CCR truths and misconceptions

[Cave Diver]

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.

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?

 

[jkaterenchuk]

One of the thoughts I had when reading some of the responses is that many divers may have gotten to secure in using computerize dive planning tools and the gas requirements that they provide. What could be simpler then putting in you RMV/SAC and poof out comes the gas needs for the dive you want to do. We need to be more thinking divers then just blindly accepting what a software package tells us.

I took my course with ANDI so I do not know what other agencies do but I was taught to use a detailed sheet in which the gas requirements were manually calculated at each depth for the entire dive and then added up. Hence, if your in a cave its going to take you 10 minutes to swim at 260ft until you can start to ascend then you need to calculate how much gas you would use for the portion of the dive. And so on at each depth you stop and for each ascent portion. It a long exercise but in the end you really understand the gas usage intervals in the dive plan.

This type of detailed manual planning has nothing to do with being CCR Cave trained. Often times you need to swim for an amount of time at a fixed depth to return to the ascent line on a wreck or if your on a multilevel reef dive.

I agree that if the diver did not completely understand this concept then his/her CCR Cave instructor would certainly ensure they did understand during the course. However, they should have covered this in any agency deco procedure class as well as in the CCR MOd 2. Using VPlanner to do the calculations is not a good substitute when learning.

John

 

[TSandM]

You're not using part of your bailout to reach your turn point like you are on OC

I think it's more that your bailout is calculated for just you, instead of carrying gas reserves for you AND your buddy.

And I guess I made an assumption in the other post that was quoted, that someone diving CCR would have come up through the OC ranks and have some idea of their OC consumption. Or does OC consumption change, once you have been habitually on CCR? My SAC rate has been pretty constant for several years now -- it varies by environment and conditions, but is pretty solidly predictable for each.

 

[wedivebc]

I think it's more that your bailout is calculated for just you, instead of carrying gas reserves for you AND your buddy.

And I guess I made an assumption in the other post that was quoted, that someone diving CCR would have come up through the OC ranks and have some idea of their OC consumption. Or does OC consumption change, once you have been habitually on CCR? My SAC rate has been pretty constant for several years now -- it varies by environment and conditions, but is pretty solidly predictable for each.

I find Mexico an excellent place to hone our skills. We know a few of the cave systems pretty well and access is a breeze so we spend a bit of time each cave season spooling up our skills. This year I intend to do a bailout from the junction between the Boch Chen and Calimba line meets the Grand Cenote line. Last year we staged a bailout tank there but I believe I can make in back on what we are carrying. My intention is to carry 3 tanks between 2 of us but attempt to make it back on 2. This will validate my bailout plan. If it doesn't work I will have to rethink my gas plan but that is how we check ours. I can't speak for other RB divers.
Last year I swam back from the monster room at Chac Mool on bailout and used exactly the amount of gas I predicted. My buddy did the same from the Lotus Room at Car Wash.

 

[Cave Diver]

I think it's more that your bailout is calculated for just you, instead of carrying gas reserves for you AND your buddy.

And I guess I made an assumption in the other post that was quoted, that someone diving CCR would have come up through the OC ranks and have some idea of their OC consumption. Or does OC consumption change, once you have been habitually on CCR? My SAC rate has been pretty constant for several years now -- it varies by environment and conditions, but is pretty solidly predictable for each.

Hopefully they would have a pretty good idea of their OC consumption. However on my last dives on OC, I found that my consumption to be much higher than it had previously been and I've heard this from others as well.

In regards to reserves, the planning somewhat depends on what type of dive you're doing. For "normal" dives we each carry our own bailout. On "bigger" dives it's not always possible/feasible to carry all our own gas, so the concepts of staged and team bailout will come into play. Having failures that render two CCR units completely inoperable is analogous to having complete loss of gas in two sets of doubles.

Also, depending on the type of failure, CCR divers have the option of going to SCR or OC. Planning is done around the OC option, but as always, plan for the worst and hope for the best.

