Among the principal hardware that divers use, the buoyancy control device or BCD has contributed greatly to divers' comfort and safety. Designed to allow a diver to control buoyancy on command, the BCD is not a life-jacket. Few, if any, will consistently float an incapacitated diver with his face clear of the water. But by reducing effort needed to maintain neutral buoyancy, even when grossly overweighted by hefty double tanks, they also reduce exertion and provide for a more comfortable and less air-thirsty dive. Descents are easily slowed to ease ear clearing, and ascents can be kept to safe rates without the need to expend energy swimming up. On the surface, a BCD floats a diver comfortably so he does not become exhausted fighting to keep his head above water. In an emergency it can be used to raise a diver to the surface, provide support for resuscitation or used as flotation for tows and pushes.

Between the 1950s and the early 1970s BCDs were actually considered optional - or even undesirable! Even now there are divers, some of them high-profile and extremely distinguished, who shun BCDs. To anyone trained in the last 20 years this probably contradicts everything they've been told.

Before BCDs, divers controlled their buoyancy as best as they could through a combination of careful weighting and breath control. Because divers wearing thick wetsuits and neutrally weighted at the surface could end up excessively heavy at depth, due to suit compression, they often dived under-weighted. If necessary, shotlines were used to control decompression stops, often with additional weights tied off that an ascending diver could clip to himself.

Divers then began adopting life-jackets used by air crew - the so-called Mae Wests. Fitted with a one-shot CO₂ cylinder, these jackets could provide support on the surface or be orally inflated underwater to achieve neutral buoyancy.

The first commercially successful BCD designed for divers was the French-made Fenzy. This horsecollar vest had a wide-bore oral inflator hose through which a diver could easily introduce or vent air for buoyancy control; and a small air cylinder for independent inflation that contained enough air to inflate the jacket about four times on the surface, or once at 30 m. An important feature was an over-pressure relief valve to protect the jacket from expansion damage if the BC was over-filled. The Fenzy was initially viewed with some suspicion - it was felt by many divers that the high speed ascent rates the Fenzy could achieve would damage any diver who lost control. By the mid-Seventies the Fenzy and its competitors had overcome these preconceptions. Training agencies such as the British Sub-Aqua Club were devising programmes to teach divers their proper use. BCDs entered a decade of innovation.

David Parker, of Cornwall's A P Valves, invented the 'Automatic Mouthpiece' that enabled a diver to breathe air directly from the BCD bladder, providing access to air from the small inflation cylinder common to European BCDs. Launched long before octopus rigs, pony bottles and other alternative air sources became popular, the Auto Mouthpiece gained widespread acceptance in the UK. Features like rapid dump valves made an appearance. One design, the Liftstop, had an automatic braking mechanism to slow a full-blown buoyant ascent in the last few metres. The next major innovation was connecting the air supply in a diver's tank directly to the BCD. Direct feeds, as they are known, were developed by a cave diver and quickly became the norm. A US manufacturer went further and designed a direct feed that automatically started to slowly inflate the BC if the diver failed to breathe regularly or the regulator started to freeflow.

Alternatives to the horsecollar BC were tried. Back-mounted BCDs provided better underwater stability and less drag. But many divers were wary of them because they tended to float a diver at the surface on his face. Scubapro countered this with the Scubapro Stabilizing Jacket which supported a properly weighted and trimmed diver head-up at the surface, while holding him horizontal and streamlines while swimming underwater.

These 'softshell' systems all used flexible air bladders requiring that the user make adjustments for depth and the changes in buoyancy. However, some companies were experimenting with hardshell systems. These used rigid chambers that could be filled with air and water, and once set for neutral buoyancy were less affected by depth changes - though suit compression and the reducing weight of the cylinder still required some occasional compensation. The British Buoyco Sub Pack and American Dacor Nautilus are probably the best-known examples. Possibly launched ahead of their time and hampered by bulk, neither survived.

So, where is BCD design going? Nowhere fast and probably in circles. There is a trend towards specialisation, but there has been little in the way of the attempts at innovation that distinguished the Seventies and early Eighties. Current BCDs are soft-shell and controlled manually.

BCDs are clearly very important to the overall safety of your diving. Like other life-support equipment such as regulators and alternative air sources, every care should be taken to ensure that you select a BCD that will measure up to your personal needs. What suits your buddy or instructor may not suit you. This guide is designed to acquaint you with some of the features offered on BCDs so that you can decide which are the ones to benefit you in the water.

