CCR truths and misconceptions

[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.