Buoyancy Compensator (diving) - Neutral Buoyancy

Neutral Buoyancy

The diver needs to be able to establish three states of buoyancy at different stages of a dive:

1. negative buoyancy: when the diver wants to descend or stay on the seabed.
2. neutral buoyancy: when the diver wants to remain at constant depth, with minimal effort.
3. positive buoyancy: when the diver wants to float on the surface.

To achieve negative buoyancy, divers who carry or wear buoyant equipment must be weighted to counteract the buoyancy of both the diver and the equipment.

When underwater, a diver often needs to be neutrally buoyant so that the diver neither sinks nor rises. A state of neutral buoyancy exists when the weight of water that the diver displaces equals the total weight of the diver. The diver uses a BC to maintain this state of neutral buoyancy by adjusting the BC's volume and therefore its buoyancy, in response to various effects, which alter the diver’s overall volume or weight, primarily:

• If the diver's exposure suit is made of a compressible gas-filled material such as foamed Neoprene, the volume of the material will change (Boyle's Law) as the pressure changes when the diver descends and ascends. The volume of air in the BC is adjusted to compensate for this.

• Gas contained in the flexible air spaces within the diver's body and equipment (including gas in the BC) is compressed on descent and expands on ascent. The diver normally counteracts this by adding gas to the space or drysuit, in order to avoid "squeeze". Gas content in the BC is adjusted to correct buoyancy if these other corrections are not enough.

• As the dive proceeds, gas is consumed from the diving cylinders of the breathing equipment. This represents a progressive loss of mass which makes the diver more buoyant; the diver’s overall buoyancy must be reduced by venting air from the BC. For this reason the diver needs to configure his equipment to be a little overweight at the beginning of the dive so that neutral buoyancy can be achieved after the loss of the weight of the breathng gas. Air or nitrox weighs about 1.3 grams for every litre at standard pressure. Thus, the magnitude of weight change from loss of air during a dive varies from roughly 4.3 kg (9.5 lbs) representing the total air content of a steel 15 litre cylinder at 230 bar/3500 psi (in practice, reserve requirements dictate that only about 8 lbs of this will be breathed), to about 5 lbs difference for the smaller 80 ft3 aluminum-80 (AL80) tank (11.1 litres internal capacity) pressurised to 200 bar/3000 psi, and again assuming that only 5/6ths of the air in the tank is used, leaving a typical safety reserve.

In practice, the diver doesn't think about all this theory during the dive. To remain neutrally buoyant, gas is added to the BC when the diver is negative (too heavy), or vented from the BC when the diver is too buoyant (too light). A feature of diving is that there isn't any automatically stable equilibrium position for a diver wearing a BC, or even simply for a diver with lungs full of air. Any change in depth from a position of neutrality and even a small changes in volume, including the simple act of breathing, result in a force toward an even less neutral depth. Thus, maintenance of neutral buoyancy in scuba must be a continuous and active procedure—the diving equivalent of balance, in a positive feedback environment. Fortunately, the diver's mass provides a source of inertia, as does the liquid medium, so small perturbations (such as from breathing) can be compensated for easily by an experienced diver.

A feature of scuba which is often non-intuitive for beginners, is that gas generally needs to be added to the BC when a diver descends in a controlled manner, and valved-off (removed or vented) from the BC when the diver ascends in a controlled manner. This gas (added or vented) maintains the volume of the gas bubble in the BC during depth changes; this bubble needs to remain at constant volume for the diver to remain even approximately neutrally buoyant. When gas is not added to the BC during a descent, the gas bubble in the BC decreases in volume due to the increasing pressure, resulting in faster and faster descent with depth, until the diver hits the bottom. The same runaway phenomenon, an example of positive feedback, can happen during ascent, resulting in uncontrolled ascent, until a diver prematurely surfaces without a safety (decompression) stop.

With experience, divers learn to minimize this problem, starting by minimizing the size of the "constant volume bubble" in their BC's. This requires learning the minimum weighting requirement needed for their system (see factors above). These techniques keep the volume of the gas bubble within the BC as small as possible at the beginning of a dive, while leaving just enough gas in the BC at first submersion to be able to compensate for the expected slow loss of diver weight as the dive progresses, as a result of gas used (in practice, about 5 to 8 lbs. lost per cylinder, as noted above).

Somewhat complex automatic reflex behaviors are also developed by experienced divers, involving breathing control and BC gas management during depth changes, which allow them to remain neutrally buoyant from minute to minute during a dive, without having to think much about it. Experienced scuba divers may often be identified by the fact that they maintain neutrality without any fin use, as fish do.