So, how do you go about selecting a BCD? Because they are becoming more specialised, it is important to think about the kind of diving you will be doing. At one extreme, a high-capacity wing model with redundant bladder is overkill for shallow Caribbean diving wearing a dive skin and a single tank. At the other end of the spectrum a tropical BC with 0 kg of lift just might not bring up your average trimix diver with two 15 litre steel tanks, a couple of sidemounts, his tools, and the binnacle he's escorting directly to the local receiver of wrecks.

We start narrowing down our choices by considering how much lift we actually need. This is basically the sum of your dead weight in the water, including all the negatively buoyant equipment you will carry and full tanks, plus a bit.

But this is not the whole story. Only the part of the BCD that is inflated and submerged provides lift. If much of that lift is concentrated in the shoulder areas, the diver's face will be close to the water level. If the BC is cut to place most of the buoyancy at waist level, the diver will float higher, which can be more comfortable and can make a diver more visible at the surface while increasing his range of view. The height at which a diver's face is supported above the water level is probably a better endorsement of a BCD than lift alone.

Some divers prefer high-volume jackets that can not only bring them to the surface, but also their buddy as well. Instructors and divemasters may have to make rescues or assist other divers by taking their weightbelts, and many choose high-lift BCDs as a safety measure. Technical divers may back up their 'main' BCD with a fully redundant, piggy-backed BCD to provide emergency buoyancy in the event a BCD is holed or otherwise fails.

To enable a diver to adapt to many different diving conditions which may require different amounts of lift, such as going from tropical to cold water diving, recreational to technical, or amateur to professional, some BCDs accept a range of bladders. The bladder with the optimum combination of adequate lift and minimum bulk and drag can then be selected and attached to the original harness.

Many BCDs are not comfortable when fully inflated. They expand and compress the wearer's torso. This means they can't comfortably be inflated to maximum capacity and claimed lift may be slightly diminished.

There are four styles of BCD. Horsecollars are now largely out of favour and today few manufacturers carry them. As soon as sir is introduced it flows around the neck and pulls the diver's head and chest upwards. This makes the diver unstreamlined so he has to exert more energy to get anywhere. It is tiring and reduces air duration. On the surface, a fully-inflated horsecollar puts a diver on his back. You have to stay vertical in order to see signals from an approaching boat, for example. Again, this is fatiguing. In their favour, the bulk of and inflated front-mounted BC makes it possible to use it to break the grip of a panicked diver who has you in a bear hug. These fit like a waistcoat and do not have breakaway shoulders. The air channels continue over the shoulders to provide additional buoyancy. These can provide as much lift as wing models and are therefore especially suitable for heavy twin-sets or heavier-built divers. Stabilisers are often overlooked in favour of quick-release shoulders which are expected to be easier to release. However, stabs are very easy to remove if you put the crook of the elbow through the armhole first and then sweep the jacket off, especially if you aren't wearing bulky wrist instruments. Stabs usually provide excellent buoyancy characteristics, supporting the diver in a horizontal position underwater and near-vertically on the surface. ADVs (a Seaquest designation that's applied universally, rather like Hoover) evolved from stabs. They have breakaway shoulders for convenience. This design usually results in lower lift, compared to stabilisers. Again, they usually hold a diver horizontally underwater and vertically at the surface. Wing's are currently in fashion. In fact, they are nothing new and can trace their roots back several decades. They were the first popular BCDs to incorporate the tank harness and provided far better underwater trim then their horsecollar rivals at that time. They lost ground after stabs were introduced, but are now in the ascendency with the emergence of technical diving. Cave divers popularised wings initially, preferring high-volume models from small, little known vendors. These were capable of supporting a cave diver horizontally when very heavily loaded with equipment. Technical wreck divers using similar equipment adopted them next, and now there are wings designed for technical divers, wings designed to look tekkie, and streamlined wings that are unashamedly aimed at ordinary sport divers.

Wings usually require heavy single tanks or doubles to counterbalance them to achieve vertical flotation on the surface.

