 A large diameter cylinder tends to move the diver's center of mass further from the centreline, making them unbalanced in the water, and a higher pressure cylinder has a similar effect, also reducing the buoyancy of the diver, due to the thicker metal required for strength.Cylinder length is also limited by ergonomic considerations in proportion to the height of the diver.A single cylinder also presents a critical single point of failure for the breathing gas supply.