The most fundamental characteristic of a buoyancy compensator (BC) is the amount of lift it provides. A BC's lift is usually given in pounds or kilograms. For example, a BC with 27 pounds of lift will increase a diver's positive buoyancy by 27 pounds when fully inflated.
When choosing a buoyancy compensator, a diver should consider how much lift he will need. The goal is to choose a BC that can comfortably float a diver on the surface when partially full. A diver using a BC with too little lift will have a hard time floating on the surface, and may have to kick to keep his head above the water. A diver using a BC with excessive lift will have to drag a larger volume through the water than is necessary, which will increase drag even when the BC is not full. Different sizes of the same BC model usually have different amounts of lift.
Children and smaller divers will generally need less lift than larger divers. Divers who use more buoyant, aluminum tanks will need less lift than divers who use less buoyant, steel tanks. Exposure protection, such as a wetsuit or a drysuit, will also effect a diver's buoyancy and therefore the amount of lift he requires. Typically, the more weight a diver has to carry, the more lift he is likely to need. Finally, dive guides and instructors require BCs with greater lift than recreational divers, because they frequently carry extra weight for clients and assist clients with weights on the surface.
