Those of you who have watched James Cameron’s cave diving movie ‘Sanctum’ will have probably left the theater going, “man rebreathers are so cool!” or had the complete opposite reaction “there’s no way I’m ever going use a rebreather to scuba dive!” A Rebreather is a complex breathing apparatus, that isn’t as commonly used as its cousin, the conventional open circuit breathing apparatus or Scuba primarily due to the cost and additional training required to use one. This has led to much mystery and fascination with the Rebreather which is why we will take a closer look at this breathing apparatus.
Conventional SCUBA diving uses an open circuit breathing apparatus, which means that the air we inhale into our lungs from the tank is dispelled through the regulator in our mouth into the water as we exhale. The problem with open circuit breathing is that it wastes precious gas. Out of the air we fill into our scuba tanks, 80% of it is Nitrogen and other gases, and only 20% of the air is Oxygen. Out of that 20% of Oxygen in our dive tanks, our lungs only absorb a quarter of the Oxygen contained in the air for each lungful we take, which means divers exhale 3/4ths of the total oxygen available to them each dive. This means despite carrying all that air in our tanks, we are only really consuming a tiny fraction approximately 5% of the total air (oxygen) and exhaling everything else.
A Rebreather – also known as a Closed Circuit Breathing Apparatus or CCUBA or a CCR (Closed Circuit Rebreather) – allows a diver to breathe the exhaled air, again and again while constantly removing the CO2 while replenishing Oxygen into the system as necessary. Oxygen or a Gas Mix, is fed electronically or using a demand valve into a closed circuit system. The air goes into the Diver’s lungs and the Carbon Dioxide rich air is breathed out into a separate one -way-valve path into a counter lung which is a bag that expands and contracts like a lung that stores exhaled air, allowing the air in the loop to remain the same. The air from the counterlung is then pulled into a Carbon Dioxide Scrubber, which is a container filled with a CO2 absorbing Chemical usually sodium hydroxide (Sofnolime). The carbon dioxide (gas) reacts with sodium hydroxide and calcium hydroxide to form calcium carbonate (solid). The Oxygen or Gas mix that is released from the tank into the loop is controlled by oxygen sensors that monitor the partial pressure of oxygen in the breathing loop and send this information to a microprocessor that controls the oxygen-delivery system.