Seven Seas | Scuba Diving Resources

Scuba Diving Resources: open-circuit and diving health questions answered: Here the diver breathes in from the set and out to waste. This type of equipment is relatively simple, making it cheap and reliable. The duration of open-circuit dives is shorter than a rebreather dive, in proportion to the weight and bulk of the set. It can be uneconomic when used with expensive gas mixes such as heliox and trimix. Most divers use standard air. The cylinder is nearly always worn on the back. "Twin sets" with two backpack cylinders were much more common in the 1960s than now; although twin cylinders (aka "doubles") are commonly used by technical divers for the increased duration they provide. Submarine Products sold a sport air scuba with 3 backpack cylinders. Sometimes cave divers have cylinders slung at their sides instead.

Closed-circuit/semi-close circuit (also referred to as a rebreather). Here the diver breathes in from the set, and out back into the set where the exhaled gas is reprocessed to make it fit to breathe again.

Since 80% or more of the oxygen remains in normal exhaled gas, and is thus wasted, rebreathers use gas very economically, making longer dives possible and special mixes cheaper to use at the expense of more complicated technology and more experience and longer training. There are three variants of rebreather: oxygen, semi-closed circuit, and fully-closed circuit rebreathers.

*source: Wikipedia, used with permission