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Another interesting law of physics divers must be aware of is Charle's law. Charle's law states that if the volume of a gas in a container remains constant, but the temperature increases, the pressure of the gas increases. The converse is also true. if the temperature of a gas decreases, the volume of the gas decreases.
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This explains why if your warm tank reads less pressure after you jump into the cool water with it. The gas is cooling and contracting and therefore you have less pressure in your tank.
To begin to understand the importance that gas laws hold to a SCUBA diver, one must realize that at the surface of the water, the diver is feeling one atmosphere of pressure. As the diver descends, however, pressure is put on him or her due to the weight of the water above. At a depth of about 10 meters in seawater, the diver is experiencing around 2 atmospheres of pressure. This is twice the pressure at the surface. This pressure continues as the diver descends.
Imagine you have a closed balloon full of air. As the balloon descends in the water column, it will shrink! This is because greater pressure is being placed on the gas and compressing it. This relation is given by Boyle's Law which states that absolute pressure applied to a gas is inversely proportional to the volume of that gas. In other words, as the pressure on the gas increases, the volume of the gas decreases. This is given by the following equation:
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As a diver, Boyle's law tells you that as you descend, gases in your body and equipment decrease in volume. If a diver descends and does not exhale any air through his or her nose, he or she may experience what is called a mask squeeze. This is when the small volume of air in the divers mask decreases with depth and causes the mask to suction to the divers face. In some cases the suction is so powerful that it can break capillaries in the eyes and face.
As a diver ascends, the pressure they experience decreases and therefore the volume of gases in their body and equipment increases. By this same law, the air in a diver's lungs increases. If this air is not allowed to escape the lungs, the small air-holding sacks, or alveoli in the lungs can burst. Therefore, the number one rule in SCUBA diving is:
Never hold your breath!