It should be pointed out that in the event of a CO2 hit, the idea that there are "sanity breaths" is a fallacy. Not only does it take much longer to recover from a CO2 hit than a couple of breaths, typically, if you have to bail out, you've bailed out for a reason and you're not going back on a compromised loop.
Because of the length of time to recover from a CO2 hit, it's entirely possible that it was hypercapnia related, the diver bailed out, but he was still overcome. CO2 is a pretty insidious thing when diving, and can occur even if you've done everything "right."
It should be pointed out that no one has suggested that sanity breaths are going to resolve a CO2 hit.
In fact it was mentioned that a significant CO2 hit may well result in the diver blowing through all his bailout on the exit - with the result that divers who have experienced a CO2 hit tend to carry lots of bailout gas relative to divers who have not.
I'll preface this by saying if a mod thinks this post should be spun off on a separate thread in the appropriate forum feel free as I don't want to further speculate about this incident.
PCO2: The Dark Matter of Rebreather Diving | DIVER magazine
That said, today I did some light research/reading on rebreathers and specifically CO2, as I do have some serious interest in diving them in the future; however, I came across the above article and series which seemed rather startling to me.
Is it safe to assume the operator is basically flying the unit blind in terms of PPCO2? Despite there being room for improvement in PPO2 sensing it appears the sensing and monitoring of PPCO2 is nearly non existent on today's CCR units. To me it seems as though the really only true indication of CO2 breakthrough is when the diver becomes symptomatic which at that point may be too late depending on the situation. I realize there is protocols and training to deal with this scenario, however, as mentioned in this thread and elsewhere, CO2 hits are no joke and even your best efforts to resolve the problem may not be enough.
I realize this article is also a few years old and perhaps technology has improved and/or this one specific source of information is flawed, but it definitely has me taking pause on the concept of diving closed circuit. Am I being too overly concerned? I always remind myself there's nothing worth dying for down there and while I realize diving in and of itself comes with risk, it would seem diving closed circuit with the "current" technology stacks the odds against you and this can be loosely quantified by accident statistics. I realize extreme sporting activitiy statistics are hard to calculate because there's little data on how often someone is participating in the activity, but it's my understanding CCR diving is safer than base jumping, but not as safe as sky diving and I don't jump out of planes or off cliffs.
I should note that I'm not at all trying to have any influence on CCR divers or argue they are not safe, more so to further discuss the challenges of CO2 and perhaps hopefully learn about manufacturers working on new technologies or implementing current technologies into their units. If what I'm understanding is accurate about CO2 monitoring I think it's grossly lacking in today's public rebreathers. And again I truly do have interest in diving them one day, but if I don't know if I'm breathing CO2 it's hard for me to consider them at this point in time.
As noted by someone above, the technology has so far bene difficult to put in the moist environment of a rebreather.
The focus is however on prevention in terms of ensuring proper maintenance, properly packing the scrubber, and paying close attention to scrubber duration and factors that may shorten it, such as work load and water temperature. Colder temperatures significantly decrease scrubber efficiency, and thus duration.
Some companies also do scrubber tests under controlled conditions to determine scrubber duration under what are pretty conservative conditions, that then produce fairly conservative estimates that can be used for planning purposes.
----
The risks are different with CCR compared to OC and whether the total risk is greater or not probably depends on your diving, as well as on some personal traits of the diver.
CCR has some advantages over OC in terms of reducing decompression relative to OC, and knowing that gas isn't an issue, and that you have much more time to work a problem if needed is a big stress reliever.
A CCR diver can also drop stages with bailout gas that are never actually used on dives that would otherwise require those stages to be used (and replaced) on each and every dive, and if you're going to be diving in a system for awhile, pre-placing the bailout gas can greatly reduce the set up required for a series of dives, compared to OC stage diving and that reduction in set up dives, reduces some overall risk.
On the other hand, if you are not meticulous about maintenance, don't bother to fully understand how the unit works, don't fully understand the possible failure modes, don't conservatively plan for possible failures, and don't have the self discipline to turn a dive early, abort a dive, or call a dive on the surface at the last minute if the unit has a failure, then rebreather diving just isn't something you should be doing.
Two people I knew personally have died on rebreathers because they dove with known flaws. In that regard, their deaths were 100% preventable, yet they show as CCR deaths in the statistics. Do you blame the rebreather and conclude that CCR is just dangerous, or do you conclude that perhaps the potentially increased danger lies in how some people choose to dive the rebreather?
A related issue is that many divers are doing dives on CCR that they would never do on OC due to the logistics involved, so in one respect CCR may be more dangerous because it makes longer, deeper dives more practical, and thus more common. In other words, if more divers are doing longer, deeper dives that are more stressful and or carry greater risk in general, as those dives are more practical on CCR, then increased accident rates on CCR may be due in part to the greater risk overall on those dives, regardless of whether they were one on OC or CCR.
