Rebreather questions

[john j]

There had recently be a lot of very heated and technical argument on the rebreather topic which I think turned many people away and we lost track of one of the main topics which was questions about rebreathers. I'd like to start discussion of that topic again and hopefully without all the flaming. So, if you have questions about rebreathers, no matter how basic the questions are, please post them. If you have a rebreather and want to tell people how and why you are using it please do so.

[Chris]

Great thread...
How about explaining the basics on how a rebreather works and perhaps some of the common terminology of a rebreather.

Personally, I know what one looks like and the general concept... that's about it.

[tcjtn0]

I second that idea !!!
I may never use one but it would be nice to know the basics...
"Just the facts , maam..."

Thanks....
Jean

[DiverMole]

OK, rebreathers 101

The basic principle is a closed circuit loop to recirculate your breathed gas. This needs an expandable area on it (the counter lung) to accomodate the lung volume going in and out. Where you place this lung is a crucial part of the design, and it can be in front, behind, over the shoulders and often is 2 lungs, one for exhale and one for inhale

Now the problem with a closed loop is twofold. For a start you use up all the oxygen in it and then turn it into carbon dioxide.

We deal with the CO2 by using a chemical "Scrubber", which can be several compounds like calcium Hydroxide (sodasorb, sofnolime) or lithium Hydroxide (used by military, very caustic). These work by the co2 dissolving in the water in the chemical to form Carbomic acid which is neutralised by the alkali scrubber. The dwell time and speed of the gas and the temperature and moisture content are all important in how much CO2 it can absorb.

The oxygen can be taken care of in several ways

a) Semi closed active addition (ie Dolphin and Azimuth). Here we leak a fixed volume of gas (nitrox) into the loop, in a quantity taht always exceeds aout respirity needs. This is obviously more gas than the loopp can hold so we need a overpressure valve that farts gas out regularly. These units therefore have a MOD set by the gas used and rely on the right volume of gas injection. A low flow rate (blockage) or a high respiration rate (surface swimming ) can kill the user with no warning. To dive one of these without an O2 measuring device is very dumb

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[DiverMole]

b) Passive Semi closed (respiritory keyed)(ie Halcyon RB80). On these units a certain percentage of every breath is separated and dumped through a pressure valce and hence replaced by fresh diluent. Again this is nitrox and again the unit bubbles and suffers from the MOD of the gas used. These units has a single point of failure in the discharge bag and valve, and although most falures give physical out of air symptoms they still need an O2 guage fitted

c) Manually CCR (Home builds), in this case we measure the O2 drop using electronic O2 meters and the user manually injects enought Oxygen to make up the amount breathed. We also have to carry diluent to make up lung volume on decent. Closed circuit units do not bubble unless you ascend and need to release air. This dissadvantage of these units is the user, forget to monito and squirt and you die

d) KISS CCR (KISS unit). In these units a smnall amount of O2 is allowed to dribble in, the user tops up the rest using teh O2 meter. The idea being that the dribble doesn't supply all the O2 you need, but will keep you alive if you forget to squirt manually. Again this unit relies on manual flying

e) Electronic CCR (Megladon, Inspiration, Prism etc), in this case a computer monitors the O2 level and injects the perfect amount of O2 to keep the unit at the set point. These units have multiple O2 cells and often multiple redundant electronic controllers, they assume the user is an idiot and will do all that is needed if you fail to monitor them. You can take over and fly manually if you wish, but the computers do a much better job at keeping a constant PPO2 than you can

Hope that helps, feel free to ask any questions, also excuse the spelling, its getting late here in England

[Groovekitty]

"they assume the user is an idiot and will do all that is needed if you fail to monitor them".



That explains a lot!

[john j]

Even though the closed circuit systems have redundant sensors and redundant electronics, they can fail. Certain types of failures can cause too much oxygen or too little oxygen in the breathing mixture. If the user dosesn't catch this and deal with the problem swiftly, it can be fatal. The training classes are intense and drill the student over and over on these problems, how to see them and what to do about it. My instructor drilled us ruthlessly during the class which is excatly what he should do. When a person dives on a closed circuit rebreather, he must frequently check the oxygen level by looking at the computer display. This is one way of making sure the oxygen level remains correct during the dive, but there are other ways too.

There is also a significant amount of set-up and calibration to do before the dive as well as more maintenance after the dive than you would be used to from open circuit. I always say that if you want to be the first one in the water, don't dive with a rebreather. What you get in return is extended bottom times and the ability to do multiple long dives without having to change tanks and get fills. Also no bubbles with a closed circuit system. I used to think so what? Now that I am used to not having bubbles I can really appreciate it.

There is a good book called "Simple Guide to Rebreather Diving" by Steve Barsky which I recommend. It explains a lot about semi closed and fully closed rebreathers and is at a level where the non-rebreather diver can understand.

[DiverMole]

Thats the way cars are going as well, ABS, Traction control, air bags. and guess what, they are a LOT safer than the old completely manual cars

Wheather you like it or not computers can do some jobs better than humans. Ceaslessly watching a readout and not getting bored or looking at fish is one of them.

You the rebreather user can also take over and fly it manually if you wish to, so its all gain, if the computers go bang, you wont die if you are doing what you are trained. Just like you should plan OC diving so you dont die if your dive computer goes or you drop your deco tables

I dive CCR with quadrouple cell redundancy, Triple PPO2 monitor redundancy and triple controller redundancy, and in case its a really bad day also carry OC bailout. Thats more cautious than most OC users, yet you tell me I'm dangerous

[DRE]

Don't wet your pants Divermole - I think it was meant as a joke...

PS: the wetting your pants is a joke as well!

[beeger]

I really wanted to stay out of this discussion, but I just can't help it. Quadruple redundancy? What the hell is that? If there is one piece of gear that is so critical to your immediate survival underwater that you have to carry four of these things and essentially use each in tandem don't you think that there is something fundamentally wrong with relying on that piece of gear in the first place? There is no single piece of gear on OC that demands that kind of redundancy. Your O2 sensor redundancy exists because if that piece of gear fails, you are seriously and immediately screwed and may not be able to tell until it is too late (hence you carry more than one to make sure everything agrees). Why don't I carry more than one extra mask on an OC dive? Why don't I have a 4th or 5th second stage? Why don't I have a second SPG? Why don't I have a 3rd bottom timer or watch? A second manifold? Because if any of these pieces of gear fail that alone will not kill me. Further, I will 1) know it immediately or at least soon enough to prevent further problems; 2) be able to move to a backup system on my gear or my buddy's gear without missing a beat.

Your argument about cars, while well intentioned, does not make sense in this context. None of the items you mention are critical to your everyday survival in the car (i.e. you don't need air bags until you have an accident). Even ABS/traction control - this only kicks in when your tires are beginning to lock up or are spinning without gripping (not a normal, everyday, every minute in the car event). When an accident occurs, these things help give you a better chance of survival, but their use is strictly for "damage control" so to speak. Your O2 sensor/computer is critical to keeping you alive during normal operation.

Okay, I’m now starting to relax. Deep breaths. Back to my happy place.