Scuba Gear I - BCD

What is a BCD or Buoyancy Compensator Device?

It is what all people call the jacket that scuba divers wear it mainly have few basic functions:

• It contains an inflatable bladder which you can add or subtract air from, allowing you to change your buoyancy in the water.
• The dive cylinder is attached to the back of the BCD using a tank strap.
• It allows the diver to carry or attach all of their accessories using its inbuilt pockets and D-Rings.

a more scientific definition ...

It is simply an air bladder that you can inflate or deflate to insure a good buoyancy throughout your dive depending on the depth you are in.

The BCD is a mandatory piece of safety equipment that every scuba diver must have. 

BCDs started out as simple life jackets designed to make a diver float face-up on the surface. They then morphed into front-mounted "horse collar" vests that tended to put divers off-balance and were hard to blow up. Today, BCs have evolved into ever more complex multi-functional systems that do all sorts of things other than just providing the proper buoyancy while diving and acting as safety flotation devices when a diver is on on the surface waiting for a pickup.

Type of BCDs 

There are a variety of different types of BCDs, each type affects underwater movement, buoyancy control and floating behavior and your ability to carry things. These different types are mainly split into 2 main categories;

Back mount BCD also called Wing or Back Flotation Jackets  have their bladders located on the back to the left and right of the tank this leave the front area uncluttered which makes the BCDs popular with photographers , technical divers, back mounts provide good stability.

vers.

Stabilizing Jackets or Vests allow air to move anywhere inside the jacket , air will gather at the highest point and therefore provide a good stability.  Stabilizing jackets have natural confortable feel and excellent buoyancy and are the best choice for most of recreational divers.

What does a BCD contain?

 Air bladder - Air bladders are made of very durable urethane plastic/polyethylene. Some designs have just a single bladder of various shapes and designs, whereas others use a twin-bag system where the inner polyethylene bladder can be replaced if it breaks or springs a leak. In twin-bag designs, the outer layer isn't necessarily a bladder; it primarily serves to protect the inner bladder. Twin-bag designs are usually heavier and bulkier and thus less convenient to transport.

Power inflator - This is a low-pressure direct feed tube from the first-stage regulator. You press a button to pump more air into the suit. Some power inflator systems have their own separate air source which might even also serve as a backup air supply. Some BCs have/had carbon dioxide cartridges for emergency inflation at the surface. These present a risk of accidental inflation and are no longer available in the US.

Oral inflator -- An inflator hose attached to the diver's left shoulder that lets you inflate the BC via blowing into a mouthpiece.

Dump valve - This is to let air out of the BC and is usually located on the left shoulder where the inflator hose is. The valve is actuated by pulling a cord. Some BCs have two or more dump valves, and they can be in varying locations.

Pressure release valve - This valve is there to keep the BC from bursting due to overinflation, which could happen if it is left sitting in the sun or during ascent. The pressure release valve and the dump valve are often combined into a single system.

Cylinder mount - Most current BCs include a tank mount in sort of a backpack type of design. The heavy cylinder is secured by strong nylon webbing and a strap with a locking buckle.
Waistband - That's the usually fairly thick "cummerbund"-style abdominal harness strap that keeps the BC in place.

Integrated weight - Some BCs have pockets for weights, making clumsy weight belts unnecessary. An additional advantage is that weights can quickly be discarded in emergencies. Weights come in different denominations, usually from one to five pounds each. Some have a hard shell whereas others are soft cushions.
Quick-release shoulder straps - Those make it easier to get in and out of the BC.
Pockets - One or more easily accessible zippered mesh accessory pockets to carry all sorts of items.
Accessory rings - Generally located in the front, these stainless steel rings let you clip on additional items.
Hose retainers - If you don't like to have hoses float all over the place, get a BC with clips and Velcro tabs to secure the hoses so you always know where they are and so they won't get in the way or caught.  

Maintenance

Just like all other diving equipements BCDs should be rinsed with fresh water then dried and stored away from sunlight in a dry cool place. However you also need to rinse the inside of the air bladders form time to time, and rinse it from time to time with warm water to dissolve any salt crystal that may have formed inside the bladder.