Surface air consumption

The surface air consumption (short SAC) describes the air volume required by a diver in one minute in relation to the water surface. Hence, it is not depending on the depth. It is a guideline for individual air consumption. The unit is liters per minute (l/min).

Basic knowledge for Open Water Diver* (OWD*)

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The surface air consumption is a key figure for your air consumption when diving. The calculation of your personal SAC is important because it can be used to calculate the air consumption and allows you to roughly plan a dive. Hence, knowing your SAC allows you to calculate the cylinder size required for the planned dive time and depth.

Terms and units

The following terms and units are necessary for the calculation:

• Cylinder size (unit l): The volume of the diving cylinder.

• Ambient pressure (unit bar): The ambient pressure at the maximum depth.

• Air supply (unit bar l): The amount of air in the cylinder minus the reserve.
  Example 10 l cylinder:

[math]\text{air supply} = \text{cylinder size} \cdot (\text{filling pressure} - \text{reserve pressure}) = 10\ l \cdot (200\ bar - 50\ bar) = 1500\ barl[/math]

• Air consumption (unit bar l): The amount of air you used from your diving cylinder during the dive.
  Example 10 l cylinder with 80 bar at the end of the dive:

[math]\text{air consumption} = \text{cylinder size} \cdot (\text{initial pressure} - \text{final pressure}) = 10\ l \cdot (200\ bar - 80\ bar) = 1200\ barl[/math]

Calculation

The SAC is the amount of air consumed per minute in relation to the water surface. It is determined by using the following equation:

[math]\text{SAC} = \frac{\text{air consumption}}{\text{ambient pressure} \cdot \text{dive time}}[/math]

If you used the 1200 barl mentioned above, for example during a dive at a maximum depth of 15 m and a duration of 15 m, your SAC for this dive calculates as follows:

[math]\text{SAC} = \frac{1200\ barl}{2.5\ bar \cdot 30\ min} = 16\ l/min[/math]

Benchmarks

The SAC fluctuates with the load between about 10 l/min in total rest to 120 l/min under heavy load. Usually, for the air consumption calculation 20 l/min is used. However, this value can increase significantly, for example due to strong current.

For inexperienced divers, values of more than 20 l/min are normal. With increasing experience and number of dives, the SAC decreases automatically.