The original kilogram may be tens of micrograms heavier than when the standard was first set in 1875.
Despite being sealed in three bell jars, the original IPK has put on weight Image: BIPM |
With many people avoiding the weighing scales after an excess of Festive Season celebrations, it appears that the kilogram itself has also put on a little weight. Worldwide, there are 40 official replicas of the original kilogram - the International Prototype Kilogram (IPK) - which were created in 1884.
However, the surface of the platinum-based weights has gained weight through the build-up of contaminants due to the effects of industrialisation and modern living, even though they are carefully stored in laboratory conditions around the world, including the original place of manufacture - the International Bureau of Weights and Measures in Paris.
In fact, the exact weight of the IPK doesn't matter so long as it is the standard used. However, the 40 replicas are gaining weight at different rates so there is no longer a single global standard. Although the weight change amounts to less than 100 micrograms, the impact of this slight variation globally has massive impact. There are cases of international trade in high-value materials or waste where every last microgram must be accounted for.
In essence, the IPK needs to go on a diet.
Using cutting-edge Theta-probe X-ray Photoelectron Spectroscopy (XPS Professor Peter Cumpson and Dr Naoko Sano from England's Newcastle University have analysed surfaces similar to the standard kilogram to assess the build-up of hydrocarbons and how to remove them.
The answer, published in a recent edition of the Journal of Metrologia, seems to be a using a mixture of UV and ozone - otherwise known as a suntan. By exposing the surface to this combination, the contamination can be removed, bringing the prototype kilograms back to their ideal weight without damaging the platinum surface.
Meanwhile several National Measurement Institutes around the world are trying to find a standardised value for the kilogram that isn't based on a single piece of metal.