Buoyancy compensator (diving)
Jacket type BC on diving cylinder
|Acronym||BC or BCD|
|Other names||Buoyancy control device|
|Uses||Ability to adjust and control the overall buoyancy of the diver|
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 containing a bladder which is worn by divers to establish neutral buoyancy underwater and positive buoyancy on the surface, when needed. The buoyancy is controlled by adjusting the volume of air in the bladder.
BCs may have the following features:
- A low pressure "direct feed" or "Power Inflator" that injects gas from a low pressure hose from the diving cylinder's diving regulator or an auxiliary cylinder to the bladder(s) of the BC, that is controlled by an inflation valve.
- A vent valve or dump valve that allows gas to be released or to escape in a controlled fashion from the bladder(s) of the BC. Most BCs have at least two vents: one at the extreme top and the other at the bottom of the BC, for use as air migrates to whichever part of the BC is uppermost, the vent situated at the shoulder is used when the diver is upright and the vent situated nearer the diver's waist is used when inverted.
- An over pressure relief valve that automatically vents the bladder if the diver over inflates the BC by ascending or by injecting too much gas. This is usually a secondary function of the vent or dump valve.
- A harness that the diver wears with straps around the torso and over the shoulders. A crotch strap may be included to prevent the harness from sliding towards the head when the diver is upright and the bladder is inflated.
- A plastic or metal backplate to support diving cylinders
- Pockets for carrying diving reel, buoys and decompression tables
- An integrated diving weighting system: 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, pressure gauge, 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 dioxide cylinder. There is a risk that an emergency cylinder is accidentally opened during a dive causing a rapid ascent and barotrauma to the diver. Carbon dioxide, being poisonous at high partial pressures, could be a dangerous gas to have in a BC because the diver may inhale it from the bag underwater., but it is unlikely that this would happen, as the diver would usually be aware that the emergency inflation has been oprerated.
There are three main types of BC:
Adjustable buoyancy life jacket
An adjustable buoyancy life jacket (ABLJ) is fitted around the neck and onto the chest, secured by straps around the waist and usually between the legs. They are sometimes referred to as "horse collars" because of their resemblance, and are historically derived from Mae West life jackets issued to World War II flyers. As they were developed in the 1960s they have been largely superseded by wing and vest type BCs, primary because of their tendency to shift the diver's center of gravity with inflation. The ABLJ's location on the diver's chest does means that it does the best job of all BC designs when it comes to floating a distressed, fatigued or unconscious diver face-up on the surface in the event of a problem.
Vest BC, stab jacket, stabiliser jacket, stab, waistcoat or disparagingly "Poodle 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 a possible hazard in an emergency. Vest BC's, whether back-inflated or wraparound, typically provide up to 25 kilograms of buoyancy (depending on size) and are fairly comfortable to wear. Vest 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" (technical and recreational) Vest BC's have the ability to carry multiple cylinders.
Backplate and Wings
Wings or BP/W 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 suitability for technical diving where they are often used, as the technical diver often carries multiple cylinders on his back and/or clipped to D-rings on the harness 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 high lift capacity (60 lbs /30 liter wings are not uncommon). Elasticated webbing around the bladder is used by some to constrict the bladder when not inflated, although there is serious dispute as to the wisdom of this addition. Heavy equipment such as diving cylinders can be fixed to or slung from the back plate.
A variation on the back mounted buoyancy compensator is used without a backplate for side mount diving This arrangement is functionally similar to wearing the buoyancy compensator sandwiched between the cylinder(s) and backplate, but there is no backplate or back mounted cylinder. The buoyancy cell may be mounted between the sidemount harness and the diver, or on top of the harness.
Some side mount harnesses are adaptable for use with a back mount cylinder as an option, without the rigid backplate.
The diver needs to be able to establish three states of buoyancy at different stages of a dive:
- 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 float on the surface.
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 a BC to maintain this state of 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 Neoprene, the volume of the material will change (Boyle's Law) as the pressure changes when the diver descends and ascends. The volume of air in the BC is adjusted to compensate for this.
- Gas contained in the flexible air spaces within the diver's body and equipment (including gas in the BC) is compressed on descent and expands on ascent. The diver normally counteracts this by adding gas to the space or drysuit, in order to avoid "squeeze". Gas content in the BC is adjusted to correct buoyancy if these other corrections are not enough.
