National Physical Laboratory

How clean is our kilogram?

Periodically the UK's national mass standard, along with those of about 40 other countries, is sent back to the Bureau International des Poids et Mesures (BIPM) for re-verification.

As part of this exercise the BIPM cleans the kilograms using a process known as nettoyage-lavage (cleaning and washing). This involves manually rubbing the weight with an alcohol soaked chamois leather, followed by washing in a jet of steam. The process is effective in removing the organic contamination that builds up on all mass standards, but relies on the skill of the operator and has proved to be difficult to replicate by other users at various National Measurement Institutes.

How clean is our kilogram

NPL, in collaboration with BIPM, has developed a novel technique for the cleaning of primary mass standards which promises to be at least as effective as the nettoyage-lavage technique, but much more reproducible. The technique involves the use of ultra violet light in an ozone rich atmosphere.

The UV 'activates' the ozone by producing atomic oxygen and also excites the organic molecules on the weight surface. The carbonaceous and hydrocarbon contamination reacts with the atomic oxygen to produce CO2 and H2O which can be easily desorbed from the surface of the mass standard. The process can be applied to mass standards of most materials (platinum iridium, gold, stainless steel, silicon, etc.) and is extremely controllable by varying the UV intensity, ozone concentration and exposure time.

Preliminary results have been very encouraging; several monolayers of contamination have been removed from the test pieces in a controllable and repeatable way. An artificially contaminated platinumiridium kilogram artefact has also been cleaned and over 70 μg of contamination removed, showing that the technique is able to cope with even badly contaminated mass standards. In 2008 a direct comparison between the new technique and the nettoyage-lavage cleaning was performed when James Berry from the Engineering Measurement team at NPL was seconded to the BIPM. The results of the comparison showed the new technique to be as effective as the existing method for cleaning mass standards.

The work for the comparison has since been published in Metrologia 48 (2011) 1-6.

The research currently under way at NPL investigating the cleaning of primary mass standards forms part of a European project 'NewKILO' to develop a practical means of disseminating the redefined kilogram.

For further information, contact Stuart Davidson

Find out more about NPL's Mass & Force research

Last Updated: 9 Oct 2014
Created: 1 Feb 2011