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The Sentinel and The Apocalypse type IV (when it ships) both have CO2 sensors - not sure on the sentinel, but the apoc it is limited to 80m/260ft, at which point they say the power draw goes too high. Not sure how they are measuring it, but it also requires a Helium sensor to verify the mix.
Aloha, Tim
Looks like the sentinel is not as established as some other rebreathers. In my area the only rebreather training I found was for inspirations. I am glad that it is getting out there though. It sounds like a successful implementation of a CO2 sensor would save some lives.