Ah, they went with the "electric kilogram". The other plan was to build up a regular structure with a known number of silicon atoms. That idea was to make a perfect crystal and count the number of atoms on each face. Apparently that's almost possible, although hard to do.
According to the article, they did both; the definition is in terms of physical constants, and the electric-scale and silicon-sphere experiments were separate ways of measuring according to this definition. This let them verify their work by checking that two separate methods of measurement came out with values that were similar to each other with enough precision.
Was that the plan then? You'd need to have one hundred percent purity nitrogen with no oxygen and no water, is it really possible to keep it at the level of purity you need?
I don't know, I was just presenting an alternative solution to the problem you were talking about. I imagine that the sides of the encasement would have to be pretty impermeable to keep oxygen out, but it's probably harder if the oxygen-containing air is pushing in and nothing is pushing out.
Playing around with the chemistry more, it might be possible to use a gas that oxidizes more readily than silicon, so that if any oxygen does get in it will be neutralized before it gets anywhere near the silicon. I'm not sure if that's possible to do with a gas, though.
