Rebreathers

[THarrison01]

What are the dangers of rebreathers? How dangerous are they?

 

[Wookie]

They are not dangerous. Rebreather operators are.

Let me explain. No rebreather diver has ever died because of a failure of the rebreather to perform, or because the rebreather broke. Lots of folks have died diving rebreathers. It's always operator error that kills the rebreather diver. They don't follow their checklist, usually. Sometimes they assembled the unit incorrectly, sometimes they put orings or the selenoid together wrong, and sometimes they dive expired or bad cells. A rebreather will kill you a hundred different ways, but it isn't the rebreather that does you in, it's you.

 

[Shotmaster]

Don't forget about channeling through the soda-lime absorbent.

 

[KevinNM]

I'm not a rebreather diver myself but Jill Heinerth wrote about a time she was on a boat with a bunch of rebreather divers and her unit failed self-test during pre-dive. Not an obvious "you're going to die" problem, but not how it should be. She couldn't get it to pass and decided to not dive. She was encouraged to go anyway by some other divers on the boat. Since that incident she has attended the funeral for at least one of the people encouraging her to dive, and they died on a rebreather that they should not have been diving on.

I also know one person who feels she survived a CO2 hit on a rebreather because her team mates convinced her to bail out and someone else on scubaboard described a similar experience.

All three of these people still dive rebreathers, but at least two of them won't do it solo.

 

[JohnnyC]

Rebreathers have a very different set of risk factors compared to open circuit. Those who are excellent risk managers will have no issues, because they do things like follow checklists, replace cells regularly, always know their PO2, and are proactive instead of reactive, even if that means bailing early and bailing often.

Those who are not excellent risk managers end up in the A&I section. If they're lucky they're telling the story themselves.

 

[Shotmaster]

Until there are better sensors the electronic controlled ccr's will continue to be unreliable and therefore very high risk. The channeling problem is greatly reduced with proper packing of the absorbent to make sure it is packed full and cannot work loose. There are many failure points, you might as well be an astronaut, the training and the attention to detail required is unbelievable when you get beyond the purely mechanical rebreathers with limited dive profiles. The weakest link is the human. One mistake can be your last. We just lost a decorated Special Forces combat diver who safely dove a mechanical rebreather his entire career to an electronic rebreather two months after his retirement. He was highly respected in our community and a fanatic for training and attention to detail. If a rebreather got him, it can get anyone.

 

[RainPilot]

Until there are better sensors the electronic controlled ccr's will continue to be unreliable and therefore very high risk. The channeling problem is greatly reduced with proper packing of the absorbent to make sure it is packed full and cannot work loose. There are many failure points, you might as well be an astronaut, the training and the attention to detail required is unbelievable when you get beyond the purely mechanical rebreathers with limited dive profiles. The weakest link is the human. One mistake can be your last. We just lost a decorated Special Forces combat diver who safely dove a mechanical rebreather his entire career to an electronic rebreather two months after his retirement. He was highly respected in our community and a fanatic for training and attention to detail. If a rebreather got him, it can get anyone.

Sorry for your loss mate. Never gets easier.

On the subject of manual vs electronic CCR, the failure modes of a CCR (as stated earlier) generally break down into what we call the 3 H's

This is Hypoxia (too low PPO2), Hyperoxia (too high PPO2) and Hypercapnia (too high CO2).

The first 2 are related in the failure scenario to malfunctioning O2 sensors, whether by user error (expired, flooded, uncalibrated) or a pure failure of a good sensor (S*** Happens Mode). A large part of CCR training is in managing the O2 readouts (what should I see, what am I seeing, what do I need to do about the difference) and that requires a lot of proactive checking and discipline to conduct checks routinely and systematically. Where a mCCR has an advantage in this aspect is that if you don't continually monitor, you WILL die even if the machine is perfect. With eCCR, they are generally pretty reliable so it is much easier to fall into the habit of trusting the machine and allowing complacency to creep in. This will get you hurt eventually.

The hypercapnia is related to the scrubber, and most of these are related to breakthrough ( overusing the scrubber so it is depleted or poorly packed leading to channeling) or bypass, where due to a change in gas path unfiltered gas is allowed to enter the diver's inhale side. These are usually related to missing or incorrectly installed o-rings or inhale/exhale valves. Again, almost all of these will be picked up by correct pre-dive checks and adherence to checklists etc.

So, the main question that, I suspect, is behind the OP, are rebreathers for you? Most of that depends on your personal mindset and attitude toward diving. Some questions for you:

You've set up all your gear on the boat, tanks open ready to don and splash. You have to go to the head, you are away from your gear for 10 minutes. Do you do a full recheck of all your gear when you get back?

You analyze your rented nitrox tanks tanks at the jetty and label them. You load the tank onto the boat and strap it down yourself. When you get to them to gear up you see that the tape has come off and there are no markings on your tank. Do you re-analyse? Even when you KNOW that tank is in the place you put it?

This should give you an idea of the mindset required to be a safe successful rebreather diver. Its no coincidence that a disproportionately large percentage of CCR divers come from an aviation background. The habit of , for example, restarting a 10 page checklist from the start because someone interrupted you in the last 2 lines, is the kind of thing that keeps you alive on the loop.

(All of the above are seriously simplified, there are a lot of other aspects to consider but these are pertinent to this discussion. Rebreatherworld.com is a good resource)

 

[RainPilot]

The main danger of rebreathers is that, unlike recreational scuba, you can have a failure that results in a non-life-sustaining gas being breathed. Preventing this, recognizing it when it happens and dealing with it is all that CCR training and technology is all about.

That being said, lying dead still in the water and having fish come peck at your suit to see what you are, all in complete silence with warm, moist gas to breathe is an experience you cant imagine until you've done it. The risks and costs and training etc are all part of the price of doing business for me.

 

[abscuba]

On CCR it is on you to keep your self alive. In open circuit if gas comes out of the regulator (long as its the right mix) things are good. If air stops it is very apparent something is wrong and you need to switch regs. On CCR it may not be so apparent (hypoxia, hypercapnia). You might just start feeling funny. I take CCR diving like I do flying, there is no way to illuminate risk only mitigate it. On the flip side I have more options when on a dive should something go wrong. The system has many redundancy and worse case I bail out and I am back to being an open circuit diver. CCR is a tool and and does allow for much longer dives, but at the end of the day they are all tools.

 

[Bobby]

Unfortunately a lot of the CCR training levels are quickly diminishing, this IMHO, makes diving CCR's exponentially more dangerous. Recently a newly minted CCR instructor called me to have a discussion because he was questioning his training. It became apparent that he didn't understand linearity or how to determine his percentage of linearity of his cells. I've heard of instructors teaching their students that they should not do a 20' (6m) check because "they will bombard their cells with O2", wtf. Very few teach to do a 20 check at the end of the dive much less why they should.

CCR's are a tool. They are more complex compared to OC however for certain dive profiles I believe they can make the dive safer. How much or even if they do or can is the debate of the ages. Without thorough understanding they are extremely dangerous. Students don't know what they don't know.