‘Fish have scales, shark have teeth and divers have bubbles. Bubbles are the ‘white noise’ of diving that goes virtually unnoticed. Like the constant drone of traffic to a city dweller. We can hardly imagine a life underwater without this ever present cacophony whizzing past our ears. That is, at least, until you make your first bubbleless dive.’ Alex Brylske, The Complete Diver
Sixty years of open circuit systems show they are safe, relatively inexpensive and overall work well. But open circuit scuba has one disadvantage: They waste a lot of gas. Humans only use a small portion of the oxygen that we inhale. At the surface only a few percent of the air we breathe in is used in our bodies and at depth even less.
Most of the air in the diver’s scuba cylinder goes unused and the exhaled air is lost as bubbles. A fit person working hard may breathe approximately 100 litres of oxygen per minute but will only metabolise a maximum of 6 litres per minute. At depth this amount decreases due to the increase of partial pressure.
Rebreathers reduce the amount of air loss through reusing the wasted oxygen. It recaptures the exhaled air, removes the carbon dioxide and prepares it to be breathed again in a closed loop, thus its reference as a ‘closed circuit system’
Although perceived as relatively new technology the rebreather was in existence long before Jacques Cousteau.
The first successful scuba was built in 1878 by an Englishman named Henry Fleuss. His equipment included a rubberised fabric hood, a breathing bag, a backmounted copper cylinder containing oxygen and a carbon dioxide scrubber. Although basic the system actually worked quite well.
In the early days of diving rebreathers took a backseat to the more familiar systems of helmets, hoses and compressors. In WWII the need for no bubble underwater operations breathed the life back into rebreathers (sorry, I couldn’t resist the pun). The system was refined by British Royal Navy researchers Kenneth Donald and J.B.S. Haldane – son of diving’s most famous decompression researcher.
Encouraged by the heroic exploits of the British and Italian frogmen, rebreather technology was well refined and much used by the time Cousteau and Gagnan perfected their aqualung. As it turns out Cousteau had nearly drowned himself whilst testing a rebreather of his own design, causing him to abandon closed-circuit systems, had this not happened the face of modern scuba could have been vastly different today. We might have all been diving bubble-free for years!
Rebreather technology is used in many different environments:
- Underwater – Known as ‘closed-circuit rebreather’ (CCR) ‘semi-closed rebreather’ ( SCR) or ‘closed circuit underwater breathing apparatus’ (CCUBA – a military term) or simply ‘rebreather’
- Mine rescue and other rescue workers such as firefighters- where poisonous gas may present itself or where oxygen may be lacking or completely absent
- Crewed spacecraft and space suits – Outer space is a vacuum without oxygen to support life. All techniques of supplying astronauts with air work exactly as a rebreather; cleaning out the carbon dioxide, reusing the air and topping up with oxygen when needs be.
- Hospital anaesthesia breathing systems – Used to supply controlled concentrations of an anaesthetic to a patient without contaminating the air the staff breathe.
- Himalayan mountaineering – High altitude reduces the partial pressure of the oxygen in the air, which reduces the ability of the climber to function effectively. Mountaineering rebreathers provide a higher partial pressure of oxygen to the climber
- Submarines, underwater habitats and saturation diving – These systems use a scrubber system working on the same principle as a rebreather.
To allow you to rebreathe your air the rebreather must do the following:
- Remove your exhaled carbon dioxide. This is done by a canister of sodium hydroxide. The carbon dioxide reacts with sodium hydroxide and calcium hydroxide to form calcium carbonate (solid)
- Replace oxygen that you have used. Small tanks of pure oxygen or mixed gases inject fresh air into the breathing loop.
- Control the oxygen level in the breathing loop. Solid state oxygen sensors monitor the partial pressure of oxygen in the breathing loop and send this information to a microprocessor that controls the oxygen-delivery system.
There are 3 different types of rebreathers available:
- Oxygen rebreathers. Carry a cylinder of pure oxygen as the only gas supply. They’re limited to ‘no decompression’ depths and carry a danger of oxygen toxicity.
- Semi-closed circuit rebreathers. Use gas mixtures as the gas supply. A diver using a semi-closed rebreather can go to greater depths without risking oxygen toxicity.
- Closed-circuit rebreathers. Carry both pure oxygen and mixed gases. They differ from semi-closed circuit rebreathers in the way that they maintain the oxygen concentration.
Rebreathers are considered to be advanced scuba gear, originally developed and typically used by the military. Advanced and commercial divers may use rebreathers, although intensive training is strongly recommended.