Scuba diving has come a long way over the years. In the early days, divers were equipped with the most basic of dive equipment, which included a cylinder with a “J” valve and no SPG. While this early equipment did allow for exploration and some basic safety protocols, there was certainly room for improvement. Along with that basic equipment was the primary knowledge that was passed on to future divers. This knowledge was gained from military and commercial applications and distilled down so it met the needs of recreational divers. Where is this short history lesson going – stressing the importance of air management!
Back to that J-valve, the basic concept was; if the J-valve was in the proper position (which was up) when the diver started the dive, once the cylinder reached a pressure of roughly 34 bar / 500 PSI, the diver would pull a bar that was attached to the J-valve down and get access to the reserve air for their ascent. Let’s just say this was not the perfect system for more than one reason. Another flaw was, it was not common for divers to be taught how to calculate their air consumption. Fast forward to today, and we have all the needed technology and the necessary information is right at our fingertips. There are dive computers that calculate our air consumption, decompression information and even our heart rate; we can’t ask for much more than that! The problem here is, not all divers dive with these types of computers. In fact, some divers don’t dive with any computer. So what is a good back-up system for divers that don’t have computers that calculate air consumption or don’t dive with dive computers? The surface air consumption or SAC rate formula.
The SAC formula helps a diver understand, on an average basis, what their air consumption will be at a given depth. This allows the diver to better plan how long their dive can be, giving two points for dive turn times: no decompression times and available air. The SAC formula is: SAC = (PSI at depth used/time) x 33/(depth+33)). To perform this in metric units, use bar instead of PSI and 10 instead of 33.
An exercise to perform which helps plan air consumption in various conditions such as currents, high workloads etc is to find a site where you can sit in one atmosphere of water (10 metres / 33 feet) and take three samplings of air consumption: resting, moderate workload and high workload. For the resting, using a slate to note your starting air pressure and sit or swim very lightly for 3-5 minutes, then note ending pressure. For the moderate workload exercise follow the same air pressure noting procedures, but this time swim at a moderate pace making sure to stay a depth of 1 atmosphere. For the high workload exercise, find an object such as a rock or wreck (something that will not move and where you will not damage marine life) note your starting pressure and place your hands on the object and try to push it while swimming, after 3-5 minutes note your ending air pressure. After performing these three exercises, you will have a clear picture of air consumption rates for low, medium, and high workloads.
The purpose and importance of incorporating air consumption into your dive plan is that the only reason you are able to stay underwater for a prolonged period of time is the air you have in your cylinder. If you do not know how long your air supply will last, or you are not tracking your air during the dive, you could run out and risk serious injury from a rapid ascent or not having enough air to perform a proper safety stop. In days past running low or out of air was a function of the equipment at the time (no SPG or dive computers) the limited knowledge available to sport divers and scuba equipment that was not built as well as it is today. Those times and reasons are far behind, and now a days there is no excuse for running out of air.
For more in-depth knowledge of dive planning and SAC, go to www.tdisdi.com/wpsite/sdi/get-certified/Deep-Diver/ to learn more and locate an SDI Dive Center near you. With some basic precautionary steps in place your dives can be fun and enjoyable.
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