Air Consumption Guide: SAC Rate, RMV and Depth

Air consumption is the rate at which you empty a cylinder, and it scales directly with ambient pressure. At 33 feet of sea water you breathe twice as much gas per minute as you do at the surface. At 66 feet, three times. At 99 feet, four times. Your lungs do not know this is happening: you take the same number of breaths at the same comfortable volume, and the cylinder simply drains faster because each breath contains more molecules. That single fact is the most useful thing a new diver can hold in their head, and it explains almost every dive that ended earlier than the diver expected.

Why does a cylinder empty faster the deeper you go?

A regulator is a pressure-matching device. Its job is to deliver gas to your mouth at whatever the surrounding water pressure happens to be, because your chest cannot expand against a pressure differential. At the surface it hands you gas at 1 atmosphere. At 99 feet it hands you gas at 4 atmospheres, which means four times the mass of gas fills exactly the same lung volume.

The arithmetic is short. Ambient pressure in ata equals depth in feet of sea water divided by 33, plus 1. Fresh water is less dense, so it takes 34 feet per atmosphere there. In metric, sea water runs 10 metres per atmosphere. At 66 feet you are at 3 ata, which means a surface rate of 0.5 cubic feet per minute becomes 1.5 cubic feet per minute in the water.

The table below runs a diver with a relaxed surface rate of 0.5 cubic feet per minute down a profile, and shows what one aluminium 80 gives them. The usable gas figure is 64.5 cu ft, which is 2,500 psi of a 3,000 psi fill with 500 psi left in the cylinder. Real gas planning reserves far more than 500 psi on a deep dive, and the reserve section below explains why.

Read the shape of that last column rather than the individual numbers. The same cylinder that supports a two hour dive at 15 feet supports about half an hour at 99 feet, and the fall is steepest in the first hundred feet. It also explains why the depth you spend most of the dive at matters far more than the depth you touched once. A dive that dropped briefly to 100 feet and then spent forty minutes at 45 feet is a 45 foot dive as far as your gas is concerned, which is why every consumption calculation uses average depth, not maximum depth. Every dive computer records average depth for exactly this reason, and the air consumption by depth chart lays the multipliers out in a printable form.

What is the difference between SAC and RMV?

SAC, surface air consumption, is how much cylinder pressure you use per minute at the surface, expressed in psi per minute. RMV, respiratory minute volume, is how much actual gas you breathe per minute at the surface, expressed in cubic feet or litres per minute. They describe the same lungs. They are not interchangeable, and confusing them is the most common mistake in recreational gas planning.

The reason is that psi is a measure of pressure, not of gas. Two thousand psi in an aluminium 40 is a great deal less gas than 2,000 psi in a steel HP117, because the second cylinder is physically bigger. If you work out that you burn 20 psi per minute and then rent a smaller cylinder on holiday, that figure will lie to you, and it lies in the dangerous direction: the small cylinder drops faster than your number predicts.

RMV travels between cylinders and SAC does not. Convert once, and you can plan any dive in any cylinder anywhere in the world. The conversion needs one number per cylinder, the cylinder factor, which is its rated capacity in cubic feet divided by its service pressure in psi.

Here is the same diver, one fixed RMV of 0.5 cubic feet per minute, expressed as a SAC figure in seven different cylinders. Nothing about the diver changes down the column. Only the psi number does, and it varies by a factor of more than two and a half.

Look at the aluminium 40, the cylinder most often slung as a pony bottle or a stage. The same breathing that reads 19 psi per minute in an aluminium 80 reads about 38 psi per minute there. A diver who memorised the first number and then watched the gauge on a 40 would see the needle fall at what looks like an alarming rate and would have no idea whether that was normal. The SAC rate calculator does the conversion in both directions for any of these cylinders.

How do I work out my own rate from a real dive?

You need four numbers off a single dive, and every one of them is already recorded by your computer: starting pressure, ending pressure, dive time in minutes, and average depth. Note the cylinder you dived, because the calculation is worthless without it.

Subtract ending pressure from starting pressure to get psi used. Divide by dive time to get psi per minute at depth. Divide that by the ambient pressure at your average depth to get SAC, your surface rate in psi per minute. Multiply SAC by the cylinder factor to get RMV. That is the whole procedure, and it takes about ninety seconds on a boat.

Do it on several dives rather than one, and do it on dives that were representative. A dive where you spent ten minutes wrestling a camera into a housing at 80 feet is not your rate, it is your rate under load, and both numbers are worth knowing separately. Most divers end up with a relaxed figure and a working figure, and use the working figure for planning because that is the honest one.

Write the numbers down at the time. Memory is unreliable about pressures, and a Scubapro Diver Logbook gives you the dive-by-dive record that makes a trend visible. A XS Scuba Folding Wrist Slate is where the numbers live during the dive itself: your planned turn pressure, your reserve, and the depths you agreed with your buddy, written down where you can read them at depth rather than reconstructed from memory at 90 feet.

