The 2019 SI redefinition, in practice: what changed for calibration laboratories

Since 20 May 2019, all seven SI base units have been defined by fixing the numerical values of seven constants — the Planck constant now defines the kilogram, the elementary charge the ampere, and the Boltzmann constant the kelvin.

For working calibration laboratories the practical effects were deliberately invisible: mass values traceable through national standards did not step-change, and no certificate needed reissuing. What changed is the architecture above the working level — the kilogram no longer depends on a single platinum-iridium artifact in Sèvres, and any suitably equipped laboratory can in principle realize the unit via Kibble balance or X-ray crystal density methods.

The durable lesson for uncertainty budgets: traceability statements now terminate in constants of nature, and NIST SP 330:2019 is the reference for how to phrase that in US practice.