This is the Poseidon SE7EN Mouthpiece Rebreather Features: Closed-circuit to open-circuit in one simple motion Mouthpiece is compact and lightweight DSV - bubble free diving with low work-of-breathing BOV - high-performance open-circuit regulator ADV - compensates for counterlung compression One of the unique components of the Poseidon rebreather is its open circuit/closed circuit switch-able mouthpiece, that incorporates automatic diluent addition (ADV) functionality.