Arguably the biggest misconception in scuba diving is the notion that the less you breathe, the better diver you are. I’ve witnessed – as I’m sure you have, too – many divers bragging about how much air they have left in their tanks after the dive and some even ridiculing other divers for having less. Today I’m going to explain why this idea breeds bad divers and can potentially cause dangerous behaviour.
First of all, let’s discuss what we mean by “use less air”…
What is SAC Rate / RMV and Why Should We Know It?
The way we measure how much gas – usually air or nitrox, but for the purposes of simplicity we’ll just call it “air” – we breathe whilst scuba diving is called our SAC rate (surface air consumption rate) or sometimes RMV (respiratory minute volume). Many divers refer to only one general SAC rate, but since we know that your air consumption will fluctuate during the course of a dive, in technical diving we often refer to three types of SAC rates that can be identified, which describe the three different levels of air consumption:
- Working SAC rate – when moving from one place to another
- Resting SAC rate – when completing a stationary safety stop or deco stop
- Stress SAC rate – when hyperventilating, usually due to an emergency or overexertion
Knowing your SAC rate is essential for gas management and dive planning, especially in decompression, cave and wreck diving, where we may not have direct access to the surface should we run low on air supply. We use the SAC rates together with the dive plan to calculate – as accurately as possible – how many litres of breathing gas we will use on that dive, which is a crucial aspect of planning any technical dive. Knowing your buddy’s SAC rate is also vital in the planning process, as although we always carry enough breathing gases in case of failures, we might also need to donate our reserves to a member of our dive team.
We don’t usually use our stress SAC rate to calculate for the average dive, however it is useful to have an idea of how much air you might use in a panic situation, to give you the knowledge of what is possible so that you stay well within your planned limits.
How to calculate your SAC rate
Your SAC rate is calculated by measuring how much air (in this example we will use the metric system) that you use at a fixed depth for a fixed period of time. You can do this by first letting your dive buddy know what you are going to do and when (the beginning of a dive is usually the safest time to complete this exercise), and making sure you have plenty of air in your tank. Then you can complete the following steps:
- Establish neutral buoyancy at a fixed depth
- Using an underwater slate, record your depth and tank pressure
- Using a timing device, swim while maintaining a constant depth
- After a minimum of 5 minutes, use the slate again to record your tank pressure
- After the dive, you can use the data you collected in the SAC rate swim to calculate your SAC rate by using the following formula:
If you used 20 bar of air from an 11l cylinder at 20m depth for 5 minutes your calculation would be as follows:
This is the total amount of air that you would theoretically use on scuba at 1 ata (sea level), so now you can apply your SAC rate to any given depth, and use it to estimate how much air you will need for any given period of time. It’s important to note that this is only an estimate, and in order to get a more accurate measurement, you should regularly calculate your SAC rate to find the average figure, and don’t forget that your SAC rate can change in different situations. Over time, you should be able to guess your tank pressure during the dive, and it should be similar to, or the same as, your actual pressure.
What Other Factors Can Affect Your SAC Rate?
Sometimes, your SAC rate may be higher or lower than your buddy’s regardless of your dive skills. Additional factors such as body size, metabolic rate and overall health can also affect your SAC rate. It makes perfect sense that a 6’4” man that weighs 200lbs would generally have a higher SAC rate than a 5’1” woman that weighs 120lbs.
How is SAC Rate Related to Experience?
It’s commonly known that people who are taking their first breaths underwater on scuba tend to take some time to get the hang of this unusual way of breathing. Often, they also need several or more dives to get the hang of perfecting their buoyancy. Both of these things factor in to increase air consumption, along with the facts that often new divers are prone to swimming faster, in a vertical position, and changing their depth more often. It’s true that all of these things combined do increase a diver’s air consumption, but a reduction in SAC rate should not be the reason why new divers should be working on practicing and improving these skills.
What Makes A Better Diver?
Strictly skills speaking, and simplified for illustrative purposes, a good diver is often defined as someone who:
- Keeps good control of their buoyancy (at the surface and underwater)
- Keeps good trim (streamlined equipment and positioning)
- Has a good understanding of decompression theory
- Communicates well with dive buddies
- Has mastered emergency dive skills and procedures
- Avoids dangerous or irresponsible practices/behaviour
So What Is So Dangerous About Decreasing SAC Rate?
You may be thinking, ‘what’s the harm in wanting to reduce my SAC rate? After all, it means that my air will last longer…’ Well, it’s completely understandable that you may want more time underwater, but divers have been known to take extreme measures to reduce their SAC rate that could be potentially very harmful. I’m referring here to skip-breathing. If you’ve never heard of skip-breathing, it’s when a diver inhales then holds their breath before exhaling, and then holds their breath again before inhaling, and continues this practice to effectively decrease the total amount of air used. I’ve also heard of divers who only inhale a shallow breath, before exhaling in steps, holding their breath for very short periods in between. There are many ways to reduce SAC rate, but these practices only increase the chance of decompression illness, and will likely decrease the control you have over your buoyancy.
Decreasing your SAC rate should not be a goal in and of itself, but it could be a side effect of becoming a better diver – saying that, if your SAC rate does not decrease over time, then perhaps it was fine to begin with. There are many ways to increase your dive time without having to reduce your SAC rate, such as using a 15l tank instead of an 11l tank, taking two 11l tanks in sidemount rather than one in backmount, or diving a closed-circuit rebreather.
So How Should You Breathe?
The ideal breathing technique in open-circuit scuba diving is to inhale a full and comfortable breath and immediately exhale, controlled but not too slowly. Make sure you don’t hold your breath, not only to avoid lung-overexpansion, but also because your exhalation will contain carbon dioxide and nitrogen, two gases that you really don’t want staying in your lungs for longer than absolutely necessary.
Overestimating the Word, “Experienced”
Experience in a field is commonly associated with a person’s knowledge and level of mastery, but it’s not always the case. It is very possible that a person with 50 dives can be a better diver than a person who has 300 dives – good divers are not always dependent on the size of their logbook. What I would say is more important, is someone who uses each and every dive to work on and improve their skills and knowledge, and who, even after thousands of dives, never thinks that they have learned everything.
There is often a stigma against divers who have “higher than average” SAC rates – some being blamed for ending the dive earlier than their buddies would have liked, and some feeling like they are put under pressure to use less air – but your SAC rate does not define how good of a diver you are, and it does not have to limit your abilities.