Diving multiple rebreathers...

dave t

In my limited experience with double rebreathers I have only been in the 60-70m with them and sanity breaths were two or three only between loops, I can see your point at 100m plus, the question is do we need to go OC in-between? I guess it depends on wether you belive a loop should be pre breathed or not! personally I would be happy to switch directly from primary to secondary loop as long as the secondary loop was maintaied correctly but there-in lies the difficulty.........maybe

Mdemon

I think the pre-breathe on the BOB was thought to be useful but not necessary to remove CO2 at bailout depths in another thread on the subject a while back. Likewise, for long dives there was no need to "re-charge" the pre-breathe after 45 minutes or so. I would do one at the surface purely to check for channeling and as part of the pre-flight checks. I'll find the link if it's needed...

As to OC gasses for bailout. As mentioned before, I dive 7 li dil tanks anyway. However, given that diving a BOB may mean removing all off-board dil, is this going to be enough to get control of the breathing at 100m+ after a CO2 hit? You can watch the needle move on every breath on a twinset OC at that depth! I guess the gas needed is a function of time taken to recover breathing rate after a hit, and time taken to deploy the BOB. I think a lot of practice/experimentation is needed here. Didn't DrJM mention that bailing out to his BOB in Dotty wasn't a pleasant experience?

There is a certain amount of redundancy built into full OC bailout - if one cylinder fails, at least you have the option to go to a higher mix and risk the tox. With the BOB, it is a matter of all the eggs in one basket. Because I think the risk of two rebreathers failing at the same time is very, very unlikely, I will be happy to take that risk, personally. The key seems to be in having enough OC gas to make the switch...if, as Dave points out, you actually need to.

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johnv

When switching CCR's underwater, especially at depth, It is wise to go semi closed for the first few minutes after the switch to let the scrubber warm up. This works and is a sensible precaution.

With regards to removing all off board diluent on a 2 ccr rig, I suggest completely the opposite plan, I have no inboard diluent. my inspiration has 2 O2 cylinders, one to the manifold, one to the second CCR, which can be cross connected if necessary.

I practice a mixed approach to diving 2 ccr's. I believe you still need some O/C as for one thing, you can't throw up into a CCR. As someone who had been forced to do some long dives with bad migraines, this is important. There are also many other unanticipated problems that make having one or more decent sized dil cylinders a good idea, regardless of how many rebreathers you carry.

John

sadave

This is a great point, and one that I definately hadn't thought of. I recall speaking to Pete Mesley and Dr Simon Mitchell about their RMS Niagara dive (125m) where on ascent, Pete had a chronic case of Inner Ear DCS, and spent a number of hours switching between his Inspo and OC, to constantly throw up.
I guess you can't plan for every contingency, but given the number of cases I know of IEDCS on sub 100m dives, I believe you would be crazy to not factor this into the equation.

Great call John.

pchanning

Right, this is gonna be a bit off-the-wall but hey it'll serve to feed the conversation, so here goes.

How about the following configuration ideas:

get two of Alex's rEVO's each with the pendulum breathing hose, one coming from each side (so it's like a normal looking Rebreather loop) and have just one BOV mouthpiece that switches between the two - his RB's have the OC built in, so that would be a result!!

Put alex's thin (110mm deep) Rebreather's back to back between a FAT twinset. So its all back mounted. I handset per RB for each arm 'a la hammerhead'.
Sounds expensive though , reliably expensive - like Alex's Rebreather and not unlike the Stella Artois

But hey, if you were going REALLLLLY deep, you could also add a stage or two


Another wackier idea, but might be attainable in the shorter-term:

2 x side monuted Rebreather's 'a la Duncan Price' with ADV's
1 x back mounted twin set with LP hoses to feed both Rebreather's
1 x fancy double loop mouthpiece BOV

Pros:
simple, legible layout - just like OC times
a useful quantity of bail gas on the back (use H-Valves if truly paranoid)
modular - use what is required for different depths

Cons:
fancy mouthpiece will be tricky and likley cost a few quid too. It will also be bulky, but could be strapped to the head or more likley built onto a FFM. A OC reg could be built into the bottom of the BOV so there is no need to remove the mouthpiece at all - it will do Rebreather 1, RB 2 or OC from the tin on the back. Gordon made a 'snog valve' for the KISS where the diver added o2 and dil using the tongue, so this sort of thing is hardly impossible, and hey, Cis Lunar built that gas-switching block (which I still cant work out) so this should be easy

Anyway, later folks and have a good xmas

cheers
Paul

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Mdemon

I hadn't thought of the throwing-up thing. Eeeuw!

What sort of size dil cylinders were you thinking of please?

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"small minds talk about people, Average Minds Talk About Events, GREAT MINDS TALK ABOUT IDEAS!"

The WRONG Attitude will get you killed.

"Once the agenda-monkeys and perfect-worlders have moved on, perhaps we can do some diving?"

johnv

It varies, I have used 18's, 15's , 12's a 10 with a Y valve, or more often a combination but I'm not a boat diver. You can do the sums, look at your RMV at your bottom depth, then multiply by the number of minutes you want on O/C, and add what you expect to use keeping the loop at ambient etc and what you need for a return on the second CCR.

I often use a couple of cylinders for a several dives so at the beginning, a big cylinder is overkill, by the end of the week using it for dil it's about right.

It's worth thinking about how you actually use your 2 rebreathers, one option I sometimes do is to stage a CCR at 21m for deco bailout and expect to use O/C to reach it. 2 CCR's with the loops "ready to go" introduce yet another air space that affects your boyancy. it's a pain.

John

johnv

this is almost exactly how the joki was built to be used, except that you have 2 seperate mouthpieces (much better) so you can hand off a CCR if necessary, and it's easier to carry each unit on it's own

John

Steve

Yet another reason for the OC/CC DSV. They make air breaks a piece of piss as well.

As for the scrubber warming up stuff. We have been over that a heap of times and I believe it is one theory that should have died a graceful death by now.

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Marc T

2 CCR's with the loops "ready to go" introduce yet another air space that affects your boyancy. it's a pain.

John[/quote]

I was thinking about a bunjee ( or perhaps something like a waterbag fited side by side with the counterlung ) to sqeeze the joki's counterlung to limit buyoancy problem when you are not breathing on it...But nether try it.
What do you think about that?

Marc T