Mass Change of Stainless Steel Kilogram Standards at NMIA

A study of the relative mass changes of eight of the kilogram secondary stainless steel mass standards kept at the National Measurement Institute, Australia (NMIA) was conducted between 1993 and 2006. Three of these secondary standards were compared directly to the International Prototype Kilogram copy no. 44 which has been the Australian national standard of mass since 1949. Two of these three standards were then compared with the other five secondary kilogram standards and also used to calibrate our two one kilogram stainless steel working standards. In this paper, the mass comparisons between copy no. 44 and eight of NMIA's secondary standards during this period are presented. The result shows that mass changes of the stainless steel secondary standards vary from 23 mu g to 126 mu g over 13 years even though they were kept under the same environmental conditions.