A divers reg doesn't "Use" air.A typical divers air cylinder runs at 232bar(Air only) .The "First stage" (the bit which fits to the cylinder)reduces this pressure to ,if memory serves me right,100bar.The "Second stage" or regulator (the bit you put in your mouth)again reduces this pressure to that of the surounding water.When you breathe in it then supplies air at this pressure.The amount of air that you breathe depends on how deep you go and how fit you are.The deeper you are the more air you will use.