Buoyancy compensator
Encyclopedia
- Stab jacket redirects here. For a type of body armor, see stab vestStab vestA stab vest, or stab proof vest is a reinforced piece of body armor, worn under or over other items of clothing, which is designed to resist knife attacks to the chest, back and sides...
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A buoyancy compensator (or buoyancy control device, BC or BCD) is a piece of diving equipment
Diving equipment
Diving equipment is equipment used by underwater divers for the purpose of facilitating diving activities. This may be equipment primarily intended for this purpose, or equipment intended for other puprposes which is found to be suitable for diving use....
worn by divers to provide:
- life saving emergency buoyancyBuoyancyIn physics, buoyancy is a force exerted by a fluid that opposes an object's weight. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the...
both underwater and on the surface. - the ability to adjust and control the overall buoyancy of the diver and the diver's heavy equipment allowing the diver to achieve neutral buoyancy, remaining at constant depth, or to descend or ascend in a controlled way.
Some types of buoyancy compensators are also built around the diver's scuba set
Scuba set
A scuba set is an independent breathing set that provides a scuba diver with the breathing gas necessary to breathe underwater during scuba diving. It is much used for sport diving and some sorts of work diving....
or are built into its harness.
Features
- A low pressure direct feed that transports gas from diving cylinder and diving regulatorDiving regulatorA diving regulator is a pressure regulator used in scuba or surface supplied diving equipment that reduces pressurized breathing gas to ambient pressure and delivers it to the diver. The gas may be air or one of a variety of specially blended breathing gases...
to the BC. - An inflation valve that allows gas from the direct feed into the bladders of the BC.
- A vent valve that allows gas to escape from the bladders of the BC. Most BCs have at least two vents: one at the extreme top and the other at the bottom of the BC. As air migrates to whichever part of the BC is uppermost, the vent situated at the shoulder is used when upright and the vent situated at the diver's waist is used when inverted.
- An over pressurization valve that automatically vents the bladders if the diver over inflates the BC by ascending or by injecting too much gas.
- A harness that the diver wears with straps around the torsoTorsoTrunk or torso is an anatomical term for the central part of the many animal bodies from which extend the neck and limbs. The trunk includes the thorax and abdomen.-Major organs:...
and over the shoulders - A plastic or metal backplateBackplate and wingthumb|right|A stainless steel backplate, wing and manifolded twinsetA backplate and wing , is a type of Scuba harness with buoyancy compensation device worn by scuba divers. Unlike most other BCDs, the backplate and wing is a modular system, in that it consists of separable components...
to support diving cylinderDiving cylinderA diving cylinder, scuba tank or diving tank is a gas cylinder used to store and transport high pressure breathing gas as a component of a scuba set. It provides gas to the scuba diver through the demand valve of a diving regulator....
s - Pockets for carrying diving reel, buoys and decompression tables
- An integrated diving weighting systemDiving weighting systemDivers wear weighting systems, weight belts or weights, generally made of lead, to counteract the buoyancy of other diving equipment, such as diving suits and aluminium diving cylinders...
, pockets for lead weights with a quick release mechanism. Integrated weights can eliminate the need for a separate weight belt. - D rings or other anchor points, for clipping on other equipment such as torches, strobes, reels, cameras and stage cylinders
- Emergency inflation cylinders. This can either be a 0.5 litre air cylinder, filled from the diver main cylinder, or a small carbon dioxideCarbon dioxideCarbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
cylinder. There is a risk that an emergency cylinder is accidentally opened during a dive causing a rapid ascent and barotraumaBarotraumaBarotrauma is physical damage to body tissues caused by a difference in pressure between an air space inside or beside the body and the surrounding fluid...
to the diver. Carbon dioxideCarbon dioxideCarbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
, being poisonous at high partial pressurePartial pressureIn a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
s, is a dangerous gas to have in a BC because the diver may inhale it from the bag underwater.
