CCR truths and misconceptions

[Cave Diver]

This thread is a by product of a recent accident discussion, 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.

[wedivebc]

The mix changes the fraction of oxygen throughout the dive in order to maintain a constant PO2. In order to maintain a constant volume in the rebreather a diluent must be added which compresses at depth so more diluent has to be added. A hot diluent is one that has more oxygen than the PO2 of your planned depth. A 10/70 diluent will not support life at the surface without added oxygen. That is something that can happen accidently and I lost a friend this year to that very thing.



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

[Dive-aholic]

Proper gas planning/bailout was covered in my basic CCR course. Knowing how to plan bailout is not unique to a cave course.

BO to ascend to the surface is a little different than BO to get out of a cave. I don't know what the plan was, or if there even was one, but I do recall reading there was 1000psi left in the dead diver's BO bottle (not plural). He went through 2/3s of his BO before going unconscious. He was recovered from 500' in at 270' depth. It's very doubtful that 1000psi would have been enough to get him out if he had stayed off the loop and not passed out. I know a lot of this is speculation, but so is everything else we're discussing in these threads.

[rjack321]

Last year I swam back from the monster room at Chac Mool on bailout and used exactly the amount of gas I predicted.

Do you remember how much volume this was? There's no right or wrong I'm just curious about the "SAC goes to hell once you've been diving CCR awhile" concept. 20% greater than never been on CCR OC divers? 50% greater?

FWIW in a wetsuit + AL80 doubles I know I can do a expeditious but non-failure-stressed exit on between 30 and 35cf (between 600 and 700psi) from the Monster House. I think my warm, relaxed SAC here in Puget Sound (0.45) and in MX (0.4) is average for experienced HWP 145lb males like myself.

[Cave Diver]

BO to ascend to the surface is a little different than BO to get out of a cave. I don't know what the plan was, or if there even was one, but I do recall reading there was 1000psi left in the dead diver's BO bottle (not plural). He went through 2/3s of his BO before going unconscious. He was recovered from 500' in at 270' depth. It's very doubtful that 1000psi would have been enough to get him out if he had stayed off the loop and not passed out. I know a lot of this is speculation, but so is everything else we're discussing in these threads.

The planning and final outcome may be a little difference, but the concept of carry enough bailout to get you from your maximum point during the dive to the surface safely still applies. On an strictly recreational OW dive to 130', this may be an AL40, while in a cave scenario it may require carrying an AL80 and stashing another along the way.

I'm not saying that there aren't any differences in actual execution, just that the basic premise remains the same. IMO, failure to carry adequate bailout is not so much a lack of cave training but a failure in basic CCR planning.

[wedivebc]

Do you remember how much volume this was? There's no right or wrong I'm just curious about the "SAC goes to hell once you've been diving CCR awhile" concept. 20% greater than never been on CCR OC divers? 50% greater?

FWIW in a wetsuit + AL80 doubles I know I can do a expeditious but non-failure-stressed exit on between 30 and 35cf (between 600 and 700psi) from the Monster House. I think my warm, relaxed SAC here in Puget Sound (0.45) and in MX (0.4) is average for experienced HWP 145lb males like myself.

About 1500psi from an Al80. But I am also dragging a RB. My SAC rate in Mexico is about mid 4s so I plan for 6s plus 1/3 and it usually does the trick. I have had one real emergency which was a CO2 breakthrough and surprisingly enough my SAC rate did not go up. It was on the way back from the monster room and that is why I decided to do the test bailout from there just to verify my range. Since that incident we start each cave trip with a bailout drill which helps keep the chattering monkeys quiet when we're a couple of km in.

[Meng_Tze]

I'm not saying that there aren't any differences in actual execution, just that the basic premise remains the same. IMO, failure to carry adequate bailout is not so much a lack of cave training but a failure in basic CCR planning.
I agree fully. I sometimes see CCR divers a fair bit into a well known system with a single AL40 .......... I know I need at least one AL80 to swim back to the entrance, not to mention the deco obligation on top of that.

