LEARNING GOALS In this chapter you will: • Learn about some of the characteristics of air and water. • Be introduced to the concept of buoyancy and how if affects you under water. • Understand the concept of pressure and how it affects the volume and density of air in a closed container. • Learn how the pressure of the water affects the air spaces in your body and how to prevent problems occurring from the pressure changes. • Be introduced to different types of lung overexpansion injuries and how to prevent them. • Learn about the indirect effects of pressure on your body by means of the gases you breathe while diving. • Understand the thermal effects of the air and water temperature and your body. The underwater world presents a new and totally different environment from the air world in which you live. As you descend below the surface, the increased pressure will have direct and indirect effects on your body. However, most of your time underwater, you will not be aware of or feel these effects. This chapter addresses physical properties of air and water and ways specific parts of your body are affected while diving. The better you understand these differences, the easier it will be for you to function as a diver. DIRECT EFFECTS OF PRESSURE When you descend in water, the force from the combined weight of air and water will increase. This force is called pressure. In this chapter, you will learn how pressure increases under water and how it affects your body. Density and Its Effects If you pick up one scuba cylinder with air in it at 1 bar (1 atmosphere) and another just like it filled with air to its rated pressure, the second is about 2-4 kg (4-8 lb.) heavier than the first. This is because the compressed air is more dense. Density is the mass of an element per unit of volume. The density of a gas depends on its pressure and temperature. The density of a liquid, like water, is constant over a wide range of temperature and pressure. As you can see from the above example, air has weight. One liter of air weighs 1.25 grams. In the Imperial Measurement System this is usually expressed as 0.08 pounds per one cubic foot of air. One liter of fresh water weighs 1.0 kilogram and one liter of salt water, because of it dissolved salts and minerals, weighs 1.025 kilograms. Again in the Imperial System these are commonly expressed as pounds per cubic foot, that is, 62.4 pounds per cubic foot for fresh water and 64 pounds per cubic foot for salt water. On the Air You Breathe Most of the time, we do not think about air, because breathing is an automatic activity and we cannot see or feel the air around us. Composition Air is a mixture of different gases. The gases in air are colorless, odorless, and tasteless. Air is composed of approximately 20.9% oxygen, 78% nitrogen, and 1.1% miscellaneous gases, such as argon and carbon dioxide (figure 4-1). NAUI Scuba Diver 94 Diving Science Oxygen (20.9%) Nitrogen (78%) Misc. Gases (1.1%) FIGURE 4-1. AIR IS A MIXTURE OF GASES.
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