Types
Wings
WingsBackplate and wing
thumb|right|A stainless steel backplate, wing and manifolded twinsetA backplate and wing , is a type of Scuba harness with buoyancy compensation device worn by scuba divers. Unlike most other BCDs, the backplate and wing is a modular system, in that it consists of separable components...
consist of an inflatable bladder worn between the diver's back and the cylinder(s). Wings are not a recent development, but have recently become popular again because of technical diving
Technical diving
Technical diving is a form of scuba diving that exceeds the scope of recreational diving...
where they are often used, as the technical diver often carries multiple cylinders on his back and/or strapped to the harness's webbing. The bladder and cylinders are fastened to a backplate which is strapped to the diver. The wing design frees the divers sides and front and allows for a large volume bladder with important lift capacity (60 lbs /30 liter Wings are not uncommon). Elasticated webbing
Webbing
Webbing is a strong fabric woven as a flat strip or tube of varying width and fibres often used in place of rope. The name webbing comes from the meshed material frequently used in its construction, which resembles a web...
around the bladder is used by some to constrict the bladder when not inflated, although there is some dispute as to the wisdom of this addition. Heavy equipment such as diving cylinder
Diving cylinder
A diving cylinder, scuba tank or diving tank is a gas cylinder used to store and transport high pressure breathing gas as a component of a scuba set. It provides gas to the scuba diver through the demand valve of a diving regulator....
s can be fixed to or slung from the back plate.
Stab jacket
Stab jacket, stabiliser jacket, stab, waistcoat, or vest BCs are inflatable vests worn by the diver around the upper torso, and also act as the cylinder harness. The air bladder may extend from the back around the diver's sides ("wraparound") or may only cover the back ("back inflation.") Wraparound bladders are favored by some divers because they may make it easier to maintain balance, both submerged and on the surface. However, some designs have had the tendency to squeeze the diver's torso when inflated. Back inflation (Wing) BC's do not have this problem but have a greater tendency to float the diver face-down on the surface, which presents an obvious hazard in an emergency. Jacket-style BC's, whether back-inflate or wraparound, typically provide up to 25 litres of buoyancy (depending on size) and are fairly comfortable to wear. Jacket-style BC's are the most common type among recreational divers because they can integrate buoyancy control, weights, attachment points for auxiliary gear, and cylinder retention in a single piece of gear. The diver need only attach a cylinder and regulator set in order to have a complete SCUBA set. Some "tech-rec" jacket-style BC's have the ability to carry multiple cylinders.
Diver's lifejacket
Adjustable Buoyancy Life Jackets, ABLJs, or Horsecollar BCs are worn around the neck and chest with straps around the waist and usually between the legs. They are historically derived from Mae West lifejackets which were cheap and available. As they were developed in the 1960s they have been largely superseded by wing and vest type BCs. Additionally, they are less comfortable because they use a crotch strap between the legs, and generally provide less total buoyancy potential (up to 15 liters) than Jacket BC or Wings, even though this amount is still generally adequate for both warm water and cold water recreational diving. Finally, because the diver must use a separate cylinder harness as a platform for the aqualungAqua-lung
Aqua-Lung was the original name of the first open-circuit free-swimming underwater breathing set in reaching worldwide popularity and commercial success...
, they are slightly more complicated to use. However, its location on the diver's chest means that it does the best job of all BCs in floating a distressed, fatigued and/or unconscious diver face-up on the surface in the event of a problem.
Neutral buoyancy
- negative buoyancy: when the diver wants to descend or stay on the seabed.
- neutral buoyancy: when the diver wants to remain at constant depth, with minimal effort.
- positive buoyancy: when the diver wants to ascend or float on the surface.
To achieve negative buoyancy, divers who carry or wear buoyant equipment must be weighted
Diving weighting system
Divers wear weighting systems, weight belts or weights, generally made of lead, to counteract the buoyancy of other diving equipment, such as diving suits and aluminium diving cylinders...
to counteract the bouyancy of both the diver and the equipment.
