BCDs- Buoyancy control Devices

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A buoyancy compensator (BC), also called a buoyancy control device (BCD) which is worn by divers to establish neutral buoyancy underwater and positive buoyancy at the surface, when needed.

The buoyancy is usually controlled by adjusting the volume of gas in an inflatable bladder, which 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.

Ambient pressure bladder buoyancy compensators can be broadly classified as having the buoyancy primarily in front, surrounding the torso, or behind the diver. This affects the ergonomics, and to a lesser degree, the safety of the unit.

The buoyancy compensator is one of the items of diving equipment most requiring skill and attention during operation, as control is entirely manual, and adjustment is required throughout the dive as weight reduces due to gas consumption, and buoyancy of the diving suit and BC generally varies with depth. Fine buoyancy adjustment can be done by breath control on open circuit, reducing the amount of actual BC volume adjustment needed, and a skilled diver will develop the ability to adjust volume to maintain neutral buoyancy while remaining aware of the surroundings and performing other tasks. The buoyancy compensator is both an important safety device when used correctly, and a significant hazard when misused or malfunctioning.

Operating principle

A buoyancy compensator works by adjusting the average density of the diver and their attached equipment to be greater than, equal to, or less than the density of the diving medium. This can be done in either of two ways:

Variable volume type

The common type of buoyancy compensator increases buoyancy by adding gas at ambient pressure to a flexible airtight bladder, thereby increasing the volume, and decreases buoyancy by releasing the gas into the water. This volume of gas will compress or expand as the ambient pressure varies with depth, following Boyle’s law and therefore the buoyancy of the system will increase and decrease in proportion to the absolute pressure variation and the volume of gas in the bladder. The variation of buoyancy for a given change of depth will be greater near the surface than at greater depth and greater for a large volume of gas than for a small volume. The range of depths for which the diver can compensate for these changes by voluntary adjustment of lung volume while breathing effectively is therefore dependent on the volume of gas in the bladder and the nominally neutral depth, where breathing at normal tidal volume of about 500 ml results in approximate dynamic equilibrium, and the diver remains at that depth without additional effort. This type of buoyancy compensator functions by increasing buoyancy from the most stable state, which is empty, so weighting is done for neutral buoyancy at the condition of least mass, which is at the end of the dive with the cylinders empty, at which point the diver should be able to stay at the last decompression stop without physical effort.

Variable density type

An alternative method of adjusting the buoyancy of the diver is by varying the density of a rigid container of constant displaced volume, by adjusting the volume of added water in a normally gas filled space. This approach can also be described as buoyancy reduction, as opposed to buoyancy addition when gas is added to a flexible ambient pressure space. Water from the surroundings is injected into the tank to decrease buoyancy by ambient pressure difference or by a pump, depending on the internal gas pressure. Water can be removed in a similar way to increase buoyancy. As the tank is rigid and effectively incompressible within the range of diving depths for which it is intended, buoyancy changes due to depth variation during the dive are negligible,and the diver only needs to adjust the buoyancy to account for gas usage and volume variation of the diving suit.

AVELO system uses this mechanism, with a rechargeable battery powered pump unit which is demountable from the cylinder.

Configurations

There are three main configurations of inflatable bladder buoyancy compensation device based on buoyancy distribution:

Wraparound buoyancy BCs

Diver wearing a stabiliser jacket, BCs are inflatable vests worn by the diver around the upper torso, which incorporate the cylinder harness. The air bladder extends from the back around the diver’s sides or over the diver’s shoulders.

Wraparound bladders are favored by some divers because they make it easier to maintain upright attitude on the surface. However, some designs have a tendency to squeeze the diver’s torso when inflated, and they are often bulky at the sides or front when fully inflated, and may lack sufficient volume to support a full technical rig with a thick wetsuit.

Vest BCs typically provide up to about 25 kilograms of buoyancy (depending on size) and are fairly comfortable to wear, if of the correct size and adjusted to fit the diver. Vest BCs 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.

