The most important component of air to us is oxygen. We cannot survive without oxygen. You can breathe pure oxygen for limited periods, but pure oxygen can be extremely hazardous above and underwater. Nitrogen is a metabolically inert gas. It does not react in our bodies when we breathe it under normal conditions. However, dissolved nitrogen is present in your body, and additional nitrogen is absorbed in your body when you are under increased pressure underwater. Some divers use special mixtures of gases, such as nitrox, for diving. These mixtures contain different percentages of oxygen and nitrogen than air. Diving with special gas mixtures requires additional training and specialized equipment. The NAUI Enriched Air Nitrox (EAN) Diver Specialty course provides the additional training you will need to dive with nitrox. Characteristics Air can be easily compressed. When pressure is applied to a volume of air, it can be forced to occupy a much smaller area. With a compressor, we can squeeze a large amount of air into a smaller space, making it denser than normal and therefore heavier than normal. For example, the compressed air in a scuba cylinder, when released, would fill a space equivalent to a small closet at atmospheric pressure. This is why a full scuba cylinder weighs more than an empty scuba cylinder (figure 4-2). The air surrounding the earth at sea level is compressed by the weight of the air above it. In other words, the air at sea level is denser than the air at higher levels. In the Water Water cannot be significantly compressed. When pressure is applied to water, the pressure is transmitted throughout the water. The density of water at any depth is the same as the density of water at the surface, though the pressure it is under will be greater because of the weight of the water above it. But water is about 800 times denser than air. This and other factors have several effects on us as divers. Water affects our vision, our ability to distinguish Chapter 4- Diving Science Diving Science Full Empty colors, our hearing, our ability to retain heat, and our ability to move through it, compared to air. Vision The human eye is designed to focus light rays in air. This is why objects appear blurry when you open your eyes under water. The mask allows you to put an air space in front of your eyes to see without the blur. As light rays pass from air to water, they slow down and bend. This bending is called refraction and it changes the way you see objects under water. Objects appear 1/3 closer and larger than they do in air. Objects are actually further away and smaller than they appear to be under water. Remember this when you are reporting the sighting of an animal or an object. As a new diver, you will find that you reach in front of an object until you learn to compensate for refraction. Colors Colors also look much different underwater. Sunlight is composed of a spectrum or a rainbow of visible and invisible colors ranging from infrared to ultraviolet. As light passes through the water, the water absorbs the colors of the spectrum of the sunlight. The first visible color to be absorbed is red, followed by orange, then yellow, then green, then blue, then indigo, and finally violet. At greater depths, the only colors you can see without a dive light are blues and purples. 95 FIGURE 4-2. AIR HAS WEIGHT AS YOU CAN SEE BY THE DIFFERENCE IN WEIGHT BETWEEN AN EMPTY AND FULL SCUBA CYLINDER.
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