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Re: [PSUBS-MAILIST] Newbie

And if you can keep your air volume constant you won't have to change your ballast too much as you descend or ascend. 
 
The buoyancy from a submerged volume of air doesn't change, unless the density of the water and/or air changes.  Which happens.  Anyway, the lifting force of air in seawater should stay close to 64 lb/cubic foot, or 1025 kg/cubic meter.
 
Now if you were talking about a balloon filled with air, where the volume could change, then you'd see the behavior you suggest, except reversed.  As the balloon goes deeper, the water pressure will compress the air, and the lift will decrease.  A cubic meter of air at the surface would compress to half its volume (and lift) at a 10m depth.
 
Any fact checking on this would be good.  :-)