This tendency of the inflated BC to shift towards the head is less of a problem when the weights are carried in integrated weight pockets on the BC, but it may then have a tendency to slide towards the head when deflated on an inverted diver underwater. This is less of a problem for the average recreational diver, who does not spend much time head down underwater, but can increase the difficulty of recovering from a dry-suit inversion where the air in the suit flows to the feet and the weights in the BC shift towards the head. A crotch strap will prevent this.

Back inflation

Back inflation buoyancy compensators are typified by the stainless steel backplate and wing arrangement popular with technical divers, but other arrangements are also available. Suitable for technical diving. The back-mount cylinders or rebreather assembly are fastened over the buoyancy bladder to a backplate which is strapped to the diver by the harness. 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). The distance between bolt holes on the center line of the back plate has standardised at 11 inches (280 mm) between centers.

Diver wearing a wing buoyancy compensator
Wings for twin-cylinder set: the cylinders are fastened together by 2 metal bands which are bolted through the wing to the backplate
Bladder of a wing buoyancy compensator, showing the side which is away from the diver. The two pairs of slits allow use of cambands to hold a single cylinder
Divers preparing for a decompression dive using backplate and wing with sling mounted decompression gas cylinders.

Sidemount BCs

A variation on the back mounted buoyancy compensator is used with or with out a backplate for 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. The sides of the bladder may be restrained from floating upwards when inflated by bungee cords clipped to the waistband in front of the diver or clipped to each other, forming an elastic belt across the front of the hips, well below the diaphragm. In this application, back mount keeps the inflated bladder from occupying the space at the diver’s sides where the cylinders are suspended.

components:

A flexible bladder to contain gas which may be added or released during the dive to control buoyancy.
A means of adding gas to the bladder, generally a low pressure direct feed or power inflator that injects gas from a low pressure hose from the diving cylinder’s regulator 1st stage or an auxiliary cylinder to the bladder(s) of the BC, that is controlled by an inflation valve, and usually an oral inflation option. These are usually at the end of a corrugated or ribbed rubber inflation hose.
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. Venting through the oral inflation system is also usually possible and may be preferred.
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, and is a necessary safety feature to prevent over-pressure damage.
A means of securing the BC to the diver to transfer buoyancy forces, and to hold the BC in the position intended for its designed function. The BC is typically secured to a diver’s torso, either with dedicated straps or as part of a multi-functional system integrated with the bladder or casing.

In addition some BCs may include other features:

A tough textile casing to contain and protect the bladder, and to which most of the other components are attached, with zippers for access to the bladders.
Straps (cambands) to secure back-mount cylinders
A Metal back plate to support back-mount Cylinders
A crotch strap may be included in the harness to prevent the BC from sliding towards the head when the diver is upright and the bladder is inflated.
A cummerbund is an alternative approach to reduce the tendency for the BC to slide towards the head by providing a close fit around the waist.
Pockets for carrying small accessories or tools
An integrated pockets for lead weights with a quick release mechanism. Integrated weights can eliminate the need for a separate weight belt.
Trim weight pockets for adjusting the position of the diver’s centre of gravity to improve trim
D-rings or other anchor points, for clipping on other equipment such as torches, pressure gauge, reels, cameras and stage, bailout, or side-mount cylinders
Reflective tape for better visibility.
Padding for comfort.
A redundant bladder with associated filling and venting components, as a backup in case of failure of the primary bladder- specially seen in Technical diving.
Alternative breathing gas regulator connected to or integrated with the inflation/deflation valve assembly.
Bungees to restrain a partially inflated wing

Size and fit

The buoyancy compensator must fit the diver comfortably and must stay securely in place without constraining the diver’s freedom of movement. There is some conflict between allowing easy adjustment to fit a range of diver builds, and setting up the harness to optimum fit for a specific diver in a specific diving suit. This is a particular problem with jacket style BCs which are inherently less adjustable for fit than backplate harnesses, which are more adjustable, but may take more time to adjust.

