page_title: Buoyancy compensator (diving)
section_title: Orientation in the water
hierarchical_section_title: Buoyancy compensator (diving) / Operation / Orientation in the water
caption_attribution_description: English: DIR Divers, Delray Ledge off of Palm Beach, Fl
context_page_description: A buoyancy compensator, also called a buoyancy control device, BC, BCD, stabilizer, stabilisor, stab jacket, wing or ABLJ depending on design, is a piece of diving equipment with an inflatable bladder which is worn by divers to establish neutral buoyancy underwater and positive buoyancy at the surface, when needed. The buoyancy is controlled by adjusting the volume of air in the bladder. The bladder is filled with ambient pressure gas from the diver's primary breathing gas cylinder via a low-pressure hose from the regulator first stage, directly from a small cylinder dedicated to this purpose, or from the diver's mouth through the oral inflation valve.
  They can be broadly classified as having the buoyancy primarily in front, surrounding the torso and behind the diver. This affects the ergonomics and to a lesser degree, the safety of the unit. They can also be broadly classified as having the buoyancy bladder as an integral part of the construction, or as a replaceable component supported inside the structural body.
context_section_description: The vertical-horizontal orientation, or trim, of the submerged diver is influenced by the BC and by other buoyancy and weight components and contributed to by the diver's body, clothing and equipment. The scuba diver typically wishes to be trimmed nearly horizontally (prone) while under water, to be able to see and swim efficiently, but more nearly vertical and perhaps partly supine, to be able to breathe without a regulator when on the surface.
  Buoyancy and trim can significantly affect hydrodynamic drag on a diver and the effort required to swim. The effect of swimming with a head up angle, of about 15° as is quite common in poorly trimmed divers, can be an increase in drag in the order of 50%.
  The static and stable orientation of an object floating in water, such as a diver, is determined by its center of buoyancy and its center of mass. At stable equilibrium, they will be lined up by gravity and buoyancy with the center of buoyancy vertically above the center of mass. The diver's overall buoyancy and center of buoyancy can routinely be adjusted by altering the volume of the gas in the BC, lungs and diving suit. The diver's mass on a typical dive does not generally change by what seems like much (see above—a typical dive-resort "aluminum 80" tank at 207 bars (3,000 psi) contains about 2.8 kilograms (6.2 lb) of air or nitrox, of which about 2.3 kilograms (5.1 lb) is typically used in a dive, although any air spaces such as in the BC and in diving suits will expand and shrink with depth pressure. Larger changes in buoyancy are possible if the diving weights are jettisoned, or a heavy object is picked up.
  Generally, the diver has a little control over the position of the center of buoyancy in the BC during a dive, the air in an incompletely inflated buoyancy compensator will rise to the shallowest part of the bladder unless prevented by a restriction to the flow. The position of this shallow point will depend on the diver trim and the geometry of the bladder. If the diver changes orientation in the water the gas will flow to the new high part if it does not have to flow down first to get there. As a result of this movement of gas, some buoyancy compensators will tend to hold the diver in the new position until actively changed. This is more likely in back mounted wing type bladders, where the gas can flow laterally to the high side and stay there. The diver can change the centre of gravity by adjustment of the equipment setup, which includes its configuration and position of weights, which ultimately influence where the effective BC lift is positioned relative to the Center of Gravity.
  Traditionally, weight belts or weight systems are worn with the weights on, or close to, the waist and are arranged with a quick release mechanism to allow them to be quickly jettisoned to provide extra buoyancy in an emergency. Weight carried on a belt can be distributed to shift the weight forward or backward to change the position of the diver's center of mass.  Systems that integrate the weights into the BCD, can provide improved comfort so long as the BCD does not have to be removed from the body of the diver, for example in an underwater emergency such as an entanglement. When a weight integrated BCD is removed, a diver wearing no weight-belt, and any type of wetsuit or dry suit, will be very buoyant.
  By inflating the BC at the surface, a conscious diver may be able to easily float face-up, depending on their equipment configuration choices. A fatigued or unconscious diver can be made to float face up at the surface by adjustment of their buoyancy and weights, so the buoyancy raises the top and front of the diver's body, and the weights act at the lower back of the body.  An inflated horse-collar BC always provides this orientation, but an inflated vest or wing may float the diver face-down if the center of buoyancy is behind the centre of gravity. This floating orientation is generally considered undesirable and can be minimized by relocation of some of the weights further to the rear, and using higher density cylinders (typically s
caption: Divers with neutral buoyancy and horizontal trim with the fins raised are less likely to touch or disturb the bottom
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