2 newbie questions that turned into 6 newbie questions...

carlthecat

Hello all. This is my first post to this site. I am an experienced OC diver who is now getting into true OC tech diving (>130' & gas switches). Recently, I watched 2 shows, one on Discovery ("Deep Sea Detectives" - when they used rebreathers to get deep into a Cozumel cave system to determine how the Mayans got pottery and human sacrifices so far into the cave system) and the other was on Discovery HD ("Florida Underwater Caves" - when a man and his support team mapped some underwater caves in north Florida). On one (or both) of the shows, someone mentioned that the rebreather gave them somewhere between 10 to 13 hours of breathing gas. That leads to my first question:

1) Does/Can a rebreather give a diver 10-13 hours of breathing gas? If so, what is the maximum (ball park is ok)? Is the amount of breathing gas variable with depth like OC systems? Which rebreather models provide the highest breathing capacities?

Jeeez... my first question turned into 4...

So, after being intrigued, I came to rebreaterworld.com and read a bunch of the "new to rebreathers" material. During my reading, the author mentioned using 2 diluents on deep dives. This leads to my second (or 5th question):

2) How can a CCR diver change from one diluent to another in the middle of a dive? All the CCR rigs I have seen have one bottle for O2 and the other for the diluent. Do they change the dilutent bottle under water or is a 3rd bottle added to the rig?

After these 2 questions are answered, I will continue reading and learning about the interesting world of CCRs.

Thanks for all responses in kind,

Carl the Cat

jhaaja

For question 1 (or 1-4):
In theory yes but in practice I would say 4-6hours. It is easy to extend the gas range but the scrubber (remover of CO2) will eventually stop working.

CCR uses more gas when you go deeper. That is because you have to maintain a volume big enough for breathing. So going from 10m --> 20m you would add say 3-4 litres of gas (depends on your normal tidal volume).

2: The diver has a stage/sling tank that he connects to the breather and flushes the breathing loop with the new gas. The tank can also be mounted on the main rig.

Hope this helps a bit.

JH

JonnyB

hi Carl, welcome to the dark side!
To answer your questions:
1. Yes you can get 10-13 hours out of your tanks, on a rebreather the limitation is the scrubber and your decompression rather than your gas supply.
2. What the maximum is, well that is very depended on the size of cylinders, the pressure in them and your metabolic rate, usually the metbolic rate can be divinded into a group of 4.
At rest-> 0.3-0.5L/min
Light to moderate work -> 1.0-1.5L/min
heavy work -> 2.0-2.5L/min
maxwork 3.0->3.5L/min

So if you have a 3L bottle at 200Bar of O2 you have 600L of O2 which should give you between 400 and 600 minutes of gas.
And on the diluent side you mainly only use it on descent, so i have gone a very long time myself between fills.
3. No the consumption is independent of depth
4. As long as we talk about CCRs they are mainy the same, the consumption depends on the diver not the brand.
5. I can change my diluent at any give time on my meg, the same goes for O2, i have an drysuit inflation inlet on both my counter lungs, one for diluent and one for O2.
6. Always carry bailout, the bailout being yours or your buddies or hanging on the line can be plugged into the rebreather.

Hope that you got answers to your questions and good luck.

/Jonny

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Ben Field

Hi Carl, Welcome on board!

:-) Don't believe everything you watch on television!

Er, yes and er... no. :-)
"Most" rebreathers on a single dive will last 3hours, a couple of extreme models 5-8 and few people have strapped two together to do longer.

The limiting factor is the CO2 scrubber which will last a limited amount of time depending on the amount of scrubbing material in the rebreather, the workrate and depth.

With full tanks the gas available is often enough for 10hours diving though.
[color=black] Er, yes and er... no. :-)
The amount of gas is pretty constant, remember we're only replacing the Oxygen here the amount of which remains fairly constant irrespective of depth.
The amount of breathable gas is limited by the life of the scrubber, you'll still have gas left after several hours but chances are the CO2 scrubber will be exhausted is very high, some rebreather have ways of measureing this, everyone else just times there dives, done 3hours on the scrubber, yes- then chuck it away.
[color=black]

Don't quote me but I think, CCR- Oruoboros, CisLunar and Mk15series can go longest, SCR- RB80 and clones carry the most scrubber. Or just carry two or more rebreathers.

Why would you? Most of the time people use one diluent for the entire dive, even very deep dives as you use the onboard O2 supply to make the hypoix mix breathable in the shallows (not much good to bailout on though!)

If you needed too, on an extremely long dive for example, then a QR on switch block can be used.
Well I think you've come to the right place, the people on here have given me alot of advice but remember, just like the TV- not everyone here knows everything, so until you hear otherwise assume everything I wrote could be wrong!

BEN

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