SI Units Explained


Posted on 2nd April 2019

In our previous article ‘What is Metrology?’, we explained what metrology is, alongside its origin, evolution and the impact it has on our day-to-day lives.

We also briefly described the defined International Systems of Units (SI for short). In this article, we will describe what the SI units are and what they entail.

What are the SI Units?

In a nutshell, the SI units are a recognised and agreed system of measurements which are adhered to around the world. The SI units are also commonly known as the metric system of measurements and are easily comparable in different countries.

The SI units are made up of seven individual types of measurement:

  • Kilogram (kg) – the unit measurement for mass
  • Metre (m) – the unit of measurement for length
  • Second (s) – the unit of measurement for time
  • Ampere (A) – the unit of measurement of electric current
  • Kelvin (K) – the unit of measurement for thermodynamic temperature
  • Mole (mol) – the unit of measurement for substance
  • Candela (cd) – the unit of measurement for luminous intensity

The above measurements are known as the base units. The base units go further to define a further 22 derived units. For example, Celsius temperature is derived from the Kelvin base unit and acceleration is derived from the second time base unit.

Standardising these units of measurement keeps them accurate and consistent for those countries who use them across the globe. They also secure an individual’s confidence when using measurements daily. For example, when we measure the ingredients for our favourite cake recipe, we rely upon the assumption that the stated measurements are correct.

How were the SI Units Defined?

On May 20, 1875, the International Bureau of Weights and Measures (BIPM) was established by the Metre Convention in France following the signing off on the metre standard. The BIPM is an organisation who act as one body on behalf of 59 member states in relation to measurements and metrology. It wasn’t until 1960 that the SI units were established by the BIPM to cover the different units of measurement.

How do the SI Units Affect Metrology?

The SI units form a fundamental part of metrology and its practices. Without the definitions that the SI units provide, we simply would not be able to accurately measure the equipment we are tasked with. Simply put, metrology would not function without the defined units. As a calibration laboratory within the traceability chain, we follow the procedures associated with metrology and the SI units to measure various equipment types with the highest standard of accuracy. The same applies when redefinitions to the SI units occur.

The SI Units – Redefined

Science is constantly evolving and being reimagined, which only emphasises the importance and demand for the most accurate results possible. Increasing the accuracy of measurements starts at the core units. The redefinition centres on the SI being defined in terms of constants of nature. Having them as constants will ensure the units will remain stable well into the future in anticipation for new technologies and advances in science.

Regarding the definition, the National Physical Laboratory states: ‘although these changes won’t be felt in everyday life, they represent a profound change of perspective.’

Essentially, the redefinition will ensure the measurements remain the same size as before, but the definitions will become more precise.

What Definitions are Changing?

  • The kilogram will be defined in terms of the Planck (h).
  • The ampere will be defined in terms of the elementary charge (e).
  • The kelvin will be defined in terms of the Boltzmann constant (k).
  • The mole will be defined in terms of the Avogadro constant (NA).

How will the 2019 Redefinition Affect CoMech?

Coinciding with World Metrology Day, from May 20, CoMech will be using the newly revised and constant measurement definitions within our procedures involving calibrations, repairs and maintenance for electrical equipment types.

The redefinition will also ensure our practices will remain unbeatably accurate for the years to come. As science and metrology are constantly evolving, so are the technologies and methods which are used. The SI units will remain to be a constant reference for all measurements we will be carrying out in the future so all our customers and suppliers can rest assured that our services will be as accurate as ever.

For more information regarding the SI units and their redefinition, follow the link to the National Physical Laboratory website here.

In the next instalment of our metrology articles, we will be exploring the reasons why metrology is used, while explaining how traceability and calibrations are used today.

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