As we celebrate the 50th anniversary of the International System of Units (SI), NPL's Jonathan Williams explains just how essential the system is to science and society alike.

Measurement today is incredibly valuable - we depend on it for almost everything. Healthcare relies upon measured dosages; food and raw materials are bought by weight or size; vehicle manufacturers need parts made in different countries to fit together - we could go on and on. Measurement is so embedded it often plays an essential but overlooked role. At the centre of it is the SI - the standard system of measurement units for scientists worldwide.

The SI: from the laboratory to business

The SI plays a large part in the work we do here, as NPL is the UK's National Measurement Institute (NMI). As a NMI, we maintain and develop measurement standards, including those of the SI units. Our role also includes providing all the measurement infrastructure scientists, manufacturers and those in research and development need, working with them to ensure the accuracies required. A large part of this is calibrating measurement equipment against our standards, to find any errors and to keep measurements across the UK linked and traceable. As measurement touches on every aspect of our lives, the quality of it often matters greatly.

Anyone who needs access to an SI unit can come to NPL. Of the SI base units - the second, the kilogram, etc. - some we do calibrations directly against. However, there are also the derived units, which are products and ratios of the base units. We package those units up and then give people calibrations of the derived units as well. For instance, force is measured in newtons, but made up of metres, kilograms and seconds; so we'd give calibrations in newtons. That's another role of the NMI - to provide all the combinations that people want to use.

Industries who need the highest quality of accuracy come directly to us for calibrations. Turning out products, day in and day out, they have to be sure everything is right all the time. NPL works directly with a diverse range of business sectors including aerospace, energy, environment, communications and healthcare.

Tracking measurement

An important part of the SI is traceability. At a basic level, this means following calibrations back to a national standards facility such as NPL, so you can see how many steps there are between an instrument's measurement and the SI national standard. Uncertainties and corrections at each step are noted. However, traceability covers a wide range of areas - it also includes the training of staff using the instruments, procedures they might use, the checking of results and the whole management infrastructure. Otherwise you can have the most accurate instrument in the world, but if you use it incorrectly, you're not going to get the right answer.

Traceability is essential for keeping measurements accurate. If it wasn't in place, at first nothing would happen, because we all have quite good standards of our own. However, eventually, things would start to drift apart. It would be like untying a boat from its mooring, gradually it will move off. That's what would happen if we stopped comparing units and staying up-to-date.

The SI and global trade

The SI is international, essentially for global trade. A car, and indeed an individual car part, might be made in different continents, so everything has to fit together. If an aeroplane needs repairing away from its home base, adjustments might have to be done, or parts replaced. These have to be the same, to the right tolerance, that they would be in the UK. As well as a technical requirement there's also a political requirement. If you want to trade in another country people ask what the source of accuracy is, and sometimes how safe it is.

The SI, its units and traceability quietly underpin many aspects of our society. From the thread of a nut and bolt on an aeroplane, to filling a car with petrol, to having an X-ray at a hospital - all these require accurate measurement that is recognised around the world. Moving out of the laboratories of the NMIs such as NPL, the SI supports our economy, industries and businesses, academia, and many aspects of our day-to-day life.

Find out more about the SI

Listen to a short podcast of NPL's Peter Whibberley talking about the SI unit the second

Download NPL's 'Units of measurement' poster

For further information, please contact Jonathan Williams