SAC, RMV, and SRMV: What Your Breathing Rate Is Actually Called

Every diver who has wondered whether their gas will last has reached for the same simple number: how much of it they get through in a minute. The number is easy to measure, and the arithmetic behind it has never been in serious doubt. What to call it is where the trouble begins. Is it SAC or RMV? In bar per minute or litres per minute? Referred to the surface, or to the depth you were at? Ask three divers and you will get four answers, and at least one of them will be stated with total confidence and be wrong. This is a small corner of diving, but it generates an unreasonable amount of argument, and the argument is almost never about the measurement itself, only ever about what to call the result. So let us settle it properly: what these terms actually mean, the maths behind them, and why I gave up on the two everyone fights about and started using a third.

One thing is worth adding before the definitions: the mess is real, not something I am inventing to sell you a tidier word. So every claim below links to its source, including the contradictory ones.

The three terms, defined

RMV stands for Respiratory Minute Volume, and that is not a diving term at all. It is borrowed wholesale from respiratory physiology, where it has meant one specific thing for over a century. Your respiratory minute volume, also called minute ventilation or minute volume, is the amount of gas your lungs move per minute. It equals the size of one breath (your tidal volume) multiplied by how many breaths you take in a minute, and for a person at rest it sits around five to eight litres per minute.^{[1 ,2 ,3 ]} Notice the units. It is a volume per minute, in litres. (It stays a litre figure even in the imperial-using world: clinical medicine quotes minute ventilation in litres per minute everywhere, the United States included. Divers working in feet and psi convert their own volumetric rate into cubic feet per minute, but the medical parent term is a litre.) It is in the name: the V is for Volume. A respiratory minute volume measured in bar, or in psi, is not a stricter or looser definition of RMV. It is a contradiction in terms: a volume is not a pressure, any more than a distance is a weight. Hold on to that, because it is the one fixed point in the whole discussion.

SAC stands for Surface Air Consumption. The useful word there is the first one, Surface. SAC is your consumption rate referred back to the surface, stripped of the effect of depth, so that one number describes you no matter how deep the dive it came from. That much everyone agrees on: the S pins down the depth. What nobody agrees on is the unit. DAN's own Alert Diver defines SAC as "a measure of your rate of gas usage expressed in pounds per square inch (psi) or… in bars of pressure", in other words a pressure drop per minute.^{[4 ]} Meanwhile Subsurface, the open-source dive log a great many people use, reports your SAC as a volume in litres per minute.^{[12 ]} Both call it SAC. One is a pressure, the other is a volume. So SAC tells you the depth it refers to and leaves the unit an open question.

SRMV stands for Surface RMV, and it is the one term here you can read off the label with no ambiguity left over. It is an RMV, so it is a volume in litres per minute, the unit fixed by the word Volume. And it is the Surface RMV, so the depth it refers to is fixed too: the surface. I will come back to where it comes from in a moment, because it is rare enough that you have probably never met it, though it is not ours. But hold the definition: SRMV is your breathing volume per minute, quoted at the surface.

The maths, which is the easy part

Start with what you actually measure on a dive. You note your starting and ending cylinder pressure, the time, and the average depth, and the pressure you used divides cleanly by the time to give a pressure drop per minute. The only complication is depth, because gas gets denser the deeper you are, so the same breathing empties the cylinder faster down low. To get a figure that does not depend on depth, you divide by the ambient pressure, which in seawater is just the depth in metres over ten, plus one for the atmosphere already pressing on the sea.

That SAC still belongs to one cylinder, though. A pressure drop only means something against the size of the cylinder it happened in: lose 1 bar from a 10-litre cylinder and you breathed 10 litres of gas, but lose 1 bar from a 15-litre cylinder and you breathed 15. So to turn your cylinder-bound SAC into a portable volume, you multiply by the cylinder's water capacity in litres. That volume is your surface RMV.

This is the step that exposes how cylinder-bound SAC really is. The same diver, breathing identically, posts a different SAC on every cylinder they pick up, while their SRMV stays put. That is the whole case for carrying the volume rather than the pressure.

Depth is the other axis, and it is where RMV alone falls apart. Your SRMV is referred to the surface, but on the dive itself the gas you draw from the cylinder each minute scales straight up with the ambient pressure. At 10 metres, twice surface pressure, you pull twice the surface-equivalent volume out of the cylinder for the same breathing; at 30 metres, four times.

Here is all of it in one place. Set a breathing rate, pick a cylinder, drag the depth, and watch which numbers move and which hold still. The SRMV never budges; the SAC moves with the cylinder; the gas drawn moves with the depth. That is the entire argument, made by the arithmetic.

Where everyone disagrees

None of the arithmetic above is controversial. The fight is entirely about the labels, and it is worth seeing how deep it runs, because once you have seen it you stop expecting the words alone to carry the meaning.

