Flood Tolerance and dewatering: what design features determine these?

[rsingler]

On the verge of going over to the dark side. Will dive as many brands as I reasonably can before buying, but travel and instructional cost are limiting factors. Recs of those I trust will play a role, no doubt.

But with a reasonable eye for the technical, help me get a feel for why posts label certain brands as:
a) flood tolerant/intolerant
b) easy/impossible to dewater.
What should I be looking for in the circuit and components?

It would seem that DSV/BOV design play a role in locking out the loop when the mouthpiece is out. And some circuit designs may have an issue with condensation in certain positions, though I'm not sure exactly what to look for. And obviously, presence or absence of a valve that vents to ambient from ?counterlung? ?exhalation side of the loop? might be a factor in dewatering.

But is there something I could look at in a CAD drawing or schematic that would enable me to say, "Aha! This one is going to be a real problem with caustic sludge if I..."

Yes, I'm going to try to be one of those CCR divers who can say, "I've never flooded a loop in my life!" But still, what do I look at and think about as I decide where to plunk my $8 grand with regard to these two factors?

And of course, with regard to these two issues, what are your favorites?

 

[rsingler]

Rebreather designs and buoyancy control strategies
This thread is what got me started thinking about all this. Without starting flame wars re: the various designs that have been dissed in this thread, help me (as a relative newcomer to CCR) to spot what design elements are being commented upon.

I get the counterlungs above the DSV thing. But what keeps the canister dry otherwise?

 

[KenGordon]

Rebreather designs and buoyancy control strategies
This thread is what got me started thinking about all this. Without starting flame wars re: the various designs that have been dissed in this thread, help me (as a relative newcomer to CCR) to spot what design elements are being commented upon.

I get the counterlungs below the SBV thing. But what keeps the canister dry otherwise?

Loose lips cost lives!

Your mouth keeps the sea out. You always close the loop at the DSV when removing it. You get condensation every where due to warm moist gas in a coldish tube. You get bodily fluids in the exhale hose and lung. If you let water past then it ends up on the exit side of the exhale side mushroom valve. This is noisy and annoying as each exhale bubbles though it. On my JJ you can persuade that water into the exhale counter lung to fix that. It and the APD (and maybe others) use the lungs as a trap for water between the mouthpiece and the scrubber. Then water has to get past the scrubber to get to the cells. The way the scrubber sits in the canister, with gaps all round plus sitting off the bottom, creates a another fair sized space for liquid to be trapped before soaking the scrubber.

On the JJ, when entirely horizontal, the lungs are above the mouthpiece.

The thing is to get to poke at the units and dive the ones which look good to you. There are other factors worth considering such as how hard it is to take the head to a safe place at night, ability to dry off electronics, whether it or the boat breaks if you drop it.

 

[rsingler]

Great info, thanks!

Could I get a rEvo diver to weigh in on why I shouldn't worry?

 

[tbone1004]

Great info, thanks!

Could I get a rEvo diver to weigh in on why I shouldn't worry?

the sidewinder has no de-watering ability either.

@paulraymaekers can try to calm your nerves on why you shouldn't worry

 

[Tassi Devil Diver]

Could I get a rEvo diver to weigh in on why I shouldn't worry?

Since you asked, you would always carry enough bailout should the loop become flooded and not breathable, pre-dive positive and negative tests will make sure the loop is adequately air tight when you build up the unit prior to a dive.

Regarding the rEvo there are a few design features that reduce the chance of a flood. The loop has only 4 user serviceable O rings to seal it (with two additional back up O rings to seal the canister lid), other units have more in T pieces etc, these are potential points of water ingress should these leak. The rEvo counter lungs are encased in stainless steel or titanium reducing the chance of tearing or puncturing the counter lungs during the dive. The only vulnerable area is the loop hose which could be punctured or torn which is applicable to any unit using rubber hoses. The DSV retaining strap, that comes as standard, reduces the possibility of the DSV being accidently removed or knocked from mouth and water flooding in during a dive.

Overall I don't have any concerns regarding the lack of flood tolerance with the rEvo or lack of ability to dewater. Approaching 5 years of diving with the rEvo and I have only totally flooded the unit once, my own fault left the DSV slight open during shallow bailout practice, otherwise the amount of water I get in the unit is negligible and only as a result of condensation. And should I ever flood the unit I always carry enough OC bailout to end the dive.

 

[NAND]

...i guess if you make it to flood your CCR there are other things that may have gone wrong- preventing you from staying on the loop anyway.

