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How would you guys recommend making a variable ballast system for an underwater robot? I was thinking about this problem earlier and I was trying to figure out if there was a way to make one that didn't require a tank of compressed air.

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Variable ballast on ROVs in practice is usually limited to just the really big ones, for several reasons:

  1. In general, you only need to vary your ballast when you're trying to get an exact amount of buoyancy at great depth -- when that amount would be different than at the surface (due to compression of various components)
  2. The pressure housing required adds a lot of size and weight
  3. The power required for the pump or compressor is significant
  4. It requires more maintenance

That's not to say it can't be done on a small scale, for slight adjustments to ballast. Underwater gliders use variable ballast as "buoyancy engines".

Depending on your specific application, you might get away with one of the following alternatives:

  1. Descending with a dropweight, then shedding it at the desired depth (repeat for rapid ascent)
  2. Having a canister of lead shot, and dropping the balls in a controlled fashion until you have reached the desired buoyancy -- useful if you are going to collect samples
  3. Use more powerful motors and actively compensate for buoyancy
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