CCR's and Cave Diving

[And]

Hi

I recently read an article on RBW which covered CCR's and cave diving which was more of a 'what' than a 'how to' article, and following Bardo's post about trying out a Boris I have a couple of questions.

Knowing you only have one scrubber, how do you determine your penetration time? Do you just pick a duration and then run thirds, similar to OC? I have heard someone say this before, maybe Zak, can't remember.

If so, then using the common duration 3-4 hours, your penetrations are restricted to 1-1 1/2 hours I guess. What do you do when you want to go further?

Andy

[NotDeadYet]

You dive within the limits of your scrubber. If you have a four hour scrubber then you do a 4hr max dive. If you want to do longer then increase your scrubber capacity... easiest way would be to buy a bigger rebreather.

I plan for 4hrs max on the scrubber: I allow 3hrs dive time + 1hr "fuck me!!!" time. The scrubber is rated for three hours though I think this is more to do with a conservative liability ass covering rating, it's done four and in an emergency I think it will do longer, especially if you go semi-closed with offboard gas. If I'm in this situation (which would imply something has gone badly wrong somewhere) then I've gone beyond "fuck me" and probably strayed into "stupid c**t" territory. Regardless, I always carry more than enough OC gas to get me out from wherever I plan to go. The rebreather to me is like a penetration stage: dive on halfs but with a little bit of padding as there is plenty of excess OC gas around to get out on.

On a lot of dives, deco time forms a huge chunk of your dive time. Normally I've been doing the whole dive on one rebreather but I have another with a 90min scrubber which is for O2 deco use (but quite probably will get converted to CCR so I can do the whole deco on it).

An alternative would be to do deco on OC. If it wasn't for the logistical aspect then I would be quite tempted to do that and save the RB purely for penetration and then emergency use (once I've reached deco). I'm not a fan of constant high pO2's during deco, I've gotten quite wheezy after a few long dives. But given the logistics, a second decobreather is a more appealing prospect.

As for dive planning, I plan according to what bail out I have. If my bailout allows a 40min penetration then I do that regardless of what the scrubber may handle in excess of that. In the water, my turn point is either this penetration limit or 1/3 of diluent, whichever comes sooner. In caves you do use a considerable amount of dil as opposed to open water. For me, availability of diluent is THE major controlling factor. I don't worry about oxygen, I don't even have a gauge on it. A lot of people will disagree with that but I don't see the point. I gauge it before I get in the water, I know what the usage rate will be and anything that will affect that will be VERY evident underwater. You can stay on the loop without oxygen in a cave but you can't get very far without diluent.

I have an offboard connection for diluent to my manifold. Other than that my KISS is a stock unit. It has no mods and no HUD. The article on RBW says this is an essential, I would rate it more a luxury. I dived hogarthian when I was OC, my attitude towards fancy bits of kit was always "if it isn't there then it can't fail and it's one less broken bit of kit to deal with".

Ramble... ramble... ramble...

I would also add that I start off planning my dive as an OC diver. I only go CCR if it will make the dive easier or safer.

And another thing, the above is probably a load of bollocks as I am neither open water nor cave CCR trained.

[Sheff diver]

The following is qualified by the fact that I am not a rebreather diver, but know people who are, and people who do "big" cave dives on RBs.

If you're diving a single RB, there is no point thinking about "thirds" or any other similar margin, as Stuart says. If you are going a long way, and deep (which to my simple mind is the main reason I might get into RBs), then it seems to me the only way to go is two rebreathers. Plenty of gas with variable O2 setpoint for deco, but probably some OC deco bailout for comfort factor (I always was a scaredy cat).

Marcus

[NotDeadYet]

Originally Posted by Sheff diver

If you are going a long way, and deep (which to my simple mind is the main reason I might get into RBs)

Same for me, it was a deep diving tool. Then it became an intermediate diving tool... I draw the line at using it shallow. That's only because I'm so tight I won't waste lime and O2 on a 10m dive

[Sheff diver]

Originally Posted by NotDeadYet

... I draw the line at using it shallow. That's only because I'm so tight I won't waste lime and O2 on a 10m dive

Wot ..... you? ..... Tight?

[Ken]

Originally Posted by Sheff diver

The following is qualified by the fact that I am not a rebreather diver, but know people who are, and people who do "big" cave dives on RBs.

If you're diving a single RB, there is no point thinking about "thirds" or any other similar margin, as Stuart says. If you are going a long way, and deep (which to my simple mind is the main reason I might get into RBs), then it seems to me the only way to go is two rebreathers. Plenty of gas with variable O2 setpoint for deco, but probably some OC deco bailout for comfort factor (I always was a scaredy cat).

