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A world-wide comparison of gauge block measurement by interferometry, decided on by the Comite Consultatif pour la Definition du Metre (CCDM), has been carried out. Four national metrology institutes, each representing one of the regional metrology organizations EUROMET, NORAMET, COOMET and APMP, measured a set of seven rectangular gauge blocks of steel with nominal lengths between 1 mm and 100 mm. This paper reports on the results of the comparison and draws conclusions with respect to the execution of future CCDM (now CCL) key comparisons.

A set of seven gauge blocks was circulated and calibrated by ten European national metrology institutes and one accredited laboratory. Measurements were made of central length, variation in length, flatness and, where possible, of the coefficient of thermal expansion. Agreement on the measurement results for the central length is excellent. It lies roughly within ±20 nm and seems to depend less on length than on the quality of the measurement faces, their wringing capability and the skill of the operator. The measurements of flatness and variation in length, on the other hand, differ substantially from one to another. A comparison of these results is difficult, however, since in practical measurements, the border zone defined by the standard is not quantitatively implemented. The few reported measurements of the thermal expansion coefficient are in good agreement. The comparison reflects the state of the art of gauge block calibration in Western European national metrology institutes. It also raises the question of whether the tolerances for flatness and variation in length can be checked interferometrically with reasonable effort.