Understanding Air Consumption & SAC
Your actual air consumption is one knowledge that is never useless, while some dive computers can provide you with it if they have a pressure sensor, is very easy to get yours. Air consumption is often measured in liters per minute and for realistic measure you can apply different methods.
My favourite is the following, it requires a dive computer as well, but not a pressure sensor.
note your initial pressure for example 200 (bars) and the capacity of your tank, for example, a 11 litre aluminium tank. with this you basicly get your amount of air, in this case 2200 litres of air, now do a dive, and here is the important part, you want to know the average depth of your dive, this is nearly impossible without a dive computer, let’s say 15 meters. thats 2.5 atmosphere aprox.
I you get out of the water after 45 minutes with 50 bar you left 11 litres tank* 50bar =550 litres on your cylinder, you consumed a total of 1650 litres in the dive, but this is not the end, since you where in an average of 2.5 atmospheres you where consuming more because of the water pressure. So (1650 litres/45 minutes)/2.5 bar= 14.67 litres consumed per minute.
Other way is to go to a pool, and start diving in 1 meter deep aprox, you dive as long as you want, the longer the dive the more accurate it will be. you go out and make the same calculation: time / air consumption. And you will get it
Why is better the first way? During a dive you make a variety of exercises, your kick, the way you move or go closer to something to take a look, the currents, even you, when you focus on breathing you alter your breathing rythm so the best is to calculate during some dives and make an average.
This way you will know your Surface Air Consumption, and you will be able to plan accurately your dives, for example, lets think, you have a S.A.C of 16 , how long can you dive on an average of 10 meters on a 11 litres tank if you start with 200 bar and you plan to be out af 50 bar?
2200 litres /16S.A.C. /2bar= 68.75 minutes