Diving Rebreathers

Excerpt

SCUBA, or Self-Contained Underwater Breathing Apparatus, has been in use by recreational, exploration, scientific, and military divers since the advent of the technology by Cousteau and Gagnon 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 make use of an umbilical to provide breathing gas to the diver, while SCUBA divers carry their life support equipment with them at all times. SCUBA systems themselves are further classified as open circuits or closed circuits. Open circuit dive equipment provides the diver breathing gas from a container of compressed gas (a SCUBA tank), and the diver exhales directly to the environment. While 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 goes unused. With a closed-circuit breathing apparatus (commonly known as a rebreather), the exhaled gas is scavenged, scrubbed of carbon dioxide, enriched with oxygen, and then sent back to the diver in the breathing loop. The oxygen consumed by the diver via metabolic consumption is the only thing replaced to continue to sustain the diver. This system is inherently more efficient (perhaps as much as 40 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 is capable of sustaining life.

Rebreathers have been in use since antiquity when man first discovered that by breathing in and out of a leather bag, he could extend the time of a free dive. More sophisticated models were developed over 100 years ago to allow miners to escape from contaminated atmospheric conditions and used with varying degrees of success. During World War II, rebreather technology was adapted to allow combat divers to work in enemy harbors without showing surface bubbles, which would give away their position. The challenges associated with rebreathers include the potential for hypercarbia, oxygen toxicity, and hypoxia. Prior to technically advanced devices, blackout due to the various gas problems was a common complication. In the 1990s, several commercial units became available on the civilian market, and this led to a significant increase in civilian diver use in the early 2000s. There are now many types of rebreathers available to meet the diverse needs of different diving environments.