[Dive-aholic]

The planning and final outcome may be a little difference, but the concept of carry enough bailout to get you from your maximum point during the dive to the surface safely still applies. On an strictly recreational OW dive to 130', this may be an AL40, while in a cave scenario it may require carrying an AL80 and stashing another along the way.

I'm not saying that there aren't any differences in actual execution, just that the basic premise remains the same. IMO, failure to carry adequate bailout is not so much a lack of cave training but a failure in basic CCR planning.

I understand what you're saying. My point is they may have only planned for bailout from the depth they were heading to without taking into account the penetration distance. Had they been CCR cave trained, this would have been taught to them. When you consider that they weren't cave trained at all, it's easier to see this was probably not a consideration they had made. I know cave divers that think even on a RB you still need to carry all the BO gas you would need if you were doing the dive on OC and don't see the point in diving RBs because of this. They don't realize they only need enough BO to get them out from the farthest penetration. As I'm sure you know, cave divers think much differently about diving than OW divers, whether recreational or technical.

[Mr.X]

I agree fully. I sometimes see CCR divers a fair bit into a well known system with a single AL40 .......... I know I need at least one AL80 to swim back to the entrance, not to mention the deco obligation on top of that.


What are these guys thinking? Hope they stashed some gas along the way. :shocked2:

X

[elan]

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.


This one is not actually clear, if you, say, observe the same rule of thirds it does not add up. Your first one third is the ccr and then you have the remaining 2 thirds in your bailout (if we take the worst case scenario when it breaks at the farthest point of the dive) which is ~40cuf. per third whereas the double AL80 diver will have 53 per 1 third. If you do not apply the rule or third in your case then you should not apply the same rule for the OC diver or it would be apple to oranges comparison.

Am I missing anything here ?

[Cave Diver]

This one is not actually clear, if you, say, observe the same rule of thirds it does not add up. Your first one third is the ccr and then you have the remaining 2 thirds in your bailout (if we take the worst case scenario when it breaks at the farthest point of the dive) which is ~40cuf. per third whereas the double AL80 diver will have 53 per 1 third. If you do not apply the rule or third in your case then you should not apply the same rule for the OC diver or it would be apple to oranges comparison.

Am I missing anything here ?

On CCR you're not diving by rule of thirds.

On OC, you use 1/3 of your gas supply in, 1/3 to exit. The remaining third is for emergency/buddy. (~53 cft in this example) Ideally, the emergency gas should never be touched.

On CCR, you use the CCR in. If you have an emergency requiring you to leave the loop you have ~80cft of gas available to you to exit with. In addition, you have another CCR buddy that also has and equal amount of bailout. Odds of two CCR complete CCR system failures is akin to having to complete OC failures. So you have ~160 cft of gas that should be available to you to make an exit on.

Additionally, you've also got the potential to go into SCR mode and to make some use of your onboard gas as well.

If you're diving with a mixed team of OC/CCR divers the gas planning is a bit different. Calculations for dissimilar tank sizes will be used to plan the dive and the lowest amount of available gas is the controlling factor.

[Camerone]

On CCR you're not diving by rule of thirds.
As a trained CCR Cave diver (IANTD) I have to disagree. You're diving by a _different_ set of thirds than an OC diver. You have four limiting factors for your loop: Oxygen (which is based on metabolic rate...effectively a constant drain), diluent (not thirds-based, but profile based on the cave), scrubber duration (pure rule of thirds - a three hour scrubber is one hour in, one hour out, and one hour to sort s*** out), and bailout (distance driven, just like regular OC air consumption).

So there is rule of thirds in there, it's just applied differently.

Bailout strategies are possible in several ways. There's the normal "rule of thirds" strategy, which requires a LOT of gas, but is safest. You take your own bailout, and you make sure it follows the rule. There's the "just enough" where you figure your bailout, and you make sure there's just enough to get you to the surface, and you rely on everyone on the team carrying "just enough" for the weakest guy, and then there's full-blown team bailout, where you figure out rule of thirds for the weakest guy and then split up the gas carrying by team so everyone takes a portion.

I prefer the "just enough" strategy. Reasonable gas carry loads, and a fairly good margin for safety.

Note that as a CCR cave diver, you can safely do things that you cannot do on O/C. Most of the time, the limiting factors for a CCR cave dive are either scrubber duration or bailout penetration distance.

