Ballast system for rov



I saw a ballast system made out of a bicycle tube, an air compressor, and some valves. Does someone knows how can i make it?


I’ve seen those devices working on RC submarines models. Its made from what looks like a syringe.
When the piston moves, air volume changes, and hence buoyancy. Anyway, the working depth range of those devices is very limited.

In order to hold a given depth, hydrostatic equilibrium must be achieved, and that only depends on the relation between weights and buoyancy. At the same time, the only way to control buoyancy with these devices is by means of changing volume.
Hence, given that water is uncompressible, the volume of the tank, and hence, internal prssure, will be the same at any depth in order to achive the hydrostatic balance.
It means that external pressure working on the tank walls and the external face of the piston will increase with depth.
Two problems arrive:
1-) Any distortion on the syringe shape will lead to buoyancy loss, water leakeages and/or to the piston to get stuck, making impossible any further manouver.
2-) If no distortion happens, force required in order to move the piston “out” of the syringe has increased proportionally to depth.

For low depths, were pressure differential can be taken as negligible, it works fine, but for the usual depth range of Rovs, I see that system possible , but not practical as there are better solutions.