Your buoyancy control device is the piece of gear which holds your tank and regulator where they belong, and which holds you where you want to be underwater. The BC (as it’s commonly called) allows you adjust (control) your buoyancy. It contains an air cell (also called a bladder), which a skilled diver will inflate just enough underwater to make him or her float weightless in the water during a dive. On the surface, inflating the air cell (or, as we tend to say, inflating the BC) keeps you safe from waves and swells, makes you more visible, and lets you rest or swim easily on the surface.

Diving in a BC you’re familiar with, and which fits you well, is a treat. It allows you to hold the tank and your weight where they need to be for good relaxed horizontal trim (posture) underwater. With practice, you develop the ability to reach for your BC controls without even looking. The gear, which at first was overwhelming, starts to fade and your focus is on the diving. Life is good.

A BC has three main parts: a harness that secures it to you, a backplate and strap to hold the tank, and an air cell and its controls. Modern recreational BCs often include pouches for your weight. We’ll look at each of these in detail.

The Harness

The harness consists of two shoulder straps and a waist strap, usually made from black webbing and plastic snaps, although metal fittings are also used. The waist strap may include a velcro cummerbund as well as the webbing strap. There may also be a small chest strap which helps to keep the shoulder straps from sliding down your shoulders.

All of these straps allow for a wide adjustment range. Some BCs have a choice of attachment points for them, which allows you to re-size the BC even further. Some BCs include a crotch strap, which keeps the BC from riding up your torso. This is especially helpful for divers who have less-well-defined waists.

Harnesses vary in how much padding they feature. Edges of the shoulder straps are typically trimmed in soft piping, to prevent chafe. Remember that every piece of padding adds a little buoyancy to the BC, which requires you to carry a little more weight. If you always dive in a wetsuit (or drysuit), you will need less padding in the BC to be comfortable. Wear something of similar thickness (a sweater or fleece top) when trying on BCs to be sure they will work over your exposure suit.

The Tank Mount

A recreational BC harness attaches to a plastic or metal backplate. It’s usually covered in padding and may even include built-in lumbar support. The back side of the plate is curved slightly, to fit snugly against a scuba tank.

The BC holds the tank with one or two tank straps. These are also made from strong webbing. The strap may use the traditional and ubiquitous three-slot tightening clip, or it might use one of several new quick-lock mechanisms. The beauty of the tank strap is that you can carry your tank at the position that works best for you. Too high, and your head tends to hit your regulator first stage when you look up. Too low, and your trim may be affected.

We should mention here that instead of a recreational BC, some divers use a simpler arrangement called backplate and wing, or BP/W. A large metal backplate holds one or two tanks with straps or bolted clamps, and a piece of webbing is woven through slots in the backplate to create a spare and efficient harness. Weight is usually worn on a belt if the weight of the backplate and tanks is not sufficient.

The Air Cell

Inside your BC is an air cell, also called a bladder. BCs can be divided into two groups based on how the air cell is designed. Back-inflate BCs use an air cell that is either horseshoe-shaped or donut-shaped, lying between your back and the tank. Jacket-style BCs use an air cell that, in addition to the portion between you and the tank, extends forward on either side of you.

Jacket-style BCs do a good job of holding a tired diver upright on the surface of the water, but some divers feel the sides tend to restrict their breathing or movement. Back-inflate BCs give you a more unencumbered feeling, but you may hear the claim that they tend to push you forward when you’re on the surface. This happens when a diver is over-weighted (which can happen to brand-new divers) and has not distributed that weight for good trim. That unfortunate diver must add a lot of air to the BC on the surface. A correctly weighted and trimmed-out diver, however, will find a back-inflate BC poses no problems.

Integrated Weight Systems

Modern recreational BCs have made the weight belt unnecessary. A variety of weight pouch systems are in use. What they have in common is a removable pouch which takes the weight, and a pocket on the BC waist strap which holds the pouch securely but allows you to remove and dump it should the need arise. These quick-release systems originally used large velcro patches, but as velcro wears out over time, high-end BCs today use specially designed clip systems.

A good BC will also have “trim pockets” behind the shoulders. These are designed to take some weight, moving up from your waist to improve your trim. That weight cannot be ditched in an emergency but it is typically less than half of what’s on your waist.

BC Controls

To control your buoyancy, you add or remove air from the BC as needed. Underwater, neutral buoyancy is the goal. Ascending and descending, being slightly negatively buoyant is ideal (one fins up rather than using the BC as an elevator).

A corrugated rubber tube is attached to the air cell at one shoulder. At its other end are your inflate and deflate buttons. A hose from your first stage connects to them, so you can use tank air to inflate the BC. You can also add air to the hose using the air in your lungs, which is handy on the surface if you’ve run out of tank air.

In addition to dumping air with the deflate button, your BC will include one or more dump valves in other places. At the top of the corrugated tube is an over-inflate valve which will open when the air cell is full, to prevent you from bursting it. That valve can also be opened manually, by pulling on the tube. Other dumps may be found on the other shoulder or the bottom of the air cell, sporting short cords and small knobs to make them easy to locate and use.

Other Features

BCs usually have at least a few D-rings or other attachment points. You’ll use these to clip off your light, camera, slate, whatever accessories you’re taking under water. They also make it easy to stow your alternate second stage (octo) and your gauges so that they’re where you want them and not dragging across the bottom as you cruise. D-rings that are bent so they don’t lie completely flat are very convenient to use, especially when wearing gloves.

Your BC may have one or more pockets, too. Some close with velcro, others use zippers. It’s not uncommon today to see the sheath for a small dive knife mounted directly on the BC, either attached to the webbing or over a weight pocket. That makes it more accessible than it would be on your leg.

Perhaps the ultimate BC accessory is an integrated air source. These go by different trade names, but the principle is the same. A regulator second stage is built into the inflator control assembly, complete with mouthpiece and purge button. This arrangement allows you to remove the octo (alternate second stage) and its hose from your regulator. While the integrated second might not breathe as easily as a conventional octo, it has proven very popular in recreational diving.

Adjusting, Cleaning, Servicing

Before we close, here are a few words about keeping your BC working at its best. Before donning the BC, loosen the shoulder straps as far as possible. Tighten and fasten the waist first. Then tighten the shoulders only until they’re snug. Over-tightening the shoulders will just result in the whole BC riding up your torso in use, ruining your trim and failing to hold you well out of the water on the surface. You should fasten the chest strap last, leaving it slightly loose.

Cleaning your BC after each use is important, unless you’re diving in clean fresh water. Both salt water and chlorinated pool water have an effect on the materials. Drain any water out of the air cell, using the deflate button with the BC held upside-down. Pour some fresh water back down the inflator tube and swirl it around in there, then drain again. Hang the BC up with some air in the cell so the inside also gets a chance to dry.

And finally, stick to the maintenance schedule (usually annual) specified in your warranty. If you have an integrated second stage, that should be serviced every time you service your regulator.