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Surface Air Consumption Rate

The compressibility of gasses is also an important consideration for divers due to its affect onhow long a diver can stay underwater. Scuba regulators are designed to deliverair to a diver at the same pressure as the surrounding water pressure, at ambient pressure. Thatmeans that when a diver fills his lungs at a depth of 33 feet, he is taking in the equivalent amountof air as two breaths at the surface. Obviously then, a tank will only last half as long at 33 feet asit would at the surface. A tank that would last 1 hour at the surface would only last 1/3 as long,or 20 minutes, at a depth of 66 feet, etc.

It can be beneficial to be able to estimate how long a scuba tank might last at a given depth whendive planning. To determine this, it is first necessary to determine a divers Surface AirConsumption (SAC) rate. For example, if you are diving at 33 feet, and use 500 lbs of air in 10minutes, it is easy to determine that you are using 50 lbs per minute. This is only true for thisdepth however. How much air would you use at 66 feet, or 99 feet?

The first thing we must do is calculate SAC rate. In the preceding example, a diver using 50pounds per minute at a depth, would use 25 pounds per minute at the surface. His surface airconsumption rate is 25 pounds per minute. With our SAC number of 25, it is easy to calculateour consumption rate for depths of 33, 66, 99 feet etc. We know we are under 3 times thepressure at 66 feet, so we would use 3 times as much air, or 75 pounds per minute at this depth.

The process becomes slightly more complex if depth consumption rate (DCR) is determined at adepth that is not in even atmospheres. (Not at 33, 66, 99 feet etc.) For this situation we use aformula that is simply an adaptation of Boyle's Law to determine our SAC rate:

SAC Rate = (DCR x 33) / (Depth + 33)

Let's look at an example. Suppose you did a 50 foot dive for 25 minutes and used 1700 poundsof air. This would mean our DCR is 1700/25 or 68 pounds per minute. Using this in our formulawe get:

SAC Rate = (68 x 33) / (50+33)

or: SAC Rate = 2244/83 or about 27 pounds per minute.

We can then turn the equation around to determine our DCR for any depth.

DCR = SAC Rate x (Depth + 33)/33

Let's assume our SAC Rate is 25 and we want to know how fast will we use 2000 pounds of airat a depth of 75 feet.

Dropping our numbers into the equation we get:DCR = 25 x (75 + 33)/33 or DCR = 25 x 108/33 or DCR = 81.81

This means at a depth of 75 feet, we will use 81.81 pounds of air per minute. Dividing this intothe 2000 pounds, we see this amount of air would last 24.4 minutes.

It is important to note that SAC Rate takes into account the assumption that you are exerting thesame amount of energy at any given depth, and you are using the same size tank as you usedwhen calculating your DCR.

For example, under strenuous diving conditions, you can consume air 4 times faster than whensitting still taking pictures. Also it is obvious that a 50 cubic foot tank would not last as long asan 80 cubic foot tank, even if they were both filled to the same pressure.


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