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A novel way of cleaning the standard kilogram is to be tested next month. Metrologists at the (BIPM) in Sèvres, France, will try using ozone and ultraviolet light to remove grime from one of the weights and compare this with the traditional method using alchohol and steam jets.

Some 100 national standard kilograms are dotted around the world and are verified against the international prototype every 50 years or so. But, no matter how well these platinum-iridium alloy cylinders are stored, they typically gather about five micrograms of dirt each year.

Cleaning the weights is problematic. Currently they are rubbed with chamois leather impregnated with alcohol, then washed with a steam jet, a method known as nettoyage-lavage. The process is so tricky that just one person does it: Richard Davis, head of mass at BIPM, who learned the technique from his predecessor. After cleaning it can take up to a year for a kilogram to become stable again.

Ozone chamber

Now Stuart Davidson of the (NPL), UK, has adapted a process for cleaning semiconductor wafers as a non-contact method for scrubbing the kilogram.

A platinum-iridium weight is placed in a chamber containing an ozone-enriched atmosphere and then illuminated with UV light. This breaks the ozone down into atomic oxygen and O₂. Dirt on the surface of the weight – mostly long-chain hydrocarbon molecules – reacts with the oxygen to produce carbon dioxide and water molecules that are readily taken up by the atmosphere, leaving the weight clean.

“The main benefit is that the method is repeatable and not user-dependent like the nettoyage-lavage process,” says Davidson. “Potentially any of the national standard laboratories can return their kilograms to the same clean state using this technique.”

Experiments at BIPM will begin in March 2008 when NPL and BIPM scientists will compare the two cleaning techniques for the first time.