- What is a SAC rate?
- The Maths
- SAC Rate Calculator
- Why Should I Know My SAC Rate?
- Things To Know
When you first start to scuba dive, it can seem like a deluge of information that you’ll never remember!
But, as you get more experienced, it all starts to feel like muscle memory.
This is often when divers start to think – how can I improve?
Improving your scuba diving can involve courses to broaden your skill set or qualifications, but one of the best ways to improve your scuba diving is to look at your SAC rate – or surface air consumption.
What Is A SAC Rate?
Ever noticed when climbing back onto the boat after a dive that the instructor or divemaster has used significantly less air than you?!
Well, that’s because their SAC rate is better than yours. Sorry!
Surface Air Consumption (SAC) – An expression of your air consumption during a dive, but with the depth taken out of the equation.
It is a number that tells you how much air you consume per minute. Think of it as a bit like MPG for a car… but for you and air!
The maths involved in calculating your SAC rate mitigates the depth, so it represents the air you would consume sat at the surface.
Normalizing the depth allows the SAC rate to be compared between dives of different or varying depths, allowing you to compare over time and use your SAC rate when planning future dives.
A SAC rate is usually given as litres per minute, or l/m. So, you might hear someone say that they’ve got a “SAC of 15”.
Calculating your SAC rate by yourself is fairly simple, and essential for most solo and technical diving courses. We will walk you through the maths behind the calculations so you can do them yourself.
We have also made a handy calculator which will do the work for you! We’ll get to that bit later.
One of the easiest ways to calculate your SAC rate is to use the data from your dive computer. In fact, if you have an air integrated computer, it will even work it out for you! It is a good idea to take the average from a few dives to get a good read.
To use a non air integrated dive computer to get your SAC rate, follow the steps below:
- Pick a nice, calm dive without too much current
2. Make a note of your tank capacity
3. Descend to a set depth. A nice round number, like 10m, is good.
4. Note your air and the time.
5. Swim at a steady, normal pace at the same depth for 10 minutes.
6. When the 10 minutes is up, make a note of your air again.
Note: In order to estimate your SAC rate when exerted, do the same test whilst swimming hard!
A Note About Atmospheric Pressure
In order to mitigate the effect of depth on our calculations, we must take it into account and understand the pressures all around us.
When we are on the surface, we are subjected to 1 bar of pressure, which is also called 1 atmosphere – because it represents the weight of the whole atmosphere pressing down on us!
Water is denser than air. As a result, the pressure increases with our depth.
Every 10m/33 feet we descend is the equivalent of 1 bar or 1 atmosphere.
So the total or ambient, pressure pressing down on us at 10m depth is 2 bar.
The table below may make this concept a little simpler!
Calculations for metric divers are simple. Let us use an example.
Using a 10-litre cylinder, you begin with 200 bar. You immediately descend to 20m and stay there for 20 minutes. When you surface, you have 60 bar.
200 bar – 60 bar = 140 bar used over the dive
140 bar x 10 litres = 1400 litres consumed over the dive
1400 litres / 20 minutes = 70 litres/min of air consumption at 20m
70 litres/min / 3 bar total pressure at 20m = 23.3 litre/min SAC rate
(At 20m, the air is 3x as dense as on the surface. Therefore, we divide the air consumption by 3)
The numbers in imperial diving are less rounded, so the calculations are more complex. Cylinder capacity is also described differently to metric cylinders. For example, an 80 cubic foot cylinder refers to the volume of gas it contains at its working pressure. Not the true internal volume at normal atmospheric pressure, which is how a metric tank is defined.
Let’s use an example:
You take a 20 minute dive to 66 feet. You use 2000 PSI when diving with an 80 cubic foot tank which is rated to 3000 psi working pressure.
(80 cubic foot x 2000 psi)/3000 psi = 53.3 cubic feet of air used
53.3 cubic feet / 20 minutes = 2.6 cubic feet/minute at 66 feet depth
2.6 cubic feet/min / 3x atmospheric density at 20m = 0.89 cubic feet/min SAC rate
Now you have your SAC rate, you can multiple it by the intended depth (in total pressure) and time of the dive to calculate the air you will use in litres or cubic feet. Of course, you should always return from a dive with at least 50 bar in the tank, so it is important to add this on.
SAC Rate Calculator
Why Should I know My SAC Rate?
Whilst some divers just use their SAC rate to brag about how great a diver they are, it can be a useful number for any diver to know.
Safety is the first and most obvious reason for learning your SAC rate. It can save your life.
Knowing your SAC rate is akin to knowing your limits. You wouldn’t go deeper than you were qualified to dive as it would invalidate your dive insurance.
You wouldn’t enter a cave if you knew that you only had a few minutes before you know you’ll have to head back to the boat.
Underestimating your breathing rate and running out of air is one of the main causes of fatal diving accidents, so having all the information you can find is vital.
Knowing your SAC rate will also allow you to better plan your dives. You will know when you need to turn around or begin your safety stop. If you are technical diving, this can be essential to know you will have enough time to complete your decompression stops.
