Bouyancy Control Devices (BCDs)
- A mechanism for establishing and maintaining buoyancy for the diver.
- A means for attaching the air / gas supply to the diver.
There is more to BCDs than just buoyancy though. The following article describes some key things that you might want to consider when selecting a BCD for recreational SCUBA.
A Brief History of BCDs
In the early days of SCUBA, a divers gas supply was secured by means of a rigid metal plate that was secured to the diver using a robust harness. Buoyancy aids were sometimes not even available and it fell to the diver to ensure that they were weighted correctly to be able to achieve neutrality underwater as well as being able to comfortably swim to and maintain reasonable buoyancy at the surface.
Adjustable Buoyancy Life Jacket (ABLJ)
As SCUBA evolved, the concept of the (sometime called the diving “doughnut” was introduced.
The ABLJ was used primarily on the surface to maintain good buoyancy and aid diver comfort.
Buoyancy Control Device (BCD)
As SCUBA became more popular, manufacturers looked for more cost effective methods of mass producing BCDs. The traditional metal “back-plate and harness” was replaced with cheaper, lighter weight components.
Most modern day BCDs are constructed with plastic back-plates as both the materials and the injection moulding manufacturing techniques are cheaper than metal. The old “ABLJ” doughnut has been replaced by integrated buoyancy bladders in the BCD.
Unlike the ABLJ, the modern BCD can be used to adjust buoyancy whilst the diver is under water.
A modern BCD may include extras such as pockets, additional D-rings, automatic inflation mechanisms and other “convenience” gadgets. These additions are generally not required by most divers and are included to provide manufacturers with a marketing edge in a fiercely competitive market place.
BCD Key Features
Th BCD is a simple device with only a handful of essential features. These are:-
- A method for attaching the SCUBA tank. For single tank set-ups, this is provided by one or more webbed bands at the back of the BCD. The system of securing the bands varies from manufacturer to manufacturer and may include either a latch or cam-band system.
- An automatic inflater mechanism consisting of a connection to the low pressure port of the regulator set. The automatic inflater is activated by a push button located on the Low Pressure Inflater (LPI) hose or elsewhere on the BCD.
- A manual inflater mechanism enabling the diver to be able to inflate the BCD orally in the event of either an equipment failure or low on air / out of air emergency.
- At least one mechanism for dumping air. Most BCDs provide more than one way to release air. The primary mechanism is usually by means of a “deflate” button attached to the LPI. Extra dump facilities may be provided in other locations such as the shoulder and rear of the BCD.
- An over pressure valve to prevent the dive over inflating the BCD.
This is really all the features that are needed to execute safe and comfortable dives. Most training agencies will teach BCD use only with the above features.
BCD “Convenience” Gadgets
Listed below are some of the extras that can be included with BCDs.
Built in Alternate Air Source
The idea of these devices is to save you attaching a seperate alternate air source to your regulators. In an air / gas emergency, the donor diver breathes firstly removes their main regulator from their mouth. Next, the donor breathes from the BCD air source and offers their main regulator to the out of air diver.
Things to Consider
- Most training agencies do not teach using these types of devices. Even if you are fortunate enough to receive training, would your new dive buddy who you just met on holiday know what to do in an emergency? Chances are, probably not. If your new (uneducated in the ways of “AIR-2″ (”AIR-2″ is the current trade name for devices provided by Scubapro)) buddy has a air /gas problem, there is a strong possibility that, in the confusion when looking at your kit, they might just grab the 1st regulator they see working. Yes, this would be the one in your mouth!
- Being a regulator, it will need regular servicing. The suggested interval for regulator servicing is one year. Most BCDs without built in regulators would not require annual servicing.
- Sharing air on a standard hose length is not comfortable. The donor diver would offer a standard length hose in an emergency. Alternate air sources attached to regulators have extra long hoses designed to enable comfort and good eye contact between divers during an emergency. In an “AIR-2″ scenario, divers would need to ascend sharing air on a short hose length. This can be uncomfortable as it makes reading of gauges and maintaining eye contact very difficult. For inexperienced divers, it might increase the risk of a less than ideal outcome from the emergency due to additional stress resulting from task loading.
Recommendation
On the face of it these devices offer “convenience” and economic (i.e. save the cost of an additional “octopus”) advantages. However, the disadvantages in terms of potential safety far outweigh any benefits from having these devices. I would not recommend built in alternate air sources.
LPI Pull Dumps
An option that many manufacturers offer is a mechanism enabling the diver to pull the LPI to dump air from the BCD. It is provided as a convenience to save the diver time / effort in raising the LPI and pressing the standard deflate button.
Things to Consider
- Pulling the LPI can result in equipment (for example the LPI hose itself) becoming detached unexpectedly. I have personally experienced this twice.
- Additional moving parts as a result of having this “convenience” mean additional probability that the BCD can “fail”. If a pull dump fails open, the diver is unable to establish or maintain good buoyancy with the BCD.
Recommendation
Dumping gas from a BCD is simple enough, just raise the LPI and push the deflate button (as divers are taught during entry level training). The extra mechanism increases the risk of BCD failure for minimal advantages. Many divers do not even realise they have a LPI pull dump! I would not recommend LPI Pull dumps.
Plethora of Additional D-Rings
The idea here is that more D-Rings are better because it offers the diver more points to secure kit and accessories to.
Things to Consider
- Your entry level diver training would have touched on good streamlining. Every piece of equipment clipped to your BCD reduces streamlining and increases drag. Therefore, more effort and less time underwater.
- Clipping excess items to a BCD can make a diver look like a “Christmas Tree”.
- Excess equipment can result in task loading and undue stress whilst diving.
Recommendation
Just take with you what is absolutely required on a dive. Stash accessories such as cameras in BCD or dry suit pockets. Torches can be mounted in other ways than clipping them to D-Rings.
Crotch Straps
These can be provided as options on some BCDs. The idea is to provide additional security and diver comfort whilst wearing the BCD.
Things to Consider
- This option is only supported on some BCDs.
- These simple optional ad-ons can greatly increase diver comfort. For a typical diver at the surface, there are two oposing forces working against comfort. Firstly, the buoyancy of an inflated BCD pushes up. Secondly, the weight of a divers weight system presses the diver down. The net result is that the BCD can ride “high” around the diver with the BCD shoulder straps aligning with the diver usually around ear level.
Recommendation
Adding a crotch strap to a BCD reduces the effects of BCDs riding “high” and moving about on the diver in general. Crotch straps are a simple and cost effective way of improving general diver comfort. I would recommend using BCD crotch straps if available.
Final Thoughts and Conclusion
Most “extras” provided by BCD manufacturers range from simply useless to arguably just dangerous. Be aware that they are offered to entice (usually inexperienced) divers to choose to buy one BCD over another.
The exception to this rule is the simple Crotch Strap that can significantly enhance diver comfort. Unlike the other “gadgets”, it is not generally used as a “unique selling point” when marketing BCDs.
My suggestion for buying a BCD is that divers go through the following list when evaluating BCDs for purchase:-
- Do NOT include built in alternate air sources (e.g. “AIR-2″).
- Do NOT include LPI Pull Dumps.
- Do NOT include excessive D-Rings.
- Do include crotch straps.
BCD
SCUBA
Rowland :: Sep.22.2007 :: Buoyancy Control Devices :: No Comments »
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