7-14 Change AU.S. Navy Diving Manual—Volume 27-4.1Duration of Air Supply. The duration of the air supply of any given cylinder orcombination of cylinders depends upon:The diver’s consumption rate, which varies with the diver’s work rate, The depth of the dive, and The capacity and minimum pressure of the cylinder(s). Temperature is usually not significant in computing the duration of the air supply,unless the temperature conditions are extreme. When diving in extreme tempera-ture conditions, Charles’/Gay-Lusac’s law must be applied.There are three steps in calculating how long a diver’s air supply will last:1.Calculate the diver’s consumption rate by using this formula:Where:C = Diver’s consumption rate, standard cubic feet per minute (scfm)D = Depth, fswRMV = Diver’s Respiratory Minute Volume, actual cubic feet perminute (acfm) (from Figure 7-6)2.Calculate the available air capacity provided by the cylinders. The air capacitymust be expressed as the capacity that will actually be available to the diver,rather than as a total capacity of the cylinder. The formula for calculating theavailable air capacity is:Where:Pc= Measured cylinder pressure, psigPm= Minimum pressure of cylinder, psigFV = Floodable Volume (scf)N = Number of cylindersVa= Capacity available (scf)3.Calculate the duration of the available capacity (in minutes) by using thisformula:Duration = Where:CD33+33----------------RMV=VaPc Pm–14.7--------------------FV N=VaC------
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