If you are diving with a group, it will allow you to plan an accurate maximum dive time at the planned depth, based on the air consumption of the fastest breather. This ensures that you can pair up with a dive buddy who has a similar SAC rate to you.
We all know that there’s nothing more annoying than having to head back early! If you know that your buddy’s SAC rate isn’t as efficient as yours, you can mitigate the impact by planning a shallower dive or ensuring that your buddy has a larger tank.
By planning to arrive back on the boat or shore, after a safety stop, with at least 50 bar left in the tank, you will help yourself and your buddy avoid potentially dangerous out-of-air scenarios.
Things To Know
What is a good SAC rate?
There is no such thing as a perfect SAC rate. It will vary from diver to diver based on physiology, experience and dive conditions. There is no magic number that you “should” be aiming for.
A SAC rate is a bit like MPG is to a car. Whilst there are improvements that can be made to make this number more efficient, there is a limit to the adjustments you can make.
For example, a tall, broad man will probably always have a higher SAC than a small, petite female diver with similar experience.
What affects a SAC rate?
Stress is a major factor when it comes to air consumption. Stress increases your heart rate and breathing rate. A new or panicking diver who is “gasping” their air, can use air up to three times more quickly than they normally would.
It is also a large part of why experienced divers use so much less air! But, even instructors and divemasters are liable to see an increase in their SAC rate when they are teaching or guiding because of the small amount of additional stress.
Also, it’s important to note that stress isn’t always bad! Positive stress, like the exhilaration of spotting some rare and magnificent sea creature, can impact your SAC rate just as much.
One way to mitigate the stress factor is to try and remain as calm as possible. Take deep, slow breaths and focus on your breathing. Another is to keep diving! The more successful dives you have under your belt, the more relaxed you will be and the better your SAC rate will be.
The amount of exertion you experience in a dive has a huge impact on your breathing rate. If you are gently drifting over the reef, your breathing will be calm and relaxed.
If you are swimming hard against the current, you will be panting as your body requires more air to transport oxygen to your muscles via the bloodstream.
Mitigating the amount of exertion on your dive can be tricky as we can’t always drift with the current. Ensuring that you are aerobically fit can help and, with careful dive planning, you can factor it in to make sure it doesn’t negatively impact your dive.
When you dive in cold water, your body pumps more quickly in order to move the blood around your body and keep you warm. This will increase your breathing rate and therefore your SAC rate.
It is vitally important to make sure that you are wearing the correct wetsuit or drysuit in order to mitigate the risk. Though your increased breathing rate will keep your core temperature up for a time, your body will eventually succumb to hypothermia if exposed for too long.
Having good buoyancy will help you to maximise your SAC rate. If you are slightly positive or negative in the water, you will struggle to stay calm and horizontal. You are also having to expend more energy to try and stay at the right depth.
If you are perfectly neutrally buoyant, you should be able to control your depth easily with your breath. If you know your buoyancy is bad, doing a Peak Performance Buoyancy course is a great way to improve your skills. Check out our weight calculator to make sure you’ve got the right amount of negative weight!
Streamlining your body to be horizontal in the water can have a big impact on your SAC rate. You are minimizing the force and exertion it takes to move your body through the water, so your muscles need less oxygen and your breathing rate can stay low.
Good buoyancy is a key factor in streamlining, as is ensuring all your equipment is properly stowed.
A good finning technique (along with a great pair of fins, of course) is a good way to conserve energy and stay streamlined. A frog kick is more efficient and less work than a classic butterfly.
Staying physically fit and healthy is a great way to optimise your SAC rate. A healthy BMI and regular exercise can not only reduce your SAC rate in normal conditions but will allow you to cope better with unexpectedly arduous conditions, like a swift current change.
Physics and the Compressibility of Gasses
Depth is a key component when determining how long you can stay underwater. Your regulator is designed to deliver air to you at the same pressure as the surrounding water – the ambient pressure. Therefore, when you take a lungful of air at 10m, you are taking in the equivalent of 2 lungfuls on the surface. As a result, air will only last half as long at 10m as it would on the surface.
Some factors are simply out of our control. Things like having to buddy breathe, constant mask purging or air leaks in your hoses are all going to negatively impact our SAC rates.
Can I improve my SAC rate?
You can improve your SAC rate by using the points above:
- Remain as calm as possible and take deep, relaxed breaths
- Keep yourself warm on a dive with a great wetsuit
- Get your buoyancy and body positioning on point
- Learn to frog kick for added efficiency
- Ensure you are doing some cardiovascular fitness outside of the water to ensure your heart is working as efficiently as possible
- Practice practice practice! If you’re diving a lot, the chances are that your SAC rate will improve over time. Until then, try not to worry about it!
Your surface air consumption is a figure that can be really helpful for your development as a diver. The figure itself doesn’t matter so much, but it can be really useful to know the number when dive planning, arranging buddy pairs and seeing your improvement as a diver!
Whilst it can seem very scientific, we hope our calculator made it a little easier for you!