We breathe cubic feet of air, not psi of air. In option 2, we are determining our SAC rate in cuft/min. Since option 1 only works if you always carry your tanks with you, even on vacation, let’s look at option number 2. To get around this dilemma we have two choices: 1) Buy and always dive with your own tanks, or 2) Learn how to convert psi/min into cuft/min. So when ever we use a different type of tank we have to recalculate our SAC rates. If this was an Aluminum 80 and we recalculated using a LP Steel 95, our SAC rate will be completely different. For this reason, our 25psi/min SAC rate works only for the specific tank we used for the calculation. The only problem is that psi is not a measure of volume, but a measure of pressure that is dependent on cylinder capacity and working pressure. Now we know that our SAC rate is 25psi/min. So for instance, if we use 500 psi in 10 minutes, that’s 50 psi per minute at 33 feet or 2 ATA’s. We dived at a certain depth for a certain amount of time, monitored how much air we used, then plugged it into a magical formula, or a SAC Rate Calculator, and got our SAC rate in psi per minute (psi/min.). Let’s first analyze the way most of us learned to determine SAC rates. Now that I have made my case, how can we go about making sense of these SAC rate numbers? Knowing our SAC rates as scuba divers is as, if not more, important as knowing a vehicles MPG rating as travelers. If you were planning a road trip in which you needed enough fuel to get to where you were going and come back, wouldn’t you want to know how far you could go? We know what our vehicle’s MPG rating is and if we know our fuel tank capacity, we know how far we can go. Many divers fail to see the value of air consumption calculations but analogize it this way. As well trained divers, we should be recalculating our SAC rates frequently in cubic foot per minute (cuft/min) and the reasons and benefits, as you will see, are numerous. How many times have you dived with someone new, and when they signaled, "low on air - let’s surface," you looked at your gauge and discovered more than half a tank left!? Scenarios like this can easily be prevented by comparing SAC rates. Knowing your individual SAC rate is a vital piece of information useful in dive planning, selecting what type of cylinder size works best for you, and helps you determine which dive buddies are most compatible with your breathing rate. The facts are, most recreational divers don’t calculate their Surface Air Consumption (SAC) rates once beyond their Open Water training (if then!). What’s your SAC rate? Pose this question to your average diver and the answers may surprise you.
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