### Surface air consumption or SAC rate

Managing your air or understanding your Surface air consumption or SAC rate will go a long way to helping you better understand how your body uses air underwater. This understanding will undoubtedly provide valuable insights on how to better manage your gas. Most fatal scuba accidents are due to out of air emergencies (OOA) and as such it becomes critical to be constantly monitoring your air supply.

The most common tool that a diver will use to measure how much gas they have in their tank is the submersible pressure gauge (SPG) this instrument connects to the first stage of your regulator and measures the pressure of the tank, in BAR or PSI. When you are diving you should be keeping a constant eye on the pressure. A common fill is usually around 200 bar and when it comes to gas management divers usually follow the rule of thirds. 1/3 out, 1/3 back and 1/3 reserve. Some computers feature air integration that will monitor your gas via your computer although many divers believe that this is just another possible failure point.

To to understand how we use our gas the first thing you will want to do is calculate your surface air consumption.

**[{(BAR Start – BAR End) x 10} ÷ (Depth + 10)] ÷ Time in Minutes = SAC Rate in BAR/min.**

There are many variables that affect your SAC, including gender, physical fitness, and experience. Let us assume that the average diver consumes around 25 liters of air whilst in the surface. Now as we know at depth due to pressure you will be consuming more air. So you must times your SAC by the atmosphere. For example, at 20 meters SAC x 3ATA = 75 liters per minute.

Now for safety we need to add some conservatism for variables such as stress for times of emergency, let’s call this the stress factor (SF). In this example lets say that you are in a strong current, you have lost your buddy and are quite uneasy so you are breathing twice as much as normal, so your stress factor is two.

((SAC x 3ATA) x 2SF) = 150 liters pm.

Now we need to calculate how long the air in our tank will last at this rate.

Imagine that you started with 200 bar, but by the time you arrived at 20 meters you were down to 170. So first we need to calculate the liters available in the tank.

Tank size (Liters) x Remaining air (Bar) = Liters available. For this example: 12ltrs x 170 = 2040 liters left.

Now we can calculate how long this air will last at this depth, all things considered, the same.

Air consumption / liters available = 150 liters pm / 2040 liters = 13.60 minutes.

As you can see that when under high levels of stress and at depth you don’t have much air in a 12-liter tank. The figures used may be aggressive, but they demonstrate this point well.

You can use the SAC calculator on this page and adjust the figures to see how different tanks sizes, depths and dive factors all influence the outcome.

I used this method to calculate how large my pony bottle should be. I initially was carrying a 2.7-liter bottle and realised that at 40 meters it was woefully inadequate, so I sold it and purchased a 7 liter that I sling when diving.

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