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 cont amination from the atmosphere. We examine the likely physical mechanis m of carbonaceous ''build-up'', and analyse historical weighing data u sing this model, to deduce the rate of increase as a function of time. Cleaning of Pt-1O% Ir reference kilograms is currently conducted at t he Bureau International des Poids et Mesures (BIPM) by a manual chamoi s-leather rubbing followed by washing in recondensed water droplets fr om a jet of steam (nettoyage/lavage). We investigate an alternative ap proach involving exposure to ultraviolet (UV) light and ozone in air a t 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 m ass, (iii) is an easily repeatable and reproducible method, since UV l ight intensity and ozone gas concentration can be measured reliably. B oth UV light and ozone in the required amounts can be produced conveni ently in situ by short-wave UV lamps which are widely available. Optim um UV intensities, ozone concentration and cleaning times are given, a nd recommendations for experiments to validate the procedure on refere nce kilograms are discussed. If implemented, the cleaning method descr ibed should improve the long-term accuracy with which the SI kilogram can be realized by national standards laboratories from similar to 1 p art 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 nat ional laboratories. This will, of course, need to be confirmed by cond ucting weighing trials on real Pt-Ir kilograms.