How much does a kilogram weigh?
A team of scientists have proposed a mechanism for cleaning the official kilogram prototype – known as the International Prototype Kilogram (IPK) – which was recently discovered to have been changing its mass by around 50 micrograms.
The IPK, which consists of an alloy of 90 percent platinum and 10 percent iridium (Pt-Ir) and has a density of approximately 21500 kg/m3, is maintained at the International Bureau of Weights and Measures (BIPM) in Sèvres near Paris. A recent examination earlier this year concluded the bit of alloy is slowly gaining weight, disrupting the world of measurement. According to scientists familiar with the matter, the kilogram had gainedthe equivalent of a small grain of sand in weight, enough to throw out calculations in everything from precision engineering to trade.
Now, scientists at Newcastle University say they developed a high-tech way to clean the standards. The newly devised system is based not on manually scrubbing the metal chunks but rather on exposing them to ultraviolet light and ozone about once per decade.
“Around the world, the IPK and its 40 replicas are all growing at different rates, diverging from the original,” says Cumpson, professor of Micro Electro Mechanical Systems (MEMS) at Newcastle. “It doesn’t really matter what it weighs as long as we are all working to the same exact standard – the problem is there are slight differences. ”
Using a state-of-the-art Theta-probe XPS machine – the only one of its kind in the world – the team have shown the original kilogram is likely to be tens of micrograms heavier than it was when the first standard was set in 1875. Working earlier this year, the team of scientists showed how the UV/ozone wash could be used to remove contamination without damaging the platinum surface, possibly providing an alternative cleaning system.
“By exposing the surface to a mixture of UV and ozone, we can remove the carbonaceous contamination and potentially bring prototype kilograms back to their ideal weight,” Cumpson noted.
Already it has the endorsement of the former head of the Mass Section at the International Bureau of Weights and Measures in France, which oversees the kilogram.
“It sounds good,” said Richard Davis, a physicist and former head of the Mass Section at the International Bureau of Weights and Measures in France. “The technique they’re proposing is something that is not that expensive and could be implemented in different places without too much trouble.”
While the change seems small, the pair of scientists warn that it could have widespread consequences. Besides, mass is “such a fundamental unit,” they point out, that even a small change could have a “significant impact” globally, including on commodity trade. “There are cases of international trade in high-value materials — or waste — where every last microgram must be accounted for,” the scientists write in a paper published in the journal Metrologia.
While the newly devised cleaning system could restore order to the world of measurements, scientists around the world have already proposed changing how the kilogram is defined. Work is underway internationally in several National Measurement Institutes to find an alternative to the IPK – a standardised value for the kilogram that is not based on a matchbox-sized piece of metal. Under a proposed deal, the weight change would not take place until 2014 at the earliest.