Old "oxygen-16" chemistry atomic mass unit = 0.9999394 to 0.9999856 (new) carbon-12 daltons. We put oxygen-16 in scare quotes because it was actually isotope-weighted. The range is because the atomic weight (equivalently isotopic concentration) varies a lot depending on what mineral or gas the oxygen is part of. The differing isotope concentrations is called the Dole effect.
oxygen-16 physics atomic mass unit = 0.9996821637225 carbon-12 daltons
We used modern measurements of oxygen.
Interestingly, the isotopically weighted old standard is closer to the new pure carbon-12 isotope standard than the old pure isotope standard. Maybe it was more important to preserve the results from chemistry? Or the chemists had more political power in defining the carbon-12 standard?
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