SAC rate and RMV calculator
How much gas do you breathe a minute? Enter one dive and see.
How it is worked out
Gas used is the pressure you used times the tank's size: 140 bar from a 12 litre tank is 1,680 litres of gas at the surface. Spread over the dive's time that is 37.3 litres a minute, at depth. Ambient pressure at an average of 12 metres of sea water is about 2.2 times the surface, so the same breathing at the surface would use 37.3 ÷ 2.18 = 17.1 litres a minute. That is your RMV. Your SAC rate is the same sum with bar instead of litres: 140 bar ÷ 45 minutes ÷ 2.18 = 1.42 bar a minute, for this tank.
The calculator uses 10 metres of salt water (10.2 metres of fresh) per bar, on top of the 1.01325 bar at the surface, and treats air as an ideal gas, which is accurate to a few per cent at normal tank pressures. In imperial, tanks are sold by the gas they hold at their rated pressure (an aluminium 80 holds 80 cubic feet at 3,000 psi), so the calculator asks for that rating too.
What the numbers mean
A lower RMV means your gas lasts longer, so you can stay longer or dive with more in reserve. It changes with effort, cold, current, stress, trim and fitness, so compare dives that are alike. Diveology works out your RMV on every dive with tank pressures, shows your average and your last ten dives against it, and draws the trend over months.
Planning with your RMV
Turn it round to see how long a tank lasts: gas available ÷ (RMV × ambient pressure). With an RMV of 17 litres a minute at 18 metres (2.8 ata), 140 bar of a 12 litre tank lasts 1,680 ÷ (17 × 2.8) = about 35 minutes. Always plan your reserve with your buddy and your training.
Questions
What is the difference between SAC and RMV?
RMV (respiratory minute volume) is the volume of gas you breathe a minute at the surface, in litres or cubic feet, so it is the same whichever tank you dive. SAC is the pressure you use a minute at the surface, in bar or psi, which depends on the tank. RMV is the better number for planning; SAC is handy for one tank you always dive.
What is a good SAC rate?
As a rough guide, relaxed experienced divers often breathe 10 to 15 litres a minute at the surface, many divers 15 to 20, and newer divers or hard working dives 20 to 30 or more. What matters most is your own trend.
Why use average depth and not maximum depth?
You breathe at every depth of the dive, so the average depth over the whole dive gives the right ambient pressure. Your dive computer shows it; Diveology works it out from the profile.
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