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Do you have enough breathing gas to complete your next dive?
Determining what breathing gas you will need is a key part of dive planning. Doing this might mean you don’t have to end a dive early or, even worst, make a hurried ascent because you are low on air.
Calculating your breathing gas volumes is something that is a must for technical divers, but also can be applied to basic open water diving.
Surface air consumption (SAC) is a measure of the rate of gas usage expressed (in metric terms) in bars of pressure.
One way to estimate your surface air consumption is averaging actual consumption at set depths:
Step 1: Descend to a depth you can maintain comfortable, we do not want a lot of variation up and down here (Depth).
Step 2: Record the depth, your SPG reading (Start Pressure), and bottom time.
Step 3: Swim around at a comfortable pace, maintaining a constant depth for 10 minutes (Time).
Step 4: Record the depth, your SPG reading (End Pressure) and bottom time.
Step 5: Calculate what pressure of gas was used (Gas Used = Start Pressure – End Pressure).
Step 6: Calculate SAC Rate using the formula: SAC = (Bar Used x Cylinder Capacity) / (Absolute Pressure x Time)
Absolute Pressure = Depth + 10m / 10 because we need to add the weight of the atmosphere too (equivalent to 10m of water).
Step 7: Repeat for multiple blocks of 10 minutes over multiple dives, and average the SAC rates to get a more accurate estimate.
So now you know how much breathing gas you use calculated as SAC. You can use your SAC rate to determine how much breathing gas you might use on a planned dive.
These calculations can be easily done on paper for a basic open water dive. If you are planning a technical dive such as a decompression dive your dive planning software can help with the calculations.