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Grateful Diver’s
School of Diving

Bob (Grateful Diver) Bailey

NAUI Instructor 41751


Understanding Gas Management

(continued)

Calculating SAC Rate

So how does this all relate to calculating your SAC rate? The amount of gas you inhale with each breath is directly proportional to the pressure of the water you are swimming in. By knowing your SAC rate … which is calculated at the surface … you can determine how much gas you are consuming at any depth. To do this you convert depth to pressure, expressed in ATA (absolute atmospheres). The relationship between depth and pressure is expressed as:

Pressure (ATA) = depth/33 + 1

In salt water, every 33-foot increase in depth increases our pressure by 1 ATA. By dividing our depth by 33, we can make a conversion from depth to pressure. We add 1 ATA because that is the pressure we are exposed to when we’re at the surface.

So let’s look at an example … if you are swimming at 60 feet, the pressure of the water on your body is:

60/33 + 1 = 2.82 ATA

In order to equalize that pressure and allow you to breathe, your regulator delivers the gas to you at 2.82 times the rate that it would if you were breathing from it at the surface.

Now that we know the relationship between depth, pressure, and gas consumption, let’s look at how this applies to your SAC rate.

Let’s suppose you are doing a dive to an average depth of 40 fsw for 30 minutes and you consume 1,600 psi from your cylinder. To find out how much you breathe per minute at depth …

1,600 psi / 30 minutes = 53.33 psi per minute

To convert this to how much gas you breathe at the surface you must determine the pressure, in atmospheres absolute, at 40 fsw …

40/33 + 1 = 2.21 ATA

And then determine your SAC rate by dividing your depth consumption rate by the pressure, in atmospheres absolute …

53.33 /2.21 = 24.13 psi per minute

To use this number for gas planning, round up to 25 psi per minute.

Pressure and volume … converting SAC to RMV

As I mentioned earlier, your consumption rate can be expressed in equivalent terms of either pressure (SAC = PSI per minute) or volume (RMV = cubic feet per minute). Both of those numbers are important, and are used for different purposes.

Although SAC and RMV are the same value expressed in different terms, there is one important difference. SAC is specific to the cylinder you are using … RMV is not. This is because your cylinder holds a certain volume of gas for every PSI it displays on your pressure gauge. To convert from SAC to RMV, you need to understand how many cubic feet of gas your cylinder holds for every PSI of pressure you can read on your pressure gauge. This is referred to as your cylinder’s “baseline”, and is expressed as follows:

baseline = volume (in cubic feet) / working pressure (in PSI)

Once you’ve determined a cylinder’s baseline you can use it to convert from SAC to RMV.

RMV = SAC x baseline

Let’s look at a couple of examples of calculating baseline for different cylinders.

So to look at an example, if you have a SAC rate of 25 PSI per minute using an AL80, you can convert this to volume as follows …

25 (PSI per minute) x 0.026 (cubic feet per psi) = 0.65 cubic feet per minute.

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