Closed Circuit Scuba (Rebreather Diving)

Closed circuit scuba, also known as rebreather diving, is a method of underwater diving that uses a specialized apparatus called a rebreather. This device allows divers to recycle their exhaled air by removing carbon dioxide and adding supplemental oxygen, providing a more efficient and environmentally friendly diving experience. Unlike traditional open circuit systems, closed circuit scuba diving is noiseless and produces no bubbles, making it an ideal choice for scientific research, underwater photography, and military operations. This entry will discuss the history, components, types, advantages, and safety considerations of closed circuit scuba diving.


The concept of a rebreather dates back to the early 18th century when Englishman Stephen Hales invented a device to recycle exhaled air. The first practical rebreather was designed in 1878 by Henry Fleuss, a British engineer who used the apparatus to perform maintenance on underwater structures. Over the years, rebreathers have evolved from rudimentary contraptions to the sophisticated devices used in closed circuit scuba diving today.


A rebreather is composed of several key components, including:

  1. Mouthpiece: The diver breathes through the mouthpiece, which is connected to the loop hose. The hose transports the exhaled air to the scrubber canister and returns the recycled air back to the mouthpiece.
  2. Scrubber Canister: This essential component contains a chemical absorbent, usually a granular substance called soda lime, which removes carbon dioxide from the exhaled air.
  3. Oxygen (O2) Cylinder: The O2 cylinder supplies the rebreather with a constant flow of oxygen to replenish the amount consumed during diving. This allows for longer dive times compared to open circuit scuba systems.
  4. Diluent Cylinder: A diluent gas, usually air or a nitrox mixture, is stored in the diluent cylinder. This gas is added to the loop to maintain a breathable mixture and counteract the compression effect of depth.
  5. Electronics: Modern rebreathers use advanced electronics to monitor and control gas mixtures, ensuring the optimal balance of oxygen and diluent.


There are three main types of rebreathers used in closed circuit scuba diving:

  1. Semi-Closed Circuit Rebreathers (SCR): SCRs provide a constant flow of gas to the diver, recycling a portion of the exhaled air while venting the remainder. This type is less efficient and produces some bubbles, but is simpler to use compared to a fully closed circuit system.
  2. Closed Circuit Rebreathers (CCR): CCRs fully recycle the exhaled air, only venting gas in cases of over-pressurization. These systems require precise control of gas mixtures, making them more complex but offering longer dive times and greater efficiency.
  3. Passive Semi-Closed Rebreathers (PSCR): PSCRs combine elements of both SCR and CCR systems, using a passive mechanism to control the flow of gas. This design simplifies the diving experience while maintaining many of the advantages of closed circuit systems.


Closed circuit scuba diving offers numerous benefits compared to traditional open circuit systems:

  1. Increased Dive Times: By recycling exhaled air, rebreathers dramatically extend the supply of breathable gas, allowing for longer and deeper dives.
  2. Noise Reduction: The absence of bubbles and exhaust noise makes closed circuit diving ideal for observing marine life, underwater photography, and covert military operations.
  3. Reduced Decompression Obligations: The controlled gas mixtures provided by rebreathers help minimize nitrogen absorption, reducing the risk of decompression sickness and shortening decompression times.
  4. Conservation: Closed circuit systems are more environmentally friendly, as they produce fewer bubbles and consume less gas compared to open circuit systems. This reduced gas consumption also lowers the cost of diving over time.
  1. Improved Buoyancy Control: The absence of bubbles and reduced gas consumption make it easier for divers to maintain neutral buoyancy, providing a more stable and comfortable diving experience.

Safety Considerations

While closed circuit scuba diving offers numerous advantages, it also presents unique challenges and potential hazards:

  1. Complexity: The management of gas mixtures and the maintenance of rebreather systems require additional training and expertise compared to open circuit diving. Divers must be prepared to handle potential malfunctions and understand the appropriate protocols in case of an emergency.
  2. Oxygen Toxicity: The precise control of oxygen levels is crucial, as exposure to excessively high concentrations can lead to oxygen toxicity, a dangerous condition that can cause seizures, unconsciousness, and even death.
  3. Hypercapnia: Failure to properly remove carbon dioxide from the exhaled air can result in a buildup of CO2, leading to hypercapnia, a condition characterized by dizziness, confusion, shortness of breath, and potentially unconsciousness.
  4. Equipment Maintenance: Rebreathers require meticulous care and regular maintenance to ensure optimal performance and safety. Divers must diligently inspect, clean, and replace components as needed, particularly the scrubber canister and chemical absorbent.
  5. Training and Certification: To mitigate the risks associated with closed circuit scuba diving, divers should undergo comprehensive training and obtain certification from a reputable organization, such as the Professional Association of Diving Instructors (PADI) or the International Association of Nitrox and Technical Divers (IANTD).

In conclusion, closed circuit scuba diving, or rebreather diving, is an advanced method of underwater exploration that offers several advantages over traditional open circuit systems, including increased dive times, noise reduction, and environmental conservation. However, the complexity and potential hazards associated with rebreathers necessitate proper training, certification, and equipment maintenance. By adhering to these safety measures, divers can enjoy the unique benefits of closed circuit scuba diving while minimizing risks and ensuring a safe and enjoyable underwater experience.