I cannot address the DIR reasons but I am not sure that the faulty computer is the answer. CCR's can have (and many do) redundacy in O2 cells (2, 3, 4) , batteries (1,2,3), computers (2,3). In addiiton the redundancy can be integrated or completely independent. The more redundancy the lower the risk similar to other OC concepts.
I suspect some of the DIR reasons might stem from the threat of Co2 overexposure.
there's a few equipment issues which make CCRs look a little sketchy:
- the O2 cells work best when not exposed to water, which is somewhat problematic when you've got a loop of air you are rebreathing with high humidity and you're diving under water.
- even with voting logic you could have 2 or more cells go bad similarly (e.g. both get wet, both are from the same batch). the voting logic itself is still a single point of failure.
- electronics don't particularly like water and should be approached a little skeptically in critical life-support underwater.
then there's the procedural issues:
- your pre-dive equipment checks must be extremely good and rigorous. screw up with equipment checks on open circuit and you usually either get no gas or lose gas. screw up on a rebreather and you go unconscious (the exception to this obvious is gas analysis in technical diving, which is why that needs to get done religiously). equipment failure is second only to buoyancy issues in rebreather fatalities and occurs at a much higher relative rate than in open circuit fatalities because rebreathers are more complicated devices than OC.
- during the dive you have to be constantly aware of your ppO2 and readouts on CCR, you can't get task loaded or space-out underwater. again, spacing out on OC is bad, spacing out on CCR and you can go unconscious and have a fatality.
- the first awareness you have that something is going wrong and you need to bailout will probably be anxiety due to either hyperoxia or hypercapnia and with hypoxia you'll probably just pass out without noticing. if you've analyzed your gases properly on OC, you shouldn't see these issues.
then there's the fact that the positives of CCR aren't really as positive as all that:
- constant ppO2 decompression goes against the theories of push-pull deco at high/low ppO2s that GUE/WKPP have developed: no backgas breaks with CCR, no high/low ppO2 sections of the deco curve, no giving your lungs and your CNS clock a rest.
- typically CCR divers run with low setpoints and gain very little decompression time advantage over OC divers doing bumps to 1.6
- given sufficient OC bailout, the weight reduction from going to CCR is reduced unless you start doing team-bailout strategies.
So, "DIR CCR" would probably looks something like diving mCCR, or diving an eCCR like an mCCR, with a diver who is extremely detail oriented on pre-dive and post-dive checks and constantly watching their ppO2 and scanning their awareness for warning signs that they're going hyperoxic or hypercapnic. There's a whole lot of minuses there and not really enough plusses.
- don't know if that's exactly in line with DIR thought, but those are my heavily DIR-influenced thoughts.
