Tools · Air consumption

How fast you breathe a tank

Two divers with identical tanks descend to the same depth for the same time, and one surfaces with half a tank while the other is nearly dry. The difference is their air-consumption rate — partly fitness, and a lot of it how efficiently they move — and you can put a number on yours. The trick is that a tank empties faster the deeper you are, so to get a figure that means anything you strip the depth back out: take the gas you used, divide by the minutes, then divide by the pressure you were under.

That gives your Surface Air Consumption (SAC) — what your breathing would cost at the surface, with the depth factored away, so it's a personal constant you can carry to any dive. Set your numbers below and read it off.

Note the "aluminum 80" holds 77.4 cubic feet, not 80 — the name is nominal. In metric it's simpler: the factor is just the cylinder's water volume in litres, so a 12 L cylinder gives 12 litres of surface gas per bar.

Two numbers come out, and the difference is exactly what trips people up. The SAC in pressure-per-minute — psi/min or bar/min — is the rate your gauge needle falls, and it's tied to the tank on your back: the same breathing drains a big cylinder more slowly than a small one, so your psi/min depends on the steel. The RMV — Respiratory Minute Volume, in cubic feet or litres per minute — is the actual volume of gas your lungs move, and it doesn't care what tank you're carrying. Convert from one to the other with the tank's size. People say "SAC" loosely for either, which is why the question keeps coming up; if you want a number you can take to any cylinder in any shop, it's the volume one — the RMV — that travels.

Why does depth cost gas at all? Your lungs move about the same volume every minute whether you're at the surface or at 30 metres — but down at 4 atmospheres, each of those lungfuls is air squeezed to a quarter of its surface size — four times denser to breathe, too — so it takes four times the gas from your tank to fill. That's the whole reason consumption climbs with depth, straight out of Boyle's law, and the whole reason we divide it back out: at depth you burn your SAC times the ambient pressure, so the same calm breathing that lasts two hours up top lasts thirty minutes at 30 metres.

Knowing your rate, you can estimate a dive: roughly, gas needed equals your RMV times the ambient pressure times the minutes. But estimate is the operative word — never plan to your last breath. Real gas planning keeps a reserve for the ascent and safety stop, enough to get a buddy up too, and a margin for the dive going sideways; the rule of thirds and "rock-bottom" reserves exist for exactly that. This tells you how fast you breathe. It does not tell you when to turn the dive — your training, your plan, and your gauge do.