National Physical Laboratory

Cleaning the kilogram for the BBC

The BBC's The One Show investigated why the kilogram is putting on weight and discovered how NPL scientists are working towards a solution to the problem. The resulting segment was broadcast on Tuesday 7 October and is available to view on the BBC iPlayer (from 16:25).

Kilogram no. 18 in the UV cleaning system at NPL
Kilogram no. 18 in the UV cleaning system at NPL

Science presenter Marty Jopson visited NPL, where he saw for himself the UK's national standard kilogram and a new system that uses a combination of UV light and ozone to clean contamination from its surface. He interviewed NPL's Stuart Davidson, who explained the history of mass measurement and why it's a problem when national standards do not agree with one another.

NPL keeps the UK's national standard kilogram, which is copy no. 18 of the International Prototype Kilogram and one of many national mass standards used all over the world. Over time hydrocarbons in the atmosphere bond to the surface of the standards causing tiny changes to their mass that can lead to significant errors further down the line.

As Marty Jopson explains in the programme:

"The measurement system is a bit like a game of Chinese Whispers: If you don't get it right at the very top, every step of the way down it gets worse and worse.

"If you have a 10 microgram error in the original Prototype Kilogram, that could lead to a 100 microgram error in the replicas. That's less than one grain of sugar. By the time you get to a factory you're looking at maybe 100 milligrams error. By the time that gets to the greengrocers, however, you're looking at an error that could be up to about a tablespoon of sugar. If you buy a tonne of the stuff, you'll end up with an extra 10 kilograms."

Traditionally, the kilogram is cleaned by rubbing it with an alcohol-soaked chamois leather, but this process is dependent on the individual doing the cleaning. The technique developed at NPL, in collaboration with BIPM, involves the use of ultraviolet light in an ozone-rich atmosphere, and promises to be as effective as the traditional technique but much more reproducible.

Find out more about the cleaning technique

Watch the clip on The One Show (from 16:25)

The international prototype of the kilogram recently celebrated its 125th anniversary. Find out why this may well be its last landmark birthday

For further details, please contact Stuart Davidson

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Last Updated: 13 Aug 2015
Created: 9 Oct 2014