Marcus

It's always a comfort to know you have enough bailout and staged gas along the way. Sometimes deep into the cave (solo), when it's just tunnel after tunnel after boring tunnel you can get carried away with thoughts of "what if". That's when it's a comfort to know that OC gas is along the way. Funny on the how mind works when it has the leisure to freely roam.

[And]

Originally Posted by Sheff diver

If you're diving a single RB, there is no point thinking about "thirds" or any other similar margin, as Stuart says.

Well, thats not quite true. Rather than using the scrubber to thirds, Stuart is using Diluent to thirds. I'm not sure I would be happy with that, but if you have sufficient OC bailout, then it is a similar logic to WKPP and a single RB80 I guess, although I think they do use thirds on the scrubber, judging by the bottom times, and have alternatives for deco, as Stuart suggested.

Andy

[Padowan]

Originally Posted by And

Well, thats not quite true. Rather than using the scrubber to thirds, Stuart is using Diluent to thirds. I'm not sure I would be happy with that, but if you have sufficient OC bailout, then it is a similar logic to WKPP and a single RB80 I guess, although I think they do use thirds on the scrubber, judging by the bottom times, and have alternatives for deco, as Stuart suggested.

Andy

On a write up of the 20-odd hour dive that JJ did recently, I seem to recall that they were planning to exhaust their scrubbers, then switch to OC during the latter stages of deco? Obviously the caveat to this would be the infrastructure that they are using with habitats to give multiple options for completing the deco obligation. Also for such a dive, it's possible that thirds may have been followed if the deco portion (from the habitat and upwards) was excluded from the equation, and if the RB was used from this point onwards I expect that it would be because it is more comfortable to breathe the warm moist gas for a long long time.

[Sheff diver]

Originally Posted by And

Well, thats not quite true. Rather than using the scrubber to thirds, Stuart is using Diluent to thirds. I'm not sure I would be happy with that, but if you have sufficient OC bailout, then it is a similar logic to WKPP and a single RB80 I guess, although I think they do use thirds on the scrubber, judging by the bottom times, and have alternatives for deco, as Stuart suggested.

Andy

Point taken. I was refering to thirds on the scrubber. My point is that if you are really going to take advantage of the RB, you will have to carry a sh1t load of OC as bailout, or a second RB. Thinking of some of the long dives at 70m or 90m (or deeper) here. Agree that dil usage can be a lot higher in caves, but you can still get an awful long way on a 12 of dil, even if you decide to work on thirds. Scrubber duration is the limiting factor, not dil (or O2), so as Stuart says, the dive should be planned according to this. When we were discussing this, and I asked about extending the duration of a scrubber, the response was "how likely are you to need more than 3 hours heading in?" You can take spare sof with you to recharge the scrubber on the other side of the sump (thinking of cavers here, so the sole aim is to pass the sump and go caving on the other side), and have OC or even a spare rebreather staged / brought in for the deco on the way out.

To be honest, the thing with this sort of diving is getting your head around it. I'm pretty sure I never will, so for now I'll stick with my squalid little sumps and a pair of threes.

Marcus

[Duncan Price]

Originally Posted by Sheff diver

Point taken. I was refering to thirds on the scrubber. My point is that if you are really going to take advantage of the RB, you will have to carry a sh1t load of OC as bailout, or a second RB. Thinking of some of the long dives at 70m or 90m (or deeper) here. Agree that dil usage can be a lot higher in caves, but you can still get an awful long way on a 12 of dil, even if you decide to work on thirds. Scrubber duration is the limiting factor, not dil (or O2), so as Stuart says, the dive should be planned according to this. When we were discussing this, and I asked about extending the duration of a scrubber, the response was "how likely are you to need more than 3 hours heading in?" You can take spare sof with you to recharge the scrubber on the other side of the sump (thinking of cavers here, so the sole aim is to pass the sump and go caving on the other side), and have OC or even a spare rebreather staged / brought in for the deco on the way out.

Essentially the duration of the rebreather (limited by scrubber or drive gas) should match the planned duration of the dive (as a minimum). Bailout should be sufficient to exit from the furthest point though there are no protocols that I am aware of to decide what excess of bailout over the "shit or bust" bare minimum required. It is common for rebreather divers in caves to pool and/or stage bailout - this means that they dive together and exit if one rebreather fails (a problem would arise if both divers' rebreathers failed...).

There comes a point when you put your trust in your kit and trim your safety margins accordingly (this is akin to diving a short sump on a single set).

I've mainly used my rebreather for dives where it gave me a lot of working time (for surveying or relining) - not far in or deep but just handy to be able to spend time doing stuff rather than watching my guages. The lack of bubbles was handy too. Many dives are best done on bailout alone - i.e. OC, and, as stated earlier, many dive profiles are not rebreather friendly.

The last couple of Wookey dives were done solo using two rebreathers because of the limitations in providing two divers with adequate bailout. In the Ressel, the divers carried in spare sorb.