underwater the air added to lift bags is more compressed than it would be at the surface, because of the increased ambient pressure. As the lift bags rise, this air expands until the bags reach the surface. So “more air” must be taken down from the surface to supply the needed volume at depth. You should remember this from your entry-level scuba course. Scuba cylinder air capacities are given in surface equivalents (that is, the volume that the air would occupy if it were fully expanded to one atmosphere of pressure). We will address this simple problem of how much cylinder air you would need to use after we have discussed pressure and the Gas Laws. PRESSURE Pressure is the force acting on a unit area. Force is any action that tends to cause motion or change in motion of an object or to distort it. Expressed as a formula: Pressure = Force Area In the SI/metric system, the standard unit of pressure is the pascal. (Pa), the pressure generated by a force of one newton acting on an area of one square meter (N/m2). (A newton is the force required to give a mass of one kilogram an acceleration of one meter per second per second). A pascal is a very small unit, and the kilopascal (kPa) is more commonly used. The pressure unit in the U.S./Imperial system is the pound per square inch (psi). For divers, the pressures most often encountered are the result of the weight (not mass) of the air (barometric or atmospheric pressure), usually plus the weight of the surrounding water (water or hydrostatic pressure). That the atmosphere exerts pressure was first demonstrated and measured by Torricelli, a 17th century Italian mathematician. It had long been known that a suction pump would not raise water more than a distance of about 10 meters (34 feet). Following a suggestion of his mentor Galileo, who was the first to show that air has weight, Torricelli filled a glass tube that was Chapter 3- Diving Physics Atmospheric Pressure closed at one end with mercury, which is 13.6 times denser than water, and inverted it into a dish of mercury. The mercury did not flow entirely out of the tube. Instead the weight of the air was able to support a column of mercury 760 mm (29.92 in) high in the tube (figure 3-6). In showing that air has pressure, Torricelli had also created the first vacuum–in the space above the mercury. He also noted that there were minor variations in the height of the mercury column in his tube as the weather changed, the first barometer. The “torr,” a unit of pressure equal to 1 millimeter of mercury, is named after him. There are several measurement units in addition to kilopascals and pounds per square inch that are regularly used to express pressure values. Millimeters of mercury (mm Hg) or torr is one standard measurement of pressure. As divers we often use atmospheres to measure pressure. One standard atmosphere of pressure (atm) is defined as a pressure equal to that exerted by a 760 mm column of mercury (760 mm Hg – Hg is the chemical abbreviation for mercury). Another unit of pressure is the bar. In meteorology millibars (mb) are commonly used. A bar is equal to 100 kilopascals. A bar is very nearly 750 mm Hg, which is quite close to an atmosphere of pressure, and sometimes “bars” are used as Diving Physics 81 Hg { 760 mmHg Hg FIGURE 3-6. THE COLUMN OF MERCURY STANDS 760 MM HIGH IN THE TUBE.
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