Exhaled gas

[junior diver]

In a rebreather (as far as I understand) gas leaves the two cyclinders, gets mixed, gets breathed, sent back along the hose and sodelyme recycles it. But you can only recycle it so much, after you've recycled it as far as you can, where does that gas go then???

 

[O2BBubbleFree]

In a rebreather (as far as I understand) gas leaves the two cyclinders, gets mixed, gets breathed, sent back along the hose and sodelyme recycles it. But you can only recycle it so much, after you've recycled it as far as you can, where does that gas go then???

I'm not a RB diver yet, but have been studying them for a while...

Sodalime doesn't 'recycle' the air, it only removes the CO2.

In a Closed-Circuit Rebreather (CCR) the breathing loop is initially filled with air. As you breathe the gas, you consume the O2 and convert it to CO2. The exhaled air passes around the loop where the scrubber (sodalime) turns the CO2 into heat and water. Then new O2 is introduced from the O2 bottle, restoring the mix back to what it started from, and the process starts over.

In a Semi-Closed-Circuit Rebreather (SCR) the CO2 is scrubbed in the same way, but instead of replacing the O2 directly, Nitrox is periodically introduced into the loop. Since N2 and O2 are being introduced, there is a buildup of N2, and the loop must occasionally be vented to expell the built-up gas.

That's the simplified explanation, but there are RB divers on this board who know far more than I.

Or, for a better description, you might try:

http://www.iantd.com/rebreather/rb.html

 

[junior diver]

Aaahhh. Thanks.

 

[caveseeker7]

In a Closed-Circuit Rebreather (CCR) the breathing loop is initially filled with air.

Actually, swimming at or diving just below the surface are among the most treacherous places to be with a RB. There is no increased ambient pressure, so your pO2 would be .21, and dropping with every breath ... until you get hypoxic. That has happened, and way too many times!

If you fill the bag with air diluent only most any CCR will add O2, provided the O2 tank valve is open and the electronics (if any) on. Ususally eCCRs have a shallow water setpoint of .7, and would inject until it is reached and then trying to maintain it.

The only time you would ever want to start with a fairly low pO2 is for a very rapid decent to great depth, which is way down the road as part of advanced training.

... Nitrox is periodically introduced into the loop. Since N2 and O2 are being introduced, there is a buildup of N2, and the loop must occasionally be vented to expell the built-up gas.

Actually, on current systems Nitrox is either continiously introduced into the loop (constant mass flow - i.e. Dolphin) or per breath (respiratory minute volume - RB80).
CMF-SCRs add more gas than the diver is supposed to use as a safety margin. The "additional" gas gets vented through an overpressure valve.
In the RMV-keyed SCR both bellows (counterlungs) contract and expand simultaneously. Upon contraction the gas in the larger bellows gets inhaled by the diver, the gas in the smaller bellows expelled by a dump valve.