Artificial gills aren't used for diving, not because they can't extract oxygen out of water, but because they won't provide enough oxygen for someone to breathe (only arround 1% oxygen in water instead of 20% in air). So they stick with oxygen tanks.

But why don't they use them both at the same time? The gills will on give a little bit of oxygen, true, but over time it will allow the diver to stay underwater longer with the same oxygen tank. If you add a rebreather to that, so only arround 5% of your air goes away every breathe, it will be even longer.

  • $\begingroup$ Because divers like reliability - breathing is a fundamental concern - do you want to volunteer to be the test pilot? $\endgroup$ – Solar Mike Apr 15 at 19:49
  • $\begingroup$ Oxygen would, at best, not be exchanged between the water and the "spent" air (which will have significant oxygen still in it), and at worst will be absorbed into the water. Do a web search on "partial pressure of dissolved oxygen". $\endgroup$ – TimWescott Apr 16 at 0:52
  • $\begingroup$ Thanks for the answers, also if you could find a way to keep the pressure in the oxygen tanks below the pressure of water, would oxygen then diffuse into the tanks through the artificial gills? (I think gills work with diffusion instead of henry's law but could be wrong) $\endgroup$ – user20123 Apr 16 at 6:06

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