You also need a pressure reading you trust. A Cressi Rugged Analogic Mini SPG Console is mechanical, so it cannot run out of battery and cannot lose a wireless pairing, and it is the backup every air integrated diver should still be carrying. If you would rather have pressure on the wrist alongside depth and time, a Shearwater Research Swift Transmitter feeds a compatible computer such as the Shearwater Research Peregrine Dive Computer , which will then log average depth, cylinder pressure and dive time together and do most of the arithmetic above for you. Air integration is a convenience with real value, not a replacement for knowing the numbers.

What is a normal air consumption rate?

Typical adult recreational RMV runs 0.4 to 0.75 cubic feet per minute, roughly 11 to 21 litres per minute. Newly certified divers frequently start around 0.8 to 0.9 and come down over the first fifty dives, almost entirely through better buoyancy control rather than anything respiratory. A larger diver breathes more than a smaller one at the same workload, so comparing your figure with a buddy of a different size tells you nothing useful.

The gap between the top and bottom rows of that table is the entire argument for working on consumption. Two divers on the same reef at the same depth out of identical cylinders can differ by more than half an hour of bottom time, and the difference is not lung capacity. It is trim, weighting, finning technique and calm.

How much gas do I have to keep in reserve?

Not 500 psi, and not on a deep dive. The reserve that matters is the gas two divers need to get from the deepest point of the dive to the surface while sharing one cylinder, at an elevated breathing rate, with a safety stop included. That quantity is usually called rock bottom or minimum gas, and it is a pressure at which the dive ends rather than a number to swim past.

The components are straightforward to describe. One minute at depth to identify and solve the problem and get on the same regulator. The ascent from depth to the 15 foot stop at no more than 30 feet per minute, priced at the average depth of that ascent rather than at the bottom depth. Three minutes at 15 feet. Then the last short ascent to the surface. All of it doubled, because two people are breathing, and all of it at an elevated rate: 1.0 cubic feet per minute each is the common planning figure rather than a relaxed 0.5, because a diver who has just run out of gas is not relaxed and neither is the one donating.

Worked at 100 feet in an aluminium 80, that comes to roughly 33 cu ft, which is about 1,300 psi. That is a long way from the 500 psi a lot of divers still treat as the reserve, and the gap grows with depth. Run your own depths and your own RMV through the gas planning calculator rather than carrying one memorised number, because the answer changes with every profile.

Turn pressure follows from the reserve. On a straightforward out and back recreational dive, the usable gas is your starting pressure minus your rock bottom reserve, and you turn the dive when you have used half of that usable gas. The rule of thirds, a third out, a third back and a third held in reserve, is the overhead environment rule rather than the open water one, and overhead diving requires specific training. It is described here, not taught here, and the technical diving page explains where that boundary sits and why this site stops at it.

How do I actually use less gas?

Fix your weighting first. It is the single largest lever and almost nobody starts there. A diver carrying six pounds too much lead has to hold that extra weight up with a partially inflated BCD, which is a bigger, draggier shape, and often with a slight upward fin angle that turns every kick into a small climb. Both cost gas continuously for the whole dive. A proper weight check at the end of a dive with a near empty cylinder is the fix, and the buoyancy and weighting guide walks through it.

Fix your trim second. Flat and horizontal presents a much smaller frontal area than feet-down, and drag rises with the square of speed, so a diver who is both badly trimmed and in a hurry pays twice. Moving a small amount of lead from the hips to a tank band, or from a weight belt into trim pockets, often changes attitude more than anything else you can do.

Then slow down. Almost every diver moves faster than the reef requires. Halving your speed cuts drag substantially, and the reef does not go anywhere. Long, slow, deliberate fin cycles with a glide between them beat continuous kicking, and a frog kick beats a flutter kick both for effort and for not silting out the site behind you.

Stay warm. Being cold raises consumption noticeably and shortens dives long before shivering starts, which is why divers so often find their gas runs low faster on the third dive of the day. Suit thickness is a gas management decision as much as a comfort one, and a suit one step warmer than you think you need is often the cheapest bottom time available.

Do not try to breathe less. Skip breathing, deliberately extending pauses to stretch a cylinder, is a genuinely dangerous habit: it drives carbon dioxide retention, which produces headaches, raises the risk of an oxygen toxicity event on an enriched air mix, and tends to end in an anxious diver breathing much harder than they would have otherwise. Breathe normally, deeply and slowly. Every legitimate gain comes from doing less work, not from moving less air.

Finally, remember that gas and nitrogen are two separate constraints and either one can end a dive. Enriched air extends the nitrogen limit and does absolutely nothing for gas consumption, so a diver on EAN32 at 90 feet still empties the cylinder at the same rate as the diver next to them on air. The nitrox guide covers which of the two constraints actually binds at a given depth, and the answer above roughly 40 feet is usually gas. A Sea Elite Cyl-Tec Aluminum 80 Scuba Tank is the cylinder almost all of these figures assume, and a Aqua Lung i300C 2-Gauge Console Dive Computer or any console unit will keep pressure, depth and time in one glance if you would rather not read three instruments.