P1 = 1 bar P2 = (25 msw x 1 bar / 10 msw) + 1 bar = 3.5 bars V1 = 2,840 L V2 = unknown T1 = 23°C + 273° = 296 k T2 = 30°C + 273° = 303 k Then apply these values to the General Gas Law formula: P1V1 = P2V2 T1 T2 1 bar x 2,840 L = 3.5 bars x V2 296 k 303 k V2 = 1 bar x 2,840 L x 303 k = 830 L 3.5 bars x 296 k U.S./Imperial A cylinder contains 95 cubic feet of air at a temperature of 84°F at the surface. What volume of gas is available to a diver at a depth of 112 feet in a cold water lake in which the temperature is 40°F. First determine appropriate absolute values: P1 = 1 ata P2 = (112 ffw / 34 ffw per atm) + 1 atm = 4.29 ata V1 = 95 ft3 V2 = unknown T1 = 84°F + 460° = 544°R T2 = 40°F + 460° = 500°R Then apply the General Gas Law formula: P1V1 = P2V2 T1 T2 1 ata x 95 ft3 = 4.29 ata x V2 544°R 500°R V2 = 1 ata x 95 ft3 x 500°R = 20.3 ft3 4.29 ata x 544°R Air Consumption As seen above, less air is available to the diver as he or she descends. The diver breathes air at the ambient pressure, and the air in every breath taken at depth is correspondingly denser than it would be at the surface. The diver’s lungs assume a nearly normal volume with each inhalation, so there are more molecules of air consumed with each breath. At 10 msw/33 fsw, where the ambient pressure is 2 ata, the diver will breathe twice as many molecules of air with each breath as he or she would at the surface, and the rate at which air is consumed from the cylinder will be twice as great. Actually, many other factors also affect air consumption rate (e.g., level of work or activity, temperature, stress, physical size, and condition), but determination of air consumption can be approximated. A rule of thumb is that a beginning diver’s surface air consumption rate (SCR) or Respiratory Minute Volume (RMV) will be about 25 liters per minute or one cubic foot per minute. (These SI and U.S. measures are not exactly equivalent, but they are nearly so and have the advantage of being easy to manipulate–a definite plus in rules of thumb.) As diving experience and personal comfort level grow, a diver’s air consumption rate will decrease, and many divers exhibit prodigiously low consumption rates, especially on relaxed, low activity dives in warm water. Sometimes a diver’s boasted air consumption is hard to believe (as it probably should be). Nevertheless a small woman, who is in good physical condition with a lot of diving experience and diving relaxed and at ease, may seem to the casual observer to use almost no air compared to her large, out-of-shape, male diving partner. The best approximation of personal air consumption comes from one’s own experience. You should measure your depth consumption rates (DCRs) on several constant depth dives at varying activity levels or workloads– remaining stationary, swimming, working moderately, etc. The longer the time period over which you measure your air consumption, the more representative will be the measurement. Then compute your SCRs for the various activities. Air consumption rate increases proportionally to the increasing absolute pressure, so your SCR (or surface RMV) is determined by dividing your DCR by the ambient pressure (in atmospheres) at which you made the measurement. NAUI Master Scuba Diver 92 Diving Physics
20047TC
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