What is a key difference between fully-closed circuit and semi-closed circuit rebreathers?

The key difference between fully-closed circuit and semi-closed circuit rebreathers lies in how they manage gas within the system, particularly oxygen usage. In a semi-closed circuit rebreather, oxygen is actively added to the system as the diver consumes it during the dive. This ensures that the diver maintains an adequate partial pressure of oxygen in the breathing gas.

In contrast, a fully-closed circuit rebreather recycles all the exhaled gases, removing carbon dioxide and only adding the necessary amount of oxygen to maintain the desired levels, without releasing bubbles into the surrounding water. This results in a less visible presence to the dive environment and can enhance dive duration and efficiency.

Understanding the operational differences is crucial for divers, as it affects aspects of dive planning, gas management, and overall diving technique. The function of adding oxygen in real-time in semi-closed systems is essential for maintaining appropriate gas levels for safe breathing underwater.