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An Introduction to Stage Decompression
Many divers are familiar with the term "no-decompression diving." In all reality there is no such thing. All dive tables set maximum ascent rates for divers which is actually a decompression procedure. When discussing dives that do not require mandatory decompression stops to avoid Decompression Sickness (DCS) a better term to use is no-stop diving. Conversely, when discussing dives requiring decompression stops a more accurate term is "stage-decompression." "Stage" refers to the fact that the divers stop at various levels or "stages" during ascent to reduce the risk of DCS.
As you recall from your entry-level scuba course, as divers expose themselves to the increased pressures of depth, inert gases such as nitrogen and helium are absorbed into the tissues and blood. No-stop divers make realitively short stays at depth in order to reduce the amount of inert gas absorbed into the body. This allows them to make a direct, controled ascent to the surface without substantial risk of DCS. Technical divers, on the other hand, frequently stay at depth well beyond the no-stop limits, forcing them to execute a series of stops to reduce the inert gas tissue loading on their way to the surface (stage -decompression).
Becuase stage-decompression divers do not have the option of making a direct ascent to the surface in an emergency they must apply special procedures and equipment to reduce their risk exposure. Protecting their breathing gas supply is obviously a priority. Typically stage-decompression divers will dive with two tanks, coupled together via a special manifold with an isolation valve between the two tanks. An independent first-stage/second-stage regulator system is routed from each side of the manifold. In the event of a regulator failure the diver can switch to their back-up, shut off the malfunctioning side, and start their ascent to the surface. Of course, like no-stop divers, stage-decompression divers may also share air with a diving partner via one of the two regulators.
Another concern for divers executing stage-decompression dives is thermal protection. As you recall water conducts heat away from the body about 25 times faster than air. Because of their extended stays on the bottom and the slow process of reaching the surface, these divers are often in the water for 60, 90, 120 minutes or more! Even in very warm water drysuits are the rule rather than the exception.
Stage-decompression divers will frequently use enriched air nitrox and/or pure oxygen to shorten the duration of their required decompression stops. By switching to a gas containing a higher fraction of oxygen and reduced percentage of inert gas, the excess gas loaded in the diver's tissues comes out of solution quicker and is expelled via exhalation. For longer or deeper dives, stage-decompression divers may make two or three gas switches on their way to the surface. For truely extreme dives even more decompression gas changes may be employed.
Properly executed stage-decompression dives probably carry about the same or less risk of DCS as no-stop dives. However, the lack of direct access to the surface and the need for using specialized equipment and procedures do increase the risk factor for this type of diving. Before attempting stage-decompression dives the diver should have a good understanding of decompression theory, diving physics and physiology. They also need to be extremely comfortable in the water and have their skills honed to perfection. Specialized training is obviously required.
You can learn more about stage-decompression diving by visiting other areas of SportDiverHQ and following some of the provided links to other websites. SportDiveHQ can also provide and/or arrange for training in stage-decompression diving.
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