If bailout is the limiting factor, when you hit your max pen distance, you can always come back and explore side tunnels until you hit scrubber thirds or another limiting factor. Coming out and going in somewhere else is perfectly fine, as long as you never exceed the distance you can swim out. On O/C, when you hit thirds, you start to exit, and you don't go meandering down side tunnels for kicks.

There's a lot of nuances to CCR cave diving, but I'm a huge fan of having the extra time and flexibility that the unit allows in the overhead environment. You just have to understand what you're doing with it and how the tool has to be applied...

[Cave Diver]

As a trained CCR Cave diver (IANTD) I have to disagree. You're diving by a _different_ set of thirds than an OC diver. You have four limiting factors for your loop: Oxygen (which is based on metabolic rate...effectively a constant drain), diluent (not thirds-based, but profile based on the cave), scrubber duration (pure rule of thirds - a three hour scrubber is one hour in, one hour out, and one hour to sort s*** out), and bailout (distance driven, just like regular OC air consumption).

So there is rule of thirds in there, it's just applied differently.


Good points, especially in regard to scrubber duration. On most CCR dives, this is going to be the limiting factor. Personally, I'd rather be about more conservative than this on scrubber duration, because the more of the scrubber bed that has been used, the higher the potential for breakthrough is. My previous response was tailored more to the gas planning questions.


Bailout strategies are possible in several ways. There's the normal "rule of thirds" strategy, which requires a LOT of gas, but is safest. You take your own bailout, and you make sure it follows the rule. There's the "just enough" where you figure your bailout, and you make sure there's just enough to get you to the surface, and you rely on everyone on the team carrying "just enough" for the weakest guy, and then there's full-blown team bailout, where you figure out rule of thirds for the weakest guy and then split up the gas carrying by team so everyone takes a portion.

I prefer the "just enough" strategy. Reasonable gas carry loads, and a fairly good margin for safety.

Agreed.

Note that as a CCR cave diver, you can safely do things that you cannot do on O/C. Most of the time, the limiting factors for a CCR cave dive are either scrubber duration or bailout penetration distance.

If bailout is the limiting factor, when you hit your max pen distance, you can always come back and explore side tunnels until you hit scrubber thirds or another limiting factor. Coming out and going in somewhere else is perfectly fine, as long as you never exceed the distance you can swim out. On O/C, when you hit thirds, you start to exit, and you don't go meandering down side tunnels for kicks.

Agreed.

There's a lot of nuances to CCR cave diving, but I'm a huge fan of having the extra time and flexibility that the unit allows in the overhead environment. You just have to understand what you're doing with it and how the tool has to be applied...
Totally agree.

Have you seen this thread? http://www.scubaboard.com/forums/cave-diving/310989-do-you-have-ccr-cave-training.html

[WD8CDH]

Really makes me wonder why bail out rebreathers aren't more common.

[Cave Diver]

Really makes me wonder why bail out rebreathers aren't more common.

Cost of owning two rebreathers would be one reason.

[wedivebc]

Really makes me wonder why bail out rebreathers aren't more common.

Bulk mainly. For a RB to work well it has to have well placed counterlungs. There is only so much real estate on a divers body to place those counterlungs so for true redundancy you have to either reduce the size of the CLs or place them in a less than optimal position. There are some ingenious solutions people are coming up with but nothing I am ready to use as of yet.

[wedivebc]

Cost of owning two rebreathers would be one reason.

I own 3 but I still don't have a BOB

[Tegg]

What happens in a max penetration issue for a two person RB team using shared bailout when one has a buddy who is either "lost" or is simply not around? (missed communication, etc)?

I personally have made the clear thought out decision to make sure I carry at least 2/3rds the "required" gas for exit. Meaning that I carry enough gas for the planned max penetration that equates to at least what I would have in gas volume if I did the same dive on OC.

On a RB, there are many issues (such as hypercapnia) that can cause one to use quite a bit of bailout gas to "calm down" and clear the head and I want to be sure that I make it out of the cave whether my buddy is around or not... Thus, if I needed a certain amount of gas in OC (2/3rds) at max penetration, why would things be different when using a tool such as a RB?

