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The compressibility of gasses is also an important consideration for divers due to its affect on how long a
diver can stay underwater. Scuba regulators are designed to deliver air to a diver at the same pressure as
the surrounding water pressure, at ambient pressure. That means that when a diver fills his lungs at a depth
of 33 feet, he is taking in the equivalent amount of air as two breaths at the surface. Obviously then, a tank
will only last half as long at 33 feet as it 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 when dive
planning. To determine this, it is first necessary to determine a diver's Surface Air Consumption (SAC) rate.
For example, if you are diving at 33 feet, and use 500 lbs. of air in 10 minutes, it is easy to determine that you
are using 50 lbs. per minute. This is only true for this depth 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 50 pounds per
minute at a depth, would use 25 pounds per minute at the surface. His surface air consumption rate is 25
pounds per minute. With our SAC number of 25, it is easy to calculate our consumption rate for depths of 33,
66, 99 feet etc. We know we are under 3 times the pressure 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 a depth that
is not in even atmospheres. (Not at 33, 66, 99 feet etc.) For this situation we use a formula 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 55 foot dive for 25 minutes and used 1700 pounds of air. This
would mean our DCR is 1700/25 or 68 pounds per minute. Using this in our formula, we get:
SAC Rate = (68 x 33) / (55+33)
OR: SAC Rate = 2244/88 or 25.5 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 we will use 2000 pounds of air at 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 into the 2000
pounds, we see this amount of air would last 24.4 minutes.
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