Platinum–iridium (Pt/Ir) kilogram mass prototypes are known to gain contamination from the environment in which they are stored. The current method of cleaning these mass prototypes is called nettoyage–lavage and involves the physical rubbing of a kilogram with a chamois leather cloth soaked in a solvent followed by removal of any solvent residue using a jet of steam water. The manual nature of the technique means the effectiveness of the cleaning process is reliant on the human operative. An alternative cleaning method involving exposure to ultraviolet light and ozone (UV/O3) has been tested on Pt/Ir foils and kilogram mass prototypes. The changes to the surface of the Pt/Ir foils as a result of this process have been quantified using x-ray photoelectron spectroscopy and have shown a clear reduction in the quantity of carbonaceous contamination. Variation of the UV intensity, ozone concentration and exposure duration enabled the optimum cleaning conditions to be established. The UV/O3 cleaning method was then used to clean two Pt/Ir kilogram mass prototypes and gravimetric weighing of the kilograms before and after cleaning gave the amount of contamination removed. These gravimetric weighing results demonstrated that UV/O3 cleaning was as effective as the nettoyage–lavage process.

A cleaning method using ultraviolet activated ozone has been developed at the National Physical Laboratory (NPL). A comparison to test the effectiveness of this cleaning method with nettoyage-lavage cleaning on platinum–iridium artefacts and kilogram mass standards was undertaken at the Bureau International des Poids et Mesures (BIPM). Three cleaning trials were performed to compare the effectiveness of the two methods at removing natural, medium and heavy levels of surface contamination.Both the ultraviolet light/ozone method and the nettoyage-lavage method successfully removed both natural levels and medium levels of surface contamination. However, in the medium contamination trial nettoyage-lavage cleaning removed additional mass from the Pt/Ir mass standard under test that could not be accounted for in terms of surface contamination.In the heavy contamination trial nettoyage-lavage cleaning successfully removed most of the contamination from the surface of the contaminated Pt/Ir mass standard. The ultraviolet light/ozone method removed about two thirds of the surface contamination and therefore for heavily contaminated mass standards it is advisable to pre-clean them with a solvent before ultraviolet/ozone cleaning.

The redefinition of the kilogram, along with another three of the base SI units, is scheduled for 2018. The current definition of the SI unit of mass assigns a mass of exactly one kilogram to the International Prototype of the kilogram, which is maintained in air and from which the unit is disseminated. The new definition, which will be from the Planck constant, involves the realisation of the mass unit in vacuum by the watt balance or Avogadro experiments. Thus, for the effective dissemination of the mass unit from the primary realisation experiments to end users, traceability of mass standards transferred between vacuum and air needs to be established and the associated uncertainties well understood. This paper describes means of achieving the link between a unit realised in vacuum and standards used in air, and the ways in which their use can be optimised. It also investigates the likely uncertainty contribution introduced by the vacuum-air transfer process.1 See Section 2 of the draft mise en pratique of the definition of the kilogram http://www.bipm.org/cc/CCM/Allowed/15/02A_MeP_kg_141022_v-9.0_clean.pdf for details of the primary methods to realize the definition of the kilogram

This paper reports work undertaken to evaluate the change in mass of platinum/iridium, stainless steel and silicon artefacts measured at atmospheric pressure and in vacuum at a range of pressures typical of those used in vacuum mass comparators and watt balances and for x-ray crystal density (XRCD) measurements. The sets of platinum/iridium, stainless steel and silicon artefacts used in this work have different surface areas and the effect of transferring them between atmospheric pressure and different levels of vacuum was evaluated by measuring the relative changes in mass between them. Reversible variations in the mass differences between the artefacts were found over the pressure range from 0.1 Pa to 100 000 Pa (atmospheric pressure). At lower pressures (0.001 Pa to 0.1 Pa) the mass differences between all the artefacts were stable and no evidence for hysteresis over this range was found when going down in pressure compared with increasing pressure. Therefore consistent results between watt balance, XRCD measurements and vacuum mass measurements can be realized providing the measurements are performed within this pressure range.

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