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The excess nitrogen passes from your body tissues into your blood. The nitrogen then passes from your blood to your lungs, where you exhale the nitrogen. This process occurs rapidly during the first few minutes after your ascent, but it takes many hours for your body’s nitrogen level to return to normal. If you reduce the pressure on your body gradually, and the reduction in pressure from the depth of your dive to the surface is not too rapid for the amount of nitrogen you have absorbed, offgassing occurs without a problem. However, if the change in pressure is sudden, the nitrogen in your tissues can come out of solution so rapidly that bubbles form in your body. These bubbles can damage tissues and cause a painful condition known as decompression sickness (DCS) or classically, as the bends. A bottle of soda can illustrate the principle of DCS. Carbon dioxide is dissolved in the soda under pressure and it remains in solution until opening the bottle reduces the pressure. The rapid drop in pressure causes the carbon dioxide to form bubbles in the liquid and the soda foams (figure 5-1). If you open the bottle slowly, reduced bubbling occurs. You must control the two factors that affect in-and offgassing: time and pressure. You do this by controlling your dive time and depth. To do this, you rely on the information provided by dive tables, dive time calculators, and dive computers. You must also consider factors that can increase your chances of suffering DCS in situations where it might not normally occur. These factors include dehydration, fatigue, injuries, hard work during or shortly after a dive, effects of drugs or alcohol, obesity, or advanced age. In any case, you must be fit for diving and dive conservatively. Bubbles form in the human body if pressure is reduced beyond a specific point. If you dive deeper than approximately 6 meters (20 feet) and then ascend, the pressure change might be sufficient for bubbles to form if you have absorbed enough nitrogen. If you dive to depths of 6 meters (20 feet) or less, DCS is not likely to occur unless you go to altitude after diving, which reduces the pressure outside your body and can create a sufficient pressure differential to cause bubbling. For depths of 6.4 meters (21 feet) or deeper, time limits called dive time limits or no-decompression limits have been established. The time spent at a given depth is not to exceed these limits, or you could experience DCS during or after your ascent from the dive. If you take special precautions, involving offgassing in a series of planned decompression stops during ascent; you lessen your risk of DCS. The United States Navy has established time limits for various depths based on research and field experience of military divers. Recreational diving uses shorter time limits than those in the US Navy tables (figure 5-2). Research and analysis by many experts determined that shorter dive time limits would reduce the risk of DCS during recreational diving. Reduced time limits have been incorporated into the NAUI Dive Tables and NAUI Dive Time Calculator. Most dive computers also use shorter time limits than the US Navy tables for a single dive at a constant depth. Any dive you make puts you at risk for DCS. However, diving within the limits set by the dive tables Decompression, Dive Tables, and Dive Computers 125 FIGURE 5-1. THIS BOTTLE OF SODA WAS SEALED UNDER PRESSURE. WHEN THE BOTTLE IS OPENED, BUBBLES FORM BECAUSE OF THE SUDDEN DROP IN PRESSURE INSIDE THE BOTTLE. Chapter 5- Decompression, Dive Tables, and Dive Computers


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