Semi-closed circuit

Semi-closed circuit rebreather systems are a type of scuba diving equipment that allows divers to recycle part of their exhaled gas. This technology is an intermediate step between open-circuit scuba, where all exhaled breath is expelled into the water, and closed-circuit rebreathers (CCRs), which recycle the entire breath. Semi-closed rebreathers (SCRs) offer advantages in gas efficiency and can provide longer dive times and reduced decompression obligations compared to open-circuit systems.

Overview

A semi-closed circuit rebreather works by partially recycling the diver's exhaled breath. The system removes carbon dioxide (CO2) from the exhaled gas using a chemical sorbent, and then adds fresh oxygen to replace the oxygen that was consumed. The mix of recycled gas and added oxygen is then available for the diver to breathe again. Unlike closed-circuit rebreathers, SCRs release a portion of the exhaled gas into the water, either continuously or at intervals, which means they do not conserve inert gases (such as nitrogen in air) as efficiently as CCRs.

Types of Semi-Closed Circuit Rebreathers

There are several types of semi-closed circuit rebreathers, categorized by their method of gas addition and exhaust:

1. Passive addition systems: These rely on the diver's breathing pattern to trigger gas addition. As the diver inhales, a slight negative pressure adds fresh gas into the system. 2. Active addition systems: These systems add gas at a constant rate or in response to pressure sensors, independent of the diver's breathing. 3. Demand-controlled systems: Gas is added to the loop in response to the diver's inhalation effort, similar to a traditional regulator.

Advantages

• Gas efficiency: SCRs use gas more efficiently than open-circuit systems, allowing for longer dive times with smaller gas supplies.
• Silent operation: With less frequent gas release, SCRs are quieter than open-circuit systems, which is beneficial for wildlife photography and avoiding underwater noise pollution.
• Warmth: The recycled gas in the loop is warmer than the surrounding water, providing additional warmth to the diver.

Disadvantages

• Complexity: SCRs are more complex than open-circuit systems, requiring additional training and understanding of the equipment.
• Partial gas recycling: Since SCRs do not recycle all exhaled gas, they are less efficient in gas use than CCRs, especially for dives requiring significant amounts of inert gas.
• Monitoring: Divers must closely monitor their oxygen levels and scrubber performance to avoid hypoxia or hypercapnia.

Applications

Semi-closed circuit rebreathers are used in a variety of diving applications, including recreational, technical, and military diving. They are particularly favored for applications where longer dive times and reduced gas consumption are beneficial, and where the silence of operation offers an advantage.

Safety Considerations

Divers using SCRs must be trained in their use and maintenance, including understanding the risks of hypoxia, hypercapnia, and oxygen toxicity. Regular maintenance and pre-dive checks are crucial to ensure the safety and reliability of the system.

See Also

• Rebreather
• Scuba diving
• Underwater diving