How great is the risk (in your perception)?

[PfcAJ]

Disclaimer: I am a CCR diver so self-justification is always suspected.

I do believe that the quote is factually correct BUT may not be the whole story. Lets take an example of another activity, mountain climbing. Lets say that there are 10 times as many fatalities among climbers using oxygen bottles as there are for those climbing without. Is that a reflection of the type of climbing that people are using O2 for, or a reflection on the dangers of the equipment?

I believe that the VAST majority of CCR dives done worldwide are technical dives, done to places where OC can't necessarily go. By the very nature of these dives, you will have many more incidents than in the big data pool of all OC dives. I firmly believe that if we could compare apples with apples (ie all dives done to 300' plus, OC vs CCR, deep cave penetrations etc) that the numbers would break down much more evenly. There may even be a slight safety increase for some types of incident ie where the extra time a CCR affords may prevent a fatality. Im sure @kensuf and @PfcAJ and @Capt Jim Wyatt etc may know of such cases.

I also strongly believe that there is a huge element of personality involved. There are people who will have no problem with CCR because they are really detail oriented and disciplined in the way they approach their diving, while others will always be at a higher risk because they are the kind of people to leave an old O2 sensor in, not restart a checklist if they were interrupted etc etc.

I'm sure there are cases where the extra time afforded by a rebreather helped.

But I think those cases are dramatically overshadowed by the instances of people drowning on rebreathers due to problems with the rb.

 

[KenGordon]

The big thing about the statistics above are to look at who is doing what kind of dives and whether the dives that are killing CCR divers are still being conducted on OC *they largely aren't*, what the incident rate would be if they were on OC *probably pretty comparable*.

The Fock paper groups dives into five catagories. The first (lowest risk) had a third of the fatalities.

1. low risk, < 40 msw, all checks and tests conducted, no wreck/cave penetration;
   
   Sadly he did not split out high risk behaviour (skipping checks, inadequate bailout) from high risk (deep or hard overhead) dives. But that a third of the deaths involve simpler dives which are being done on OC would be worrying.
   
   The main flaw for me is using the Divelife database and making up the numbers of users and dives done. At least some attempt was made to clean it up.
   
   It is also worth remember that the data is now quite old. Maybe people are learning?
   
   

 

[tbone1004]

Makes sense. On a second note, how do rebreathers behave in ice conditions? Open circuit is prone to free flow (piston regulators more than sealed diaphragms but sealed diaphragms are also not completely reliable.) Are rebreathers affected the same way? Thanks.

the regulators themselves are subject to the same failures but the loop is not at risk of freezing since the scrubbing reaction is exothermic. The run time is reduced due to the ambient temperature though as scrubber duration is tied to temperature.
What makes regulators freeze is the adiabatic cooling from the pressure differential. The rebreather loop itself is at ambient pressure all the time so there is no cooling from pressure drop outside of the injection points. The injection points are fully enclosed similar to a double hose regulator so those are fine, and the first stages are prone to freezing technically but they don't get near as much use in terms of frequency or duration *of flowing gas* since the loop addition is minuscule compared to a normal breathing rate. Even SCR's are only injecting about a third of the gas that you would be breathing on OC

 

[Capt Jim Wyatt]

The risk is about the same as with OC.

The difference is the amount of understanding of the equipment a user has, how well the user REALLY understands how to properly use it, and the amount of time the user is willing to put in maintaining the gear.

Everything being equal with attitude I just don’t see a big difference.

 

[DepthCharge]

The greatest risk is the operator. Complacency is the main cause. As long as you remember that the RB is trying to kill you, and you have to be on the lookout constantly you will be alright.
I was recently diving on a mixed boat, as I was setting up my RB, I was answering question about the RB. I WAS NOT FOCUSED. I'm getting ready to dive and sensors were reading .20 .21 .21. Shot in some O2 same readings (pre-breath). Knew immediately what the problem was. I had switched the tanks. So I switched the tanks to the correct position. The boat captain offered to drop me on the site (drift dive), but I declined. I needed to refocus. I screwed up setting it up, I don't give fate a second chance. I made the other dives without issue.
I varied from my usual routine. I usually have the RB fully set up prior to getting on the boat. The shop brought my tanks late to the boat, I let myself be distracted. So after diving these thing for 17 years you can still make mistakes. You just have to be able to recognize them.
Later,
John

 

[CAPTAIN SINBAD]

To all those who are saying that the greatest risk is a) operator error or b) complexity of the dive itself, how do you approach the following piece of information ...

