Errors due to magnetic effects in 1 kg primary mass comparators

A revised draft of OIML Recommendation 111 has been approved recently. Among the many notable changes to the previous edition are specific requirements for both the magnetic susceptibility and residual magnetization of weights used in legal metrology. The highest class of such weights is E1, for which a 1 kg weight has a manufacturing tolerance of ±0.5 mg. It therefore follows that better magnetic properties might be needed for weights having mass uncertainties substantially better than E1 tolerances. The work presented below shows how this problem was approached at the BIPM and supports our conclusion that alloys meeting the new Class E1 specifications for 1 kg weights have magnetic properties sufficient for the needs of the BIPM.