Monitoring of the Drift of the Pt–Ir Kilogram Prototypes of KRISS, Traceable to BIPM As-Maintained Mass Unit

The Bureau International des Poids et Mesures (BIPM) announced recently that its as-maintained mass unit had been changed as the result of Extraordinary Calibration. Following that amendment, the mass change of the national prototype of the kilogram (NPK) of Korea, No. 72, did not show the positive rate of drift shown in other Pt–Ir prototypes. Before the amendment, the drift rate was incorrectly evaluated to have a positive slope as a time evolution. However, we have collected measurement data for two other prototypes, Nos. 39 and 84, for about 15 years in an annual comparison with the NPK. Our periodic monitoring found drifts of a few micrograms in transit as it crossed international borders. Inspired by the new amendment of the reference prototype, our analysis of linear and exponential modeling with the least square method indicated that the modeling results of the mass change of prototypes Nos. 39 and 84 were very similar to those of earlier studies, regardless of the way we assign reference values. Prototype No. 39 notably showed unexpected behavior in the third verification: It recovered to about 1.0 μg/year, the normal rate of mass change. Different from our intuition, the mass drift of the prototype did not seem to be heavily influenced by a physically poor surface. However, in assigning the reference value in an interpolated manner, this study gives the lowest uncertainty of fitting parameters considered as evidence to prove the validity of the last evolution of the BIPM as-maintained mass unit.