Scuba Diving Rebreathers – Two Types of Rebreathers

I’ve always wondered why Jacques Cousteau chose the wrong title for his first book, The Silent World. The sea is anything but quiet. Whales are so booming loud that they can communicate with one another across hundreds of miles of ocean. Dolphins ping the void with sonar to “see” underwater. The ocean is full of crashing waves, fighting fish and, increasingly, chugging motorboats.

It wasn’t until I used a rebreather for the first time, and experienced how it silently purifies and recycles exhaled gasses rather than dumping them out as noisy bubbles, that I finally figured out why Cousteau had gotten his title so wrong: His Aqualung-grandaddy to all of todays regulators-made so much noise that it masked the sea’s other sounds. Like us, he could only hear the sound of his own breathing.

The solution to the noise problem – experienced by every diver who has tried to sneak up on skittish fish while exhaling a storm of bubbles – has been around since before Cousteau’s first scuba dive. Military and commercial divers have used rebreathers since the turn of the century. Interest in rebreathers among sport divers has snowballed recently as advances in electronics and computing have made possible designs that are safer and more functional than ever before. The result may be nothing less than a revolution in the way we dive. At least for some of us.

Two Types of Rebreathers

For each of these goals, there is a corresponding rebreather configuration. All rebreathers can be grouped into two primary design categories: closed-circuit rebreathers (CCRs), which release bubbles only on ascent, and semi-closed rebreathers (SCRs), which release an intermittent stream of small bubbles.

Both systems employ a chemical scrubber to remove carbon dioxide (CO2) from a diver’s exhaled breath, which is then returned to a breathing bag. A modern CCR unit uses sensors to maintain a constant pressure of oxygen in the breathing bag by adding small amounts of oxygen or a diluent gas (usually air) as needed.

An SCR, on the other hand, feeds premixed nitrox (oxygen-enriched air) into the system either at a constant rate (active addition) or at a variable rate keyed to the diver’s breathing rate (passive addition). An SCR “burps off” the excess through a discharge valve. Because most of the gas is being recycled, the amount of gas released as bubbles is much less than with open-circuit scuba – usually around one-fifth – and a cylinder of gas therefore lasts correspondingly longer.

While the two systems are easier to operate and much safer than earlier designs, both require rigorous training, self-discipline and a significant cash outlay. Clearly, rebreathers are not for the great majority of recreational divers. But there is enough demand from certain segments of the diving community – scientists, photographers, cave divers, extended-range divers and gadget-heads – that several manufacturers have decided to make “recreational” rebreathers available to sport divers.

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