Flood Tolerance and dewatering: what design features determine these?

[wedivebc]

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

I guess I'm going to have to disagree on a couple of points.
Depending on the position of the diver a leaking hose past the T piece will still drain into the front mounted counterlung where you could dewater the unit. A leak on a rEvo that enters the counterlung is a non-issue unless you alter your position in the water to allow that moisture to infiltrate your cells which are exposed in the counterlung. This can easily happen in a cave dive scenario where sometimes you have to roll over or stand on your head to get through a passage. That will basically knock out your O2 monitoring system and render the RB inoperable.
Having said that I can't over stress the point you made about bailout. No amount of flood tolerance should lead one to point of carrying inadequate bailout no matter how reliable your unit may seem.

 

[The Chairman]

I'm certified on 4 breathers and the only Caustic Cocktail I got was on the rEvo. By far, it seemed to be the wettest of all of them when I took it apart after the dive. I was surprised at how little water was needed to get it to go sour on me. I had a much longer out of my mouth situation with the SF2, put my butt down, hit the dil until it burped out it's butt. No problems, what so ever. When I pulled it apart after the dive, I was surprised how dry it was. Yeah, there's the usual condensation in the loop hoses, but that's easy to clear.

 

[mmadiver]

Flood tolerance and ability to dewater are the last of my considerations when assessing a CCR. However, Novice CCR divers tend to be preoccupied with the subject.

In 11 years of CCR diving I've never flooded a rig. The rig floods that I have witnessed were all attributable to operator error - incorrect assembly, no pos/neg checks, or poor DSV management. As a casual observation, those floods happened to divers with undisciplined personalities and probably have no business diving CCRs anyway.

My advice is to look for a CCR that is well engineered with extremely tight loop integrity, that can hold a negative for days, and has as few connections/orings as possible

 

[Tienuts]

I guess I'm going to have to disagree on a couple of points.
Depending on the position of the diver a leaking hose past the T piece will still drain into the front mounted counterlung where you could dewater the unit.

I'll stipulate to that and stand corrected, obviously it depends on the location of the leak. My only point being the flood tolerance does not apply to the entire loop.

A leak on a rEvo that enters the counterlung is a non-issue unless you alter your position in the water to allow that moisture to infiltrate your cells which are exposed in the counterlung. This can easily happen in a cave dive scenario where sometimes you have to roll over or stand on your head to get through a passage. That will basically knock out your O2 monitoring system and render the RB inoperable.

Of all the CCR's I have the most time diving a rEvo. Probably close to 800 hours. Over this time I've been filming in all sorts of water, in all sorts of environments. When filming I end up in every position you can think of. I have had dives with literally liters of water in my counterlungs. I never once lost a sensor because of water in the lungs.

I wrote a big long post discussing the various positions of the rEvo and how water collects in it and affects it's components. But then I erased it, as it's rambling, hard to explain without visual aids, and a moot point, as all of us are in total agreement that no amount of flood tolerance or flood recovery is a substitute for bailout.

I agree with Pete. The rEvo is the wettest loop I've ever seen. I can only write to my experience, which is that the water in the loop was never a big deal.

Flood tolerance and ability to dewater are the last of my considerations when assessing a CCR. However, Novice CCR divers tend to be preoccupied with the subject.

In 11 years of CCR diving I've never flooded a rig. The rig floods that I have witnessed were all attributable to operator error - incorrect assembly, no pos/neg checks, or poor DSV management. As a casual observation, those floods happened to divers with undisciplined personalities and probably have no business diving CCRs anyway.

My advice is to look for a CCR that is well engineered with extremely tight loop integrity, that can hold a negative for days, and has as few connections/orings as possible

This entire post I agree with 100%.

To the OP, my advice:

Remember, everyone you speak to (instructors and divers) have some degree of bias to "their" unit, no matter how impartial they try to be. I try to be completely unbiased, and give a good pro/con argument about every unit, yet I still have a bias. It's human nature, and much more prevalent in some.

Don't speak to just one instructor, speak to 15 instructors. Learn as much as you can about every unit. Handle them if you can.

Be wary of pool-only "try dives." 15 minutes in the pool is not representative of how a rebreather dives, and does not give you the experience of the actually owning it. Things like the build process, the disassembly/cleaning/storage process are not in a pool try dive. Often the harness won't fit properly, it won't be rigged to suit you, and you'll be in shallow water, which is the hardest environment to dive a rebreather. My analogy is if you were going to buy a Porsche, and asked to test drive it, and the salesman let you only back it out of the parking space without adjusting the seat or the mirrors. That's not really the experience of driving or owning a Porsche.

If you can find someone who does an actual "rebreather experience" which consists of a build, dive in open water, and breakdown, pay this instructor for their time and do it.

Once you narrow it down, find an instructor that will rent you unit and take the class before you buy it. You'll learn the questions you didn't know to ask when you take mod1.

Most importantly, get as much info as you can and make your own informed decision. Don't buy what someone tells you to buy.