Last Updated on August 19, 2021 by Dan
A buoyancy compensator device, commonly known as buoyancy compensator, BCD or BC for short is a piece of equipment worn by divers to help them maintain their buoyancy underwater. It is usually buoyant and provides positive buoyancy in water so that the diver does not have to rely on swimming skills alone.
For experienced divers who are diving at relatively shallow depths and in water conditions that are considered relatively benign, scuba diving buoyancy compensators serve merely as back-up for controlling buoyancy. In these situations, the buoyancy compensator is mainly used during inhalation to ensure that the air supply last long enough and do not become too buoyant due to high-pressure effect when ascending fast. Experienced divers can control buoyancy accurately by adjusting their buoyancy compensator during exhalation.
For inexperienced divers, or for dives in water conditions that are considered to be potentially dangerous, buoyancy compensators allow uncontrolled buoyancy changes to be countered by inflating or deflating the buoyancy compensator automatically. The automatic adjustment is achieved through the hydrostatic switch which senses the difference in pressure between a gas-filled bladder and the ambient water pressure. If the buoyancy of the diver increases due to an increase in depth (without counteracting efforts being made), then additional gas will be added into the bladder until it reaches equilibrium with water again. This process happens constantly while diving. If you have too much weight on your back or very low lung volume while ascending, buoyancy compensator will automatically provide buoyancy until you reach the surface. The buoyancy compensator is required to be worn at all times during scuba diving activities by law in many countries unless an exemption has been granted for research or training purposes.
Diving buoyancy compensators consist of a buoyant bladder which may either be inflated manually through valves on the buoyancy compensator, or by gas from a high-pressure gas supply regulator. When fully inflated with no more room for further inflation, the buoyancy compensator supports positive buoyancy of approximately 1 kg per litre of air. This amount can vary considerably depending on factors such as insulation and materials used. They are closed-circuit breathing apparatus so that exhaled air and buoyant gas do not escape directly to the water.
The buoyancy compensator absorbs the difference in pressure between ambient pressure, i.e., sea level (atmospheric pressure), and the pressure inside of it. For example, if a diver is at a depth of 10 meters where atmospheric pressure is 2 bar absolute, then there will be an internal (inside) air pressure of only 1 bar in the buoyancy compensator bladder because this is equal to 2 bars minus 10 meters of water. Thus when returning from 10 m below surface back up to 20 m below surface, with no further adjustment being made to buoyancy controls, air has to be let out of buoyancy compensator until it reaches a neutral buoyancy state, requiring a reduction in buoyancy of 20 meters.
The buoyancy compensator does not only provide buoyancy but also acts as a lifejacket in case of an emergency or a diver may black out from lack of oxygen. It is also designed to keep the head out of the water under normal conditions and may be equipped with various accessories such as lights, pockets, dump valves etc.
With the buoyancy compensator controlling buoyancy and maintaining positive buoyancy at all times, divers are able to continue their activities while submerged without having to exert themselves much during exhalation which allows them to conserve energy for other things such as exploring the underwater environment for longer periods when compared to negative buoyancy diving techniques where divers tend to be much more active during exhalation.
Buoyancy compensators are usually buoyed up with air when the buoyancy of the diver increases due to an increase in depth (without counteracting efforts being made), then additional gas will be added into the bladder until it reaches equilibrium with water again. This process happens constantly while diving. If you have too much weight on your back or very low lung volume while ascending, buoyancy compensator will automatically provide buoyancy until you reach the surface.
If there is too little buoyant gas left in the buoyancy compensator, then this may allow the buoyancy compensator to sink gradually without warning and before sufficient breathable gas becomes available from a high pressure source that can inflate buoyancy compensator. The buoyancy compensator may also sink if no gas remains inside it after decompression stops at the surface.
Divers must make sure that buoyancy compensation devices are kept inflated during ascent, descent or when changing depth which is why they should be filled with sufficient buoyant gas before starting a dive and not rely on buoyancy control devices alone to keep them buoyant while diving. Depending on the size of the bladder, they will start to deflate from as little as 1 bar down to as much as 5 bars of pressure differential which would lead to an unsupported weight increase of approximately 8 kg per bar per 10 m of water (60 kg in total) This causes buoyancy compensators to become unusable for buoyancy control if not properly filled with buoyant gas before entering the water. Even a buoyancy compensator with overfilled bladders may lose buoyancy due to excess air escaping through pressure relief valves which can lead to the diver becoming unconscious underwater while continuing on their way towards the surface without being able to breathe after surfacing until fresh gas becomes available in another high pressure source from where it can be recharged into buoyancy compensators.
Clear and unambiguous markings of depth, date, time (UTC), and dive counter are usually marked on best buoyancy compensators so that this information is available at all times even when the diver is submerged under water and they serve as a means of recording dive data by making notes directly onto them.
These buoyancy compensators are usually made of neoprene which becomes less buoyant at higher temperatures. Neoprene based buoyancy compensators can be cooled with water before diving to prevent the bladder from inflating until it is cool enough for the gas inside to liquefy and provide extra buoyancy thus allowing divers to reach greater depths than would otherwise be possible. Other buoyancy compensator materials include PVC, vinyl, polyamide, and silicone rubbers
Some buoyancy compensators have a ‘neutral’ buoyancy level when in an upright position where other models will keep you at a constant ‘negative buoyancy’ underwater – meaning that you can more easily achieve horizontal orientation or remain stabilized horizontally without having to stay in perpetual motion.
Buoyancy compensators are meant to reduce buoyancy problems arising from lung inflation during buoyant ascent, descent or when changing depth. This is achieved by creating a negative buoyancy situation underwater where the buoyancy compensator will automatically float you more easily towards the surface with minimum effort using buoyant gas supplied from a high-pressure source which can be either your tank (if it is still functioning) or another external source of buoyant gas such as your buddy or an alternative air supply.
If you are unable to reach the surface while wearing buoyancy compensators during zero buoyancy situations, then there might be a problem with your primary breathing gas supply that you need to address immediately because this might become life-threatening if left unattended.