The physical properties of gasses are a fundamental safety concern when scuba diving. Gasses behave quite differently than does water; for example, gasses compress easily under pressure while water does not compress at all. The behaviors of oxygen and nitrogen in particular, which make up the bulk of the air we breathe, are important whether you are diving in cold water or in warm, tropical water. Four physical gas laws help explain the significance of how gasses behave when scuba diving and highlight safety factors to consider.
Boyle's Law
Boyle's law states that the pressure and volume of a gas are inversely proportional. In other words, if one increases, the other decreases. This law describes how, under the increased pressure your body experiences beneath the surface of the water, nitrogen compresses. This compression results in more nitrogen absorbed by your body's tissues, with more absorbed the deeper you dive. Up to a certain point, the extra nitrogen in your system causes no problems. However, when you ascend to the surface, in both cold and warm water, the excess nitrogen must be released at a certain rate. If you ascend too quickly, large nitrogen bubbles in your body may cause health issues as drastic as death.
Charles' Law
According to Charles' law, the volume of a gas under constant pressure is proportional to its temperature, meaning that gas expands as it heats and contracts as it cools. Stated differently, compressed gas in a scuba tank is forced to remain at a fixed volume and therefore will increase in pressure when heated. Conversely, when this gas releases from the scuba tank as you breathe it through the scuba cylinder, the decreased pressure allows the gas to cool. In cold-water diving, this cooling may be sufficient to freeze the regulator, interfering with normal underwater breathing.
Dalton's Law
Dalton's law describes the partial pressure of gasses, or the relative contribution of each gas to the total pressure in a mixture of gasses. For example, at atmospheric pressure, air comprises approximately 21 percent oxygen and 78 percent nitrogen, so oxygen supplies 21 percent of the gas pressure and nitrogen supplies 78 percent. With increasing pressure, as when you dive in cold water, the partial pressures of these gasses change and may cause physiological damage to your body, depending on the depth to which you dive.
Henry's Law
Henry's law concerns the rate at which a gas dissolves in a liquid. At a given temperature, it depends on the partial pressure of the gas in contact with the liquid. This matters in cold-water diving because at greater diving depths, and therefore greater partial pressures of oxygen and nitrogen, more of these gasses will dissolve in your body tissues and will later release from your tissues as you ascend to the water's surface. Because different body tissues absorb and release gasses at different rates, and because people are of different sizes and body compositions, gauging the safe rate of ascent after a dive may be difficult.
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