Page "Scuba set" Paragraph 81

A semi-closed circuit rebreather may have an endurance of about 3 to 10 times that of the equivalent open circuit dive, and is less affected by depth ; gas is recycled but fresh gas must be constantly injected to replace at least the oxygen used, and any excess gas from this must be vented.

Although it uses gas more economically, the weight of the rebreathing equipment means the diver carries smaller cylinders.

Still, most semi-closed systems allow at least twice the duration of open circuit systems ( around 2 hours ) and are often limited by scrubber endurance.