BUOYANCY

    As applied to scuba diving, the term buoyancy describes a diver’s relative floatiness¹.  The principle that defines relative floatiness is Archimedes principle².  This principle states that a body immersed in a fluid is buoyed up by a force equal to the weight of the displaced fluid. In the case of scuba, the fluid in question is water.  It’s important to note that salt water is heavier than fresh.  If the weight of the water displaced is greater than the weight of the object, the object will float. If the weight of the water displaced is less than the weight of the object, the object will sink. If the weight of the object equals the weight of the water displaced, the object will be suspended in the water.

     In short, if a diver weighs 250 lbs fully geared up and his body/gear makes a hole in the water that would normally contain less than 250 lbs of water, that diver will float. If that diver shaped hole in the water would normally hold more than 250 lbs of water, our diver will sink. If the diver shaped hole would normally contain exactly 250 lbs of water, our diver is suspended in the water column. A diver who floats is called positively buoyant. A diver who sinks is negatively buoyant. The suspended diver is neutrally buoyant.

     In scuba, there are two pieces of gear specifically designed to affect buoyancy, the buoyancy control device (BCD), and the weight system. The purpose of the weight system is to overcome the positive buoyancy of the body and such equipment as wetsuits or drysuits and allow the diver to sink. The BCD allows the diver to adjust his buoyancy by adding air to or removing air from the BCD’s bladder and displacing more or less water as necessary.  By removing air from the BCD bladder, a properly weighted diver will become negatively buoyant and sink. Adding air to the BCD allows the diver to achieve neutral buoyancy to hover or positive buoyancy to rise in the water column.