The diver has 182 bars of air available at depth (232 bars total minus 50 bars reserve). Duration of the cylinder at depth is equal to the available pressure divided by the DCR in pressure units per minute: Duration at depth = 182 bars ÷ 6.4 bars/min = 28.4 minutes The Maximum Dive Time for a 30 msw dive on the NAUI Dive Tables is 22 minutes. Using NAUI Tables to plan the dive, the diver will have extra air remaining at the completion of the “table-driven” dive. U.S./Imperial What is a diver’s SCR (surface RMV) for a moderate workload dive in cubic feet per minute (cfm) if the diver consumes 600 psi in 5 minutes using an 80 ft3/3,000 psi aluminum cylinder at a depth of 34 ffw under the same workload. Would a 71.2 ft3/2,475 psi cylinder be sufficient for the diver to perform a moderate workload dive to 80 fsw for the Maximum Dive Time (NAUI Dive Tables) if the diver must begin her ascent to the safety stop with 500 psi remaining in the cylinder? As in the above SI/metric problem, there are several distinct questions to be answered. (1) What is the diver’s SCR in cfm? (2) What would be the duration of the 71.2 ft3 cylinder at the surface? (3) What will be the cylinder duration at 80 fsw? (4) What is the Maximum Dive Time for a dive to 80 fsw? Since there is only a moderate workload involved, the dive need not be considered as “strenuous.” (5) Will the cylinder duration be greater than the Maximum Dive Time at 80 fsw? (1) Find the diver’s air consumption rate at depth: DCR = 600 psi/5 minutes = 120 psi/minute The absolute pressure at 34 ffw is 2 ata. The diver’s surface air consumption rate in psi/minute is: SCR = 120 psi/min = 60 psi/min 2 ata This is the diver’s SCR for an 80 ft3/3,000 psi cylinder. Converting this SCR to cubic feet per minute: SCRcfm = 60 psi/min x 80 ft3/3,000 psi = 1.6 cfm (2) Determine the cylinder duration at the surface. The diver has 1,975 psi available (2,475 psi total minus 500 psi reserve). Find the number of cubic feet this represents by converting psi to ft3. The available volume equals the available psi times the cylinder conversion factor of 71.2 ft3/2,475 psi. Vavailable = 1,975 psi x (71.2 ft3/2,475 psi) = 56.8 ft3 The surface duration equals the volume available divided by the SCR. Remember that we must work in volumes not pressures because we are diving with a different cylinder. Surface duration = 56.8 ft3 = 35.5 minutes 1.6 ft3/min (3) Determine the duration at depth. Depth duration equals surface duration divided by absolute pressure (in ata) at depth. The ambient pressure at 80 fsw is: Pdepth = ( 80 fsw ) + 1 = 3.42 ata (33 fsw/atm) Duration at depth = Surface duration = 35.5 minutes = 10.4 minutes Pdepth 3.42 ata (4) The Maximum Dive time for an 80 fsw dive is 35 minutes on the NAUI Dive Tables. (5) The diver has very limited air available for the dive and will have to turn the dive well before reaching the Maximum Dive Time. Remember that your air consumption rate will change as your diving experience grows and workload and conditions vary. You should reevaluate your air consumption rate occasionally. Gas management is a critical matter in technical diving, and technical divers make a point of being knowledgeable about their consumption rates. Henry’s Law In discussing the kinetic theory of gases, we said that gases are composed of molecules that are in constant motion. The movement of the molecules is completely random, and any gases will mix or diffuse into NAUI Master Scuba Diver 94 Diving Physics
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