I'm thinking there is a way to improve the dump section. In reality I think it will take some skillful grinding, drilling and honing to make the trigger operate smoothly. It's going to weigh very little underwater since its just slightly negative, so if the trigger isn't just right its going to the bottom and waiting there for eternity. A better trigger needs to be thought up that takes little effort to hold a significant spring pressure. Think about a mousetrap. Enough force to kill a mouse, but triggered with a nibble. Also it would be nice if the rod goes up through the center instead of the side. Besides looking better, as it rises the triggerplate will act like fins in the rear or a weathervane and hopefully make it rise as straight as possible. If it's out to the side it might ascend with a crazy spiral.

With the air chamber on one end and the permanent ballast on the other end, I think it should stay reasonably straight up and down. The air chamber could be permanently sealed. Before you glue the end cap on though, what if you drill a nice smooth round hole and pull an automotive (or semi-truck) valve stem through it. Then, you could pressurize the chamber. Schedule 40 PVC holds what, 300psi? Thats about 655feet! And even at that depth, the pressure is only equalized, you could dive much deeper still! If you went that route though, pressurization would be required if you went to any depth or else the valve stem would be sucked (or blown or whatever) inside the chamber. Here's a waterdepth/pressure calculator for salt water:

[link=http://www.gazza.co.nz/waterpressure.html]Pressure Calc.[/link]

I was thinking of a way to record the maximum depth. How about this:

[link=http://i5.photobucket.com/albums/y177/dustyatticx/Depthrecorder.jpg]Depth Recorder[/link]

Its just a pressure gauge from 0-50 psi or whatever range you plan on diving in. The face is removed and attached to the needle is an extension arm which has a felt tip marker on the tip. The rear of the gauge has cardboard attached. As the gauge moves, its marks the cardboard. In the pic that you see, you have just opened the instrument package and see that the capsule has decended to XXX depth by the red line.

The cost should be minimal. Whats a joint of PVC cost? Primer and glue, some fittings. Some aluminum rods and stuff. Any local hardware store should have everything needed. If I come up with a trigger that I am satisfied with I might make one of these things to see what happens. I just need someone with a boat to go to the lake with to test it.