It is critically important that the fully inflated buoyancy compensator can support the diver with the maximum equipment load on the surface at the start of a dive, and with maximum suit compression at the maximum depth before much gas is used up. There have been fatalities due to overloading the BC. On the other hand, buoyancy control is easiest with the lowest practicable volume of gas in the BC and dry suit, as these volumes change with depth changes, and must be adjusted to remain neutral.

It must be possible to remain neutrally buoyant at the end of the dive, at the shallowest decompression stop, when almost all the diver’s breathing gas has been used up.

Weighting must be sufficient to allow the diver to stay at the shallowest stop with almost empty cylinders, and available buoyancy volume must allow the BC to support the full cylinders.

An unnecessarily large volume BC constitutes a greater risk of loss of control of ascent rate, particularly when combined with carrying more weight than is necessary to allow neutral buoyancy at the end of the dive with empty cylinders. On the other hand, a large volume gives greater comfort and security when floating at the surface before and after a dive.

Operation

The inflatable buoyancy compensator is operated by adjusting the volume of gas contained in the bladder, using an inflation valve to inject gas and one or more deflation valves, or dump valves to release gas. The gas is usually supplied from a low pressure port of the diving regulator on a breathing gas cylinder, or orally, as exhaled gas, though dedicated gas cylinders can be used. At the surface, the bladder is inflated to provide positive buoyancy, allowing the diver to float in a preferred orientation, or deflated to let the diver start to sink to initiate a dive.

Buoyancy control

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. Recreational divers seldom need much buoyancy deficit, but commercial divers may need to be heavy to facilitate some kinds of work. A feet first descent may make ear equalisation easier for some divers, and this is difficult unless buoyancy is slightly negative.
neutral buoyancy: when the diver wants to remain at constant depth, with minimal effort, and no other support. This is the desired state for most of a recreational dive, and allows trim which minimises environmental impact. This state is also optimal for a number of professional diving activities.
positive buoyancy: when the diver wants to float at the surface.

A feature of diving 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 vented (removed or dumped) from the BC when the diver ascends in a controlled manner. This gas (added or vented) maintains the volume of the gas in the BC during depth changes; this bubble needs to remain at approximately constant volume for the diver to remain even approximately neutrally buoyant. When gas is not added to the BC during a descent, the gas in the BC decreases in volume due to the increasing pressure, resulting in a decrease in buoyancy and faster descent with greater depth, until the diver hits the bottom.

The same runaway phenomenon, can happen during ascent, resulting in uncontrolled ascent, until a diver prematurely surfaces without a safety (decompression) stop. This effect is greatest near the surface where volume change is greatest in proportion to depth change. With practice, divers learn to minimise this problem, starting by minimizing the volume of gas required in their BCs.

Skilled scuba divers may be identified by their ability to maintain constant depth in horizontal trim, without fin use. Ease and accuracy of buoyancy control is affected by awareness of changes of depth. Precision control is relatively easy while there is a clear visual reference, but more difficult when the only reference is instrumentation. The most difficult circumstances for most scuba divers are during ascent in low visibility in mid-water without an ascent line, a time when depth control is most important for decompression safety.

Orientation in the water

Divers with neutral buoyancy and horizontal trim with the fins raised are less likely to touch or disturb the bottom

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%, which will adversely affect gas consumption.

The static and stable orientation of an object floating in water, such as a diver, is determined by its centre of buoyancy and its centre of mass. At stable equilibrium, they will be lined up by gravity and buoyancy with the centre of buoyancy vertically 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 & Wet 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.

Generally, the diver has a little control over the position of the centre 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

Weight carried on a belt can be distributed to shift the weight forward or backward to change the position of the diver’s centre of mass. Systems that integrate the weights into the BC, can provide improved comfort so long as the BC does not have to be removed from the body of the diver. When a weight integrated BC 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.