Cave divers are not taught to "share" gas reserves when using a DPV with a buddy, why do it with a breather?

[Cave Diver]

I own 3 but I still don't have a BOB and this wouldn't fit in most places I go.
If you wanna donate one to me, then we can both have redundancy. :D

[nadwidny]

I own 3 but I still don't have a BOB and this wouldn't fit in most places I go.


Dave, you need to resize this picture to something smaller. It's messing up the format of the forum.

[wedivebc]

Dave, you need to resize this picture to something smaller. It's messing up the format of the forum.

Yeah I see. It was ISCs picture so I had to remove it

[Cave Diver]

Yeah I see. It was ISCs picture so I had to remove it
From rebreatherpro.com:

http://rebreatherpro.typepad.com/.a/6a00e54fbce237883300e553b13b918834-320wi

[WD8CDH]

Bulk mainly. For a RB to work well it has to have well placed counterlungs. There is only so much real estate on a divers body to place those counterlungs so for true redundancy you have to either reduce the size of the CLs or place them in a less than optimal position. There are some ingenious solutions people are coming up with but nothing I am ready to use as of yet.
One choice is OTS and back mount counterlungs. That is the way my RB with integrated BOB is configured. My other BOB is slung, about the size of an AL80 with a built in CL.

[wedivebc]

One choice is OTS and back mount counterlungs. That is the way my RB with integrated BOB is configured. My other BOB is slung, about the size of an AL80 with a built in CL.

Have any pictures?

[WD8CDH]

Most of my gear is currently packed up because I am preparing for heart surgery, but I think I have some pictures around.

[Cave Diver]

Here is a good write up on CCR's:

Rebreather Fatality or Simple Pilot Error :: Seaduction ® (http://seaduction.com/scuba-diving/rebreather-fatality-or-simple-pilot-error/)


By Mike Ange

This is an opinion piece and the views are solely those of the author. The views expressed are in no ways indicative of the views of this publication or its principles.

Rebreather fatalities are largely caused by primary equipment failure.

Now that I have the attention of every avid rebreather diver (and personal injury attorney) out there – BEFORE you get the tar and feathers out – let me clarify that in accident after accident the primary equipment failure generally lies directly between the auditory input sensors on the diver’s head! I recently had a conversation with a reporter for a popular news source that sent me over the edge – when he asked why no one had taken steps to make this horribly dangerous technology illegal in the US. I have known it for a while but this “interview” drove home an important point. Rebreathers like American made cars, generally get a pretty shoddy deal from the diving media and the media in general. I think this occurs because deaths on this “exotic technology” are more sensational than the “garden variety accidents” that occur on scuba and because in the US we live with a mentality that no one is required to actually have personal responsibility. In this article, we will look at the technology very briefly for the uninitiated and the realities of why most accidents occur on this technology that predates regular SCUBA by more than 50 years.

What is closed circuit breathing? Rebreathers are a form of scuba – self-contained underwater breathing apparatus – that attempts to make breathing underwater more efficient by recycling used gas. The average adult inhales approximately two liters of gas every time he breathes. But, in reality, he only uses about 4% of that gas. The other 96% is exhaled as a waste product. On the surface, this is no big deal as the exhaled gas mixes back into the atmosphere and is available for use again. But underwater we have to carry our gas supply with us and to make matters worse, the deeper we go, the higher the volume of gas we waste with every breathe. As you know, according to the principles of Boyle’s Law, a diver at 33 feet inhales with every breath, twice as much gas as he did at the surface. However, metabolically, he still consumes approximately the same amount of oxygen. So, at 33 feet, 98% of the gas used is wasted and the deeper you go the more pronounced this waste becomes leading divers to use increasingly heavier and larger cylinders in order to complete their dives.

