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SAC VS. RMV

As you become a more advanced diver and want to start getting into gas planning, you’ll hear the phrase “SAC rate” a lot. “You can do XYZ dive if you have a good SAC rate”, or even just “What’s your SAC rate?”. When we look at our computers or manuals though, many will refer to an RMV rate. But what are these abbreviations, how do they compare and what do they have to do with how you breathe? 

Depending on where you go and who you talk to, you’ll get different answers to these questions. Some people say one is represented in metric vs imperial, while others say it's how much you breathe underwater vs on the surface. But let’s break it down and find out the truth. 

RMV 

RMV is your Respiratory Minute Volume; so how much gas you breathe in one minute. It is represented in the volume of gas breathed per minute, cubic feet or liters. RMV is independent of tank size, so regardless of what tank you’re using your RMV will be the same, as it doesn’t factor in a tank...all it is is how much air you breathe. It will however change with depth. This is due to Boyle’s Law - the volume of a given mass of gas varies inversely with the pressure - the deeper you go the more you breathe because of the increased pressure, and therefore the higher your RMV is. 

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SAC 

A SAC rate is your Surface Air Consumption rate. Your SAC rate is dependent on tank size, as it how much of your tank’s pressure you’re breathing. Here is where the real “arguments” start. Depending on how SAC is being calculated it may be represented in the pressure you breathe per minute, so PSI or bar, or the volume you breathe per minute, so cubic feet or liters.  

In its most basic form a SAC rate is the amount of pressure you’d breathe if you were at the surface. Because it relies on pressure, it will vary based on tank size. Think of a 40 cubic foot (5.5 L) tank vs a 130 cubic foot (16 L) tank. Even if they’re both filled to 3000 PSI (200 bar) you have significantly more gas in the 130. A basic SAC rate will be expressed in pressure (PSI or bar) per minute. While knowing how much pressure you breathe at the surface is a helpful tool for gas planning, it alone isn’t going to allow you to plan your dive. Because of this, many SAC rate calculations include what is called a “tank factor”. 

A “tank factor” is, as the name suggests, factoring in the size of your tank into the calculations. So instead of having a SAC rate for a 40 cubic foot tank, and another for a 130 and so on, just have one rate that works on all tank sizes. Including a tank factor allows this number to be expressed in volume (cubic foot or liter) making it much easier for gas planning purposes. 

Calculating SAC Rates 

To calculate your SAC rate, you need to gather some data first. You'll do so during a SAC rate swim. Pick a depth, the record your pressure when you are about to start your swim. This will be your starting pressure. Swim however you would regularly, while maintaining your depth for an allotted amount of time. I usually use ten minutes. At the end of your decided upon time, record your ending pressure and ensure you note the depth and time you chose. Once you’re done your dive and ready to start calculating, subtract your ending pressure from your starting pressure to get your pressure used. Now you have all the data to start your calculations. 

Below is a formula for calculating SAC with a tank factor.

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By following this formula, we can establish the volume of gas we breathe at ambient, or surface, pressure, so we can then use this for gas planning. In the above formula PSI Used is the pressure you used during your SAC swim. Working Pressure in PSI is the working, or fill, pressure of your tank, so for a HP steel it would be 3442 PSI, for an aluminum it would be 3000 PSI, and so on. Total Cylinder Capacity is your tank size, so an 80 cubic foot, or a 130, or so on. Depth in Feet is the depth you swam at for your SAC swim, and Min is the length of time - in minutes - your swim was. 

Some of you may already notice that the bottom part of the formula looks familiar. That is because it is; it is simply converting our depth from feet to atmospheres. By filling out this formula we are then left with our SAC rate adjusted for a tank factor. With this number we can then go about gas planning without having to worry about tank size. 

To plan a dive, all we need to do is take our adjusted SAC, multiply it by how long we want to be underwater, and multiply that by the depth we want to go to (in atmospheres) and we can see the volume of gas we need for a dive. From there we can choose what type of tank to use. 

Can My Computer Calculate This? 

Air integrated computers usually provide some form of gas planning information, either SAC or RMV. The problem now lies again with terminology. As said at the beginning of this article, there are many different answers to what SAC and RMV are, so even if you ready your computer’s manual, you still may be unsure of what it is really giving you. However, hopefully this article has shed some light on what the information you’re receiving is telling you, so what terminology is being used isn’t as important. 

For example, Shearwater gives you a number expressed in pressure per minute. From just that knowledge, we know that it is giving us a non-adjusted SAC rate. To use this number for gas consumption calculations, we simply have to add a tank factor, so we would multiply our non-adjusted SAC rate by our tank factor, which is the tank’s volume divided by the working pressure of the tank you used.  

Suunto however requires you to add a tank size, so with that information we can assume it is giving us an adjusted SAC rate already. 

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What Affects SAC Rate? 

Whether we use the word SAC or RMV, either way we’re talking about how much gas we’re using. The better our SAC and RMV rates are, the less gas we use, which is what we want. So how do we go about improving our SAC or RMV? Well, there are a few different ways. 

  1. Buoyancy control – the more we adjust our buoyancy throughout our dive, the more gas we’re wasting as we add and remove gas from our system, this increases our SAC rate.

  2.  Finning techniques – the more efficiently we kick, the less effort we need to put into moving through the water. The less effort we use, the less we breathe. Think of running compared to walking. We breathe more while running because we’re working harder. Same if we’re flailing all over underwater while kicking. 

  3. Calm and in control – when we’re cool and calm, we breathe less. Again, think of when we’re running around doing errands stressing about everything vs when you’re just chilling on the couch. You breathe a lot less when you’re just sitting there relaxing than when you’re all over the place. 

  4. Trim – the trimmer and more streamlined we are, the less we’ll drag in the water allowing us to use less effort while moving around. The less effort we use the better! 

Another thing that affects SAC, that we generally can’t control is size. The bigger you are, the bigger your lungs are. The bigger your lungs are the more gas you need to fill them. You could be the calmest diver out there, but you’ll almost always have a higher SAC rate than a person half your size simply because your body needs more gas. 

While there are always going to be different opinions on the exact explanation of SAC and RMV, hopefully now you have a better understanding of what each one is calculating, and how it is used. In my opinion the exact wording being used is less important than the understanding behind it. Take a look at your computer and see what it calculates for you, then go do a manual SAC swim and see how it compares! 

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