The international prototype kilogram is a cylinder of platinum and platinum-iridium alloy, which is kept at the International Bureau of Weights and Measures (BIPM) near Paris.

The international standard, a cylinder-shaped hunk of metal that defines the fundamental unit of mass, has gained tens of micrograms of mass from surface contamination, according to a new study.

As a result, each country that has one of these standard masses has a slightly different definition of the kilogram, which could throw off science experiments that require very precise weight measurements or international trade in highly restricted items that are restricted by weight, such as radioactive materials.

But ozone and ultraviolet light could be used to clean the kilograms without damaging them, the research suggests.

The cleaning technique, described in the January issue of the journal Metrologia, may eventually be widely adopted as a way to keep the fundamental unit of mass more consistent.

In addition, since it uses “equipment that one could come by without too much investment of time and money,” it is a practical technique that could be widely adopted, said Richard Davis, a metrologist who consults for the International Bureau of Weights and Measures in Paris, who was not involved in the study.

The kilogram was first adopted as an international standard at the Convention of the Meter in 1875.At that time, scientists were frustrated that there was no consistent, standard way to measure mass with high precision.

To solve the problem, scientists created a cylindrical hunk of mass called the international prototype kilogram (IPK) from platinum and platinum-iridium alloy. The cylinder, which weighs approximately 2.2 pounds (1 kilogram), is the definition of the kilogram. The kilogram is one of seven standard international base units of measurement.

In the 1880s, about 40 of these prototype kilograms were distributed to countries that signed the Meter Convention, said study co-author Peter Cumpson, a metrologist at Newcastle University in the U.K.

cutting-edge X-ray Photoelectron Spectroscopy (XPS)
Peter Cumpson has come up with a simpler, more reproducible way to clean standard kilograms. Here, he’s pictured with the device used to measure contamination on the surface of metals.

Now Professor Peter Cumpson and Dr Naoko Sano have used cutting-edge X-ray Photoelectron Spectroscopy (XPS) to analyse surfaces similar to the standard kilogram to assess the build-up of hydrocarbons – and how to remove them.
Ozone and ultraviolet light could be used to clean the kilograms without damaging them, LiveScience reported.

“Statute decrees the IPK is the kilogram,” explained research lead Peter Cumpson, Professor of Micro Electro Mechanical Systems (MEMS) at Newcastle University.

“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. Around the world, the IPK and its 40 replicas are all growing at different rates, diverging from the original,” Cumpson said.

“We’re only talking about a very small change – less than 100 micrograms – so, unfortunately, we can’t all take a couple of kilograms off our weight and pretend the Christmas over-indulgence never happened,” Cumpson added.
“But mass is such a fundamental unit that even this very small change is significant and the impact of a slight variation on a global scale is absolutely huge. There are cases of international trade in high-value materials – or waste – where every last microgram must be accounted for,” he said.

“What we have done at Newcastle is effectively give these surfaces a suntan. 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,” he added.

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.

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