The upgrade of NIM-2 joule balance since 2017

In 2017, we published the first measurement result with the NIM-2 joule balance and the relative standard uncertainty was 2.4 × 10-7. Since then, a series of improvements have been carried out to upgrade the NIM-2 joule balance towards the 10-8 level relative uncertainty. The main improvements include a permanent magnet, as well as a two-layer magnetic shield to decrease the main uncertainty contribution from the external magnetic field, a parasitic displacement measurement system and a new suspension for improving alignments, and a new mass translation stage constructed with all non-magnetic materials. In September 2019, when the NIM-2 joule balance was used to calibrate a 1 kg mass, the type A relative standard uncertainty had been reduced to 3 × 10-8(k= 1) and the combined relative standard uncertainty was less than 1 × 10-7(k= 1). In October and November 2019, a 1 kg Pt/Ir mass and a 1 kg stainless steel mass were measured with the NIM-2 joule balance, and the relative standard uncertainties were 5.2 × 10-8 and 6.5 × 10-8, respectively. The relative difference between the measured values and the calibration values obtained from the mass group were 4 × 10-9 and 3 × 10-8, respectively.