A closed circuit or rebreather uses a cylinder of which substance in its operation?

In a closed circuit or rebreather system, the primary substance used is oxygen. These systems are designed to provide breathable air to the user for extended periods without releasing used air into the environment. Instead of using compressed air, which contains nitrogen and other gases, rebreathers recycle exhaled gases, removing carbon dioxide while replenishing the oxygen that the user consumes.

The device works by having the user inhale oxygen from a cylinder, which is then mixed with the user’s exhaled air. The exhaled air passes through a scrubber that removes carbon dioxide, and the oxygen content is monitored to ensure adequate levels remain for breathing. This design allows for a much longer operational duration compared to traditional systems that vent exhaled gases, making it suitable for applications such as diving, firefighting, and search and rescue operations in environments where air supply is limited.

The other substances, like carbon dioxide and carbon, do not serve the purpose of supporting human respiration in a rebreather context, while compressed air is used in open circuit systems rather than closed circuits.