The kilogram was headed for a new definition, not an instant improvement in every weighing instrument. On November 16, 2018, delegates meeting in Versailles, France, voted to replace its physical reference with a definition based on natural constants. The change, alongside new definitions for the ampere, kelvin and mole, was scheduled to take effect on May 20, 2019. The U.S. National Institute of Standards and Technology (NIST) said consumers and most industries would not notice immediate effects. For businesses relying on precise measurements, the distinction was between improving the reference system and demonstrating what a particular instrument could measure. [S1][S3]
The existing kilogram depended on a platinum-iridium object kept near Paris, known as Le Grand K. NIST described the problem with making that artifact the final reference: its mass could change as it absorbed material from the air or lost material during cleaning. The approved replacement tied the kilogram to the Planck constant, together with the definitions of the metre and second. The aim was a reference grounded in natural phenomena rather than the condition of one manufactured object. [S1]
Putting that definition into practice would still require an experiment. NIST described a Kibble balance as a route to the new kilogram: it links the balancing of a mass to electrical measurements. That method also needs a precise measurement of local gravity, which varies with location. A universal definition therefore does not make every local measurement identical; the apparatus and the measurements used to operate it still matter. [S1]
A laboratory comparison reported before the vote made that practical distinction visible. In a March 15, 2018 update, NIST summarized a pilot study completed in late 2016, whose final report appeared in June 2017. The primary realizations of the kilogram, including NIST’s, agreed within 20 micrograms of each other. That was a result for the participating realization experiments, not a promised accuracy for a factory balance or every future measurement. NIST said the comparison was dominated by the experiments’ performance and, to a lesser degree for stainless-steel standards, by the behavior of the travelling masses. [S3]
Even moving a mass standard could affect the result. NIST’s account said transport appeared to cause larger mass changes in the stainless-steel masses than in the silicon and platinum-iridium masses. Those observations concern the physical objects used to carry measurements between laboratories. Replacing an artifact-based definition did not make the handling and comparison of such objects irrelevant. The experiment and the transfer of its result remained separate parts of making a shared unit useful. [S3][S1]
For a business evaluating a precision claim, the useful questions were consequently different: what defines the unit, how a laboratory realizes it, and what evidence establishes the performance of the measurement the business actually uses. The vote answered the first question for the revised system; the pilot comparison supplied evidence about particular realizations and transfers. NIST also reported scientists’ expectation that the changes would eventually reduce calibration costs. That was a prospective benefit, not a measured saving or an accuracy certificate for a customer’s equipment. [S1][S3]
Source documents
- Historic Vote Ties Kilogram and Other Units to Natural Constantshttps://www.nist.gov/news-events/news/2018/11/historic-vote-ties-kilogram-and-other-units-natural-constants
- Comparison of Future Realizations of the Kilogram - Final Reporthttps://www.nist.gov/news-events/news/2018/03/comparison-future-realizations-kilogram-final-report