Stability of reference masses .4. Growth of carbonaceous contamination on platinum-iridium alloy surfaces, and cleaning by UV/ozone treatment

Platinum-iridium mass artefacts are known to pick up carbonaceous contamination from the atmosphere. We examine the likely physical mechanism of carbonaceous ''build-up'', and analyse historical weighing data using this model, to deduce the rate of increase as a function of time. Cleaning of Pt-1O% Ir reference kilograms is currently conducted at the Bureau International des Poids et Mesures (BIPM) by a manual chamois-leather rubbing followed by washing in recondensed water droplets from a jet of steam (nettoyage/lavage). We investigate an alternative approach involving exposure to ultraviolet (UV) light and ozone in air at room temperature and pressure. This process (i) is shown to be more effective than the cleaning/washing procedure for removing hydrocarbon contamination, (ii) involves no physical contact with the reference mass, (iii) is an easily repeatable and reproducible method, since UV light intensity and ozone gas concentration can be measured reliably. Both UV light and ozone in the required amounts can be produced conveniently in situ by short-wave UV lamps which are widely available. Optimum UV intensities, ozone concentration and cleaning times are given, and recommendations for experiments to validate the procedure on reference kilograms are discussed. If implemented, the cleaning method described should improve the long-term accuracy with which the SI kilogram can be realized by national standards laboratories from similar to 1 part in 10(7) to similar to 3 parts in 10(9), much the same as would be achieved if a BIPM nettoyage/lavage could be applied regularly by national laboratories. This will, of course, need to be confirmed by conducting weighing trials on real Pt-Ir kilograms.