Begin with the one claim that is not a matter of opinion. RMV is a medical term for a volume, full stop. On ScubaBoard, where this gets argued at length, an instructor put it bluntly: "the definition of RMV is not up for debate and is strictly a volumetric measurement… anyone who votes for pressure on RMV is 100% incorrect."^{[7 ]} He is right, and the physiology backs him with no asterisk.^{[1 ,2 ]} If someone gives you an RMV in bar per minute, they have used the wrong word.

Everything past that point gets slippery. The same instructor, a few lines on, also wrote that SAC and RMV "should be used interchangeably", which is a fairly common position and not a crazy one, given that in practice context usually tells you what is meant.^{[7 ]} But interchangeable is exactly what they are not, if you take the words seriously, and other regulars in the very same thread use them to mean different things. One lays out a clean split in which SAC is the surface-referred figure and RMV is the raw at-depth one, so that "your RMV at 33 feet deep is double your RMV at the surface."^{[7 ]} Read that twice. In his usage RMV is the number that changes with depth, the opposite of the portable constant the first diver meant. Both are experienced, both are sincere, and they are using the same three letters for two different quantities, on the same page.

The agencies are no tidier, and you do not have to go to different agencies to see it. DAN's Alert Diver defines SAC as a pressure and RMV as the volume you convert it into.^{[4 ]} DAN Southern Africa, the same organisation, opens its gas-planning guide by calling RMV, SAC and surface consumption rate the same thing, all measured in litres per minute.^{[5 ]} A diving blogger who tried to reconcile the agency definitions back in 2009 gave up with the observation that "there is no agreement among agencies as to the exact definition" of either term, and not much has changed since.^{[6 ]} When a careful cave-diving instructor introduces the topic by admitting that "few topics seem to generate more confusion among divers and instructors", he is not being modest.^{[10 ]}

Even the software you plan with disagrees, which is the part that pushed me to pick a side. MultiDeco, the technical planner many divers trust, asks for your RMV, and to its credit it pins the meaning down in the manual: the input is a volume "at 1 ATA", which is to say a surface RMV in all but name. The same program also carries a separate SAC figure as a pressure per minute.^{[11 ]} Subsurface, as noted, books the same kind of quantity under the name SAC and prints it as litres per minute.^{[9 ,12 ]} So the two tools sitting on a technical diver's laptop use SAC and RMV to mean nearly opposite things, and both are internally consistent. The imperial world adds its own layer, where SAC is often a pressure in psi per minute and you convert to a volume with a "tank factor", the cylinder's rated cubic feet divided by its rated psi; useful, and one more reason two divers comparing numbers can both be right and still not match.^{[4 ,13 ]}

The honest summary is that the field never standardised, and context usually rescues you. When MultiDeco asks for your RMV, you know it wants the surface figure, because that is the only one that makes sense as a planning input. When your buddy says their SAC is "about twelve", you can usually guess from the number and the accent whether they mean bar, litres or psi. Usually. The trouble with leaning on context is that it fails exactly when you most need it to hold: teaching a new diver, comparing two people's figures, writing a number into an app that has to do something definite with it. That is the situation I was in.

Why Dive Kit settled on SRMV

Building Dive Kit's tools, I had to put a single label on the breathing-rate field and have it mean one thing, on every screen, for every diver, with no surrounding conversation to lean on. SAC was out: I would have had to specify the unit every time, and half the world would still have read it as the other one. Plain RMV was better, because at least the unit is fixed, but it left the depth dangling, and "RMV" with no depth is the number two ScubaBoard veterans used to mean opposite things. So I reached for the term that closes both gaps at once. SRMV is a volume, because it is an RMV, and it is quoted at the surface, because of the S. Cylinder-agnostic, depth-agnostic, one reading only.

I want to be straight about the term, because I care more about being right than about looking original. "SRMV" is not in common use, and most divers have never met it. But it is not ours. SDI's training materials use this exact label, Surface Respiratory Minute Volume, for what IANTD, NASDS and TDI all simply call RMV, and divers have been turning the term over on forums since at least 2010.^{[15 ]} What Dive Kit added is a decision, not a word: faced with SAC, RMV and SRMV, we argued it out and settled on SRMV as the one label across the app, because it is the only one of the three that fixes both the unit and the depth at once. The idea behind it was never novel either. MultiDeco already defines its RMV "at 1 ATA"; DAN explicitly brings its figure "to the surface"; GUE and others talk about a surface consumption rate.^{[11 ,4 ,5 ]} Everyone serious is already quoting a surface-referred volume. They just call it half a dozen different things and usually leave the "surface" part unsaid. SRMV is the habit of saying it out loud, adopted rather than invented, so I do not have to write "RMV at the surface" on every screen.

So the next time this argument starts, and it will, here is the short version you can send instead of having it again. RMV is a volume; that part is medicine, not opinion. SAC is referred to the surface; that part is in the name. Neither word fixes both at once, which is why divers have been talking past each other about it for decades. If you want one number that travels with you and not with your cylinder or your depth, it is your surface RMV, and you may as well call it that.