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I actually do not know off the top of my head. We often work in 'gauge pressure', setting 1 atm as the 0 point. The trivial answer I guess, is 0. Looking up the answer, it's a little over 10 1/3 m.

that is a big head :D

In hindsight, I should've known this both because it's the same as the limit for sucking water up a tube in a vacuum (at 1 atm) and the rule of thumb that pressure increases by 1 atm every 10 m.

This is why I actually suggested 10 in the very beginning. This being said, the next episode is finally out :D