Also, As you get into the static diving subs, there are two different ways to do this. Either a gas system (like Propel) that uses a can of compressed gas that can be used to clear water from a ballast tank with an opening at the bottom and a valve that lets the air out up top. And a closed tank with a piston that moves water in and out to change the ballast level.

Pros and cons to both systems, but you can get a sub set up with either system.

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