Buoyancy compensator (diving)
A buoyancy compensator, also called a buoyancy control device, stabilizer, stabilisor, stab jacket, wing or adjustable buoyancy life jacket, depending on design, is a type of diving equipment which is worn by divers to establish neutral buoyancy underwater and positive buoyancy at the surface, when needed.
The buoyancy is usually controlled by adjusting the volume of gas in an inflatable bladder, which is filled with ambient pressure gas from the diver's primary breathing gas cylinder via a low-pressure hose from the regulator first stage, directly from a small cylinder dedicated to this purpose, or from the diver's mouth through the oral inflation valve. Ambient pressure bladder buoyancy compensators can be broadly classified as having the buoyancy primarily in front, surrounding the torso, or behind the diver. This affects the ergonomics, and to a lesser degree, the safety of the unit. They can also be broadly classified as having the buoyancy bladder as an integral part of the construction, or as a replaceable component supported inside the structural body.
The buoyancy compensator requires a significant amount of skill and attention to operate, because control is entirely manual, adjustment is required throughout the dive as weight reduces due to gas consumption, and buoyancy of the diving suit and BC generally varies with depth. Fine buoyancy adjustment can be done by breath control on open circuit, reducing the amount of actual BC volume adjustment needed, and a skilled diver will develop the ability to adjust volume to maintain neutral buoyancy while remaining aware of the surroundings and performing other tasks. The buoyancy compensator is both an important safety device when used correctly and a significant hazard when misused or malfunctioning.
The ability to control trim effectively is dependent on both appropriate buoyancy distribution and ballast weight distribution. This too is a skill acquired by practice, and is facilitated by minimising the required BC gas volume by correct weighting.
Function
The buoyancy compensator is used by ambient pressure divers using underwater breathing apparatus to adjust buoyancy underwater or at the surface within the range of slightly negative to slightly positive, to allow neutral buoyancy to be maintained throughout the depth range of the planned dive, and to compensate for changes in weight due to breathing gas consumption during the dive. Where staged cylinders are used, it may also be used to compensate for weight changes when dropping and retrieving these cylinders. Variations in the buoyancy of wetsuits depend on the volume and density of the suit and the ambient pressure, but for thick suits at depth it can be in the order of 10 kg. Variations in the buoyancy of dry suits should be compensated by maintaining a constant volume of gas inside the suit, by manual addition and a combination of automatic and manual dumping, independently of the adjustments to the buoyancy compensator made to compensate for gas usage.Scope of application
The buoyancy compensator is a standard item of scuba diving equipment, though not always necessary, and an optional item for surface-supplied diving, where neutral or positive buoyancy may not be necessary or desirable. Breathhold divers do not have a gas supply to operate a buoyancy compensator, so they cannot use them, though they may wear an inflatable vest lifejacket for positive buoyancy at the surface. Atmospheric pressure diving suits may use a trim tank similar to that on a submarine for small adjustments, but can be ballasted to be almost precisely neutral, and are virtually incompressible within their designed operating range.Accurate and reliable depth control is necessary for safe decompression. The surface-supplied diver has the option to use the umbilical for depth control with the assistance of the line tender, and a tethered scuba diver can use the lifeline in the same way. Similarly, any diver using a shotline or jackstay to navigate between the surface and the work site can use it for depth control, making a buoyancy compensator non-essential provided the diver can find the shotline when needed.
In most recreational and professional scuba, neutral buoyancy during most of the dive is necessary or desirable, as it gives the diver enhanced mobility and maneuverability, and allows the diver to avoid contact with delicate benthic organisms, and to fin without disturbing sediment which can rapidly reduce visibility. For this function a buoyancy compensator is necessary.
Positive buoyancy at the surface is a safety requirement for any diver who must swim to or from the point of descent or surfacing, but this does not need to be precisely controllable buoyancy.
The buoyancy compensator is intended to control buoyancy of a diver and their personal diving equipment, including stage and bailout cylinders, and for minor additional equipment such as reels, cameras and instruments that are lightweight or near neutral buoyancy. It is not a buoyant lifting device for heavy tools and equipment. If a diving task requires the diver to work heavy, it is almost always better, and always safer, to use surface supplied equipment.
If used by saturation divers to allow mid-water work, precautions must be taken to limit possible uncontrolled upward excursion. This may be possible by limiting excursion umbilical length or using an underwater tending point.
Operating principle
A buoyancy compensator works by adjusting the average density of the diver and their attached equipment to be greater than, equal to, or less than the density of the diving medium. This can be done in either of two ways:- Variable volume, or inflatable: The volume of a flexible device can be varied by adding or removing ambient pressure gas, which has a relatively low density, or
- Variable density or compressible: The density of a rigid device can be varied by compressing or expanding the internal gas by adding or removing incompressible diving medium, which has a relatively high density.
