"

 Short-term instability - possible mistake 

""What is known specifically about the IPK is that it exhibits a short-term instability of about 30 µg over a period of about a month in its after-cleaned mass.[18]"" - from what I think is the BIPM's source for this, http://www.bipm.org/utils/en/pdf/Monographie1990-1-EN.pdf, it seems to be more like 4 µg over a period of about four months for the IPK (figure 14 in that document). The proceedings linked in footnote 18 give no source for the much higher value."