Could I get a rEvo diver to weigh in on why I shouldn't worry?

Technically the unit can flood quite easily- thats somehow the nature of it. Such event would be nasty and annoing, force you on bailout. All inner parts are waterproof- you would lose scrubber and sensors- simply said all wear and tear had to be replaced. If the flooded scrubber could create heat damages cant be answered from my side.

It however is (at least for me diving a rEvo) very unlikely to flood. Imagine you breath from a plastic bag all the time- when adding water traps etc.pp. you will increase the WOB- increasing CO2 retention. So everything comes at a cost.

I believe its safe to say the concious diver (who is on the loop and not on bailout practise etc.) wouldnt drop the loop unintentionally. Yes, you may bite the mouthpiece off or cut the loop-hoses- but cmon- there is quite something needed for this to happen. The counterlungs are (compared to other CCR) very well protected within steel casing- I dont really see a likelyhood of a puncture by the external environment.

Most likely, a diver would surface and forget to create positive boyancy when dropping the loop for whatever reason, leading to a rapid descent. There is not much different to any OC diver spitting out his Reg drowning head down on surface- except for loss of your loop as a floatation device. It is however emphasised in training and drilled into your mind to not allow this to happen and not to misuse the loop this way.

Another scenario would be the unconcious diver who lost control over mouthpiece or strap. Here, I see quite a hazard of drowning- but again- every OC diver would suffer the same effects of breathing water- no matter if next to him floats a flooded loop or any OC reg...

 

[kensuf]

Since you asked, you would always carry enough bailout should the loop become flooded and not breathable, pre-dive positive and negative tests will make sure the loop is adequately air tight when you build up the unit prior to a dive.

Don't just assume a positive/negative test is all you need to do. While 99% of the time this is correct, a tear in the mouthpiece will allow you to pass a positive/negative yet still cause water to come into the loop.

 

[LiteHedded]

Don't just assume a positive/negative test is all you need to do. While 99% of the time this is correct, a tear in the mouthpiece will allow you to pass a positive/negative yet still cause water to come into the loop.

i struggled with that for months and months on my rb80. my mouthpiece wasn't torn, it was a seacure that just would seal right no matter how tight i made it. you would have to bend it JUST the right way to get it to leak. anyway, it would pass all tests and leak like a sieve every dive.

 

[Tienuts]

I've been diving rebreathers for over 10 years, and I've been certified on, and owned quite a few of them. Some of these were back mounted lung units like the rEvo, while others were OTS lung units with good 'flood tolerance' like the Meg or the Opima. A few thoughts:

The most water you're going to find in the loop is going to come from your own drool, and the reaction of the scrubber. As pointed out, a positive/negative check will abate most (not all) leaks and sources of water and leaky mouthpieces (if you have one) will be the largest cause of water intrusion into the loop. Installing a seacure on a rEvo is a pain in the ass, and requires patience and lots of sealant. I've done it enough. I've also had severe water intrusion from leaky mouthpieces and done multi-hour dives on the unit. Water gets in, and you blow the water into the exhale lung, and you know what happens? Nothing. Most of the water stays in the lung. The exhale scrubber gets wet, and that's about it. My point here, is minor amounts of water from respiration, scrubber reaction, loose lips, or leaky mouthpieces are generally OK. It's not ideal, but you're not going to die.

My second thought - many who dive OTS lungs I feel walk around with a false sense of security when it comes to de-watering, and flood tolerance. Yes their loop may be flood tolerant and any water in the loop may be drained from the exhale lung, so long as that source of water is before the t-pieces. If your leak is after the t-pieces, there's nothing you can do. That being said, I've seen a number of leaks occur near the head. Several from rebreathers that tipped over and fell on the bench or dive boat. These resulted in cut loop hoses near where the loop met the head. In some cases I've seen a careful rebreather diver do another negative check and catch the problem. In other cases sadly, I've seen flooded loops that were discovered underwater. One notable incident is a well known cave instructor who was diving an Optima in Ginnie Springs. He became stuck, and rather than thinking his way out of it, he bulled his way out of it, and tore the loop in the process. My point here, is flood tolerance only works on the half of the loop that's before the t-pieces, and can't be relied on for all loop flood scenarios.

Now if you put a gun to my head and said I had to dive Eagle's nest in a rebreather with no bailout, I'll take something with OTS lungs to increase my odds somewhat. But since we need to carry enough bailout to get to the surface anyway, it's sort of a moot point. A loop flood is just one of those scenarios we train for.

Tony