The upgrade of NIM-2 joule balance since 2017

Abstract

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.

A method for evaluating the force factor in oscillating Kibble balances with application to the Planck-Balance

“…Here, the coil of the voice coil actuator is moved with a (usually) linear trajectory through the magnetic field of a permanent magnet (see, e.g. [5][6][7][8][9][10]). The induced DC voltage is measured synchronously to the position of the coil (from which its velocity is derived).…”

UME Kibble balance operating in air

“…Moreover, the primary realization of the unit is separated conceptually from the definition so that the mise en pratique may be improved as science and technology develop. Currently, two complementing primary realization methods have the capability for realizing the kilogram with the required uncertainty: the Kibble balance (KB) [2][3][4][5][6][7][8][9][10][11][12][13] and the x-ray crystal density [14][15][16]. Although the target uncertainty was achieved by the individual experiments, the submitted Planck constant data to CODATA in 2017 was not statistically consistent [17].…”

Automatic alignment technique for the suspended coil in the joule balance

“…From May 20, 2019, the kilogram, the unit of mass, was redefined based on the Planck constant [1]. The joule balance, derived from the Kibble balance, is one of the methods used to measure the mass from the Planck constant at 10 −8 relative uncertainty level [2,3]. The basic principle of the Kibble balances and the joule balance is to use the electromagnetic force to balance the gravity of the measured mass [4][5][6][7].…”

“…In the second generation of the joule balance NIM-2, fine-tuning mechanics have been used to adjust the horizontal position and posture of the suspended coil, and this process could be performed in a vacuum [26]. Although this development can largely reduce the alignment uncertainty to less than 5 × 10 −8 [3,27], the adjustment was still complicated to achieve due to the strong coupling between the horizontal forces and torques. To solve this problem, this study proposes an automatic technique and system to minimize horizontal forces and torques.…”