Two platinum-iridium kilogram standards are being used for this project (No. 651) owned by NPL and the other (No. 55) on loan from the PTB. After careful monitoring of their stability the circulation of the standards started in February 2001. The two standards are currently on the third leg of the comparison, which involves five legs in all – the weights returning to NPL between each leg. Each participating laboratory has one month in which to perform the measurements. The comparison is slightly behind schedule but it should be possible to make up the time when the weights next return to NPL. Nine of the seventeen participants had completed their measurements.
The comparison is due for completion early in 2003.
An alteration to the schedule of the comparison was agreed at the contact person’s meeting in Bratislava and the amended timetable has been circulated to all participants.
The main objective of this comparison is to estimate the stability of the platinum-iridium national primary kilograms and to evaluate the mass gain models used by the participants to correct for the contamination of platinum-iridium kilograms after cleaning by the BIPM. Initial results suggest that the stability of the national primary kilograms is a significant contribution to the uncertainty in the dissemination of the unit of mass, the spread on the results to date being of the same order as that seen on the EUROMET stainless steel comparison (project 215).