The screw on the front of the Argonaut DSV serves two purposes. Prevents lateral movement and is a positive stop when it is either fully open or fully closed.
I've had several requests from divers wanting to lock their DSV either the open or closed position. In the picture below is an inexpensive and easy solution. It's simply a nylon thumbscrew with a #8 O-ring on it. Put it in place of the phillips screw that came on your DSV originally and by putting more tension on the thumbscrew it will be much more difficult to turn. All new Argonaut DSV's and Argonaut Kraken regulators will have one included with them as an option for the diver if they want to use it.
Existing DSV Divers can get one simply by asking for it in the notes section of their next order though the website store.
In "theory" and if one was so inclined.. You could easily put shallow round divots at the fully open and closed position for an absolute positive lock....Of course this would be up to the diver to do and no DSV will be shipped with divots in it.....IF I were doing it, I would be 110% sure the diviots were in the correct place to have the mouthpiece fully open or closed when the screw was turned in or the DSV will leak like a screen door on a submarine!
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Doing it right should include some common sense, not just blindly following specs and instructions. .Gary D, AWAP on SB