When underwater, a diver often needs to be neutrally buoyant so that the diver neither sinks nor rises. A state of neutral buoyancy exists when the weight of water that the diver displaces equals the total weight of the diver. The diver uses the BC to maintain this neutral buoyancy by adjusting the BC's volume and therefore its buoyancy, in response to various effects, which alter the diver’s overall volume or weight, primarily:
- If the diver's exposure suit is made of a compressible gas-filled material such as foamed NeopreneNeopreneNeoprene or polychloroprene is a family of synthetic rubbers that are produced by polymerization of chloroprene. Neoprene in general has good chemical stability, and maintains flexibility over a wide temperature range...
, the volume of the material will change as the pressure changes when the diver descends and ascends.
- Gas contained within the air spaces within the diver's body and equipment is compressed on descent and expands on ascent, resulting in volume changes similar to those caused by exposure suit compression, but of a greater magnitude. Gas contained inside a drysuit is also compressed as the diver descends; however, the diver normally counteracts this by adding gas to the drysuit, not the BC, in order to avoid "squeeze", a mild barotraumaBarotraumaBarotrauma is physical damage to body tissues caused by a difference in pressure between an air space inside or beside the body and the surrounding fluid...
to the skin.
- As the dive proceeds, gas is consumed from the diving cylinderDiving cylinderA diving cylinder, scuba tank or diving tank is a gas cylinder used to store and transport high pressure breathing gas as a component of a scuba set. It provides gas to the scuba diver through the demand valve of a diving regulator....
s of the breathing equipment. For open circuit SCUBA, this represents a progressive loss of mass from the diver, which makes him or her more buoyant; the diver’s overall buoyancy must be reduced by venting air from the BC. For this reason the open circuit diver needs to configure his equipment to be a little overweight at the beginning of the dive so that neutral buoyancy can be achieved after the loss of the weight of the air. Air or nitrox weighs about 1.3 grams for every litre at standard pressure. Thus, the magnitude of weight change from loss of air during a dive varies from roughly 4.3 kg (9.5 lbs) representing the total air content of a steel 15 litre cylinder at 230 bar/3500 psi (in practice, reserve requirements dictate that only about 8 lbs of this will be breathed), to about 5 lbs difference for the smaller 80 ft3 aluminum-80 (AL80) tank (11.1 litres internal capacity) pressurised to 200 bar/3000 psi, and again assuming that only 5/6ths of the air in the tank is used, leaving a typical safety reserve.
In practice, the diver doesn't think about all this theory during the dive. To remain neutrally buoyant, gas is injected into the BC when the diver is too heavy, or is sinking, and vented from the BC when the diver is too buoyant, or is rising. A feature of diving is that there isn't any automatically stable equilibrium position for a diver wearing a BC, or even simply for a diver with lungs full of air. Any change in depth from a position of neutrality and even a small changes in volume, including the simple act of breathing, result in a force toward an even less neutral depth. Thus, maintenance of neutral buoyancy in scuba must be a continuous and active procedure—the diving equivalent of balance. Fortunately, the diver's mass provides a source of inertia, as does the liquid medium, so small perturbations (such as from breathing) can be compensated for with ease by an experienced diver.
A feature of scuba which is often non-intuitive to beginning students of diving, is that gas generally needs to be added to the BC when a diver descends in a controlled manner, and valved-off (removed or vented) from the BC when the diver ascends in a controlled manner. This gas maintains the volume of the gas bubble in the BC during depth changes; this bubble needs to remain at constant volume for the diver to remain even approximately neutrally buoyant. When gas is not added to the BC during a descent, the gas bubble in the BC decreases in volume due to the increasing pressure, resulting in faster and faster descent with depth, until the diver hits the bottom. The same runaway phenomenon, an example of positive feedback
Positive feedback
Positive feedback is a process in which the effects of a small disturbance on a system include an increase in the magnitude of the perturbation. That is, A produces more of B which in turn produces more of A. In contrast, a system that responds to a perturbation in a way that reduces its effect is...
, can happen during ascent, resulting in uncontrolled ascent, until a diver prematurely surfaces without a safety (decompression) stop.
