Here is the ultimate answer to all those who ask about safety:
as far as I know in all the time rebreather-SCUBA-diving is done (since1876) there was NO fatality that happened to rebreather-divers who observed those rules:

1. Pay attention to be adequate trained and always dive with an adequate comrade.
2. Always be able to start, with a single second of action, a process that will bring you to the surface, pull you in a face-up position and show a (not to small) signal-colour-surface.
3. Necessarily know your ppO₂ and dive within save limits.

Dive Safe,

          Karl

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http://Rebreather.de/rebreather/safety.htm © Karl Kramer, 1.Feb.1999