Your surface consumption rate, or SAC
To plan dives and reserves you need one number: your surface consumption rate, SCR, the litres you breathe in a minute at the surface. It is also called SAC, surface air consumption, though the gas is not always air. Any dive gives it, without hard maths, and this episode works it out in five steps on one example dive, then gives an easy planning rule.
What is said
To plan dives and reserves you need one number: a key part of gas planning.
Your surface consumption rate, SCR: the litres you breathe in a minute at the surface. Also called SAC, surface air consumption, but the gas is not always air: nitrox, trimix.
An example dive: twenty metres average depth, thirty-five minutes, an eleven point one litre aluminium tank, two hundred bar down to sixty. Step one: two hundred down to sixty bar. A hundred and forty bar used. Step two: a hundred and forty bar times eleven point one litres. Fifteen hundred and fifty-four litres.
Step three: over thirty-five minutes. Forty-four point four litres a minute at depth. Step four: twenty metres is twenty over ten, plus one. Three atmospheres. Step five: divide by that pressure. Fourteen point eight, rounded up to fifteen litres a minute at the surface. That is the rate for this dive.
Over several dives, easy and hard, plan with the higher figure, rounded up. For an easy rule, plan with twenty litres a minute: in bar, every five minutes. Breathe more? Use your own number. Breathe less? Extra margin.
Some computers track it with a transmitter; for Shearwater divers, Dive log shows it for each dive, and soon over time. Free on the App Store. diver.today.