Of all the equipment divers use to carry them into the wonders of the underwater world, the buoyancy control device (BCD) may be the one that is most taken for granted. After all, it’s just a fancy vest that holds a scuba tank and gives us a handy place to clip on lights and cameras, right?
This article explores the components of a modern BCD and what they do and explains the importance of devoting just as much care and maintenance to your BCD as you do to your regulator, scuba cylinders, and underwater camera.
The modern BCD originated with a device that early skin-diving books called the “safety vest”. Constructed out of rubberized fabric, its distinctive shape gave the vest its common nickname, the “horse collar” buoyancy compensator. The diver placed the device over his neck—in those days, scuba was assumed to be a masculine sport—and strapped it around his chest in a style similar to the preflight safety demonstrations everyone has seen during airline briefings. Some models employed a small CO2 cartridge for inflation since the purpose of the safety vest was not to adjust buoyancy underwater so much as to float the diver face-up at the surface in an emergency.
Over time, the safety vest was modified to allow divers to add and vent air underwater, permitting buoyancy adjustments throughout the dive. Although the compressed air scuba cylinder was still attached to the diver using a separate harness system, the forerunner of today’s vest-style BCD was born.
Sometimes referred to as a buoyancy compensator (BC), the dive industry seems to use the terms BC and BCD interchangeably. The modern BCD has evolved into a sophisticated piece of diving equipment that performs many functions for the diver. The dive cylinder attaches to it, and there is an internal air bladder that provides surface flotation, while at the same time allowing the diver to trim their buoyancy underwater through the course of the dive. As gas from the tank is used, the tank becomes lighter and more buoyant. Likewise, as the diver descends and ascends during the course of the dive, the gas in the BCD contracts and expands, requiring the diver to make a series of adjustments to maintain proper neutral buoyancy.
Every new diver receives training on the proper assembly and use of a BCD, but many divers—and even instructors—fail to realize how many important functions the BCD performs, Here’s a quick run-down of what you’ll see on almost every style of BCD:
- Backplate—The rigid or semi-rigid internal stiffener that gives the BCD strength and shape to support the dive cylinder
- Tank strap—The webbed belt and cam buckle that holds the dive cylinder firmly in place, and which can be adjusted to accommodate different sized tanks
- Interior flotation bladder—The internal compartment which holds air to provide buoyancy at the surface, and which can be adjusted during the dive to maintain neutral buoyancy
- Power & manual (oral) inflator valve—The button-activated valve that sends pressurized air from the dive cylinder into the BCD, which is combined with a small mouthpiece allowing the diver to inflate the bladder manually by exhaling into the BCD
- Deflator button—The valve that opens the air bladder and allows gas to escape, permitting the diver to adjust buoyancy during ascent
- Over-pressure relief valve—The valve that allows air to escape when the internal bladder becomes too full, either as a result of over-filling with the power inflator or in a rapid ascent where the diver has not manually vented the expanding air
- Dump valve (includes cord & handle)—Sometimes called a manual dump valve, most BCDs are equipped with a cord-activated valve which allows the diver to vent air from the BCD during ascent, and to adjust buoyancy during the dive. Primarily used while the diver is in a horizontal (swimming) position, so they do not have to become vertical in the water column to maximize the “air dump” using the hose-mounted deflator
- Inflate/deflate hose—The corrugated hose that connects the power inflator/deflator to the air bladder. A low-pressure hose from the first stage regulator attaches to the inflator with a quick-disconnect fitting
- Cummerbund & straps—The adjustable waist straps which serve as the primary attachment between the diver’s body and the BCD, along with shoulder straps and, on many models, an adjustable chest strap
Additional features on some models of BCDs:
- Shoulder strap snap release—Some BCD styles include a quick-release snap on one or both shoulder straps, allowing more comfortable donning and doffing of the device
- Weight integration release system—For BCDs which include an on-board weight system—known as a weight-integrated BCD—the weight pockets, snaps, straps and Velcro fasteners that together both hold the weights in place, while also permitting the diver to ditch the weights if needed
- Pockets—Most BCDs include pockets which can be used to hold additional equipment such as submersible marker buoys (SMBs), flashlights, small cameras, and the like
- D-rings—Either plastic or metal, external rings on the BCD make handy attachments for cameras, second stage regulator clips, compasses, and other items that need to be secured, yet accessible to the diver
In the second part of this article, we’ll look at some of the things that can go wrong with a poorly-maintained BCD, and talk about what goes into a professional BCD service or overhaul.
