Surface Air Consumption (SAC)
As we progress as scuba divers, a good indicator of how well we are doing is our surface air consumption (SAC) rate. How many of us completed our basic open water training, went on a diving holiday and then looked in amazement at the divers that were getting back onto the boat with half their air left? I remember thinking as I looked at my 50 bar that these people must be super fit marathon runners.
Good breathing and air consumption are one of the easiest ways to spot an experienced diver, and there are lots of factors that can influence our air consumption under water:
The deeper we go, the more compressed the air becomes and so we consume more of it for each breath. At the surface, we might consume 1.5 litres of air in a breath, but at 20 meters deep this has turned into 4.5 litres.
The biggest influence, as we get stressed, we subconsciously increase our breathing rate and heart rate. Unfortunately, this can be a vicious circle as your nervous system becomes more active and your awareness decreases. This means you can get more anxious and the cycle keeps on going. A stressed diver could be breathing three times as much as a relaxed diver.
A basic requirement is that as we exercise aerobically, our muscles need the oxygen to convert stored sugar and fatty acids to create energy. More exercise means more oxygen required and more air breathed in to provide it. Fin against a current for a while and you'll quickly start to see your air consumption go up. Do a drift dive where you are gliding along and it is totally the opposite.
This is related to exercise. Divers who have lots of dangling kit or who progress through the water at an angle are constantly being resisted by the body of water they are trying to move through. A streamlined diver who has tucked all un-needed equipment away, and is trying to stay as horizontal as possible, is having to work a lot less and so needs less air to supply less oxygen to the muscles in the body.
Getting your buoyancy right can have huge impacts on your air rate. If you are either slightly positive or negatively buoyant, you are going to find it hard to stay horizontal and flat, so your streamlining is bad. Worse still, you are having to expend energy either keeping your self up or down. A totally neutral buoyant diver has none of these problems. This is why you should never begrudge the air you have to put in your BCD as you descend. Get this right and you will save loads more by being able to float properly with less energy consumption. If the conditions allow, a good tip is to try and do a quick buoyancy check when you've finished descending, and fine tune your BCD inflation before starting the main part of the dive.
The colder you are, the more air you breath. If you are cold, your body tries to protect you by raising your internal core temperature, and the only way it has to do that is burn some stored fuel (sugars and fatty acids), which in this case requires oxygen to metabolise. So your heart rate and breathing rate go up to supply the oxygen. Wearing the right exposure suit for the conditions is always a good idea and will also help you with your air consumption.
Your Body Mass Index (BMI) and overall level of health will affect how much oxygen you need. More body mass = more oxygen. However, this factor can be less important than the ones above.
Some things like air leaks or mask purges are going to lose you air and there is nothing you can do about it.
Calculating Air Consumption:
The first factor, depth, means that to measure our air consumption we need to do it in relation to the surface or we are comparing apples with oranges for every dive we do. If we did two identical dives one at 10 meters and one at 20 meters then we should expect to use 1.5 times the amount of air for the 20 meter dive. By converting our air consumption to an equivalent value as if we were standing on the surface, we can compare different dives and see if we are generally improving or not.
The calculations for metric divers are fairly straight forward. Take a 20 minute dive to 20 meters where you used 140 bar and you were diving with a 10 litre cylinder:
140 bar x 10 litre cylinder = 1400 litres consumed during the dive.
1400 litres / 20 minutes = 70 litres / min air consumption rate at 20 meter depth.
At 20 meters, the air is 3 times denser than at the surface so we divide by 3 to get a 23.3 litres/min Surface Air Consumption Rate.
For imperial divers, the calculations get a little more complex with the addition of the way cylinders are described. In this case, an 80 cubic foot cylinder is the volume of gas it contains at it's working pressure. Not it's true internal volume at normal atmospheric pressure which is how metric cylinder are defined.
So lets take a 20 minute dive to 66 feet where you used 2000 psi and were diving with an 80 cubic foot cylinder rated to 3000 psi working pressure.
80 cubic feet 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.
At 66 feet the air is 3 times denser than at the surface and so we divide by 3 to get a 0.89 cubic feet per minute Surface Air Consumption Rate.
When writing up your log, having a calculator to hand is not always possible, which is why we created the DiveLogs SAC calculator chart. Using a simple chart you can now find your SAC rate in a few seconds and minimal brain cells. You can give it a go here:
The SAC calculator chart is included in all our SAC log sheet packs as well as being engraved onto our placeholders. All you need is a straight edge. It is available in metric and imperial units as well as for select common cylinder sizes.
Using Surface Air Consumption Rate:
There are two good reasons for becoming comfortable with measuring SAC rate. Initially, it helps you identify when you had a bad dive and help you improve (the big tips are: sort out your buoyancy, relax, tuck everything away, keep your arms folded, and use just your fins to swim).
In the log chart on the right you can see the time I completed my advanced open water course in Sharm El Sheikh in Egypt. There were two dives where my air consumption went up like crazy, and I clearly remember both dives even today. The first one was a complex (for me at the time!) drift dive around Ras Muhammad and I was super stressed! I did the 5 minute safety stop at the end of the dive hanging onto the instructor upside down on her backup air supply! My buoyancy was shot and she still surfaced with 100 bar! A good example of what stress and non-stress looks like.
The second was a drift dive further north. Went well to plan, but at one point I ended up towing my wife against the current as we came round a headland before turning back. I have never seen an air gauge move so fast!
In the same trip we did a photo elect speciality course and got so focused on what we were doing and getting our buoyancy right for photo taking that we surfaced with 100 bar after a one hour dive! Totally different SAC rates and all driven by the three main factors of stress, exercise and buoyancy.
You can also use your typical SAC rate to back calculate what size cylinder and fill level you should have for a particular dive profile.
Many people wonder what the ideal SAC rate is and the answer is that there is none. Everyone is different and we all have different oxygen needs. What's important is to try and improve and get used to what a good comfortable dive looks like by logging them - then you'll find out what your personal optimum SAC rate should be.