British scientists face up to a weighty problem

The nation's standard kilogram ingot needs to slim down

The two men boarding the Eurostar to Paris this Wednesday will be unremarkable, save for the metallic suitcase they keep in their sight at all times. Their fellow passengers could be forgiven for shuffling away as they eye the words DO NOT DROP in red on the side of the case.

Far from transporting some doomsday device, the two British scientists will be carrying a kilogram of harmless metal. But not just any kilogram: it is the UK standard kilogram, used as the ultimate reference for everything from the accuracy of a grocer's scales to the ingredients in a pill.

Actually, it weighs a fraction more than a kilogram. And that is a problem: as the years go by, it is gaining weight. Scientists from Britain's National Physical Laboratory (NPL) hope that cleaning the weight with a revolutionary new method, invented in the UK, will bring it back closer to its true mass. Long-term comparisons suggest that other nations' benchmark kilos are also gaining weight compared with the prototype in Paris – the definitive kilo based on the weight of a litre of water at 4C – which was cast in 1879.

While every other standard unit is derived from a constant that is the same anywhere – one second, for instance, is the time it takes for a caesium atom to vibrate 9,192,631,770 times – the kilogram is decided simply by the mass of that one object, even if its weight changes.

"It's amazing that this thing was made in the 19th century and it's still being used now," said Richard Davis, head of mass at the Bureau International des Poids et Mesures (BIPM), the world authority on weights and home of the prototype.

As they change in weight, the standard kilos around the world differ. No one can tell for sure whether the differences are because the original is losing weight, or the others are gaining. One theory is that, despite the care taken to protect the cylinders, tiny particles of dust accumulate over time.

The last time the kilos were compared was in 1992, and the next weigh-in is due soon. But the previous cleaning process, which involved wiping the kilos with a chamois leather and then dousing them in steam, is so prone to human error that only the man who devised the process, George Girard, has ever done it with the reference weights. He has retired and scientists around the world have been working on a new method since 1995. British efforts have come to fruition, with a method that involves bathing the kilogram in ozone gas, and then shining a powerful UV light on it. The ultraviolet rays trigger a reaction between the oxygen in the ozone and the carbon and hydrocarbon deposits, turning them into carbon dioxide and water, which are easily removed.

In the long term, the BIPM is keen to replace the prototype. "This kind of work is tedious," said Richard Davis. "What you're studying is dirt. It'd be nice to put our energies into something more interesting."