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Surface Air Consumption (SAC) Rates; Breathing…Underwater!
brokenogre - 6/29/2013 4:18 AM
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Surface Air Consumption Rates are the rate that a diver depletes his/her cylinder (regardless of the mix), and although the majority of divers will never use it, there is actual math to determine a diver’s rate. Most dive computers today already calculate this, and some even display the SAC rate inside one of the modes of the computer, most are simply downloaded with all other pertinent information of the dive onto a personal computer.
Regulators are designed to give the contents of the cylinder at the surrounding pressure. Since Boyle’s Law affects the contents of the cylinder at any given depth, then the SAC rate will change depending upon depth, temperature, and ambient surrounding pressure. The formula for determining a SAC rate is thus:
[{(P1 - P2) 33} ÷ (D + 33)] ÷ T = SAC
P1 = PSI start P2 = PSI end D = Depth T = Time in minutes SAC = SAC rate in PSI
Keep in mind, this is on the Imperial scale; not metric. Metric conversions replace the atmosphere of 33 feet with 10 meters, and the SAC rate solution in Bar as well as the P1 and P2 variables also in Bar. Metric formats are just as simple to complete and solve. The “T” variable should reflect the dive time, and the “D” variable should reflect the average depth of the entire dive from start to finish whether the formula is Imperial or Metric. Calculating an exact SAC rate is difficult without an air-intergrated computer since the formula cannot take into account exertion and cylinder size. There are other laws and formulas that govern the mathematical matrixes behind these calculations, best left to your dive computer in my opinion, but there are several sites on the net that go into those formulas for use by chemists and gas blenders in great detail. At the recreational level all you need to know is the basics behind why and how the formula works; and then do what I do, leave it up your dive computer to calculate.
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