Each manufacture basically has their own version of these and some are available aftermarket from secondary manufacturers.
DSV = dive surface valve
the mouthpiece of a CCR that you breath on, has a switch to close it so if the loop doesn't flood with water if your remove it from your mouth
With a DSV, you close the loop completely and have to physically unclip your bailout reg from your right shoulder strap and swap it to your mouth *over* your DSV and put it in your mouth. If you can't breathe momentarily while task loaded, it's not the quickest solution to get to your next breath. Valuable skill to learn? Yes. Of course. Not the most efficacious way to do it, however.
BOV = bailout valve
incorporates an open circuit regulator into the dsv, switches from rebreather loop to open circuit with a knob without taking the mouthpiece out of your mouth
Yes! My yankee-doodle
Hammerhead Extreme came with a BOV. The HH rig I trained on in Mod1 had a DSV. Much better way to go, but comes with its own potential issues, particularly if you are using 2 litre cylinders. Not much breathing gas (dil) available and lessens considerably with depth. So, turn the knob, breathe dil for one nice deep breath *while* unclipping your bailout reg and getting on it asap. It minimizes the risk of flooding your loop and adds a component of relaxation to the bailout switch that is not present when switching from a DSV. But, if your rig doesn't come with a BOV, then it's an expensive upgrade to a Golem Shrimp BOV.
ADV = automatic diluent valve
automatically adds diluent to the loop as it gets crushed by increasing depth
As a novice, I did not like this on the HH. The way it was set up on the HH, an out of trim angle of attack (pitch) would sometimes activate it in the most unwanted situations. Roll axis would also fire it off sometimes too. I discussed this with my instructor when configuring the order for my unit and elected not to get it. I had asked about putting an inline shutoff between the manifold and the ADV, so I could easily disable it and fly the dil manually to start out, slowing learning to work with the ADV as I became more stable with my trim (pitch and roll). Ultimately, I just went with MAV's for both dil and O2. I think I remember reading that Randy from HH uses MAV and thinking what's good for the goose is good for the gander. But Everyone here will have an opinion and preference regarding this one... all of them valid...
MAV = manual add valve
a button to either add o2 or diluent to the loop manually. commonly the dil MAV is on one side of your body and the O2 MAV on the other so its harder to get them mixed up.