Inflation gas supply and consumption

The usual inflation system is through a low-pressure hose from the primary breathing gas supply, but a dedicated direct feed pony bottle was common on early buoyancy compensators, and remains an option for some models. Most BCs allow oral inflation both underwater and on the surface. This could theoretically reduce gas consumption, but is generally not considered worth the effort and the slight additional hazard of taking the Regulator out of the mouth underwater, and possibly having to purge it before breathing again. Oral inflation is, however, an effective alternative inflation method in case of a failure of the pressurised inflation system.

Gas consumption varies depending on the dive profile and diver skill. The minimum consumption is by a diver who uses the correct amount to neutralise buoyancy and does not waste gas by overfilling, or by excessive weighting.

Hazards and malfunctions

Although a correctly fitted and competently operated buoyancy compensator is one of the most important items of equipment for diver safety, convenience, and comfort, particularly for scuba divers, it is also a significant hazard if used wrongly or in case of some kinds of malfunction

There is a risk of Malfunction of LPI during a dive causing rapid inflation of BCD, leading to a rapid ascent and Barotrauma to the diver.
Redundant bladders may be inadvertently filled, either by unintended action of the diver, or by malfunction of the filling mechanism, and if the failure is not recognized and dealt with promptly, this may result in a runaway uncontrolled ascent, with associated risk of decompression Illness. There is a risk that the diver will not recognize which bladder is full and attempt to dump from the wrong one. The risk can be reduced by ensuring that the filling mechanisms are clearly distinguishable by both feel and position, and not connecting a low pressure supply hose to the reserve until needed, so it is impossible to add gas by accident. Another strategy for avoiding the problem of confusion between bladders in use is to strap the inflator valves together and assume that both are always in use. For this to work reasonably reliably the dump valves must also always be operated together.
Catastrophic bladder failure due to puncture, tearing, or failure of the dump valve or inflation assembly can leave the diver with inadequate buoyancy to make a safe ascent, particularly if diving deep with large gas supply and insufficient ditch able weight. The risk can be mitigated by diving in a dry suit, which can be inflated to add buoyancy in an emergency, by carrying a DSMB, which can be deployed to provide a surface float, and by using distributed ditch able weights – ditching the whole weight belt or too much weight may result in the opposite problem of excessive buoyancy and the inability to maintain neutral buoyancy at decompression stops.
Ineffective or poorly adjusted cam-bands may let the cylinder slip and it may fall off the harness. Twin cam-bands provide redundancy against a cam-band being inadvertently released.
Excessive gas volume, to compensate for over-weighting or carrying heavy equipment, may increase in volume during ascent faster than the diver can vent and result in a runaway ascent, particularly with large volume BCs. This is avoided by using a bladder volume which matches the buoyancy requirements, and avoiding over-weighting.
A loose-fitting BC without a crotch strap may slide up the diver and fail to keep their head out of the water at the surface.
Insufficient buoyancy to achieve neutral buoyancy at maximum depth of a dive due to mismatch of BC volume with weighting and wetsuit compression. This can be caused by excessive weighting or by an undersized BC. A larger volume is needed with large or multiple cylinders to compensate for the greater mass of gas which may be used during the dive.

CLEANING BCD & AFTER CARE:

Rinse the BCD after the usage with fresh water
Wash the bladder inside with flushing of fresh water thru the opening in Hose, and orally inflate the BCD also
Tumble the BCD and it rinses inside the bladder and let it water flow thru the Dump valves and Inflator hose
You may use BCD cleaner also to rinse inside the bladder, which will remove the salt crystals from inside by de solving them.
Soak the BCD in fresh water with fabric conditioner for about 2-3 hours, to remove the smell of sea water and salt
Hang the BCD in shaded area on a BCD hanger