The main advantage of the rebreather over open circuit breathing equipment is
economical use of gas. With open circuit scuba, the entire breath is expelled
into the surrounding water when the diver exhales. A breath inhaled from an open
circuit scuba system whose cylinders are filled with ordinary air is about 21% oxygen.
When that breath is exhaled back into the surrounding environment, it has an
oxygen level in the range of 15 to 16% when the diver is at atmospheric pressure.
This leaves the available oxygen utilization at about 25%; the remaining 75% is
lost. As the remaining 79% of the breathing gas (mostly nitrogen) is inert, the
diver on open-circuit scuba only uses about 5% of his cylinders' contents.
At depth, the advantage of a rebreather is even more marked. The diver's
metabolic rate is independent of ambient pressure , and thus the oxygen
consumption rate does not change with depth.
The production of carbon dioxide does not change either since it also depends on
the metabolic rate. This is a marked difference from open circuit where the
amount of gas consumed increases as depth increases since the density of the
inhaled gas increases with pressure, and the volume of a breath remains almost
Long or deep dives using open circuit scuba equipment may not be feasible as
there are limits to the number and weight of diving cylinders the diver can carry.
The economy of gas consumption is also useful when the gas mix being breathed
contains expensive gases, such as helium. In normal use at constant depth,
only oxygen is consumed: small volumes of inert gases are lost during any one
dive, due mainly to venting of the gas on ascent. For example, a closed
circuit rebreather diver effectively does not use up any diluent gas after
reaching the full depth of the dive. On ascent, no diluent is added, however
most of the gas in the loop is lost. A very small amount of trimix could
therefore last for many dives. It is not uncommon for a 3 litre diluent
cylinder to last for eight 40 m dives.