You can work out your gas consumption by calculating your SAC. This is simple, and it allows you to estimate gas usage on your dives and the length of time the cylinder will last. SAC is calculated by taking a set time underwater at a set depth and recording your pressure usage from the cylinder. Then, you bring that number to the surface by dividing the amount of pressure in atmospheres you are under.
You can work out how long your gas supply will last underwater by using your SAC rate at depth. We need to know the following information.
- SAC rate
- Depth in atmosphere
- Cylinder usable gas volume (total minus reserve)
As an example, we will plan a dive for 18 meters/60 feet for 40 minutes. We will work with 20 liters per minute/0.7 cubic feet per minute so that metric and imperial measurements are covered.
How much gas pressure is available?
Typical Full Cylinder Pressure: 200 bar/3,000 psi
Reserve Pressure (you must be at the surface with): 50 bar/750 psi
A typical cylinder you use in the tropics is 11.1 liters or 80 cubic feet in volume. To work out how much gas you have available on the dive, you use the following formulas.
Use the following to determine the reserve volume.
How long will your cylinder last?
To calculate this time, use the following information.
- SAC: 20 L p/m or 0.71 cu. ft. p/m
- Depth: 18 m/60 ft.
- Usable Gas Volume: 1,665 L/60 cu. ft.
Now, you need to work out your consumption at depth. This is simply your SAC rate multiplied by the depth in atmospheres, and it is called Respiratory Minute Volume (RMV).
Depth: 18 m/60 ft. = 2.8 atm (1 for the air and 1.8 for the water)
Now you can use your RMV with the available cylinder volume from above to work out how long it will last.
This means the 40-minute dive we wanted to do will be limited by our gas consumption. Again, we need to adjust depth or time to be within our limitations. You will find that as you get more experienced your gas consumption really drops because you become more efficient in the water.