How people or objects float is significant to divers. The term BUOYANCY refers to the ability to float. When an object sinks it is NEGATIVELY BUOYANT, when it floats it is POSITIVELY BUOYANT. Objects which neither float nor sink, are NEUTRALLY BUOYANT. Divers attempt to be neutrally buoyant, but if something catches their eye, they can generate negative buoyancy by venting air from the BCD. By adding or releasing air the diver changes his VOLUME and DENSITY so he can either sink or float or remain neutral.
BUOYANCY CONTROL is possibly the single most important diving skill. Functioning together, the weight belt and BCD regulate buoyancy. How the weight belt is prepared will affect diver comfort and safety. Divers must have on the precise amount of weight and have it balanced on the belt.
What the diver is doing with his BCD are changing his density and volume to change his buoyancy. This is known as ARCHIMEDES' PRINCIPLE. Archimedes was a Greek mathematician and physicist who legend has it that while taking a bath he observed that as he got into his tub, water spilled out. After doing tests he discovered that when a body is placed in a liquid, buoyant forces act upon it. The direction of this force is opposite to the gravitational force acting on the object in relation to the displaced fluid.
Archimedes' Principle: A body immersed in a liquid either wholly or partially, is buoyed up by a force equal to the weight of the displaced liquid.
To ascertain the buoyancy of a submerged object subtract the weight of the object from the displaced liquid.
Fresh water weigh's 62.4 pounds per cubic foot whereas seawater, due to its dissolved salts and minerals, weighs 64.0 pounds per cubic foot. Thus, it can be seen that a body will be buoyed up by a greater force in seawater than in fresh, and divers must wear more weight because of this in seawater. Likewise cold water is denser than warm, due to the molecules being closer together, making the water more buoyant due to increased density.
LIFT is the amount of force needed to bring an object up from the bottom, and is determined by using Archimedes' Principle. If an object in seawater displaces two cubic feet, then the lift needed to float that object to the surface will be: 2 X 64, or 128 pounds. The way the object is lifted is with lifting devices such as bags or barrels.
Lift bags are rated by their lifting capacity. In choosing a lift bag it is a prudent idea to choose one with a dump valve. A dump valve functions to vent off excess air as the bag rises. Using large bags for small jobs is acceptable as long as the bag has a dump valve. Oil drums are extremely strong and useful for lifting objects requiring their strength, but keep in mind there is no way to vent excess pressure (air).


FRESH WATER: V cu. ft. X 62.4 = Lift

SEAWATER: V cu. ft. X 64.0 = Lift

National Association of Rescue Divers

P.O. Box 590474, Houston, Texas 77259-0474

Site Made & Maintained by: RescueDiver.org

Any Inquires Should Be Made To: WEBMASTER@RescueDiver.org

Copyright 2006 National Association of Rescue Divers ©