In preparation of the redefinition of the kilogram, the official copies of the international prototype of the kilogram and the working standards of the BIPM were compared with the international prototype of the kilogram [1-3]. These extraordinary calibrations have shown a significant decrease in the mass of the working standards of the BIPM since the 3rd periodic verification of the national prototypes of the kilogram (1988–1992), which has led to systematic deviations in the dissemination of the unit of mass by the BIPM. The required corrections for the national prototypes of the kilogram are dependent on time. They amount to approx. -4 µg for calibrations performed in 2003 and change approximately linearly by up to -35 µg for calibrations in 2013.
These changes have also been disseminated to and by PTB. If no more than five years have passed since your last link-up with mass standards of PTB, corrected certificates can be issued for link-ups with relative expanded measurement uncertainties (k = 2) in the range of U(m)/m ≤ 8 · 10-8 upon application and submission of the original of the respective certificate. For the last link-up of your standards, issuing of a corrected certificate is, under the conditions mentioned, free of charge. If the respective link-up is older than five years, we recommend a recalibration.
References
[1] Stock, M.; Barat, P.; Davis, R.; Picard, A.; Milton, M. J. T.: Calibration campaign against the international prototype of the kilogram in anticipation of the redefinition of the kilogram part I: comparison of the international prototype with its official copies. Metrologia 52 (2015), pp. 310 - 316 (Open access: http://iopscience.iop.org/0026-1394/52/2/310/)
[2] http://www.bipm.org/en/news/full-stories/2015-03-ipk-results.html
[3] CCM Recommendation to NMIs on managing the consequences of the corrections to the BIPM as-maintained mass unit (http://www.bipm.org/cc/CCM/Allowed/15/17_agenda6&10_CCM-Recommendation.pdf)