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What is the level of accuracy they are aiming for? If it entails have some uncertainty over the precise number of atoms in the silicon sphere, then how did they choose this level of accuracy?

The possible accuracy is limited by uncertainty over mass of IPK(International Prototype Kilogram), which is 20 ppb. You cannot measure the number of atoms in a kilogram of silicon better than that, because a kilogram is defined by IPK.

20 ppb translates to uncertainty of about 10000 trillion atoms. The number will be set to a round number, because any number within trillions is compatible with the legacy standard.

I don't know what fundamentally sets the accuracy, but it's definitely a lot coarser than the number of atoms in the silicon sphere. G is known to 4 or 5 significant digits, while atomic accuracy would be 26 significant digits.

G doesn't matter if you use dual-pan balances, and they do.

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