- As the dive proceeds, gas is consumed from the diving cylinders of the breathing equipment. This represents a progressive loss of mass which makes the diver more buoyant; the diver’s overall buoyancy must be reduced by venting air from the BC. For this reason the 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 breathng gas. 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 added to the BC when the diver is negative (too heavy), or vented from the BC when the diver is too buoyant (too light). 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, in a positive feedback environment. 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 easily by an experienced diver.
A feature of scuba which is often non-intuitive for beginners, 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 (added or vented) 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, can happen during ascent, resulting in uncontrolled ascent, until a diver prematurely surfaces without a safety (decompression) stop.
With experience, divers 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, 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 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 center of buoyancy and its center of mass. At equilibrium, they will be lined up under gravity with the center of buoyancy vertically on a line with, (and preferably 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 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 buoyancy are of course possible if the weight belt 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 buoyancy center by control of 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.
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, such belts have the advantage of being able to be shifted fore and aft so as to change the diver's center of mass. BCD systems that integrate the weights into the BCD, can provide added 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 on the surface a diver wearing no weight-belt and any type of foamed neoprene wetsuit will remain very buoyant.
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 Horsecollar always provides this orientation, but an inflated Vest, and all styles of BP/w generally float the diver face-down because the center 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 center 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 center 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 cylinders, the type of diving suit, 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.
In 1957, F. G. Jensen and Willard F. Searle, Jr began testing methods for manual and automatic buoyancy compensation for the United States Navy Experimental Diving Unit (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 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 inflation cartridges (a holdover, for surface use, of the Mae West flyer's lifejacket). This was abandoned when valves that allowed divers to breathe from the BC's inflation bag were introduced. Since 1969 most BCs have used for inflation mainly gas from one of the diver's main cylinders, and an oral inflation tube have been generally retained for abnormal situations (no high pressure gas left, malfunction of an inflator hose) both underwater and on the surface. In 1972, Watergill developed the Atpac wing, the first wing-style BC and in 1978, ScubaPro released the Stabilizer Jacket, the first Vest 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. Innovations for backplate and wing include redundant bladders, stainless steel backplates, lightweight soft nylon 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:
- European terminology
- North American terminology
- http://www.emedmag.com/html/pre/tox/0500.asp CO2 Toxicity
- Historicizing Lifestyle: Mediating Taste, Consumption and Identity from the 1900s to 1970s (Hardcover), Raptures of the Deep: Leisure, Lifestyle and Lure of Sixties Diving by Bill Osgeby ISBN 978-0-7546-4441-5
- Discussion of Bungied Wings, aka Bondage Wings
- Kakuk, Brian; Heinerth, Jill (2010). Side Mount Profiles. High Springs, FL: Heinerth Productions. ISBN 978-0-9798789-5-4.
- Williams, Guy; Acott, Chris J (2003). "Exposure suits: a review of thermal protection for the recreational diver". South Pacific Underwater Medicine Society journal 33 (1). ISSN 0813-1988. OCLC 16986801. Retrieved 2009-06-13.
- Fead, L (1979). "Is dropping your weight belt the right response?". South Pacific Underwater Medicine Society Journal (reprinted from: NAUI News, September 1978) 9 (1). ISSN 0813-1988. OCLC 16986801. Retrieved 2009-06-13.
- Swimming Position Centroid Calculation Methodology illustration from huntzinger.com
- Surface Buoyancy Moment Arm illustration from huntzinger.com
- Jensen, FG; Searle, Willard F (1957). "Buoyancy Control of Open Circuit Scuba". United States Navy Experimental Diving Unit Technical Report. NEDU-RR-8-57. Retrieved 2009-06-13.
- Janney, G. M; Hanger, G. W (1960). "Walter Kiddie and Co. - Buoyancy Compensating Tank". United States Navy Experimental Diving Unit Technical Report. NEDU-Evaluation-7-60. Retrieved 2009-06-13.
- Acott, Chris J. (1996). "An evaluation of buoyancy jacket safety in 1,000 diving incidents.". South Pacific Underwater Medicine Society journal 26 (2). ISSN 0813-1988. OCLC 16986801. Retrieved 2009-06-13.
- Buoyancy compensator evaluations hosted by the Rubicon Foundation
- McLean, David: History of Buoyancy Compensators