Slashdot Mirror
The Future of the Kilo: a Weighty Matter (theguardian.com)
A lump of metal in a building near Paris has long served as the global standard for the kilogram. That's about to change. From a report: Later this month, at the international General Conference on Weights and Measures, to be held in France, delegates are expected to vote to get rid of this single physical specimen and instead plump to use a fundamental measurement -- to be defined in terms of an electric current -- in order to define the mass of an object. The king of kilograms is about to be dethroned. And crucially much of the key work that has led to the toppling of the Paris kilogram has been carried out at the National Physical Laboratory where the late Bryan Kibble invented the basic concepts of the device that will replace that ingot in the Pavillon de Breteuil. The Kibble balance works by measuring the electric current that is required to produce an electromagnetic force equal to the gravitational force acting on a mass. A second stage allows the electromagnetic force to be determined in terms of a fundamental constant known as the Planck constant which will, in future, be used to define a kilogram. These machines will provide the standard for weighing objects -- and that means no more dusting of old lumps of alloy to ensure they stay pure and accurate.
[...] "One key reason for doing this work is to provide international security," says Robinson. "If the Pavillon de Breteuil burned down tomorrow and the kilogram in its vaults melted, we would have no reference left for the world's metric weights system. There would be chaos. The current definition of the kilogram is the weight of that cylinder in Paris, after all." [...] Another major motivation for the replacement of le grand K is the need to be able to carry out increasingly more and more precise measurements. "Pharmaceutical companies will soon be wanting to use ingredients that will have to be measured in terms of a few millionths or even billionths of a gram," says Prior. "We need to be prepared to weigh substances with that kind of accuracy." Suggested reading: A thread on Twitter which discusses SI units and the redefinition of the kilogram.
[...] "One key reason for doing this work is to provide international security," says Robinson. "If the Pavillon de Breteuil burned down tomorrow and the kilogram in its vaults melted, we would have no reference left for the world's metric weights system. There would be chaos. The current definition of the kilogram is the weight of that cylinder in Paris, after all." [...] Another major motivation for the replacement of le grand K is the need to be able to carry out increasingly more and more precise measurements. "Pharmaceutical companies will soon be wanting to use ingredients that will have to be measured in terms of a few millionths or even billionths of a gram," says Prior. "We need to be prepared to weigh substances with that kind of accuracy." Suggested reading: A thread on Twitter which discusses SI units and the redefinition of the kilogram.
If the Pavillon de Breteuil burned down tomorrow and the kilogram in its vaults melted, we would have no reference left for the world's metric weights system. There would be chaos.
That would absolutely be inconvenient, because it is the master reference.
However, other reference kilograms exist, for example, the US National Institute of Standards and Technology has a kilogram and a meter. These secondary references are sometimes used to compare against the primary reference kilogram to ascertain drift.
It would be an annoyance to lose the master, but not a disaster.
Anyway it will soon be redefined in terms of nonphysical objects so the window of problem is small.
You keep using this fundamental units, it doesn't mean what you think it means.
The SI system is a complete clusterfuck of "fundamental units":
* Amp depends on the definition of kg
* candela depends on the definition of kg
* Kelvin depends on the definition of kg
* Mole depends on the definition of kg
These units should be ORTHOGONAL; not dependent on one another.
And anyway, isn't 1 kilogram already defined at 1 L of pure H2O?
No, it is not. RTFA. It is defined as the mass of a slug of platinum-iridium alloy in Paris.
Where are you going to get a liter of pure H2O? Water contains about 0.1% deuterium and three different stable isotopes of oxygen, all in varying concentrations depending on the source of the water. You could distill it, but never get it completely pure. And how are you going to determine the purity? By weighing it?
Using water as the basis is way worse than using metal, because water evaporates, absorbs gases from the air, absorbs ions from the container, etc.