The Fock paper groups dives into five catagories. The first (lowest risk) had a third of the fatalities.

1. low risk, < 40 msw, all checks and tests conducted, no wreck/cave penetration;

If CCR dives to 40 meter are resulting in significantly higher number of diver fatalities, then complexity of the dive can be ruled out as the main factor here. Operator error is also ruled out because all checks and tests were done as per the rule book.

Thoughts?

 

[tbone1004]

@CAPTAIN SINBAD how do you know that all checks and tests were conducted?
There are CCR Instructor Trainers that don't know how rebreathers work, many of them. That is why there is so much voodoo surrounding CCR's with most of it centered around the cells. If the instructor trainers don't actually understand how the cells function, how can you expect the divers to understand? It's something that I know @Bobby has been fighting for years.

You can claim all checks were conducted, but what if the diver was taught specifically not to conduct a 1.6 ppO2 check on descent *yes instructors teach this*, and then when they end up on decompression and go to a setpoint of 1.6 for deco, they get oxtox'd because the cells are limited and O2 keeps getting injected? All the checks and tests were conducted officially, but they still died.
What happens to the guys that died of hypercapnia which may be from improper packing? There is no pre-dive check to validate that because the pre-breathe does nothing for CO2 detection, most CCR's don't have CO2 sensors on them, and if they do, their accuracy is debatable.

Let's look at the list
181 total
15 are heart attack so that goes away
22 are decompression related
17 to hypercapnia which is either overbreathing the scrubber or some sort of breakthrough
38 to some sort of O2 problem with more being hypoxic than hyperoxic. That one is operator error, 100%
The rest are mostly "other" that we don't really know what happened or it's not relevant to this.

So 20% are written off because you'll get bent/heart attack/AGE regardless of OC vs. CCR
10% are scrubber related, most likely due to improper packing or people pushing scrubber duration. Neither of which would be detectable in "proper predive checks"
20% are due to ppO2 issues that are operator error, and depending on what it was, may have been a possibility with OC though it is unlikely. OC ppO2 issues are due to improper gas switches which can happen on CCR, but these are most likely due to rapid ascents and descents with improper loop management.

30% of them we can pinpoint on operator error, but 44% of the numbers are in "other" and "scant data" where it was probably operator error, but it is unknown. Point is, that only 10% of the dives were put in the "high risk" category and I didn't bother finding out the breakdown of how many were because it was super deep vs. being dumb. As shown above, more than 10% were due to operator error, so whether you did pre-dive checks or not, the brain still malfunctioned causing the fatality. The risk "rating" that he gave is pretty irrelevant since pre-dive checks can't do anything to tell you about the scrubber, and they can't check for current limiting. Pre-dive checks do nothing to protect from operator error with ppO2 issues on ascent and descent.

Are they inherently more dangerous than OC? I don't think so.
Do they require that the operator be an active participant in the dive vs. inhale-exhale-repeat as necessary? Absolutely, and being a passive participant in the dive is what killed at least 20% of the divers in that study and I'd be willing to bet that if we had all of the information for "other" and "Scant data" that most all of those fall into that category

 

[KenGordon]

To all those who are saying that the greatest risk is a) operator error or b) complexity of the dive itself, how do you approach the following piece of information ...



If CCR dives to 40 meter are resulting in significantly higher number of diver fatalities, then complexity of the dive can be ruled out as the main factor here. Operator error is also ruled out because all checks and tests were done as per the rule book.

Thoughts?

