The recent redefinition of the International System of Units (SI) shifts the definition of the unit of mass, the kilogram, from a physical artifact to the fixed value of the Planck constant. Utilizing the new SI, mass measurements can be carried out with Kibble balances, which directly realize mass through electromagnetic force measurements ultimately traceable to quantum electrical standards with accuracy at the parts per hundred million (108) level.
The 1-kg Kibble balances at national metrology institutes are the size of a room, extremely expensive to build, and require technical experts to operate. However, calibrations laboratories and metrology institutes are showing increasing interest in directly measuring gram-level masses. NIST is developing an instrument for that purpose that is much smaller, cheaper and simpler to operate: the tabletop Kibble balance.
NIST scientists initially built a proof-of-concept tabletop Kibble balance, KIBB-g1, which can realize gram-level masses with parts per million (106) accuracy — on par with the accuracy of OIML Class E2 masses. It operates on the same physical properties as the 1-kg devices, including the overhead wheel (and supporting air bearing) connecting the main-mass side to the counter-mass side, but with a footprint of about one-quarter of a square meter excluding signal-processing electronics.
That endeavor recently caught the attention of the U.S. Department of Defense, and a collaboration was formed for developing a more compact, robust second-generation instrument, KIBB-g2, geared toward calibrations laboratory use.
For KIBB-g2, the design was revised. The air bearing linear guidance mechanism — the largest source of uncertainty in KIBB-g1 — was replaced with a flexure-based mechanism that allows motion in only a single dimension. Research is also currently underway in exploring alternative displacement sensors to the interferometer, such as an optical encoder, for measuring the motion of the coil.
It is compulsory for calibrations laboratories to regularly calibrate their mass sets by physically shipping them to primary laboratories such as NIST. This process is logistically and financially inefficient, as well as time-consuming. Within this calibration infrastructure is a convoluted dissemination chain involving the relative comparisons of artifact masses.
Gram-level mass determinations have historically been realized via subdivisions ultimately traceable to a primary kilogram standard. The further you scale away from the 1-kg benchmark, the more uncertainties you accrue along the way.
The new, fixed value of the Planck constant is scale-invariant and allows mass to be directly realized at any level, making the historical 1-kg standard no longer unique. A tabletop Kibble balance can directly realize a 1.52834-gram mass as easily as a cardinal 1.00000-gram mass. Furthermore, it allows for calibrations labs to truncate this outdated mass traceability chain and directly realize gram-level masses on site with traceability to voltage and resistance standards, and ultimately to the Planck constant.
The end goal of this project is to develop a commercial Kibble balance comparable to standard mass comparators in terms of usability, robustness, cost and size.
I.A. Robinson and S. Schlamminger. The watt or Kibble balance: a technique for implementing the new SI definition of the unit of mass. Metrologia. Sept. 28 2016. DOI: 10.1088/0026-1394/53/5/A46
L. Chao, F. Seifert, D. Haddad, J. Pratt, D. Newell and S. Schlamminger. The performance of the KIBB-g1 tabletop Kibble balance at NIST. Metrologia. May 14, 2020. DOI: 10.1088/1681-7575/ab507d
Y. Cao, S. Schlamminger, D. Newell, J. Hendricks, B. Goldstein and L. Chao. The design and development of the second generation tabletop Kibble balance at NIST. euspen’s 22nd International Conference Proceedings. June 2022.
L. Chao and J. Pratt. Absolute Mass Balance. United States Patent US 11187571B2. Nov. 30, 2021.