The science of measurement: A history

February 4th, 2019, Published in Articles: EngineerIT, Featured: EngineerIT

Metrology is the science of measurement and the technology behind the quality assurance processes in manufacturing that ensure a car runs the way it should, the computer’s processor works properly, aircraft stay in the air and more aspects of daily life that we take for granted – until something goes wrong.

In manufacturing, hundreds or thousands of parts are produced each week – most of these by machines that are programmed by people. In modern manufacturing, every one of these parts must be produced to precise specifications to ensure a quality finished product. This where metrology fits in, to make sure a millimetre is a millimetre, an Amp is an Amp and a kilogram a kilogram. In fact, it checks all the measurements in the SI system. SI is an abbreviation of the French Système international (d’unités), the modern form of the metric system, and is the most widely-used system of measurement. It comprises a coherent system of units of measurement built on seven base units, which are the ampere, kelvin, second, metre, kilogram, candela, and mole.

One of the very first applications of metrology dates back to ancient Egypt and the building of the pyramids, where a “cubit” was used as the length measurement unit. A master cubit was made from marble; equal to the length of the forearm (measured from the elbow to the tip of the middle finger) of the reigning Pharaoh. Foremen of the pyramid building sites had to have their working cubit sticks calibrated regularly against the master cubit, and failure to do so was punishable by death.

The International Bureau of Weights and Measures (BIPM) defines metrology as “the science of measurement, embracing both experimental and theoretical determinations at any level of uncertainty in any field of science and technology,” which is a broad description to match the broad amount of industries, technologies, and applications it holds in today’s world.

Metrology has a long history in South Africa. It dates back to when the National Physical Research Laboratory (NPRL) as a division of the Council of Scientific and Industrial Research (CSIR) was formed in 1946 under the leadership of Dr Meiring Naude. The history of the National Metrology Institute of South Africa (NMISA), however, officially started in 1947 when the first calibration records were issued at the then CSIR offices in Visagie Street, Pretoria. By 1950 the NPRL was responsible for the ultimate standards of length, mass and pressure, as well as fundamental electrical and temperature scale standards. Calibration services were offered for electrical sub-standards and instruments, thermometers and pyrometers, electro-acoustic instruments, microphones and loudspeakers, with standards for colorimetry and photometry in development. The laboratories relocated to the current CSIR Scientia Site in 1957.

In 1961, the new Act 32 of 1962 made provision for the CSIR to be responsible for the measurement standards. South Africa eventually became a signatory of the Metre Convention and a member of the BIPM in 1964. Despite restructuring, work focussed on the development of primary standards and in 1967 South Africa was appointed as a member of the Consultative Committee on Photometry.

A new act, the Measuring Units and National Measuring Standards Act 76 of 1973, was promulgated, establishing the National Metrology Laboratory, and Dr Dick Turner, the then director, had the vision that funding was to be secured from the Department of Trade and Industry (unconnected to the CSIR’s parliamentary grant) to grow the NML to an independent entity. Finally, on 1 May 2007, with the promulgation of the new Measurement Units and Measurement Standards Act (Act 18 of 2006), the National Metrology Institute of South Africa was established as a new public entity independent of the CSIR. The NMISA is, with the SABS, SANAS and the NRCS, now part of the DTI’s Technical Infrastructure family​.

NMISA provides South African industry and environmental, health and safety sectors with fit-for-purpose measurement standards and measurements. This is achieved by keeping and maintaining the national measurement standards and units to an acceptable international standard; and by disseminating traceability to the South African industry.

A core concept in metrology is metrological traceability, defined in the International Vocabulary of Metrology as “the property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons, all having stated uncertainties.” Traceability is most often obtained by calibration, establishing the relation between the indication of a measuring instrument and the value of a measurement standard (the NMS). These standards can either be primary (established through a realisation of the unit and the traceability is to the NMI) or secondary, where an artefact calibrated against a primary standard at another NMI is maintained as the NMS (and the traceability is to the NMI with the primary standard).

The official definition of a kilogram is currently determined as the mass of a metal cylinder called the International Prototype of the Kilogram, but that will change on 20 May 2019. This is a replica of the IPK at the Cité des Sciences et de l’Industrie in Paris.

South Africa is a signatory to the Metre Convention, a treaty dating back to 1875. Under this convention the International Bureau of Weights and Measures (BIPM) was created to act in matters of world metrology, particularly concerning the demand for measurement standards of ever increasing accuracy, range and diversity, as well as to address the need to demonstrate equivalence between national measurement standards. The SI was also established under the Metre Convention and is overseen by the International Committee for Weights and Measures (CIPM). The whole system is governed by the General Conference on Weights and Measures (CGPM), whose members are the states that signed the Metre Convention.

In 1999, the BIPM and the National Metrology Institutes (NMIs), with the consent of their governments and thus the CGPM, created the CIPM Mutual Recognition Arrangement (MRA). The CIPM MRA gives users reliable quantitative information on the comparability of national metrology services and to provide the technical basis for wider agreements negotiated for international trade, commerce and regulatory affairs. It is the basis for the international acceptance of national measurement standards and for calibration and measurement capabilities (CMCs) and calibration and analysis certificates issued by NMIs.

As the custodian of the South African NMS, NMISA develops and maintains primary and secondary standards (chemical and physical quantities) for South Africa and compares those standards with other national standards (and the international standard for mass, the International Prototype of the Kilogram or IPK) to determine their equivalence and ensure global comparability. These standards are disseminated to the South African industry through a range of services and products and in the case of a measurement dispute, reference analyses are provided to ensure conformity.

Technological advances over the past decade are placing stringent demands on metrology. New areas in metrology, such as nanotechnology, optical techniques, quantum-based technologies, and material sciences are developing rapidly and require new measurement methods and measurement standards. In response, NMISA is investing more funds into research activities and are actively pursuing opportunities for collaboration with their peers in order to pool resources. NMISA thus engages in research towards the improvement of existing standards and to facilitate the development of new measurement standards to address emerging national needs.


Revised definitions for four SI standards

In a landmark decision, the BIPM’s 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.

This 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. While definitions are new, the actual units will not change, i.e a litre of petrol will stay a litre of petrol (and just as expensive!).

In the revised SI, four of the SI base units – namely the kilogram, the ampere, the kelvin and the mole – are redefined in terms of constants. The new definitions are based on fixed numerical values of the Planck constant (h), the elementary charge (e), the Boltzmann constant (k), and the Avogadro constant (NA), respectively. Further, the new definitions of all seven base units of the SI are also uniformly expressed using the explicit-constant formulation. A document will be drawn up to explain the realisation of the definitions of each of the base units in a practical way. The new definitions will come into force on 20 May 2019.


 

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