Measurement reproducibility of PTB's sphere interferometer after more than a decade

The volumes of two silicon spheres that have been calibrated 15 years ago for density determinations, have now been re-measured in PTB's sphere interferometer. Since the first measurement, the configuration of the measurement system has been developed considerably. The values of the mean diameter could be reproduced for the two spheres on the level of 10
^-9
, so that the reproducibility is of one order of magnitude less than the presently estimated uncertainties.