Another comment; if you insist on a variable style dive, we came up with an easy method, which doesn't involve any weights. You need three divers to do it.
You need a piece of rope about three or four metres long, with loops on each end.
An experienced diver holds one end of the rope. The novice diver holds the other end (this is the diver who wants to go deep).
Then, the experienced diver swims down and PULLS the novice down to about 20m (this take lots of energy!) Then, the novice lets go of the loop and continues to depth. The experienced diver goes back up, and the 3rd diver at the surface checks to make sure the experienced diver doesn't black-out. Then, the 3rd diver descends to spot the novice diver coming up from deep.
In this way, the novice diver gets a 'variable' weight style dive without the problems of ballast.
Still, I don't recommend this or any variable weight system for the same reason I said before.
Eric Fattah
BC, Canada