Comment by jasonmp85 | Hacker News Reader

Comment by jasonmp85 | Hacker News Reader

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I'm not actually sure what you're insinuating here. Previously the standard WAS the "international prototype kilogram", all of which have drifted since creation (see here: https://en.wikipedia.org/wiki/Kilogram#Stability_of_the_inte... )

The problem with this is it isn't even fragile, it's _not reproducible_. The worst part is that other SI units depend on the kilogram, so they all change as this drift occurs! After a certain amount of time, how will we know what a "real" kilogram is? See here for discussion of this SI problem: https://en.wikipedia.org/wiki/Kilogram#Dependency_of_the_SI_...

The point of the watt balance approach is to make it possible for a lab to reproduce the kilogram without another kilogram reference. That's what this is about, and I don't really think it's embarrassing. I think it's very, very hard.

Why do we need these rare metals? Why not just use a block of ice generated from a known quantity of water, for example?

The prototype kilogram was made of platinum-iridium because they are very nonreactive and fairly hard, so once the prototype was made it would be unlikely to be accidentally modified.

If you tried to use water ice as a standard, you'd find that its mass was changing all the time as the ice sublimated away, or humidity condensed onto its surface. It would be impossible to make precise measurements.

I'm not sure why ice would matter given that it should weigh the same as in the liquid form, but...

How do you measure a known quantity of water to within a few tens of parts per billion? Do you specify the proportion of light to heavy water? (For 'normal' water about 1 in 41 million.) What about contaminants in the water? There are so many uncertainties that it's impossible for all practical purposes.

The whole point of using 'rare' silicon is that we already have commercial processes for producing ultrapure, defectless, monocrystals of the stuff (for use in computer chips), so it's easy to reuse that technology for defining a kilogram.

Because you don't want that lump of mass to change in any way while it is being used as a reference.

Are you aware of all the matter exchange processes that are continuously going on at the surface of water ice under normal conditions?

Necroposting, I know, but under what definition is silicon a "rare metal"?

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