With the exception of horsecollar jackets, BCDs come in a variety of sizes. It is imperative that you have your correct size. Despite the fact that a manufacturer may offer a range of sizes, that is no guarantee of the perfect fit. BCDs that are too small may restrict breathing, lack adequate buoyancy and be unstable. Over-sized BCDs encourage tank shifting and instability, and the diver may find the BCD rides high at the surface while he dangles well below it and uncomfortably close to the water.

Oversized BCDs should not be selected for young divers in an attempt to cut costs ('room to grow'). The cost in safety terms is too high. BCDs with modular harnesses that can be changed to allow for growth while using the original aircell offer a cost-effective and safe alternative.

BCDs are not complicated, but as usual a basic understanding of how they operate and some of the options on offer will help you make an informed choice. Discuss your shortlisted BCDs with your favourite dive store and be open to advice. BCD bladders are either single or double-bag. Single bag jackets must both contain the air and be tough enough to resist punctures and abrasion common to the sport. A material called Hypalon was popular, but is very expensive. Many brand-names use it, and many of the jackets from the early Seventies were constructed from this, and even now, remain serviceable. The more usual choice today is nylon for ruggedness, internally coated with polyurethane for airtightness. This is much more economical. Single-bag jackets are usually a little more streamlined and quicker drying than double-bag designs.

In the late Seventies double-bag jackets became popular for their low cost. A tough outer bag protects a urethane inner which contains the air. The jury remains out on which is better, and many manufactures offer both. Those who don't, offer convincing arguments to support their position!

Bladders and nylon outers are normally constructed from 420, 840 or 1100 denier fabrics. The higher the denier number, the heavier and tighter the weave, increasing abrasion and puncture resistance. Increased wear comes at the expense of decreased suppleness and higher cost. In truth, many dive centres use 420 denier jackets for rental where they survive far more abuse than most recreationally used jackets ever get.

Backpacks divide into soft and hard versions. Soft packs are common on BCDs designed for travelers to warm water destinations. Soft packs reduce weight and bulk and are comfortable for diving without the protection of a thick suit. Usually these packs are designed to use only a single tank and do not prevent tank roll as efficiently as a hard pack. Hard packs are bulkier and heavier, but hold cylinders very securely against the diver's back when the BCD is correctly sized and adjusted. Hard packs will usually adapt to twin cylinder sets fairly easily, though it is important the BCD has sufficient lift to offset the additional dead weight. Retaining bands should be easy to adjust on purpose - and difficult to release by accident. Padding is often offered for use with hard packs to increase comfort. This can be especially beneficial if the pack projects beyond the bag and juts into your lumber region. This is exacerbated by the tank pivoting backwards if the jacket is too large or the shoulder straps are not cinced up, and can rapidly start to hurt. The harness needs to hold the jacket and tank to your body securely so as to ensure proper flotation at the surface and stability underwater. Otherwise the tank will roll you sideways and hit you on the head when you fin down. Load-bearing straps are wide to provide friction and stop the BCD riding up. Modern harnesses are often routed to allow an inflated BCD to expand away from the diver - and to minimise that feeling of being gripped in a vice and unable to breathe properly. Crotch straps are sometimes employed, especially with heavy technical cylinder sets and high-volume BCDs to further reduce tank shifting and BCD ride-up. There are usually up to four options for inflating BCDs. Mouth inflation is standard but very few divers would now choose to use it routinely. It is not complicated to accomplish, but it is inconvenient. Direct feed, push button control from your tank is far better and does not involve negotiating cold lips round a pipette at night with your torch getting entangled in your hoses. Drysuit divers using twin scuba rigs often route direct feeds from independent regulators and tanks to their BCD and drysuit to ensure and air failure still allows one or other system to be inflated. Direct feeds sometimes incorporate a safe second. Some European BCDs carry a small (80-100 l) air cylinder that can be recharged from your normal tank. These provide independent inflation in the event your direct feed fails or there really isn't enough air in your main tank to drive it. Controversially, they may be used to breathe from. However, concerns about the quantity of air available, the ability to follow the required drill in an emergency and health risks from inhaling whatever bacteria inhabit the dark, damp fissures of your BCD bladder have seen this idea largely abandoned. Outside of Europe, BCDs have traditionally used one-shot carbon dioxide cylinders to provide emergency inflation. Because of their limited capacity, they would not necessarily provide enough lift to bring a diver up to the surface from depth. The firing mechanism needed careful maintenance and the pull toggle that detonated the cartridge was prone to catching and going off unexpectedly. Today carbon dioxide cartridges are out of favour and are only fitted on request. All BCDs have dump valves to allow easy venting of air. At least one will incorporate an automatic over-pressure relief valve so that a BCD cannot over-inflate and burst.

