CHAPTER 8 — Surface Supplied Air Diving OperationsChange A 8-3= 28.38 scfAir required for ascent to reach the first stop: For this example, you need todetermine ascent time and average depth. Ascent time is 7 minutes (rounded upfrom 6 minutes 20 seconds) from 190 fsw to the surface at 30 feet per minute.Average depth is calculated as follows:= 38.01 scfTotal V_{r} = 28.38 + 38.01= 66.39 scf2. The next step is to convert the required scf to an equivalent cylinder pressure inpsig. In this example, we are using an 80 ft^{3} aluminum cylinder to support thisdive. Refer to Table 7-1 for cylinder data used in this calculation:Where:FV=Floodable Volume (scf) = 0.399 scf14.7=Atmospheric Pressure (psi)P_{m}=Minimum cylinder pressureMinimum Cylinder Pressure = First stage regulator setting + bottompressure at final stop: [135 psig + (0 fsw x 0.445 psi)] = 135 psig8-2.2.2Flow Requirements. When the MK 21 MOD 1 system is used, the air supplysystem must be able to provide an average sustained flow of 1.4 acfm to the diver.The air consumption of divers using the MK 21 MOD 1 varies between 0.75 and1.5 acfm when used in a demand mode, with occasional faceplate and maskclearing. When used in a free-flow mode, greater than eight acfm is consumed.NOTEWhen planning a dive, calculations are based on 1.4 acfm.To satisfactorily support the MK 21 MOD 1 system, the air supply must:!Replenish the air consumed from the system (average rate of flow)!Replenish the air at a rate sufficient to maintain the required pressure!Provide the maximum rate of flow required by the diveraverage depth1902---------95fsw==Ascent V_{r}9533+33------------------1.47=psig required V_{r}FV--------14.7P_{m}+=66.390.399-------------14.7135+=2580.95(round to 2600 psig)=