[eszed]

With real-time detection (I'd expect that exists, but don't know for sure) you can divert some of the airstream through this scrubber to target a particular concentration of CO2.

[telgareith]

You don't need any CO2. Oxygen bottle, co2 scrubber, pure oxygen purge prior to diving... and you've got yourself a rebreather.

https://en.m.wikipedia.org/wiki/Rebreather