 

[Nailer99]

Or does OC consumption change, once you have been habitually on CCR? My SAC rate has been pretty constant for several years now -- it varies by environment and conditions, but is pretty solidly predictable for each.

I can't speak for everyone, but many CCR divers, including myself, will tell you that their SAC rates went to hell shortly after they switched to CCR. I compensate by bumping my rate up when I punch a dive out in vplanner.

 

[Cave Diver]

BTW Lynne, since we're talking about breathing, here is something you may find interesting.

When we took our class, our instructor told us that there was a certain percentage of people that were not compatible with CCR diving. I think he said that around 1% of the population were excessive CO2 producers. As a result, they would "burn through" a scrubber and use it up much faster than the average person. We dumped and analyzed our scrubbers after the first dive to make sure that we didnt have this contraindication to CCR diving. It also allowed me and my buddy to see that our scrubber durations were "matched" almost as well as our OC SAC rates had been.

 

[jkaterenchuk]

BTW Lynne, since we're talking about breathing, here is something you may find interesting.

When we took our class, our instructor told us that there was a certain percentage of people that were not compatible with CCR diving. I think he said that around 1% of the population were excessive CO2 producers. As a result, they would "burn through" a scrubber and use it up much faster than the average person. We dumped and analyzed our scrubbers after the first dive to make sure that we didnt have this contraindication to CCR diving. It also allowed me and my buddy to see that our scrubber durations were "matched" almost as well as our OC SAC rates had been.

Not saying that you or your instructor is wrong.

The literature I have read indicates that CO2 retainers are much more at risk as a group when diving CCR.

John

 

[Keith & Sharon]

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.

First, thanks to Cave Diver for providing a separate thread to discuss for non-CCR divers as I have a few questions as I have followed along on the original thread.

What is meant by "a hot dil" and "dil flush"? I am assuming it regards the dilution of the trimix, but please explain what is happening. I am not trimix trained, so the next question is the 10/70 being a hypoxia mix at the surface. With the CCR, is the mix changed on the fly, or is constant throughout the dive.

Just trying to follow along with the issue raised as a potential cause of death.

Thanks,
Dive within your training.

Keith

 

[Cave Diver]

First, thanks to Cave Diver for providing a separate thread to discuss for non-CCR divers as I have a few questions as I have followed along on the original thread.

What is meant by "a hot dil" and "dil flush"?

CCR is basically an onboard nitrox mixing station. Diluent is the gas that you add to your breathing loop in order to maintain volume as you descend to depth. You then add O2 to maintain a constant PO2 once you've hit depth. Maintaining a constant PO2 is extremely important for a CCR diver. Too much or too little can be fatal.

A "hot dil" is a diluent that has too high a PO2 for the depth you're diving at. For example, 36% nitrox has a PO2 of 1.45 at 100'. Diving that at 130' would be considered a "hot mix."

A dil flush means exactly what it sounds like. You empty your loop and refill it with a known diluent. That brings the PO2 of your loop back into line and gives you a reference point to compare your readout with to make sure you're getting accurate results from your cell readings.

I am assuming it regards the dilution of the trimix, but please explain what is happening. I am not trimix trained, so the next question is the 10/70 being a hypoxia mix at the surface.

10/70 is ten percent O2, 70 percent helium, balance is nitrogen. 10% oxygen is not sufficient to support conciousness/life at the surface so it is considered a hypoxic mix. Once you start your descent, and the partial pressure increases, it becomes breathable.

With the CCR, is the mix changed on the fly, or is constant throughout the dive.

Just trying to follow along with the issue raised as a potential cause of death.

Thanks,
Dive within your training.

Keith

CCR's use a constant PO2 by changing the FO2 of the loop relative to the depth. In other words, you add as little or as much oxygen or dil, as appropriate, to the loop in order to keep the PO2 where you want it, regardless of the depth you're at.