The purpose of the DSV (Dive/Surface Valve) is to produce a one way flow of gas through the breathing loop. This is accomplished through the use of two one-way mushroom valves. When the diver inhales, the exhale valve closes and the inhale valve opens. When the diver exhales, the inhale valve closes and the exhale valve opens. It is extremely important that the integrity of the mushroom valves be maintained. These can quickly and easily be inspected between dives.
Any time the diver is not actively breathing on the loop, the DSV should be closed by pulling down on the lever arm until it is rotated to point straight down. It is important to close the valve before removing the mouthpiece from your mouth while underwater, or on the surface, to prevent water from entering the breathing loop and flooding the unit. Minor amounts of water in the loop can be dealt with by flushing it out of the exhalation counterlung OPV (this is a skill you will learn in class), however a major flood will not be recoverable during the dive and will require bailing out to an independent open circuit system.
If you are switching back to the DSV from a different breathing source while underwater, you will need to clear the small amount of water in your mouth and the mouthpiece before opening the DSV. Do this by exhaling through the DSV mouthpiece for several seconds before opening the lever. There is a small purge hole in the bottom of the DSV that allows removal of this water before opening (See Figure 31).
You will hear or feel air bubbles escaping once the water is cleared. Then you can open the DSV by rotating the lever until it is fully opened and pointing straight forward.