Humidity and Temperature Humidity is the amount of water vapor in the air. The higher the temperature of the air, the more water vapor it can hold. If the air containing the water vapor is cooled, the water vapor in the air condenses. When you enter the water and descend, the air inside your mask cools. This causes the water vapor in the air to condense, which fogs the lenses in your mask (figure 4-36). This is known as condensation. When water condenses on the lens of a mask, it forms beads of water. If you use a defogging solution you reduce the surface tension of the water in the mask. When the surface tension is reduced, any moisture that does condense spreads out in a thin film instead of forming drops that can block your vision. The air inside your scuba cylinder also warms and cools. In extremely cold water, condensation can cause scuba regulators to freeze and freeflow unless a cold-water diver takes special precautions with their regulator. Normally, the air in scuba cylinders has had almost all of the water removed from it as the air is compressed into the cylinder. This is necessary to keep moisture out of your cylinder to prevent corrosion. Because the air you are breathing is so dry, your body must heat and humidify it as you breathe in. Having to humidify the air causes dehydration, which is the loss of body fluids. You must drink plenty of fluids before, between, and after dives to replace the lost fluids. Dehydration decreases your ability to exercise at full capacity and makes you more susceptible to decompression sickness. Anything you drink before diving must be non-alcoholic and should not contain caffeine. Caffeine is a diuretic, which means it will cause you to urinate and lose additional fluids. You should drink decaffeinated coffee, caffeine-free soft or sport drinks, decaffeinated tea, juices, and water. Scuba Cylinders and Temperature When a container filled with a gas is heated, the gas molecules inside the container become more active. If the container is flexible, the container will expand. If the container is not flexible (for example, a scuba cylinder), the pressure inside the container will rise (figure 4-37). Cylinders should be kept from extreme heat after they are filled. If the cylinder is subjected to changes in temperature once it is full, the pressure inside can vary by several bar (several hundred psi). The pressure will increase or decrease by approximately 0.6 bar for each change of 1°C (5 psi for each change of 1°F) even though no air has been added or lost. This change occurs because the molecules of air speed or slow their activity as the temperature rises or NAUI Scuba Diver 120 Diving Science Increased pressure from heat FIGURE 4-37. WHEN THE TEMPERATURE OF A SCUBA CYLINDER RISES, THE GAS MOLECULES INSIDE THE CYLINDER ALSO BECOME MORE ACTIVE. THIS CAUSES THE PRESSURE INSIDE THE CYLINDER TO RISE. FIGURE 4-36. YOUR MASK FOGS WHEN THE WATER VAPOR IN THE AIR IN YOUR MASK CONDENSES.
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