Calculating Surface Air Consumption (SAC)

This slate may be a little hard to read, but the key numbers are 150 and 75, and 12 and 26. The 150 was my gauge reading at 12 minutes into the dive, when we reached 30 meters. The 75 was my gauge reading at 26 minutes, when we turned back.

So, my Surface Air Consumption calculation is (75 BAR used times the 11 liters in my cylinder) divided by the 14 minutes it took to breathe the 75 BAR, and divided by the absolute pressure of 4 at 30 meters, which equals roughly 15 liters per minute.

We could then use my SAC (15) to calculate our 40-meter dive:

1. Out of gas at 40 meters means (SAC x 2) for stressed breathing = 30, times 2 for air sharing = 60, times 2 minutes calming down = 120, times 5 absolute pressure = 600 liters of air needed.

2. (SAC x 2) for air sharing = 30, times the 4 minutes it takes to ascend from 40 meters to 5 meters at a rate of 9 meters per minute = 120, times the 3.25 average absolute pressure ((40 + 5) รท 2 = 22.5 meters) during the ascent = 390 liters needed.

3. (SAC x 2) for air sharing = 30, times 3 minutes for our safety stop = 90, times the 1.5 absolute pressure at 5 meters =135 liters needed.

4. 600 + 390 + 135 = 1,125 liters needed, divided by the 11 liters in our cylinders = 102.3 BAR required.

So, you’ll see that 120 is double-underlined. For safety, we decided to turn the dive at 40 meters at whichever came first, a 2-minute warning on our No Decompression Limit (NDL) or 120 BAR remaining in our cylinders.

In case you’re wondering, my instructor’s SAC is only 10, so multiplying my higher SAC of 15 for his air consumption was done for additional safety.

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