With experience, divers eventually learn to minimize this problem, starting by minimizing the size of the "constant volume bubble" in their BC's. This requires learning the minimum weighting requirement needed for their system (see factors above). These techniques keep the volume of the gas bubble within the BC as small as possible at the beginning of a dive, while leaving just enough gas in the BC at first submersion to be able to compensate for the expected slow loss of diver weight as the dive progresses, as a result of gas used (in practice, about 5 to 8 lbs. lost per cylinder, as noted above).
Somewhat complex automatic reflex behaviors are also developed by experienced divers, involving breathing control and BC gas management during depth changes, which allow them to remain neutrally buoyant from minute to minute during a dive, without having to think much about it. Experienced scuba divers may often be identified by the fact that they maintain neutrality without any fin use, as fish do.
Orientation in the water
The vertical-horizontal orientation 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 diver typically wishes to be positioned nearly horizontally (prone) while under water, to be able to see and swim usefully, but more nearly vertical and perhaps partly supine, to be able to breathe without a regulator when on the surface.The orientation of a static and stable object in water, such as a diver, is determined by its centre of buoyancy and its centre of mass. At equilibrium, they will be lined up under gravity with the centre of buoyancy vertically on a line with, (and preferably above) the centre of mass. The diver's overall buoyancy and centre of buoyancy can routinely be adjusted by altering the volume of the gas in the BC, lungs and diving suit
Diving suit
A diving suit is a garment or device designed to protect a diver from the underwater environment. A diving suit typically also incorporates an air-supply .-History:...
. 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 200 bar contains about 2.8 kg (~6 lbs) of air or nitrox, of which about 2.3 kg (~5 lbs) 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. Large changes in bouyancy are of course possible if the weight belt
Diving weighting system
Divers wear weighting systems, weight belts or weights, generally made of lead, to counteract the buoyancy of other diving equipment, such as diving suits and aluminium diving cylinders...
is jettisoned, or a heavy object is picked up.
Generally, the diver has minimal control of the relative position of the center of buoyancy in the BC during a dive, but only its quantity. However the diver can change the bouyancy center by control his equipment setup, which includes its configuration and weighting locations, which ultimately influence where his effective BC lift is positioned relative to his Center of Gravity
Center of gravity
In physics, a center of gravity of a material body is a point that may be used for a summary description of gravitational interactions. In a uniform gravitational field, the center of mass serves as 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. Current BCD systems usually integrate the weights into the BCD, providing some comfort and safety advantages, so long as the BCD does not have to be removed from the body of the diver, for example in an underwater emergency situation. When a weight integrated BCD is removed on the surface a diver wearing no weight-belt and any type of foamed neoprene wetsuit will remain very bouyant.
By inflating the BC at the surface, the conscious diver may be able to easily float face-up, depending on his equipment configuration choices. A fatigued or unconscious diver can be made to float face up on 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 on the lower and back of the body. An inflated ABLJ always provides this orientation, but an inflated stab, and all styles of wing BC generally float the diver face-down because the centre of buoyancy is too far from the diver's front. This floating orientation is generally considered undesirable and can be minimized pre-dive by relocation of some of the weights (perhaps of quick release type) further to the rear (such as in pockets close to the diver's cylinder), and avoiding the use of aluminum tanks and using higher density cylinders (typically steel), which similarly moves the centre of mass to be further behind the diver, and thus, behind the center of buoyancy. The BC type can also be selected with this factor in mind, selecting a style that moves the centre of buoyancy forward, as this accomplishes the same net effect. Any or all of these options can be utilized to trim the system out to its desired characteristics and many factors can contribute, such as the number and position of diving cylinder
Diving cylinder
A diving cylinder, scuba tank or diving tank is a gas cylinder used to store and transport high pressure breathing gas as a component of a scuba set. It provides gas to the scuba diver through the demand valve of a diving regulator....
s, the type of diving suit
Diving suit
A diving suit is a garment or device designed to protect a diver from the underwater environment. A diving suit typically also incorporates an air-supply .-History:...
, the position and size of stage cylinders, the size and shape of the diver's body and the wearing of ankle weights, or additional dive equipment. Each of these influence a diver's preferred orientation in the water (horizontal) and on the surface (vertical) to some degree.
