Recreational skin divers do not have to make decompression stops because of the absence of adequate support facilities. However, divers taking long recreational dives should carefully control their ascent to reduce the risk of decompression sickness.

Technically, decompression is any procedure for the relief of pressure or compression. I feel it has become seriously misunderstood. Decompression is a straightforward, natural procedure. Any ascent from a dive is decompression. Part of the problem seems to be in the use of some of the thumb rules that have evolved. A review of what decompression is will help divers understand, and safely apply, any decompression required in their diving plan.

Pressure is physical stress exerted uniformly in all directions. It is measured by the force exerted times the unit of area. Divers use several kinds of measurement of force and pressure – pounds per square inch, pounds per square foot and, less frequently, atmospheres of depth. There are, of course, several others.

Compression is a reduction in the volume of a substance owing to the application of pressure. A diver, under pressure on a dive, has a fixed volume. No reduction in volume can normally take place. Compression, in this case, is the increase in pressure of the breathing gas needed to equalize internal pressure with ambient (surrounding) water pressure at depth.

Decompression & diving1

DECOMPRESSION AND THE RECREATIONAL DIVER

Recreational divers should remember that diving is recreation, enjoyment and socializing. To enhance these aims, it should be as free of potential hazards as possible. To maximize safety and enjoyment, recreational divers normally avoid decompression diving. When considering the minimal support available in most recreational diving, there is no valid reason to undertake a complex, planned staged (stop) decompression dive. Finally, a diver in decompression ties up the entire operation.

Commercial, military, technical and advanced specialty divers may need to make decompression dives. They plan decompression to permit safe completion of the mission. To safely accomplish staged decompression schedules they require additional training, support equipment and personnel dedicated solely to their needs. An important fact to remember is that each additional step in a dive profile increases the potential for an accident. However, this does not mean staged decompression dives are unsafe. Obviously, staged decompression procedures increase the cost of diving with no material reward. There is nothing more monotonous or lonely than decompression time.

Although recreational divers do not normally make decompression dives, they know that under some conditions these may become necessary. Thus, all divers should know the principles and the effects of staged decompression.

Divers developing a plan will probably remember the laws of gases from their training. One law states that a gas under pressure in contact with a fluid will be absorbed by the fluid until internal pressure is equal to the surrounding pressure. However, the rate of absorption will depend on the differential in pressure, the viscosity of the fluid, the temperature and several other factors. In diving it can be even more complex.

As divers descend, their scuba, regardless of tank pressure, delivers air at a pressure equal to the surrounding (ambient) water pressure. Theoretically, tissues immediately start to absorb the air at ambient pressure. For the moment we will call this in-gassing.

Body tissues use some, but not all, of the oxygen (O2 ) in metabolic processes. In this process part of the used O2  is converted to carbon monoxide (CO2 ). At least part of the remaining O2  goes into solution in tissues. It has long been assumed that nitrogen serves only as a diluent for the concentration of O2 . However, there is some indication that, for complete body functioning, some of the nitrogen is used. As the dive progresses there is an increase in the concentration of CO2 , nitrogen, excess O2  and small percentages of other gases in the tissues. The longer divers stay under pressure, the higher the concentration of gases in the tissues. If they stay at depth (under pressure) for a long time, their tissues will no longer absorb additional gas – they are saturated for that depth.

There are many kinds of tissue in a diver’s body. There are also many rates of saturation for these tissues. Some are fast and become saturated and desaturated – decompressed – quickly. Some slow tissues saturate and desaturate very slowly. Tissues also absorb gas at different rates because of several other, sometimes changing, variables. The temperature of the water surrounding the diver apparently has some effect. How hard the diver works may also play a part. The age and weight of the diver probably has minimal influence.

During a slow ascent, gases in the tissues are believed to come out of solution and return to the lungs where they are exhaled. If the ascent is too rapid gases apparently come out of solution and form bubbles within the tissues. These may cause various symptoms and signs categorized as the bends. Several decompression tables and, more recently, a plethora of dive computers, have become available. These provide information that help divers keep the rate of ascent under control. Gases then return to the lungs for exhalation rather than forming bubbles.

It is not possible to develop a decompression schedule for each of the many tissues. Also, it is not practical to use a table providing decompression schedules for the slowest – or the fastest – tissues. In actual practice the table or computer provides a decompression schedule that exactly matches only one set of tissues. The safest decompression schedule would match the slowest tissue but such a schedule would very nearly prohibit diving.

At any time during a dive, the gas pressure in a diver’s tissue is equal to a given pressure (depth). A diver who has ascended to this equalization point would experience no in-gassing or out-gassing and, what is more important, no bubble formation. Safe decompression would be easy if it were possible to accurately determine the point of equalization of pressure. Unfortunately, finding this neutral point in the in-gas/out-gas phase is probably not possible.

Gas absorption is a function of both the depth (pressure) and the duration of the dive. Gas absorption is also a relatively slow process. For several reasons, ambient pressure can be reduced a certain amount without bubble formation and bends symptoms. This is exactly what a diver can safely accomplish with a decompression table or a computer. With proper training and some experience, the diver can also safely schedule decompression stops if they become necessary.

I have safely used perhaps a dozen different decompression tables and a few decom meters and computers. I feel the most important element in safe decompression, whether with or without stops, is the rate of ascent. Unfortunately, the prescribed rate of ascent will vary with different tables and computers.

THE SAFETY STOP

Divers should be particularly alert to the fact that on completion of most recreational dives they will have absorbed very little excess gas. They will still be in-gassing during part, probably most, of the ascent.

It is important that the prescribed rate of ascent be followed for the table in use. Know your tables or computer and follow instructions.

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