In general, rebreather technology seeks to correct this problem by capturing the used gas and recycling it. But there are a couple of issues with this. The waste gas provides a vital function for the diver – it provides the volume necessary to inflate the lungs so that they can properly function to exchange the minimal amount of gas actually required for body function. In addition to bringing good gas in, the lungs also expel waste gas, most notably carbon dioxide, and this is the primary problem that the rebreather must solve. If the carbon dioxide levels in your lungs are allowed to build, hypercapnia or carbon dioxide poisoning will result. The other problem that divers have is that as they recycle the air in and out of their lungs, the oxygen content naturally falls so that the second vital function of a rebreather is to replenish the oxygen supply so that the diver is breathing a mixture safe for vital life functions.

http://seaduction.com/wp-content/uploads/2009/11/dolphin-schematic-w-labels.jpg

Courtesy Dräger Dive Americas

Contrary to common logic, the CO2 is the more critical piece of the puzzle. A diver can die from CO2 poisoning much more rapidly than he will die from hypoxia (breathing a low concentration or partial pressure of oxygen). This math is not exact, but for simplicity let’s assume that for every liter of oxygen consumed the diver produces one liter of carbon dioxide. Therefore, if a diver on the surface uses 4% of the total gas inhaled and all of that gas is oxygen, his exhaled breath would contain approximately 4% of CO2. If the diver is breathing a Nitrox mixture of say 40% oxygen, he could re-breathe the same breathe between 4 and 5 times and still have an oxygen level that is equivalent to the level usually breathed on the surface at sea level. (A 4% decrease with each breathe X 5 breathes = 20%, 40 – 20 =20 or slightly less than atmospheric air.) Unfortunately, that 4% gas is also being replaced in our example with approximately 4% CO2. A CO2 concentration of 3% will create symptoms of hypercapnia. Sustained breathing of concentrations even slightly higher than 3% can be fatal. So using our example, the diver’s recycled breath could be dangerous to re-breathe after only one breath. Rebreathers deal with this problem by passing the exhaled air through a powdered or granular mixture commonly called scrubber. There are a number of different chemical compounds that will accomplish this and the compounds in use vary slightly from manufacturer to manufacturer and they change over time. However, the general mechanism is the same. The gas is passed through a granulated material where the CO2 bonds with the grains releasing heat and moisture in the process. The clean, warm, moist air is then passed out the other end of the scrubber material into a breathing bag or counter lung. This bag pressurizes the gas to the same level as the surrounding water pressure so that it can easily be inhaled by the diver.

The other issue that the rebreather must deal with is the depletion of oxygen and there are three different methodologies for dealing with this issue. Rebreathers are classified generally into three broad categories: semi-closed circuit (SCR), oxygen closed circuit (O-CCR), and electronic closed circuit system (E-CCR or just CCR). O-CCRs use, as the name implies, pure oxygen as the only gas and they are basically volume controlled. The diver breathes pure 02 and as the O2 is used up by the diver, a demand valve very similar to a regulator second stage kicks in and replenishes the supply of gas to maintain the volume of the counter lung. Because the divers are using pure oxygen and oxygen becomes toxic at very shallow depths, these rebreathers are generally limited to military or public safety applications and dive depths in the 6 to 9 meters (20 to 27 feet) range. Semi-closed circuit systems work on a constant flow methodology where a very slow trickle of gas is “leaked” continuously into the breathing loop to replenish the O2 content. Although much more efficient than open circuit, these systems are not quite as efficient as electronic units and they do produce a small discharge of bubbles throughout the dive. These units are a good choice for recreational divers because they do deliver very efficient gas use with durations of two and one half to four hours depending upon the unit, depth, etc on tanks containing as little 27 cubic feet (3 liter cylinders) of gas. They do require the use of a pre-blended nitrox mixture which means that the dive depth must be planned before the dive commences. A simple formula or table is used to predict the gas mixture that will actually be inhaled by the diver during the dive. Because they are relatively simple mechanical devices, SCRs are relatively easy to maintain and much cheaper to operate than electronic units.

