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Chapter 4- Diving Science Diving Science 101 necessary. If you were too buoyant, you need to add weight. If you were too heavy, you need to take weight off your weight belt. If you change equipment or if you lose or gain weight (more than 2.4 kilograms or 5 pounds), you must recheck your buoyancy before you start your next dive. Verify What You Have Learned Review the following questions about buoyancy: 7. The three factors affecting buoyancy are _____________, _____________, and _________________. 8. You can change your buoyancy by _____________________, ___________________, or _____________________. 9. An ocean diver needs to ____________ weight to dive in fresh water. 10. During descent, a diver’s buoyancy tends to __________________________. WHAT IS PRESSURE When you descend in water, the force from the weight of the air and water above you affects you. This force is called pressure and is measured in bar or pounds per square inch (psi). Air If you weighed a column of air that was 1 centimeter by 1 centimeter and extended all the way to the edge of the atmosphere, you would find that it weighed about 1 kilogram . If it were 1 inch by 1 inch, it would weigh about 14.7 pounds. In other words, this column exerts a pressure of about 1 bar or 14.7 pounds per square inch (psi). This constant pressure is called 1 atmosphere of pressure (figure 4-13). This atmosphere of pressure is constantly pushing on your body from all directions. However, you usually do not notice it for two reasons. First, your body is Atmosphere Water pressure is consistent Earth Decreases Air Pressure Increases FIGURE 4-13. ONE ATMOSPHERE OF PRESSURE IS ABOUT 1 BAR (14.7 PSI). composed of mostly fluids, which cannot be compressed. Second, most air spaces in your body, such as your lungs and sinuses, are open to the surrounding atmosphere and its pressure. As long as the pressure in a body’s air spaces matches the surrounding atmospheric pressure, the pressure is equalized and you do not feel any effects from your surrounding atmospheric pressure. Fresh Water If you took a column of fresh water that was 10.3 meters (34 feet) tall and weighed it, you would also see that it exerted a pressure of about 1 bar (14.7 psi). This equals the pressure exerted by the atmosphere at sea level. Therefore, 10.3 meters (34 feet) of fresh water is also equivalent to 1 atmosphere of pressure. Because water is not compressible and transmits pressure freely, pressure in water increases at a constant rate, and pressure is cumulative. In other words, 10.3 meters (34 feet) of fresh water is 1 atmosphere, 20.7 meters (68 feet) of fresh water is 2 atmosphere, 31 meters (102 feet) of fresh water is 3 atmosphere, and so on. Salt Water If you took a column of salt water that was 10 meters (33 feet) tall and weighed it, you would also see that it exerted a pressure of about 1 bar (14.7 pounds per square inch). This equals the pressure exerted by the atmosphere at sea level. Therefore, 10 meters (33 feet) of seawater is also equivalent to 1 atmosphere of pressure.


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