It doesn't rule out operator error, just says that proper predive checks were done. They can still mess up in the event of a failure. Also some issues, like a badly packed scrubber, will not show up in those checks.

 

[CAPTAIN SINBAD]

@CAPTAIN SINBAD how do you know that all checks and tests were conducted?
There are CCR Instructor Trainers that don't know how rebreathers work, many of them. That is why there is so much voodoo surrounding CCR's with most of it centered around the cells. If the instructor trainers don't actually understand how the cells function, how can you expect the divers to understand? It's something that I know @Bobby has been fighting for years.

You can claim all checks were conducted, but what if the diver was taught specifically not to conduct a 1.6 ppO2 check on descent *yes instructors teach this*, and then when they end up on decompression and go to a setpoint of 1.6 for deco, they get oxtox'd because the cells are limited and O2 keeps getting injected? All the checks and tests were conducted officially, but they still died.
What happens to the guys that died of hypercapnia which may be from improper packing? There is no pre-dive check to validate that because the pre-breathe does nothing for CO2 detection, most CCR's don't have CO2 sensors on them, and if they do, their accuracy is debatable.

Let's look at the list
181 total
15 are heart attack so that goes away
22 are decompression related
17 to hypercapnia which is either overbreathing the scrubber or some sort of breakthrough
38 to some sort of O2 problem with more being hypoxic than hyperoxic. That one is operator error, 100%
The rest are mostly "other" that we don't really know what happened or it's not relevant to this.

So 20% are written off because you'll get bent/heart attack/AGE regardless of OC vs. CCR
10% are scrubber related, most likely due to improper packing or people pushing scrubber duration. Neither of which would be detectable in "proper predive checks"
20% are due to ppO2 issues that are operator error, and depending on what it was, may have been a possibility with OC though it is unlikely. OC ppO2 issues are due to improper gas switches which can happen on CCR, but these are most likely due to rapid ascents and descents with improper loop management.

30% of them we can pinpoint on operator error, but 44% of the numbers are in "other" and "scant data" where it was probably operator error, but it is unknown. Point is, that only 10% of the dives were put in the "high risk" category and I didn't bother finding out the breakdown of how many were because it was super deep vs. being dumb. As shown above, more than 10% were due to operator error, so whether you did pre-dive checks or not, the brain still malfunctioned causing the fatality. The risk "rating" that he gave is pretty irrelevant since pre-dive checks can't do anything to tell you about the scrubber, and they can't check for current limiting. Pre-dive checks do nothing to protect from operator error with ppO2 issues on ascent and descent.

Are they inherently more dangerous than OC? I don't think so.
Do they require that the operator be an active participant in the dive vs. inhale-exhale-repeat as necessary? Absolutely, and being a passive participant in the dive is what killed at least 20% of the divers in that study and I'd be willing to bet that if we had all of the information for "other" and "Scant data" that most all of those fall into that category

It sort of makes sense but leads one to conclude that CCR is an enormously complex unit requiring such a high level of expertise that even after getting all the training that is possible, most divers are still inadequately trained to the point where fatalities are significantly higher than OC.

The fact that deaths were a result of user error does not mean the technology is totally forgiven. It just means that CCR is user unfriendly to the point where the level of expertise required to stay alive on those things is beyond the capacity of most people certified to use it.

Agreed?

 

[RainPilot]

The only 2 fatalities on the Poseidon that I know of were both in shallow water on very simple dives. The only issue was both of them were not trained on the units and just jumped in. One had a low battery, ignored all the warnings and did 2 dives with the unit. He died from hypoxia after the unit shut down everything but the solenoid, eventually there was no power for that either.

The other guy jumped in with the O2 tank off, the unit fired up and tried to keep him alive. He descended so the loop PPO2 stayed breathable until he got to depth, by the time he paid attention to all the alarms and flashing lights, he reverted to his OC background and swam straight up. He died when the PPO2 dropped on the ascent. If he had turned the BOV he would have survived.

These accidents are part of that “recreational dive CCR fatality” statistic.