How risky are rebreathers as compared to open circuit at technical depths (below 130)? What is the greatest risk? Unit flooding? O2 sensors going wonkers? If someone could offer insights into the risk of rebreather diving in comparison with open circuit technical diving what would be great!
Not all rebreathers are the same and the failure modes and vulnerabilities of each can be slightly different.
If you dive an eCCR, the risk is complacency in letting the unit manage the PPO2, which works great until the unit fails and either stops adding O2 or adds to much. If the diver isn't also monitoring the machine, the results can be fatal, and unless something has changes, the accident rate on e CCRs is higher than the rate on mCCRs - although you have to also control for the greater depths where leaky valve mCCRs are used not used as much given the modifications needed to operate much below 250'-300'. Still, the smarter eCCR divers I know dive them manually, by manually maintaining a set point higher than the PPO2 the unit is maintaining, which means the unit'sPPO2 monitoring is just a back up. This provides a safety net if the diver is distracted, and preserves battery life as there is no need for a solenoid to fire.
If you dive an mCCR, there are no on board batteries (outside the dive computer) and no solenoids to fail, and since you have to maintain the set point manually, there is both a reduced risk of complacency and a better/faster understanding of the conditions that will affect changes in PPO2, as well as an awareness when the unit is still operating, but is not operating optimally - i.e. identifying when an imminent failure or problem is headed your way. But, if there is a problem, (such as a partially plugged orifice, or a much higher than normal workload) and the diver is also distracted to the point of ignoring and failing to monitor the PPO2 for more than 4 or 5 minutes then the results can be fatal.
The key piece of accident prevention with either mCCR or eCCR is the diver. Things that make CCR diving safer for a diver are:
- better understanding of the theory of CCR diving;
- better understanding of the specific failure modes of his or her specific CCR;
- better understanding and experience with different failure recovery options, both short of and including bailout;
- more experience diving his or her specific CCR in demanding and varied condition;
- more experience with real world problems and failures on their unit before progressing to edge of the envelope situations; and
- careful attention to detail in maintaining, checking and cross checking the CCR both before, during and after the dive.
Few of the above items are obtainable in class (at least to an adequate degree), some of them come with experience, but most of them require a degree of independent research and critical thinking that a fair percentage of CCR divers just don't have.
Which means three things:
1) CCR diving is not for everyone;
2) some CCR divers are at greater risk of killing themselves on their unit than others; and
3) those most at risk are also the least likely to know it.
The most at risk in number 3, are those divers who know what they know solely based on what someone in authority has told them, and have not added to this level of knowledge by applying the scientific method (through their own experience and testing), logic, or the application of philosophy. All four ways of knowing something are important in decision making when you encounter pros and cons, and comparative risks and benefits that must be balanced to produce an optimal outcome.
