Summary

A Buoyancy control device (otherwise known as a BCD, or Buoyancy compensator) is a piece of equipment used in SCUBA Diving.

What is it used for

The BCD is used to control your buoyancy in water when SCUBA diving by filling or deflating with air. This allows you to alter your depth under water without having to "swim" up or down.

By careful, or learned manipulation of the amount of air inside the BCD, you can achieve a state of neutral buoyancy, which is as close to being weight-less as you can be unless you are a pilot (or know one), or an astronaut. (Or in the case of recent events, have a lot of money and can afford a trip on a Russian rocket into space). Once you have achieved neutral buoyancy, you can then concentrate on looking around rather than staying down near the bottom (or up slightly off the bottom).

The BCD can also be fully inflated on the surface to bring you higher out of the water, for visibility, safety and comfort reasons.

How does it work?

The BCD contains a bladder which can be manually or mechanically inflated. When manually inflating the BCD, you blow air through a one way valve into the bladder. Mechanical inflation is done by releasing air from your air tank into the BCD through a connection from your primary/first stage regulator.

Air is released from the BCD by a simple controlled purge valve. Though many BCDs' have an emergency purge or dump valve that rapidly deflates the BCD. This valve is usually located on the shoulder, or the lower back of the BCD and is useful if you are rising too fast or are trying to purge the last amount of air from your BCD (an indication you need more lead weights).

The usual position for the manual and mechanical inflation and the controlled purge device is in a tube which extents from the shoulder down.

Types of BCDs

There are three main types of BCDs today, although there have been many in the past, and there are a number of variants available today. These types are the front mounted, back mounted and jacket style.

Front mounted

Although they tend to be cheaper than the other styles they are usually the least preferred style. The air tank is mounted separately using a harness, and the BCDs bladder is on your chest. This style has the major disadvantage of its tendency to flip you on your back, which while moving underwater is controllable, while stationary can be rather frustrating. On the surface is allows you to swim on your back which although is more comfortable, doesn't allow you to see where you are going.

Back mounted

The back mounted design has the bladder located on your back, usually on either side of the air tank. The advantage is it properly rights you underwater so you are on your stomach, however the disadvantage is it can cause problems on the surface. Back mounted BCDs tend to flip the swimmer on their stomach on the surface when unconscious, which can lead to drowning. (Think of the case where a diver runs out of air and does an emergency ascent). There are a large number of people who swear by back mounted BCDs, and as is the case with everything; to each his own.

Jacket style

The jacket style is the most common style available today. It is worm much like a jacket, and has the bladder located primarily on the sides of the BCD (Where the pockets of a normal jacket would be). The jacket tries to offer the best of both worlds from the Front and Back mounted designs. It doesn't have the tendency to flip you over underwater, and allows both swimming on your back and front on the surface. It also doesn't force a swimmer on their front. (though unlike with the back mounted style, it can still happen).

Features

Since the most common BCD today is the jacket style, I will discuss features found on it. The other styles do share some of these features. All BCDs have a bladder which can be filled and emptied of air. This is the primary purpose of a BCD. The methods of inflating and deflating are discussed above.

Air tank attachment

All BCDs (except the front mounted style where the air tank is worn separately) have an attachment for the Air tank. This is pretty much the most important feature of a BCD after the bladder, after all, without air, you aren't SCUBA diving, you are free diving. The method of attachment is usually a ratchet style material belt which goes around the tank and uses friction to stop the tank from slipping out. The other attachment method is a small loop that goes around the top of the tank which is used in conjunction with the main method as a backup. If you were purchasing a BCD, make sure that it has padding between the tank and your back.

Pockets

Very useful since wet-suits don't have any (usually, though I have seen one or two that do!). Used to store anything from torches (flashlights), emergency sausages, knives, your car keys (oops, don't loose those 30ft. down), etc. Not much to note other than the pockets have a mesh component to avoid collecting water.

Attachment points

These are clips, D rings, etc that allow you to attach various things to your BCD, usually your inflation control, gauges, dive computer, knife, torch, pony bottle etc. Generally anything that won't fit in a pocket, or needs to be seen/reached easily.

Inflation control

Generally a tube/hose leading from the shoulder that contains a few buttons. One will inflate the BCD from the air tank, another will purge air from the tank, and a third (sometimes found) will open a one-way valve to allow manual inflation. (If that button doesn't exist, you blow into the tube while depressing the deflate button). Some BCDs have an integrated inflation control which is located near the bottom of the BCD rather like a small arm rest.

Integrated weights

Some BCDs have special pockets for lead weights to be placed. If you are lucky enough, you can carry enough weights in these pockets such that you don't need to wear a weight belt. However it is important to use only these special pockets or a weight belt to store weights, as they are designed for rapid release in the event of a problem. The disadvantage of integrated weights is it makes the BCD even heavier. The BCD is already attached to the air tank, so another 12kg of lead makes it more difficult to lift the BCD out of the water.

Dump valves

Most BCDs have at least one dump valve that will rapidly deflate the BCD. The most common location is on the inflation control tube, by pulling on the tube a valve on the shoulder opens up. However, please check before you dive your BCD has such a feature. Ripping the inflation control off your BCD while underwater can have undesirable effects!

The other location for dump valves is the opposite shoulder to the inflation control and on the lower back. This is a New Zealand divers favourite dump valve since we usually are upside down head first in a hole looking for Crayfish, and by reach back and dumping you can stop yourself from floating feet first out of the hole.

Overpressure relief valve

If you over inflate a BCD, almost all modern BCDs have a relief valve that will dump the excess pressure.

Harness

The harness is used to attach the BCD to your person. It should be comfortable, easily readjustable, and able to fit when wearing a wetsuit (7mm adds a lot of bulk)


Sources: The PADI Open Water diving manual, numerous web sites, personal experience, and conversations with other divers.

Disclaimer: The above represents my opinion on BCDs. Although I speak from some experience, and have referred to multiple sources when creating this node. I may be wrong. As such, please feel free to /msg me any corrections.

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