SAC vs RMV

Posted 17 June 2026 · Field Note #10

SAC vs RMV, know the difference

Two divers describe their air consumption, one in bar per minute, one in litres per minute, and they're measuring genuinely different things. Confusing them quietly wrecks your gas planning, so it's worth getting straight.

SAC, surface air consumption, measures how fast you burn pressure, in bar per minute, normalised to the surface. It's an honest number, but it's tied to one specific cylinder. The catch is that a bar of pressure represents a different amount of gas depending on tank size. One bar drawn from a big twinset is far more actual gas than one bar from a small aluminium cylinder. So your SAC figure only means something when paired with the exact tank you measured it on. Swap cylinders and the number no longer applies.

RMV, respiratory minute volume, fixes that by converting consumption into an actual volume of gas, in litres per minute, independent of tank size. It's the volume of gas you breathe per minute, expressed at surface pressure. Because it's a true volume rather than a pressure reading, RMV is a property of you, the diver, not of any particular cylinder.

That distinction is the whole point. Your RMV travels with you. Work it out once from real dives, and you can plan gas for any cylinder, any depth, any dive, by scaling that volume for tank size and ambient pressure. SAC alone simply can't do that; it's locked to the tank it was measured on. This is why serious gas planning, rock-bottom reserves, turn pressures, team gas matching, is always done in litres via RMV, not in bar.

A further trap: don't plan from your best-ever, relaxed-day consumption. Plan from a realistic, even slightly stressed, RMV, because emergencies are exactly when your breathing rate spikes.

Practical takeaway: log real dives, depth, time, and pressure used, and let your true RMV emerge from the data. Then build every gas plan from that volume, not from optimism, and not from a SAC tied to a tank you may not be diving today.

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⚠ Educational only — not a substitute for training or a dive computer. Verify every number independently and dive within your certification.