The nanometric roughness of mass standards and the effect of BIPM cleaning-washing techniques

Abstract

A method based on light scattering has been used to characterize the surface roughness of four standard kilograms made from XSH alacrite, in order to determine the root-mean-square height delta and the correlation length sigma. The values of these two parameters, which have been found to lie between 2 nm and 5 nm and between 170 nm and 250 nm, respectively, show the high surface quality that it is possible to obtain by mechanical polishing of XSH alacrite. Cartographies made by light scattering, as well as delta and σ values, have shown that in spite of identical manufacturing conditions the kilograms present a significant difference in surface roughness. We have observed that the lower bases were in general rougher than the upper ones. This was explained by the different histories of the upper and lower bases. On the other hand, the same value of σ was found for both bases of a given kilogram. We have also found that the smoother the kilogram, the smaller the mass loss. This clearly shows the effect of surface roughness on the stability of mass standards. Finally, significant variations in the values of and delta and σ have been observed after the cleaning-washing process at the Bureau International des Poids et Mesures. We have also shown that the variations noted after several successive cleaning-washings are mainly the result of the first of these. This agrees with observations made during earlier gravimetric studies.