This thread is a by product of a recent accident discussion (http://www.scubaboard.com/forums/accidents-incidents/310700-eagles-nest-cave-diver-death.html), to address some of the questions raised by non-CCR divers.
Proper gas planning/bailout was covered in my basic CCR course. Knowing how to plan bailout is not unique to a cave course.
I pretty well agree with this entire post.
Another post I agree with entirely.
Cave training has little bearing on most of the points made by John above. Many of them could have applied equally well in an OW scenario.
Initial, or full reports for that matter, unfortunately take a long time to see the light of day and in many cases raise more questions than they answer.
One thing to keep in mind, is that the bailout gas planning under CCR scenario is different than an OC scenario. You're not using part of your bailout to reach your turn point like you are on OC. Rather, your dive is planned by the range that the amount of bailout can safely allow you to return from. An OC diver using double AL80's has 160 cft of gas. Their maximum turn point is after using 1000 psi, or approximately 53 cft of gas. A CCR diver carrying an AL80 bailout bottle has approximately 80 cft of gas available. So a CCR diver could travel further with a single 80 bailout than an OC diver with doubles.
One of the things we were taught is that we need to periodically check our OC gas consumption by practicing bailout procedures. That way we keep an accurate idea of what our true consumption is. It's also a good skill to practice for the obvious reason of "in case we need to bailout." Another issue is that once someone becomes accustomed to diving CCR, it's been noted that their breathing patterns change and they are much less efficient when they switch back to OC.
Mostly based on experience and knowing what your SAC rate is. If in doubt, carry or stage additional bailout.
I'd say this could be handled by proper gas planning and management procedures. I would personally plan a dive requiring a hypoxic mix to use that as offboard gas and plug a normoxic mix or nitrox in after I started ascent. It's really not much different than how you'd handle gas switches on OC.
Basically accurate. Recommendation is to test the reality from time to time as I stated above.
Other questions/comments?
My understanding is they only had one bailout each. If that's the case, they didn't have enough bailout for the dive. Had they taken a CCR cave course they would have known this...
Proper gas planning/bailout was covered in my basic CCR course. Knowing how to plan bailout is not unique to a cave course.
How do you know this?
What was the BO gas?
What was the dive plan? Team BO or individual?
What was the size of the BO tank, start pressure?
Where there any other tanks staged? Top of mound? Log at 20ft?
If staged tanks what were the sizes, psi and gas?
What is the diver SAC/RMV?
These are the questions that would occur immediately to me if someone told me that they only had one BO tank and before I would make the judgement.
Also, they should have learned how to calculate gas requirements much earlier than on a Cave CCR course.
If they truly had only two OC tanks available as a team then I am lost for words. I cannot imagine a scenerio in which they could have gotten out running out of gas or missing deco with planning a total CCR failure at maximum penetration.
John
I pretty well agree with this entire post.
To have correctly planned and executed the dive at hand in this case, one needed to be Cave, CCR and Trimix trained. Lacking any one of these would raise the risks exponentially, especially diving a CCR and the depth.
I have dived EN on both OC and CCR. Each time I did, it was planned in advance, with the correct gases and volumes, RT plans and contingencies. Always a fresh scrubber in the breather and cells that tracked correctly.
I am not saying these two divers did not plan it correctly, all I am pointing out is that this is not a dive you 'just go do'. By not crossing all T's and dotting all i's, having even the slightest issue will put you over the line of no return.
Another post I agree with entirely.
To have planned this, they would have had to have been CCR caved trained . . . which, as the answer is 'not', is the unfortunate answer to all your questions above.
Has a formal 'initial report' been given on the incident?
Cave training has little bearing on most of the points made by John above. Many of them could have applied equally well in an OW scenario.
Initial, or full reports for that matter, unfortunately take a long time to see the light of day and in many cases raise more questions than they answer.
I don't think this is true. All cave divers are trained to calculate needed gas for exit -- and technical divers are trained to calculate needed gas for gas loss or deco gas loss. One of the most defining things about technical or overhead diving is that you have to solve such problems where you are. If they were not carrying enough bailout (and I don't know this and haven't seen any informed report anywhere) then either they just didn't think about it, or they made some very questionable decisions about how much was enough.
However, nothing I have read says that anyone ran out of gas here, so whether the bailout was adequate or not does not appear to have had a chance to play a role, unless it was simply by increasing the victim's anxiety.
There are many unanswered questions in this accident, but the big one is why he had already bailed out, and why he went back on the loop. We may be lucky enough, as we were with Richard Mork's death, to get a report on the condition of the equipment, but more likely, we won't, and we will never know.
Rebreathers continue to scare me.
One thing to keep in mind, is that the bailout gas planning under CCR scenario is different than an OC scenario. You're not using part of your bailout to reach your turn point like you are on OC. Rather, your dive is planned by the range that the amount of bailout can safely allow you to return from. An OC diver using double AL80's has 160 cft of gas. Their maximum turn point is after using 1000 psi, or approximately 53 cft of gas. A CCR diver carrying an AL80 bailout bottle has approximately 80 cft of gas available. So a CCR diver could travel further with a single 80 bailout than an OC diver with doubles.
The BIG difference is OC divers use the gas consumed on entry to gauge the gas required to exit (1/3rds) CCR divers have to rely on whatever OC experience they have to gauge predicted gas requirements and I suspect many fall short of their estimation of gas consumption.
One of the things we were taught is that we need to periodically check our OC gas consumption by practicing bailout procedures. That way we keep an accurate idea of what our true consumption is. It's also a good skill to practice for the obvious reason of "in case we need to bailout." Another issue is that once someone becomes accustomed to diving CCR, it's been noted that their breathing patterns change and they are much less efficient when they switch back to OC.
How do they do that, Dave? Is it based on time? I mean, I can imagine with a CCR that you have to use some other kind of metric than gas consumption to set a turn parameter for the dive -- whether that's just penetration time, or how much deco you want to rack up, or whatever. And if you set that parameter, and you know what depth you're doing, it shouldn't be too hard to calculate OC gas consumption. At least, I had to do those calculations in my Cave 1 class.
Mostly based on experience and knowing what your SAC rate is. If in doubt, carry or stage additional bailout.
No, really, some have argued for having a hot dil so that in a dil flush you'd get a ppO2 of 1.8 or something like that. I think the argument flared up after there was an accident or two which was possibly attributed to hypoxia due to doing a dil flush near the surface with something like 10/70.
I'd say this could be handled by proper gas planning and management procedures. I would personally plan a dive requiring a hypoxic mix to use that as offboard gas and plug a normoxic mix or nitrox in after I started ascent. It's really not much different than how you'd handle gas switches on OC.
Not a rebreather diver so take this for whats it is worth:
Basically comes down to penetration distance,depth and SAC rate. I normally figure I swim at around 50 feet/minute in a large cave with no flow. If you know depth profile and SAC rate it would be pretty easy to calculate the necessary bailout.
Basically accurate. Recommendation is to test the reality from time to time as I stated above.
Other questions/comments?
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