Warp schrieb:

>   Why can't the kilogram be defined as the weight of exactly 1 litre of
> pure water at a certain temperature? After all, that has been the de-facto
> definition for forever.

That was only the /initinal/ definition.

What reason would be there to stick to it - or choose a different 
definition, for that matter?

There's only one: Reproducability.

If you can find a definition for a kilogram that /matches/ the initial 
definition, but can be reprocuded with /higher precision/, then that 
definition is both superior and "backward compatible".

It turned out that measuring the mass of 1 litre of water at a certain 
temperature was subject to more error (possibly due to issues with 
producing /really/ pure water, and reproducing /exactly/ the desired 
temperature) than "copying" the mass of some sample entity made of a 
robust, non-corrosive material.

Note that while it was found that the "primary copies" of the kilogram 
exhibit a "drift" relative to the Grand K, but nobody is presently able 
to tell whether the Grand K also exhibits an /absolute/ drift. If  pure 
water would provide a reference of adequate precision (or even anywhere 
close), I guess scientists would already have checked for absolute 
drifts using this method.


Also note that there is no such thing as a "de-facto definition"; there 
are "practical realizations" that differ from the official definition, 
but I guess none is based on the initial definition these days, except 
maybe as makeshift references for low-precision applications.

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