History
In 1957, F. G. Jensen and Willard F. Searle, JrWillard Franklyn Searle
Capt. Willard Franklyn "Bill" Searle Jr. USN was an American ocean engineer who was principally responsible for developing equipment and many of the current techniques utilized in United States Navy diving and salvage operations.-Background:Searle was born 17 January 1924 in Columbus, Ohio...
began testing methods for manual and automatic buoyancy compensation for the United States Navy Experimental Diving Unit
United States Navy Experimental Diving Unit
The United States Navy Experimental Diving Unit is the primary source of diving and hyperbaric operational guidance for the US Navy...
(NEDU). In their early tests, they determined that manual systems were more desirable due to the size of the automatic systems. Later that year, the Walter Kiddie and Co. sent a prototype buoyancy compensating tank for use with two cylinders to NEDU for evaluation. The valves of this aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
tank system leaked and testing was delayed until 1959 when it was recommended for field testing.
The ABLJ was developed by Maurice Fenzy in 1961. Early versions were inflated by mouth underwater. Later versions had their own air inflation cylinder. Some had carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
inflation cylinders, a development which was abandoned when valves that allowed divers to breathe from the BC's inflation bag were introduced. Since 1969 most modern BCs have used inflation gas from one of the diver's main gas cylinders, in addition to an oral inflation tube which is used at the surface in the event the diver has no high pressure gas left (oral inflation can also be used at depth, in cases where a BC inflator hose develops a leak or problem and requires disconnection). In 1972 Watergill developed the Atpac wing, the first wing style BC and in 1978, ScubaPro released the Stabilizer Jacket, the first jacket-style BC.
More recent innovations for jacket BCs include weight pouches to adjust attitude underwater, putting the weights on the BC rather than on a weightbelt, integrated regulators, heavily reinforced 1050 denier ballistic nylon
Ballistic nylon
Ballistic nylon is a thick, tough, synthetic nylon fabric used for a variety of applications. Ballistic nylon was originally developed by the DuPont corporation as a material for flak jackets to be worn by World War II airmen...
. Innovations for backplate and wing
Backplate and wing
thumb|right|A stainless steel backplate, wing and manifolded twinsetA backplate and wing , is a type of Scuba harness with buoyancy compensation device worn by scuba divers. Unlike most other BCDs, the backplate and wing is a modular system, in that it consists of separable components...
include redundant bladders, stainless steel backplates, lightweight soft nylon
Nylon
Nylon is a generic designation for a family of synthetic polymers known generically as polyamides, first produced on February 28, 1935, by Wallace Carothers at DuPont's research facility at the DuPont Experimental Station...
backplates, and 85 lb lift bladders.
Dive Rite pioneered the first wing for diving twin cylinders in 1985. Competitors in tech diving include Ocean Management Systems. Other SCUBA manufacturers include Sherwood, Zeagle, scubapro, and cressisub.
Other buoyancy equipment
There are other types of equipment worn by divers that affect buoyancy:- backplateBackplate and wingthumb|right|A stainless steel backplate, wing and manifolded twinsetA backplate and wing , is a type of Scuba harness with buoyancy compensation device worn by scuba divers. Unlike most other BCDs, the backplate and wing is a modular system, in that it consists of separable components...
- diving cylinderDiving cylinderA diving cylinder, scuba tank or diving tank is a gas cylinder used to store and transport high pressure breathing gas as a component of a scuba set. It provides gas to the scuba diver through the demand valve of a diving regulator....
- diving suitDiving suitA diving suit is a garment or device designed to protect a diver from the underwater environment. A diving suit typically also incorporates an air-supply .-History:...
- diving weighting systemDiving weighting systemDivers wear weighting systems, weight belts or weights, generally made of lead, to counteract the buoyancy of other diving equipment, such as diving suits and aluminium diving cylinders...
Further reading
- Buoyancy compensator evaluations hosted by the Rubicon FoundationRubicon FoundationRubicon Foundation, Inc. is a non-profit organization devoted to contributing to the interdependent dynamic between research, exploration, science and education. The foundation, started in 2002, is located in Durham, North Carolina and is primarily supported by donations and grants. Funding has...