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NAUI Scuba Diver 102 Diving Science 1 ata (Air Pressure) Two atmospheres absolute pressure Three atmospheres absolute pressure Four atmospheres absolute 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 meters (33 feet) of salt water equals 1 atmosphere, 20 meters (66 feet) of salt water equals 2 atmosphere, 30 meters (99 feet) of salt water equals 3 atmosphere, and so on. Units of Pressure You can refer to pressure in two different ways: absolute and gauge. When you dive, the pressure of the water as well as the pressure of the atmosphere push on you. At sea level, you are already under 1 atmosphere of pressure absolute (ata). At a depth of 10 meters (33 feet), you are under 2 atmosphere of pressure absolute: 1 atmosphere from the air plus 1 atmosphere from the salt water. At 20 meters (66 feet), you are under 3 atmosphere of pressure absolute (ata): 1 atmosphere from the air plus 2 atmosphere from the salt water. Because atmospheric pressure is nearly constant at sea level, most diving depth gauges (pressure gauges) are adjusted or calibrated to read zero at sea level. When a depth gauge indicates 10 meters (33 feet) in salt water, it is saying that the pressure is 1 atmosphere greater than it was at the surface. The pressure on such a gauge is called gauge pressure because it ignores the 1 atmosphere from the air above the water (figure 4-14). You always use absolute pressure when determining the total pressure being exerted on your body at any depth. For example, if you descend from the surface to 10 meters (33 feet) of salt water, you are doubling the pressure on your body. If you continue to 20 meters (66 feet), you are tripling the pressure on your body. As a diver, you must be concerned with pressure and how it can affect the volume of your air spaces under water. Verify What You Have Learned Review the following questions about pressure: 11. One atmosphere of pressure is equal to about ______ meters (_____ feet) of fresh water or _____ meters (_____ feet) of seawater. 12. The pressure is ______ times greater at 40 meters (132 feet) in salt water than it is at the surface. 13. The absolute pressure is _____ ata at 20.7 meters (68 feet) in fresh water. How Pressure Affects Volume To understand the direct effects of pressure, you must consider the effects of pressure on an open system and on a closed system. Open System If you invert a bucket, force it below the surface of the ocean, and take it to depth, the pressure surrounding the bucket increases and compresses the air in the bucket. The water level rises into the bucket. As the air compresses, its volume decreases. No air is lost from the bucket. When you take the bucket back to the One square inch 1 atm (Gauge Pressure) 0' (0 m) 33' (10 m) 66' (20 m) 99' (30 m) FIGURE 4-14. WATER PRESSURE ALONE IS CALLED GAUGE PRESSURE. WATER PRESSURE AND ATMOSPHERIC PRESSURE COMBINED ARE CALLED ABSOLUTE PRESSURE.


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