One dump will be built into the oral inflator. Some instructors teach students to use this valve to control normal and controlled buoyant rescue ascents. Being able to operate both inflator and dump with one hand is a tremendous benefit and is one reason for ensuring you can grasp both controls simultaneously and have the required strength to press the inflator button. An additional dump may be built in where the oral inflator hose joins the jacket. Pulling down on the mouthpiece releases the air. This allows you to rest your hand on the oral inflator and easily control inflation and deflation withone hand. Other common positions for dumps are on the shoulder opposite the oral inflator and on the lower back. To work efficiently, dumps need to be at the highest point in the water so that air can escape freely. The lower dumps can be helpful when a diver is head-down or horizontal. Often it helps to lean slightly to evacuate all residual air.

Dump valves are designed to vent air faster than the direct feed can deliver it so that a 'sticky' inflator, which might lead to an out of control ascent, can be easily braked. Non-return valves minimise the amount of water that can flow back in to the BCD after dumping. The oral inflator dump does not have a non-return valve and it is easy to introduce a lot of water in to the BCD through this. This can reduce lift and damage dump valves by weakening the springs so that they vent early. It is best to use the dedicated dumps for routine venting.

Weight systems are integrated into some BCDs. Normally block weights or packets of shot are loaded into pockets that incorporate quick release systems. Such systems can be very convenient and comfortable because the weight is spread over a large area - a combination of long dives and heavy belts can hurt your back. However, the total weight of BCD, regulators, tanks and ballast requires careful handling out of the water. Try on the BCD over your normal exposure suit, whether a drysuit or sharkskin. Fit a cylinder to it to test for comfort. Cinch up the straps. If the straps, buckles or backpack dig into you now, imagine how uncomfortable you'll feel after a few minutes of standing around at the dive site. Ask someone else to try to lift the BCD by the tank valve, if the BCD lifts or shifts easily, try a smaller size.

Half inflate the BCD and adjust the straps for fit. Fully inflate the bag and check that your breathing is not restricted and that there is no other type of discomfort. Sweep your arms in an arc to ensure your reach isn't impaired. If you want to be able to reach your own tank valves, try this too. You may need to adjust the height of your tank or tanks, and it's not unusual to have to pitch forward and even loosen straps to get the cylinders to shift enough for you to get a firm grip on the taps.

Consider the harness layout. Women may very well discover at this point that most BCDs are designed by men - try a BCD designed specifically for females as a comparison. And some BCDs can cause problems with your drysuit valves, so this is another point to check. Try the release action. Think about using those releases in an emergency with frozen hands or thick mitts. Check out the pockets. Are they purely decorative? Or have you by chance happened upon a BCD that actually has pockets that work?!

You want to be able to control your BCD. Try the direct feed. Can you grip it comfortably (perhaps with gloves) and control both inflate and dump valves at the same time? If not, try a smaller inflator. Some inflator buttons can be hard to depress. Having to use both hands to inflate your BCD is bad news. Wrist-mounted accessories such as bags, torches and cameras may get tangled up around it. Similarly, check that you can easily operate emergency inflation air or carbon dioxide cylinders. Dump valves should be easy to locate by touch.

Owning a BCD does not confer expertise of use, nor make up for inadequate buoyancy skills. Many diving accidents result from the premise that it does.

• Always weight yourself correctly - with a full tank at the beginning of your dive and holding a normal breath, you should float at eye-level.


• Add just a little weight so that at the end of your dive with a near-empty tank you can make a safety stop with almost no air in your BCD.


• During descents add a little air to your BCD regularly. Little and often allows you to maintain a slow controlled descent. This is easier on your ears and minimises the danger of picking up speed and hurtling into deep water, as well as buddy separations.


• Be ready to dump air as you begin your ascent. Use an ascent meter to control your ascent rate. Skilled BCD users barely have to fin up, reducing effort.


• Use a properly-sized BCD. A major drawback to renting one is that you may not be able to obtain a correct fit.


• Know where all controls and releases are and ensure you and your buddy know how to operate them.


• Take a precision buoyancy control workshop. Especially if you think you don't need to.


• Have your BCD serviced annually.