Advanced 9 minutes
Gas management: consumption, reserves, and the rule of thirds
Calculating your own consumption turns a vague guess into a number you can actually use
"I don't use much air" is diving's most common and least useful claim. Not much compared to what, at what depth, doing what level of exertion. With twenty minutes of simple arithmetic, that sentence becomes a number you can actually plan with.
In this article you'll learn how to calculate your personal consumption, project it to any depth, decide how much reserve to leave, and understand when the rule of thirds applies and when it doesn't.
Running out of gas is one of the most preventable causes of incidents there is, and the one that responds best to a bit of prior calculation.
Calculating your consumption
There are two ways to express personal consumption, and it's worth understanding the difference, since they're often confused.
SAC, surface air consumption, is normally expressed in bar per minute and is tied to your specific cylinder. It's easy to calculate and perfectly useful, with one limitation: if you change cylinder size, the number stops working.
RMV, respiratory minute volume, is expressed in litres per minute and describes how much gas you move, independent of the container. It's the figure that travels with you even if you change equipment.
The procedure to get it is simple. During a dive, hold a reasonably constant depth for a stretch of at least ten minutes, swimming normally. Note the pressure at the start and at the end, the elapsed time, and the average depth.
With that data: the pressure consumed divided by the minutes gives you consumption per minute at that depth. Divide that result by the absolute pressure at that depth (depth divided by ten, plus one, as you saw in the pressure article) and you get your surface-equivalent consumption. To convert it to litres per minute, multiply by the cylinder's internal volume.
It's worth repeating the calculation several times, under different conditions, and keeping the highest value for planning. Your relaxed consumption on a calm reef isn't the same as your consumption swimming against a current or under some stress.
Keep reading with full access
This article is part of the full Encyclopedia. A donation from USD 10 unlocks every article, every species photo and the downloadable app, and helps sustain the Scuba Sign Paperless Diving Community project.