The ABCs of BCs

Understanding the primary functions of today's modern buoyancy compensator (BC) will make it easier for you to make the choice of the right BC for you. A BC's functions can be broken down into two basic categories: 1) provide buoyancy and 2) provide a means for organizing and supporting your dive gear.

BUOYANCY

The buoyancy function of the BC is for fine tuning of buoyancy at depth, usually to compensate for the compression of an exposure suit and to provide for emergency buoyancy.

Surface Flotation

The first BCs were nothing much more than inflatable life vests meant to provide buoyancy on the surface, emergency or otherwise. Surface buoyancy, particularly emergency buoyancy, is a primary function of a BC. As such, where that buoyancy is applied to the body can be of some importance. How a relaxed body floats on the surface is due, however, only in part to the BC. Distribution of the lead weighting is also an important factor. Other factors include the tank buoyancy, or lack thereof, as well as exposure suit buoyancy distribution.

It can get complicated. Although the debate rages, it is generally thought that a relaxed diver on the surface should float in a vertical, upright position. Two types of BCs have evolved to provide for these needs. The now classic BC "Jacket" places most of the air capacity under the arms against the sides of the body. This is an excellent strategy, as with this type of BC fully inflated the diver will be lifted head and shoulders out of the water. The side chambers can, however, get in the way and press in on the diver when fully inflated.

Back-mount BCs, sometimes called "wings" are the next most popular design. They tend to be more streamlined in the water and less bulky. Being on the back, they are completely out of the way but they may not always float the diver in the desired position. Much of this, however, depends on the placement of the weights.

Ultimately, the best BCs are now offering hybrids of both designs, offering underarm flotation with back-mount flotation.

Lift Capacity

Lift capacity is a number of little importance to the ordinary sport diver. As long as the number exceeds the amount of lead you will be wearing during a dive, you are fine. For technical, cave, wreck, and other divers in extraordinary situations, as well as instructors, this number could be important as they may need the added buoyancy for carrying extra gear, heavy tanks, recovery, and more.

Bag Types

Air containment bags are constructed in one of two ways, both equally effective. The most popular currently is the single bag construction. Heavy nylon is lined with polyurethane for air-tightness and welded seams. The heavier the nylon the tougher the BC, but also stiffer. Single-bag BCs should be of a gusseted construction that allows the bag to expand away from the body rather than constricting against it.

A dual bag system uses a nylon shell and an inner polyurethane bag. The shell can be somewhat elastic allowing for compactness. Also, the inner airtight bag can be replaced if it fails. You will find the dual bag system used more often on back-mount BCs.

Air Control

How the air is put into and released from the BC is quite important. There should be at least two ways each of putting air in and venting it out should one way fail.

Power inflators with low pressure air supply hoses make inflating and deflating BCs easy. The inflation and deflate buttons should be of different shape, feel and color so that they can be distinguished even in difficult situations. The inflate button should be "soft-touch" so that the diver can carefully control just how little or how much air is allowed in. Backup to the power inflator button is the manual oral inflation housed in the same assembly. Also in the module is a manual exhaust. Most BCs now incorporate a pull dump exhaust on the shoulder that is activated by pulling on the hose to which the power inflator is attached. This is an excellent design feature.

Either at this shoulder dump or elsewhere on the BC it is desirable to have an overpressurization valve. This prevents the BC from rupturing should it become over-inflated. It may be incorporated in a lower back dump valve. Lower back dump valves are desirable because they allow an upside down diver to vent air from the BC quickly.

ORGANIZATION & SUPPORT OF GEAR

The BC is a platform from which to assemble much of your dive gear and then in turn hang it on your body.

Tank

Along with your lead, this your heaviest piece of gear and it goes right on your back by way of the BC. Large nylon band and plastic cam buckles hold the tank in place. Single tank bands will work fine in nearly all situations, if installed properly. Dual tank bands simply give you some peace of mind. Pre-wetting of the nylon bands is critical as this gives the strap a chance to stretch. Afterwards adjust and retighten.

Two additional straps on the back of the BC are helpful in handling the tank. A tank positioning strap helps you place the tank in the same correct position each time by looping it over the base of the tank valve. Another strap in about the same location is meant as a carrying handle and makes it much easier to move the assembly about.

Lead

Weight integrated BCs have become increasingly popular in the last several years. It should go without saying that any weight integration should have the capability of being released all or in part with one hand, in one motion. BCs that can accommodate both lead bricks and shot packets give you more flexibility.

While hanging your lead from your BC does have the advantage of consolidating gear into one package, you have to be able to handle that package. A tank and BC assembly with lead included can top out at 50, 60 even 80 pounds. Can you lift and handle that size package?

The "Backpack"

Before an air bladder was attached, the tank was supported by a "backpack" assembly. This is still very much part of, if not the heart of, the BC. Nearly everything comes together at this point - tank, straps, bladder, etc. At one time there was a very clear distinction between a "hard pack" and a "soft pack" backpack BC. Not so anymore. A hard pack uses a rigid frame to support the tank and straps. It is more secure and stable but can be bulky. A soft pack folds neatly away and is often quite streamlined but provides little support. Hybrid backpacks use a small, compact and lightweight frame for support. Padding against the back is important for comfort. It should, however, be of material that does not affect buoyancy dramatically and dries quickly.

Harness & Straps

Depending on whether your BC is weight integrated or not, the BC will be responsible for holding 40 to 80 pounds of gear and supporting it properly on your back. The configuration of your straps have a lot to do with how that weight is distributed. The cummerbund is the wide waist strap and one of the least understood but the more important of the straps. Properly adjusted, the cummerbund can take much, but not all, the load away from the shoulder and transfer it to the hips. Cummerbunds should be adjustable and elastic. Shoulder straps should be wide with both being adjustable and with quick releases. Sternum straps cross the upper chest help keep the shoulder straps from separating and pulling the shoulders back, but a well configured BC does not always need one.

Hose Control

A good BC will help you keep control of all those hoses you have coming off your regulator. Properly placed 'D'-rings allow the placement of clips and retractors to hold octopus regulators, consoles and the like in their proper position. Be careful on the 'D'-ring over-kill, however. It is not a critical feature and too many BCs today are unnecessarily decorated like Christmas trees. Small plastic 'D'-rings are for lightweight items like octopus regulators and small lights. Stainless steel rings were introduced by technical divers to clip off extra bottles of breathing mix. But they are also good for heavy lights and other heavy accessories. What kind of diving are you going to do?

Pockets

If possible, California kelp divers should avoid clipping off accessories on the outside of a BC. These just turn into kelp snag points. Rather, use pockets for items such as spare knives and small lights. Pockets can become a very handy feature that you'll come to appreciate.

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It's interesting to think about the fact that buoyancy compensators were an unknown piece of dive gear when scuba was young, some 40 years ago. Divers with excellent skills made up for the lack of the BC with superior buoyancy skills and simple backpacks held all the gear together on your back. It was a delicate and often difficult balance. Today's BCs make diving so much easier and safer.