 2019 the CIPM made some bold changes to the definitions of four of the seven base SI units. Speaking as the president of the CIPM, Dr Louw said that the decision to redefine the SI was a long time in coming but necessary to bring the SI into the modern era and for the future, when technologies we may not even have thought about today may require more accurate measurements.

“The time had come to move from artefact standards based on natural phenomena and introduce a new approach to articulating the definitions of the units in general, and of four of the seven base SI units in particular. The new definitions are based on universal physical constants and the units are described by fixing the numerical values of ‘defining’ constants. Among them are the Planck constant (mass), the Boltzmann constant (temperature), the elementary charge (current) and the Avogadro number (mole) (the other three units are already defined in terms of physical constants whose numerical values are now also fixed, namely the speed of light, the hyperfine transition frequency of caesium-133 and the luminous efficacy of radiation of frequency of 540 x 1012 Hz), so that all seven definitions are based on and represent the current understanding of the laws of physics”.

I asked Dr Louw reasons for the move from physical standards, to a complicated physics formula. “I believe it is the quite the opposite. For many decades scientists have known that there was a problem with the artefact standards and started to replace these with definitions based on universal constants. An example is the metre whose original definition was based on 1 ten millionth of the distance from the North pole to the Equator and was represented by a metal metre length, that was replaced in the 1970s by the distance that light travels in vacuum in a period of time. The last artefact standard, the International Prototype of the Kilogram (IPK), based on the mass of a cubic decimetre of pure water as represented by a metal block stored under controlled conditions in Paris was showing drift against the 50+ national copies routinely compared to it. The definitions of three other units were also linked to the IPK. We came to the realisation that technology had developed to such an extent that the possible drift had become significant and that we had to improve and make the definitions fit for purpose again. Perhaps the new way is more scientifically challenging but we now have unit definitions that are independent from any artefact. We have effectively fixed the values of the units in time according to our current knowledge. As our knowledge increase, we can further refine the values of the defining constants.”

My next question was: how could the kilogram be redefined in terms of an invariant of nature and scaled up or down conveniently, accurately and repeatedly? Dr Louw explained that after decades of debate and experimenting, the international measurement science community has chosen to answer that question by using Planck’s constant, relating the energy of a quantum of electromagnetic radiation (or photon) to its frequency. “For many observers, the connection between the kilogram on the scale of the mass of a litre of water and a constant derived from the very earliest days of quantum mechanics may not be immediately obvious. The scientific context for that connection is suggested by a deep underlying relationship between two of the most celebrated formulations in physics. One is Einstein’s famous E =mc2, where E is energy, m is mass and c is the speed of light. The other expression, less well known to the general public but fundamental to modern science, is E = hν, the first ‘quantum’ expression in history, stated by Max Planck in 1900. Here, E is energy, ν (for the Greek letter “nu”) is frequency and h is what is now known as the Planck constant. We can therefore state that hv = mc2, thus we have the relationship between mass and Planck’s constant”.

Measurement standards for trade

Coming back to a practical level, we know how to derive and maintain standards, but how does it impact on our everyday lives and maybe more importantly, how does it impact on trade? Dr Louw said that is exactly the point. “There comes a time where countries that maintain measurement standards will no longer trade with those countries that do not maintain standards, or if they do, they will do it on their terms. You therefore are ‘dependent’ on them. Also, there are many practical examples where accurate traceable measurements are required, for example in law enforcement, where speed enforcement and breathalyser equipment require to be calibrated in a traceable way to the national measurement standards. If you cannot show traceability the measurement result will not stand up in court.”

In 1970 the introduction of the mole was a huge improvement in chemical metrology. The mole is defined as exactly 6,02214076×1023 (the Avogadro number) – entities that can be atoms, molecules, ions or electrons. The mole is widely used in chemistry as a convenient way to express amounts of reactants and other chemicals.

“Perhaps the two aspects where metrology is most relevant today are food safety and environmental monitoring. International limits of pesticides in the food chain are continually reducing and require more accurate and traceable measurement. Failing that would mean the blocking of export of our products.”

“Our goal is to have primary realisations of all seven SI base units (thus independent to the measurement standards of another country). NMISA already realises length, temperature, luminous efficacy and time, primary in terms of the SI definitions. Currently we also have a primary standard for mass in terms of the ‘old’ definition, as we have a copy of the IPK, but with the revised definition, in future a primary realisation of mass would be in terms of the Planck constant. NMISA is developing a Kibble (or Watt) balance to realise the kilogram according to the new definition. The ampere can be realised in two ways, the first (and historic way) is in terms of Ohm’s law, ampere=voltage/resistance, and the new way would be to measure the elementary charge (of an electron or proton). Currently, NMISA has a primary standard for voltage but we are now in the process of setting up a new primary standard for resistance to have a primary standard for current in terms of Ohm’s law. We also have a research collaboration with the University of Cape Town on how to accurately measure the elementary charge.”

“Other developments include the upgrade of our realisation of time to be fit-for-purpose for the local industry and over the past few years we upgraded the equipment from an accuracy of 1000 nanoseconds to better than 10 nanoseconds as required for the Square Kilometre Array (SKA). The revised definition of the kelvin also introduced new ways of realising temperature in terms of the relation between the energy and temperature of a gas, and a development project is underway to explore primary gas thermometers.”

Currently, no country performs a primary realisation of the mole as the old definition was based on mass (a mole was defined as the entities in 12 gram of carbon). The new definition in terms of the Avogadro constant makes it possible to realise a mole and NMISA will follow the advice of the CIPM to in future realise the mole.”

SA’s role in international metrology

“In summary, South Africa is well placed internationally with membership of nine of the ten consultative committees of the CIPM and holding its presidency, also allowing for attending the tenth committee (for units, the CCU). This places us amongst the best in the world. NMISA also provides traceability to most of Sub-Saharan Africa. However, we need more education in both the private and government sectors to underline the importance of metrology to support our export potential. Metrology is not a luxury; it is a necessity.”

“My vision for the next four years is to make the international system more inclusive and find innovative ways to ensure that more countries from emerging economies participate in the activities of the Metre Convention. There are encouraging developments to bring these countries into the fold of the official metrology system, and as president of the CIPM but most importantly a founder member of the Intra-African System of Metrology (AFRIMETS) I have the responsibility to bring these developments into fruition.”

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