The KRISS Kibble balance realized the kilogram for the first time in 2019–2020 with a combined relative standard uncertainty of 1.2×10−71.2\times10^{-7} (Kim et al. 2020), and KRISS itself has since identified the moving mode of that first instrument as a major source of measurement uncertainty in designing its successor (Kim et al. 2025); the alignment-related systematics of the moving and weighing modes (parasitic moving-mode voltages, weighing-mode force bias from misaligned electromagnetic and gravitational forces, Abbe offset, laser verticality) are what that successor is designed to suppress. A target relative uncertainty of 1×10−81\times10^{-8} matches the CCM coordinated-dissemination criteria, under which realization experiments must achieve key-comparison relative standard uncertainties of 40 parts in 10910^9 or better, with at least three at 20 parts in 10910^9 or less (Stock, Davidson and Fang 2022). The open problem is the practical and metrological route by which this class of two-mode Kibble balance — through KRISS's successor apparatus KB-2 or otherwise — can suppress these alignment-related systematics sufficiently to realize and disseminate the kilogram reproducibly at the 10−810^{-8} level. As of 2025 the successor apparatus exists only as a validated prototype, no improved realization has been published, and KRISS — which took part in the first CCM key comparison of kilogram realizations (CCM.M-K8.2019) at the 10−710^{-7} level — did not participate in the second (CCM.M-K8.2021).

Background

A Kibble balance (formerly watt balance) links macroscopic mass to the Planck constant hh by equating mechanical and electrical power in two phases. In the weighing phase, a mass mm in the local gravitational field of strength gg is balanced by the Lorentz force on a current II in a coil immersed in a magnetic field, mg=BL Img=BL\,I, where BLBL is the magnetic flux-integral (geometric factor) of the coil. In the moving phase, the same coil is moved vertically at velocity vv through the field, inducing a voltage U=BL vU=BL\,v. Eliminating BLBL between the two equations gives mg=UImg=UI, i.e. a comparison of mechanical power to electrical power, so mass is determined from voltage, resistance, velocity and gravity — quantities traceable, via the Josephson voltage standard and the quantum Hall resistance standard, to hh and the elementary charge. Since the 2019 revision of the International System of Units (SI), which fixed the numerical value of hh, Kibble balances are one of the two primary methods (with the X-ray crystal density method using silicon spheres) for realizing the kilogram, and national metrology institutes operate them to realize and disseminate mass independently of the retired International Prototype of the Kilogram (IPK). Uncertainty evaluation follows the metrological convention of type A components (statistical, from repeated observations) and type B components (evaluated by other means, e.g. corrections and calibration limits). Kibble balances are especially vulnerable to alignment errors: if the coil velocity is not perfectly vertical and collinear with gravity, or the laser interferometer axis is offset from the coil's center of mass (Abbe offset) or not exactly vertical, parasitic voltages appear in the moving phase and parasitic forces and torques appear in the weighing phase, biasing the result. The Korea Research Institute of Standards and Science (KRISS) reported the first kilogram realization of Korea with its Kibble balance (Kim et al. 2020): two 1 kg artefacts, a platinum–iridium cylinder (No. 111) and a stainless-steel cylinder (S1), were measured under vacuum (pressure 3×10−33\times10^{-3} to 4×10−34\times10^{-3} Pa) between December 2019 and January 2020, with a combined relative standard uncertainty (coverage factor k=1k=1) of 1.2×10−71.2\times10^{-7}, consistent within that uncertainty with the values obtained by the previous artefact-based traceability to the IPK (differences of 2 µg and 56 µg respectively). KRISS has since identified the moving mode of that first instrument as a major source of measurement uncertainty in developing its successor (Kim et al. 2025), and the alignment-related terms of the moving and weighing modes — parasitic voltages generated by horizontal coil motion in the moving mode, force biases in the weighing mode from imperfect coincidence of the electromagnetic and gravitational force directions, Abbe offset, and laser verticality — are the systematics whose suppression that work targets; this term taxonomy matches the alignment literature cited by Kim et al. 2020 itself (alignment of the NRC and NPL balances, and in-situ correction of Abbe offset error). Reaching a target relative uncertainty of 1×10−81\times10^{-8} for kilogram realization and dissemination corresponds to the CCM dissemination criteria (Stock, Davidson and Fang 2022). This defines the problem addressed here: the instrument-specific question of which alignment terms, mechanical couplings and correction strategies limit or enable the step from a demonstrated 10−710^{-7} realization to a reproducible 10−810^{-8}-level realization.

Problem Statement

By what apparatus refinement, alignment measurement and correction strategy can the KRISS Kibble balance — the existing instrument or its successor, the KB-2 apparatus KRISS has been developing since 2024 to succeed it — suppress the alignment-related type-B uncertainties of its moving and weighing modes (parasitic voltages from horizontal coil motion in the moving mode; force and torque biases in the weighing mode from imperfect coincidence of the electromagnetic and gravitational force directions; Abbe offset between the interferometer axis and the coil center of mass; non-verticality of the laser beam) so that the kilogram is realized with a combined relative standard uncertainty at the target level of 1×10−81\times10^{-8} (k=1k=1), reproducibly and at dissemination grade — the latter including verification of the realized mass values by demonstrated consistency with the key comparison reference value through participation in CCM key comparisons of kilogram realizations (Stock, Davidson and Fang 2022)?

References

  1. Dongmin Kim, MyeongHyeon Kim, Minky Seo, Byung-Chill Woo, Sungjun Lee, Jong-Ahn Kim, Dong-Hun Chae, Mun-Seog Kim, In-Mook Choi, Kwang-Cheol Lee, 'Realization of the kilogram using the KRISS Kibble balance', Metrologia 57 (2020) 055006. DOI: 10.1088/1681-7575/ab92e0, https://doi.org/10.1088/1681-7575/ab92e0
  2. MyeongHyeon Kim, Dongmin Kim, Minky Seo, Sung Wan Cho, Jinhee Kim, Kwang-Cheol Lee, 'Design of the KRISS Kibble balance II: Focus on the moving mode', 2024 Conference on Precision Electromagnetic Measurements (CPEM 2024). DOI: 10.1109/CPEM61406.2024.10646015, https://doi.org/10.1109/CPEM61406.2024.10646015
  3. Sungwan Cho, Myeonghyeon Kim, Minky Seo, Kwang-Cheol Lee, Jinhee Kim, Dong Min Kim, 'Progress of Optical Measurement System Design for KRISS Kibble Balance', 2024 Conference on Precision Electromagnetic Measurements (CPEM 2024). DOI: 10.1109/CPEM61406.2024.10645992, https://doi.org/10.1109/cpem61406.2024.10645992
  4. MyeongHyeon Kim, Dongmin Kim, Minky Seo, Sung Wan Cho, Jinhee Kim, Kwang-Cheol Lee, 'Development of KRISS Kibble balance II', in: 150th anniversary of the Metre Convention — From Units to the Universe (2025). DOI: 10.59161/fjlv9070, https://doi.org/10.59161/fjlv9070
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  6. M. Stock, S. Davidson, H. Fang, 'Dissemination of the kilogram following its redefinition', IMEKO 24th TC3, 14th TC5, 6th TC16 and 5th TC22 International Conference, Cavtat-Dubrovnik, Croatia (2022). DOI: 10.21014/tc3-2022.011, https://www.imeko.org/publications/tc3-2022/IMEKO-TC3-2022-011.pdf