Ballast tank with water pumps instead of compressed air

[EliotBR (Eliot)]

Hi @ninja, welcome to the forum

@ninja

To resurface, I plan to just open a valve that is placed under the tank so the compressed air inside would push the water out and the buyoancy would increase for robot to rise.

As the vehicle dives the pressure outside increases, so if you take in some water near the surface to reduce buoyancy, then sink a bit, it’s very likely the pressure outside the tank will be greater than the pressure inside, so opening a vent/valve will just take on more water (rather than pushing out the existing water).

If by “compressed air inside” you mean there’ll be an additional pressurised canister of CO₂ gas or something then bursting that could help to increase the pressure in the tank, but the larger your tank is and the deeper the vehicle is the more CO₂ you would need to be able to push water out through a valve.

If you haven’t already I’d recommend reading through this thread from the start - the discussion covers several aspects of the practicalities of a pump-based buoyancy/ballast system.

[Mudfoot]

Physics side of me wants to say this should be possible “if” you can start off with a perfect vacuum in your tank/tanks and have a pump that is capable of pumping fluid and also being that same pump to pull back down to its original perfect vacuum you started with after you pump in and out your ballast fluid. I would have to try it myself to prove it but seems achievable in my mind.