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The DSV stands for Diver Supply Valve and in simple terms is the mouthpiece for the loop. There will always be some sort of lever that opens the mouthpiece to the loop (on closed circuit) and shuts it again to prevent water leaking into the unit as part of all rebreather components. The DSV can also be integrated with a BOV (Bail Out Valve) which connects the mouthpiece directly to an open-circuit system. If a diver suspects problems with the quality of gas they are breathing they can switch the lever into an open-circuit mode which will connect directly to the diluent tank and allow them to breath as if they were on normal scuba. More and more rebreather models are entering the market with a BOV integrated into the DSV.
Rebreather Component 6: The BOV
Although a Bailout Valve (BOV) was not one of the standard rebreather components on past models of CCRs, newer models offer them as part of the regular package. The BOV is a second stage regulator integrated into the DSV with a direct hose line to the diluent tank on the rebreather.
In an emergency situation, when a CCR diver experiences problems with the gas quality in his/her loop, the CCR diver needs to switch over to open-circuit and start to head for the surface. Traditionally, rebreather divers have carried along one or more bailout tanks with a normal stage regulator attached. When suspecting a problem on the loop, the CCR diver would have to deploy the regulator out of the bungee on the tank, close the rebreather DSV, and put in the second stage regulator. More and more research into CCR emergencies show this complex bailout strategy being too much for a CCR diver breathing a bad gas. Coordination and muscle dexterity are some of the first reflexes lost when poisoned by deadly gases, such as carbon dioxide or oxygen. Therefore switching only the DSV to a closed position and having a direct bailout regulator on the mouthpiece reduces diver effort and gets them breathing a safe gas faster.
The BOV doesn’t reduce the need for a stage regulator on a bailout tank, as this is needed for other emergency situations, but it does put the CCR diver one step closer to a safe breathing gas.