Diving Rebreathers

Open-circuit self-contained underwater breathing apparatus (SCUBA) has been used by recreational, exploration, scientific, and military divers since the advent of the technology by Cousteau and Gagnon shortly after World War II. At the most fundamental level, an underwater breathing apparatus can be delineated into surface-supplied and self-contained units. Surface-supplied divers use an umbilical to provide breathing gas to the diver, whereas SCUBA divers always carry their life support equipment with them. SCUBA systems are further classified as open-circuit or closed-circuit types. Open-circuit dive equipment provides the diver with breathing gas from a container (or tank) of compressed gas, allowing the diver to exhale directly into the environment. Although simple and easy to maintain, this system is inefficient, as a large volume of inert gas is exhaled to the environment, and a substantial amount of oxygen remains unused. With a closed-circuit breathing apparatus, commonly known as a rebreather, exhaled gas is collected, carbon dioxide is removed, oxygen is replenished, and the refreshed gas is recirculated back to the diver through a breathing loop. The oxygen consumed by the diver through metabolic consumption is the only thing that must be replaced to continue to sustain the diver, whereas carbon dioxide must be removed. This system is inherently more efficient, up to 50 times more efficient for a given gas supply, but also more complex, requiring constant monitoring of the gas in the system to ensure it can sustain life. Rebreathers have been used since ancient times, when humans first discovered that they could extend the duration of a free dive by breathing in and out of a leather bag. Over 100 years ago, more sophisticated models were developed to allow miners to escape from contaminated atmospheric conditions and were used with varying degrees of success. During World War II, rebreather technology was adapted to enable combat divers to operate in enemy harbors without releasing surface bubbles that could reveal their position. The challenges associated with rebreathers include the potential for hypercarbia, oxygen toxicity, and hypoxia. Before more technically advanced devices became available in the 1980s and 1990s, blackouts due to various gas problems were a common complication. In the 1990s, several commercial units became available on the civilian market, leading to a significant increase in civilian diver use in the early 2000s. Today, a wide variety of rebreather systems is available, designed to meet the diverse demands of various diving environments and applications.