The electronic CCR is the state of the art, at this time, and it works exactly the same way as a semi-closed unit with one exception: instead of injecting a continuous flow of gas, the rebreather uses electronic technology and sensors to continuously adjust the gas mixture in the breathing loop. The E-CCR uses two cylinders of gas instead of one. One cylinder is pure oxygen and depending upon the dive and the unit used, the other cylinder, commonly called the diluent cylinder, can hold a number of different gases ranging from air to trimix or heliox. Gas from the oxygen cylinder is passed through an electronically controlled valve into the breathing loop whenever the sensors indicate that the O2 levels have dropped below the set point prescribed in the E-CCR’s computer system. Therefore you only use an amount of gas equivalent to the metabolic consumption of the diver for most of the dive. Depending upon the level of sophistication, the E CCR will either use a manual push button valve, a demand valve, an electronic control valve or some combination of these to add gas from the diluent tank to the breathing loop when required. Generally, this occurs when the volume of the loop drops (during descent or after mask clearing for example) and whenever the oxygen content in the loop is higher than the unit’s pre-programmed set point.

So, that is the two minute and much simplified tour of rebreather technology. For safety, it is vitally important that the diver understand the pre-dive safety checks and the specific nuances of the operating system used by the rebreather. That is why every rebreather requires a distinctive certification course before the diver can use the system. It is true for all kinds of diving, but rebreather diving is more unforgiving and as a result safety shortcuts kill here more quickly than with open circuit systems. Here is the primary difference between open circuit and closed circuit technology: if your open circuit system fails, you will either get an uncontrolled violent flow of air or nothing to breathe at all. Given those two possible scenarios, any type of catastrophic equipment failure is certain to meet a concerted and immediate response from the diver. The rebreather diver, however, doesn’t have the luxury of a kick you in the teeth warning. The gas delivery systems on any of the rebreathers can quit functioning and the diver will continue to have something to breathe. Unfortunately, the oxygen supply in the loop will continue to drop by about 4% with every breath. It is possible for the diver to continue breathing until he becomes hypoxic, looses consciousness and drowns without ever realizing that the system has malfunctioned. Likewise, the diver can wind up with CO2 buildup causing hypercapnia if the scrubber container on the unit is not packed and used properly or if the recommended duration of dive time for the scrubber material is exceeded. If the scrubber is not packed properly, the granules inside the canister can settle making channels where the breathing gas can pass through the canister without being properly cleaned.

Any of these conditions can create significant and serious safety issues. If the diver runs out of gas, if the electronic control system were to fail or any other problem were to occur that prevented the rebreather from adding gas to the loop, the diver can continue to breathe possibly without a clue that he has a problem occurring. It is for these reasons that rebreather divers have to be meticulous in the set up of their unit, the completion of pre-dive safety checks and monitor the unit’s displays and warning systems routinely throughout the dive. It is rare that all of these precautions are taken in accordance with the manufacturer’s specifications and an accident still occurs. Yet we never read in the popular media about the true cause of many of these accidents. If the semi-closed rebreather diver failed to check the flow rate as required in the setup procedures and the flows are not adjusted properly when the accident occurs, the equipment failure is not in the rebreather – it is in the diver. If a rebreather diver packs his scrubber canister improperly or too soon before the dive and channeling occurs, it is not a rebreather failure, it is a diver failure. If a rebreather diver runs out of gas while he is playing with his video system, does not realize it and dies, that is not a rebreather failure, that is a diver failure.

In most of the western world, we live in free and open societies where, generally speaking, people are free to kill themselves through recklessness and carelessness anytime they desire. Don’t get me wrong there clearly have been rebreather failures that caused accidents, but in comparison to the diver errors that have caused rebreather accidents the number is very small indeed. Undertaking dives on closed circuit technology requires that the diver gets proper training and takes responsibility for his or her own safety. If you are not ready to accept that responsibility, do not dive a rebreather. If you do take that responsibility, do not run to the media or your corner ambulance chaser to blame the rebreather for your breach of personal responsibility. It is a wild and woolly world out there and diving, especially technical diving, is an adventure sport. So either grow up and deal with it or else stay at home on your couch and watch Sea Hunt reruns on T.V.

For those of us in the media, we have a responsibility to express opinions as opinions and to be careful with what we reflect as factual. If you cannot verify that the technology failed in a diving accident do not imply that some piece of technology is responsible. Find out what really happened, limit your facts to what you actually know or go write for a supermarket tabloid! AND YES – maybe the American cars do deserve the bad rap but that is an article for another time and venue.