At its meeting in September 1997, the Consultative Committee for the defenition of the metre (CCDM, today called Consultative Committee for Length, CCL), has identified several key comparisons in the field of dimensional metrology.
These key comparisons involve a relatively small number of laboratories but widely distributed around the world so they have to be completed by regional key comparisons under the frame of regional metrology organisations such as EUROMET.
Laboratories participating in both, the Consultative Committee/BIPM and the EUROMET comparisons, establish the link between these comparisons and assure their equivalence.
In this way a larger number of laboratories will be involved in these international comparisons so that the equivalence of national standards could be established world wide.
This comparison will be parallel to the Consultative Committee/BIPM key comparison piloted by the Swiss Federal Office of Metrology (OFMET) starting in spring 1998.
The set of gauge blocks has exactly the same composition as for the CCDM/BIPM comparison and the measuring instructions will also be the same as for the document prepared by the working group from BIPM in order to avoid any distorsion between the two comparisons.
Final report, 20 December 2004
This comparison on short gauge blocks measured by interferometry is an RMO Key Comparison and is parallel to the CCL Key Comparison CCL-K1.
Participation of LNE, METAS and NPL ensures the link to the CCL-K1 for the purposes of the MRA.
The set of gauge blocks has the same composition as for the CCL-K1, i.e. ten steel gauge blocks and ten tungsten carbide gauge blocks.
The comparison lasted from October 1998 to October 2000 and involved 17 National Metrology Institutes.
The draft A of the final report was discussed at the EUROMET contact persons meeting in Maribor (10-2001).
A few laboratories have withdrawn their participation, for various reasons, with the agreement of all other participants.
This comparison pointed out that the some of the gauge blocks were not stable and required a drift correction in the calculation of the deviation from the reference values. Consequently the uncertainty on the deviation from the reference value for each laboratory is increased compared to the CCL-K1 comparison.
In addition it appears that surfaces rapidly suffered from the repeated wringings, specially for steel gauge blocks, so that some laboratories, at the end of the loop, could not measure both faces and had some difficulties in determining the phase correction. Consequently, these laboratories increased their uncertainties of measurement compared to their best measurement capability.
The measurement results show a good agreement between laboratories. The deviation from the average is, except very few results, smaller than the stated standard uncertainty: one result for steel gauge blocks and nine for tungsten carbide gauge blocks on 160 measurements are unsatisfactory with respect to En values (
Except for a few laboratories, the uncertainties given by all participants are rather coherent both in the components and in the quantification. This can be explained by the fact that many laboratories have the same type of interferometer.
Following these conclusions, an additional comparison (EUROMET project # 643, EUROMET L-K1a) was organized for those laboratories which have withdrawn and for those laboratories which have found explanations for any significant offset of their results and improved their process since the publication of the draft A of this report.
The set includes 10 steel and 10 tungsten carbide gauge blocks from 0,5 mm to 100 mm