Oddly enough, the thing that kills most rebreather divers is.... you guessed it, not turning their O2 on.
Many more things kill CCR divers.
Just to cite an example, a rather common one, is failure of the galvanic O2 Sensors.
Then there is failure of the electronics.
These failures can be intermittent.
Then there is failure of the CO2 scrubbing system.
The key is that most failure modes in OC are detectable by the diver and the diver can take corrective action. Where the diver takes no action or the wrong action (i.e. panic), then this can and usually does result in fatality of injury.
Conversely, when a CCR fails, the diver, we see this in the pattern of the fatality, becomes unconscious and drowns, unable to effect self-rescue, and the buddies, despite their proximity, are unable to help.
We see this also in the most recent CCR fatality not yet picked up by the Scubaboard accident discussions:
"At the time of the accident George was at a depth of 14-15 meters decompressing from a dive in the 60m range. George suddenly became unconscious. He was spotted within seconds and the closest team member got to him immediately. He was completely unresponsive and not breathing. As this happened just below the habitat we moved him into the habitat and tried to revive him with no success. We recovered George later in the night."
The above was from a recent post on CCRX.
Rebreathers are more complex than OC and a relatively new technology. The technology is new for all intensive purposes when galvanic O2 sensors and electronics were added to a traditional rebreather.
This was back in 1998 with the Electrolung: see
Electrolung
Since then, everybody who tried struggled to make the system reliable enough.
There are various ways to measure risk.
One is statistical looking at the fatality rates. Although we do not have precise data (i.e. the denominator), it is strikingly obvious that CCR fatality rates are very large.
This is what led A. Fock to conclude in his RF3.0 paper and presentation that CCR is 10 times more risky than OC - see also
Rebreather diving: ?Killing Them Softly? | Diver Magazine .
Then there is the engineering approach which can use reliability measures or SIL Levels.
SIL Levels measure Probability of Failure and in particular the Probability of a Dangerous Failure.
If you have two products, one with a SIL Level of 1 and the other with a SIL Level of 4, then you can say that the SIL 4 product is safer than the SIL 1 product because measured in accordance to the methodology required in the international standard EN61508 (that is what is referenced in the rebreather standard Clause 5.13.1 of EN14143:2003) the SIL 4 product has a lower Probability of a Dangerous Failure.
So, SIL 1 is more risky or more dangerous according to this methodology than SIL 4.
"Functional Safety" is applied to many product in our daily life. It is just not stamped on the product, but where a product can endanger life it is a means of ensuring a reasonable or acceptable level of safety.
Rebreather can kill without warning. A very dangerous failure mode.
A SIL 4 rebreather (the best level of Functional Safety) would still be not 100% safe, but it would offer a very high level of safety and protection to the user.
A SIL 4 rebreather would be safer/better (and likely more expensive) than a SIL 1 rebreather.
Unfortunately, despite Clause 5.13.1 of EN14143:2003 requires a rebreather to attain at least a SIL 1 level of Functional Safety, the rebreather industry has to this date been unable to deliver to the general public one such electronic rebreather - that is a rebreather with at least a SIL 1 level of Functional Safety.
Rebreathers do not meet the standard EN14143:2003 against which they are benchmarked and certified (hard to believe YES, but this is true).
There was a blunder, an innocent blunder some time ago, and this one slipped through.
Based on the intuition and work of Dr. Fock and the application of engineering SIL level calculations, the conclusions are the same.
Rebreather are very dangerous machines. There is no two ways about it.
They are fun to dive, but they are dangerous.
A poster asked what needs to be done to make them at least SIL 1 or safer?
The industry cannot deliver a SIL 1 electronic rebreather. The technology is not there for this at this point in time.
That is where we are now. We can only face the fact and accept it. No solution.
Rebreathers are not a tool safe enough for the typical OC recreational diver. Stick to OC Air and Nitrox for dives in the recreational range (safer and cheaper).
