THE BUOYANCY-COMPENSATING DEVICE
(BCD)
One of the most important things to learn about scuba diving is to maintain proper buoyancy. When a diver's buoyancy is out of control it can lead to over-exertion, ear problems, lung damage, decompression sickness, drowning, etc. A diver should be neutrally buoyant in the water. They should start to float on the inhale and should sink on the exhale. Going to the bottom or the surface too fast must be avoided. A diver's buoyancy changes for a variety of reasons. Scuba tanks get lighter as the air is breathed out of them. As mentioned in the Wet Suit Chapter, as one goes deeper the suits get thinner. Thin suits don't require the weight -belt load thick suits do. So, at depth the diver becomes negatively buoyant and tends to sink. Negatively buoyant divers find it hard to go toward the surface and may be hitting the bottom (Fire coral!?) constantly. Your lungs are a big factor in buoyancy. To float in the pool the lungs will be inflated.
The buoyancy-compensating device (BCD or just BC) is a piece of life-support scuba equipment that allows the diver to adjust their buoyancy. It consists of an inflatable bag that attaches to the scuba tank and then to the diver. Usually there is a button that controls the flow of air from the scuba tank into the bag (bladder). If the diver feels they are getting heavy the bladder is inflated until neutral buoyancy is regained. If the diver feels they are light and begin to float, air may be removed using a second button. The BCD will allow compensation for changes in buoyancy due to things such as suit compression, and tank air removal.
Some BCD's have a hard pack that is connected to the air chambers. Some have a soft pack. Hard packs tend to have better stability because the tanks don't roll around as much. To compenstae for that, the soft pack vests may have 2 straps for tank attachment, whereas the hard packs have only 1. Soft pack BCD's are easier to pack for travel because they require less space. Tanks may be carried by the handle found on top of the hard packs. Hard packs are hollow and take a minute or so to fill with water. That will change a diver's buoyancy by about 3 lbs.
New divers are oftentimes unaware of changes in their buoyancy. They might be diving along the bottom headed toward the shore. As they slowly get shallower the qwet suit expands as does the air in the BCD. As the diver gets more buoyant the fins are elevated to more the diver down as well as forward. As ascent continues the diver gets out of control and may be forced toward the surface. By the time the diver comes to an upright position to vent the air from the BCD the surface is reached. The ascent was could be dangerously fast. That must be avoided.
There is also a delayed reaction to buoyancy changes. For example, a new diver may find they have become too heavy because of increasing depth. They add air to the BCD in order to increase the buoyancy. Since there is a delay to the change if too much air is added the diver starts rising too fast. Or, a diver decides to start to the surface. They add a little air to the BCD. On the ascent the air expands and increases the diver's speed upward. If the air is not vented the diver may get out of control. Sometimes the air is vented and too much is released. The diver may start to sink toward the bottom requiring more air to be added only to have the process repeat itself. It requires practice, and lots of it, to be able to properly use a BCD.
It is important to come up in the water no faster than 1 foot per second (60'/minute) and even less above 20' (1' every 2 secouds). Usually the smallest bubbles a diver can see in the water rise about 1'/second. Rising faster than that, especially if you have been diving below 30', can create decompression illness and/or lung damage. The ascent must be controlled by properly using the BCD. A direct ascent to the surface requires the diver to be in the "standing up" position, with the left hand on the BCD hose extended upward, the index finger on the exhaust button, and the thumb on the inflate button. The right hand might be extended over the head to prevent hitting objects on the surface.
If the ascent does get out of control from an over-inflated BCD, a stuck inflation button, or a lost weight belt, the diver should immediately lay out flat, with the face toward the surface, and the arms and legs extended. The BCD should be dumped. The breathing should be normal. This will slow the ascent as much as possible. It is called, "the flare back." If the uncontrolled ascent is caused by a stuck BCD inflation button the hose from the regulator to the BC should be disconnected. The BCD should be serviced regularly so that the inflator is kept lubricated and leaks are detected.
When on the surface the BCD should be partially inflated. This allows the diver to float comfortably without exertion. This should be done during the pool training as well. Whenever you are instructed to remain on the surface the BCD should be inflated. There should be no concern about using the air from the tank for this purpose. It takes so little air for buoyancy control on a normal scuba dive the difference in dive time would be less than 1 minute. Even when training in the pool the excessive use of air for surface buoyancy takes little away from the air needed for breathing.
BCD's are also used for surface swims. Either with a snorkel with the diver's face underwater water, or swimming on the back, the surface swim is a snap with a partially inflated BCD. Swimming long distances is easy. Rescues of another diver using both BCD's is very easy compared to a swimming rescue. Carrying a diver for long distances, even while administering artificial resuscitation, is possible and not hard to do. Relieving a leg cramp on the surface using the BCD is easy as well.
You will be instructed to properly put a BCD on the tank and connect the hose from the regulator to it. You will be shown how to donn the BCD with the tank attached, and how to adjust the straps. One important note: Do not pull on the shoulder straps with the full weight of the tank on the back. The quick-disconnects cannot take the force. Either have the bottom of the tank resting on something, have someone hold up the tank, or lean over so the weight of the tank is resting on top of the back, and then pull the shoulder straps tight. You will also learn how to put a BCD on, and take it off in the water while breathing from the regulator.
The whistle on the BCD is for surface use only. Some may be used underwater. They are used to get the attention of another person such as the captain of the dive boat or your fellow divers. Not having a whistle can be dangerous. If a diver surfaces in rough water they might not be seen for the pick up by the boat. If an emergency occurs no one may notice unless the whistle is used to get attention!
On some older BCD's there may be a device used to puncture a CO2 cartridge called a detonator. Many emergency flotation vests found on airlines have a similar device. The CO2 cartridge is similar to those found for making seltzer water. A string hangs down from the detonator. If the vest needed to be blown up in an emergency, the string would be pulled, a needle would puncture the cartridge and the liquid CO2 would instantly turn into a gas filling the BCD. These devices were very popular in the past. Unfortunately, some divers accidentally detonated the cartridge which would send them to the surface in a dangerous uncontrolled ascent. They are not used on most scuba BCD's today.
The BCD should never be used as a lifting device for objects over 5 to 10 lbs. If you see an outboard motor and decide to bring it to the surface using the BCD to provide buoyancy, it could lead to a disaster. If the motor is dropped during the ascent, the upward force on the diver might be so great a dangerous trip to the surface would take place.
It is important for your buddy to know what type of BCD you are using. Where is the inflation button? Does it have weights that are added to into the vest (integrated weights), and how do you dump them? How is the vest removed from the diver? Answers to questions such as these are very important in order to avoid potential problems in an emergency.
After use the BCD should be emptied of any water that may have entered the bladder during the dive. You will be instructed as to how to do this. All the straps should be connected. It should be rinsed if it has been use in salt or dirty water. Rinsing the bladder of salt water is important. It should be blown up and allowed to dry. Than it may be deflated and stored in a place that will not have other objects on top of it to protect the bladder from damage. The BCD should be kept from freezing so ice does not open the bladder's seams!
According to Rick Layton in the July/August 2009 issue of DAN's Alert Diver, The five most common BCD problems (with % in parentheses) are: Stuck inflator valve (26%), Accidentally operation/button confusion (20%), Accidental loss of weights - weight integrated BCDs (17%), Leaking hose/fittings (14%), and Hose/connector failure (8%).