Sport divers are instructed on the use of open circuit SCUBA. Rebreathers are a bit of an enigma. Commercial units are expensive, complicated and get pretty weird looks on the diving boat. Surprisingly, rebreathers have been around for longer than modern open circuit SCUBA diving. Today in SCUBA Theory, we're covering basic rebreather theory.

How it works

During respiration, oxygen is absorbed and carbon dioxide is expelled. Atmospheric air is only about 21 percent oxygen. The rest of the gasses are inert and simply pass in and out of the lungs. (The body can and does absorb inert gasses, it just doesn't metabolize them) In open circuit SCUBA diving, used air is exhausted into the water.

 In a closed rebreather system, the carbon dioxide is removed from the expelled air, and oxygen is reintroduced in it's place. The inert gasses are recycled.
The fundamental operation of a rebreather is really very simple. The complications come in the form of construction and use.

Oxywhat?

The amount of oxygen in the circulating air must be kept at a safe level. Too much means oxygen toxicity, too little means hypoxia. Both conditions are difficult to safely manage underwater. Remember Boyle's Law? As pressure increases, the volume of a gas decreases. As someone diving a rebreather ascends and descends, the inert gas volume and partial pressure of oxygen both need to be adjusted accordingly. Just clearing a mask now has new implications on the divers air management. To take a rebreather system to any real depth (over 20 feet) the system needs an additional source of inert gas.

Micro-management is good

Modern rebreathers cost thousands of dollars. Oxygen levels are constantly monitored with electronic sensors. These systems certainly well designed, but require quite a bit of knowledge and training to safely use them. The safety of using a rebreather is greatly increased once the diver can monitor the amount of oxygen in the system. However, if your SCUBA diving computer fails, you've only lost your diving history. If the electronics on a modern rebreather experiences failure, serious situations could result. (Of course, they have manual controls and spare gauges for backup) But because the system must be very robust, the cost of the system increased dramatically. Today, a low-end rebreather costs about as much as a full set of some of the best SCUBA gear you can buy.

Cheaters!

OK. It's not really cheating, but there are advantages to using rebreathers.  By controlling the amount of inert gas and the amount of oxygen in the system, the rebreather now has deeper operating depths. Thanks to the closed system, they can dive longer with much smaller tanks. With the use of an inert gas that doesn't become an anesthetic at depth, nitrogen narcosis is taken out of the equation. With control of the gas mixture there is now the possibility of reducing decompression times.

Oh, and don't forget that the guy diving the rebreather isn't releasing a torrent of bubbles to scare away the fish. The lack of bubbles seems to be the favorite reason for diving rebreathers.

Limits

Lots of people are curious about depth limits of rebreathers. The rated depth for each device is completely dependent on it's design. People on the bleeding edge (people tend to die trying this) are saying near 200 meters of depth is possible.

If you're really interested in the various types of rebreathers, there's a short writeup explaining the differences between oxygen only, closed and semi-closed circuit rebreathers. Bill Reals recommends Mastering Rebreathers for further reading. If you're looking for more information online, check out A Learners Guide To Closed-Circuit Rebreather Operations by Richard Pyle at Rebreather World.