Long-term study of gauge block interferometer performance and gauge block stability

Abstract

We report on a successful 20-year study, involving a national metrology institute and a commercial instrument manufacturer, in the field of gauge block measurement by optical interferometry. The National Physical Laboratory (NPL) has made measurements of length artefacts on its own equipment and on 30 copies of this instrument that are now in use worldwide. Several NPL length artefacts have been measured on all 31 instruments including repeated re-measurement of the artefacts on the same equipment at NPL, over a period of 20 years. This pool of data therefore allows for quantitative examination of the temporal stability of these standards as well as the long-term performance of the instruments and of the evolving instrument design. Many of the laboratories which now own these instruments have taken part in international comparisons under the Comité International des Poids et Mesures' Mutual Recognition Arrangement. Combining the data from these key comparisons with the data obtained by NPL allows for a novel analysis—these data can be regarded as coming from an international key comparison, where both the artefact and the measuring instrument are circulated amongst the participants—the remaining variability being due to operator skill at the different laboratories, and the quality of their independent traceability chains.