mass - Redefining the kilogram using Planck's constant instead of the density of water among other examples

The kilogram is in the process of being redefined in terms of Planck's constant so as to eliminate its dependence on a physical artefact. Since the length and temperature units are already precisely defined, why not just calculate the density of some substance, say water, at a particular temperature and use that as a standard for mass? Sounds simpler to me.

Answer

Since the length and temperature units are already precisely defined, why not just calculate the density of some substance, say water, at a particular temperature and use that as a standard for mass?

Water is a lousy choice. The initial proposal for the French metric system used the mass of a cubic decimeter of water. Measurement issues resulted in this being changed to a prototype-based system in just a few years. Issues with those initial prototypes resulted in the current prototype masses. Issues with those newer prototypes are part of what motivated the physics-based redefinition of the International System (SI).

Water is a bad choice, but what about some other substance? The problem with this is that it flies in the face of one of the key goals of the proposed redefinition of the SI, which is to define the base units of time, length, mass, current, and temperature solely in terms of fundamental physical constants. The mole is also being redefined, from the number of atoms in 12 grams of ¹²C to a specified number.

Other key goals are that the changes should represent improvements and that the redefined base units must be consistent with the past. Those latter two have always been goals. Using a fundamental physics-based approach is new, or almost new. The definitions of the second and meter are fundamentally-based. The improvements that these redefinitions enabled were a strong motivator to continue this process to the remaining three physical units, and to the mole as well.

That said, using a carefully measured quantity of some substance is close to one of the two approaches being used to establish the exact value of Planck's constant. Those two approaches are the Kibble balance (formerly Watt balance), which carefully compares electrical power to mechanical power, and the Avogadro technique, which carefully calculates the number of atoms in a carefully measured sphere of nearly pure ²⁸Si.

The deadline for measurements by multiple groups using these two techniques to estimate Planck's constant passed on July 1. The requirement by the International Bureau of Weights and Measures (BIPM; the acronym is French) was to have at least three experiments with an uncertainty of 50 parts per billion (ppb) or better and at least one with an uncertainty of 20 ppb or better. Previous failures to meet that goal is the key reason the SI redefinition is not yet in place. That goal has now been met. There are multiple experiments with much better than the requisite 50 ppb uncertainty and three with better than 20 ppb.