Variable volume type
The common type of buoyancy compensator increases buoyancy by adding gas at ambient pressure to a flexible airtight bladder, thereby increasing the volume, and decreases buoyancy by releasing the gas into the water. This volume of gas will compress or expand as the ambient pressure varies with depth, following Boyle's law, and therefore the buoyancy of the system will increase and decrease in proportion to the absolute pressure variation and the volume of gas in the bladder. The variation of buoyancy for a given change of depth will be greater near the surface than at greater depth and greater for a large volume of gas than for a small volume. The range of depths for which the diver can compensate for these changes by voluntary adjustment of lung volume while breathing effectively is therefore dependent on the volume of gas in the bladder and the nominally neutral depth, where breathing at normal tidal volume of about 500 ml results in approximate dynamic equilibrium, and the diver remains at that depth without additional effort. This type of buoyancy compensator functions by increasing buoyancy from the most stable state, which is empty, so weighting is done for neutral buoyancy at the condition of least mass, which is at the end of the dive with the cylinders empty, at which point the diver should be able to stay at the last decompression stop without physical effort.A few illustrative examples are presented here. They are simplified but numerically realistic:
Variable density type
An alternative method of adjusting the buoyancy of the diver is by varying the density of a rigid container of constant displaced volume, by adjusting the volume of added water in a normally gas filled space. This approach can also be described as buoyancy reduction, as opposed to buoyancy addition when gas is added to a flexible ambient pressure space. Such variable buoyancy pressure vessels are used by submersibles and submarines for fine buoyancy and trim control. Water from the surroundings is injected into the tank to decrease buoyancy by ambient pressure difference or by a pump, depending on the internal gas pressure. Water can be removed in a similar way to increase buoyancy. As the tank is rigid and effectively incompressible within the range of diving depths for which it is intended, buoyancy changes due to depth variation during the dive are negligible, and the diver only needs to adjust the buoyancy to account for gas usage and volume variation of the diving suit.One way this can be done is by pumping water into a scuba cylinder, using a flexible bladder to keep the gas and water separate, which requires a cylinder made for this purpose, with a water inlet to the space around the internal bladder, connected to a high-pressure pump and control valve system. If the weights have been optimised for the equipment, and the diver is nearly at neutral buoyancy at the start of the dive, very little water needs to be added at the start of the dive, so the gas pressure is not greatly increased. More water is pumped in during the dive to compensate for the mass of gas used, but by this time the pressure will have dropped considerably. A small amount of residual gas pressure on surfacing will be enough to eject the ballast water to establish positive buoyancy. If this system is used with additional sling mounted bailout or decompression cylinders a larger volume of water will be needed to compensate the additional gas usage, and the gas pressure in the buoyancy compensating cylinder will rise a bit more. The Avelo system uses this mechanism, with a rechargeable battery powered pump unit which is demountable from the cylinder.
This system is inherently more stable with hydrostatic pressure variation, and decreases buoyancy from the initial state, which is with a full cylinder of gas at the start of the dive. To minimise the pressure rise caused by pumping ballast water into the cylinder when it is full, weighting is done for near neutral buoyancy at the start of the dive, with just enough positive buoyancy to safely swim at the surface with a full tank, and pump in a relatively small volume of water to descend, which is periodically increased during the dive to compensate for mass loss of breathing gas. After surfacing, the added mass of water is released to give a comfortable positive buoyancy and minimise equipment weight when getting out of the water. If using a dry suit the initial positive buoyancy at the surface could be controlled by suit inflation in excess of the amount needed for undergarment loft, allowing descent by dumping from the suit.
The depth range in which effectively stable neutral buoyancy can be maintained is inversely proportional to the volume of ambient pressure gas spaces in the diver's equipment. When an incompressible buoyancy compensator is used, almost all of the variable volume is in the diving suit, and the depth range of effectively neutral buoyancy is maximised. A diver without a diving suit would be effectively neutrally buoyant over the full depth range of the dive, and only need to adjust buoyancy for mass loss as gas is used.
A superficially similar system was used in the Dacor system of the 1970s, where the volume of ambient pressure gas in the rigid shell was maintained by a demand regulator automatically sensing a pressure deficit between the internal and external pressures and an automatic dump valve to release internal overpressure, much like the volume control of a rebreather loop by automatic diluent valve and overpressure valve, but this reduced buoyancy by flooding the shell with water and increased the buoyancy by adding gas at ambient pressure from the breathing gas supply, rather than reducing the stored gas volume by compressing the gas. Water was added to or removed from the shell to compensate for suit compression and gas use by a manually operated valve. An inherent problem with this system is that the diver must still manually compensate for changes of buoyancy due to suit compression and expansion when changing depth, so the advantages are less marked when used with thick, compressible, diving suits.