Magnetic Uncertainties for Compact Kibble Balances: An Investigation

Abstract

The Kibble balance has become one of the major instruments for realizing the mass unit, the kilogram, in the revised international system of units (SI). Researchers at about a dozen national metrology institutes are actively working with Kibble balances that are capable of weighing masses with nominal values from 10 g to 1 kg. In the future, the design of smaller Kibble balances will play a more significant role. Smaller Kibble balances require smaller magnet systems, and here we investigate the scaling of systematic uncertainties with the size of the magnet system. We describe the size dependence of three magnetic effects: the coil-inductance effect, the yoke nonlinear effect, and the thermal effect. The analysis shows that the relative systematic effects become increasingly larger with smaller sizes. For small magnets the thermal effects become dominant and, hence, a good thermal design is imperative.

A novel magnet system—which enables uniformity of radial flux density by means of a parabolic yoke—applied in the Planck-Balance

“…The consequence is a low induction voltage and thus an unfavorable signal-to-noise ratio of the voltage measurement. Furthermore, a small coil-magnet system is also more prone to ohmic heating, as the current density in the coil is comparatively high and the heat capacity of the system is low [10]. An additional aspect that must be taken into account is the coil-current effect.…”