[ieeepkew] Old and new atomic mass units

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?