Diver Vickee uses an aluminum 80 cubic foot cylinder for her SAC finding mission. She puts on her mask (for accuracy), pops a regulator into her mouth and watches a show about diving on cable TV. When the show begins, her spg reads 2800 psi. After 22 minutes, she has breathed the pressure down to 2500 psi. The pressure drop is 300 psi. The conversion of pressure to volume is a little more complex for Vickee than it was for Jack. The formula to convert psi to cubic feet is rV/WP = ft3 per psi (rV = tank's rated volume; WP = tank working pressure). So in this example 80/3000 = 0.02666 cubic feet per psi (this is called the one psi baseline for the tank). Vickee used 300 psi so to find how many cubic feet that is we now multiply our one psi baseline by the total psi to find the volume of gas consumed. That's 0.02666 * 300 = 7.99 cubic feet: let's call that 8 cubic feet. Finally we divide the total volume of gas used by number of minutes to find SAC (8 / 22 = 0.36 cubic feet per minute). Vickee's SAC is slightly below average.
Pressure alone does not tell us gas volume. We have to know the size of the cylinder to work out how pressure equates to volume.
For example 100 bar in a 10 litre scuba cylinder represents 1000 litres of gas, but 100 bar in a 7 litre cylinder is only 700 litres. Similarly, 1000 psi in an 80 cubic foot cylinder with a working pressure of 3000 psi equals a little more than 26.5 cubic feet, while 1000 psi in aluminum 50 with the same working pressure is only 16.6 cubic feet.
Gauging" Required Gas Volume
To calculate how much gas we need on a real dive, we take our SAC rate and work some simple arithmetic with it. The first step is to multiply our SAC by our target depth in atmospheres. (To convert depth to atmospheres add 1 and divide by 10 (metric) or divide by 33 and add 1 (imperial)). Once that's completed, we have to factor in environmental stressors such as temperature, visibility, surface conditions, how well-rested the diver is etc., and take into consideration the expected workload on the planned dive, such as swimming against a current. To make calculations somewhat simpler, we can collectively refer to the factors for environment, stressors and for workload as the Dive Factor. A typical dive factor for an easy wet suit dive in tropical water with good vis and minimal current is 1.5. For a dive in cold water wearing a drysuit, carrying a camera, working with a current and being a little stressed because there are hammerheads in the water, could have a dive factor of 3 or more! Finally, we multiply this figure by our bottom time, which for this exercise is the total elapsed time from leaving the surface to getting back to our safety stop somewhere between six and three metres or 20 and ten feet.
Here is a metric example using the average SAC of 14 litres. (You'll find an imperial example further down the page. Work through both if you have time.)
Solo diver A plans a dive on a local ship wreck to 27 metres for 25 minutes to shoot video. Diver A is an SDI computer Nitrox diver and is going to use an EAN36 for this dive which keeps her 25 minute profile well within the NDL for this depth. Her depth in atmospheres will be 3.7 and her SAC is 141/min. She multiplies her SAC by her depth and arrives at 51.8 litres per minute. She thinks the dive factor for this dive will be
about 1.5 (it's an easy dive) and so multiplies 51.8 by 1.5 to arrive at 77.70. This figure represents the volume of gas in litres she will consume each minute during the dive. Since her dive is planned to be 20 minutes, she calculates her gas needs at 77.70 multiplied by 20, which equals 1554 litres. Her main cylinder is a 12 litre model which is charged to 232 bar giving her a starting volume of approximately 2784 litres. She considers her gas needs and her starting volume compatible.
Solo diver B plans an air dive to a local kelp bed at 80 feet for 20 minutes to take still photographs of sea-life. His SAC is 0.5 cubic feet. His target depth is (80 / 33) + 1 or 3.42 atmospheres. Multiply that by his SAC and we arrive at 1.71 cubic feet per minute. This is going to be a moderately challenging dive because of currents so diver B uses a dive factor of 2 and gets a per minute volume of 3.42 cubic feet. Since he intends to stay for 20 minutes, he multiplies 3.42 by 20 which means his consumption on this dive equals 68.4 cubic feet. Diver B dives with a large volume, low-pressure steel cylinder that has a rated volume of 104 cubic feet. He considers his gas needs and his starting volume compatible.
We could of course work these calculations backwards from the starting volume to find out what volume of gas we are "allowed" to spend on the dive: using the Rule of Thirds for example, we could consume two-thirds of the starting volume on the dive. Once we have the "allowable useable volume" we can use it to calculate the maximum number of minutes for our dive.
As an example, let's use solo diver A and suggest that her cylinders are only filled to 200 bar. That's a starting volume of 2400 litres. Her allowable useable volume is two-thirds of that or about 1600 litres. Since she is going to use 77.7 litres per minute at depth, her total dive time is going to be shortened to approximately 20 minutes (1600 / 77.7 = 20.59).
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