Surface Air Consumption Rate Calculator
Before we give you all the details below, here is a little calculator I built for you.
Just choose a unit system and let yourself be guided 🙂
What is surface air consumption rate (SAC) ?
When it comes to evaluating your skills and plan your dives safely, understanding how much air you are using is fundamental.
You might have noticed that the more divers are experienced, the more they can extend their air reserves for a longer time.
But air consumption depends greatly on the depth at which you are diving.
When breathing air under higher pressure, you breathe more air molecules for the same volume. These extra air molecules are packed into the same breath you would take at the surface, but you are actually breathing x times more of them the deeper you go.
Because of this, it can become tricky to understand how efficient you are with your air reserves, and that’s where the Surface Air Consumption Rate (SAC) comes into play.
By taking the depth of your dive out of the equation, the SAC allows you to build a baseline idea of how much air you are consuming.
With this number in mind, you can then understand your current level and reverse how much air you should need for a particular dive, which is a necessary safety planning dimension for deep dives or cave diving.
How to calculate your surface air consumption?
Now that we understand what Surface Air Consumption (SAC) is let’s dive right in and learn how to calculate it.
1) Choose a dive with a constant depth
First, you will need to gather some reliable info:
- Your depth
- The amount of air you started with
- The amount of air you have left at the end
- The time you spent at the given depth
- What size is your tank?
For the calculation to be the most accurate possible, you should choose a dive profile that keeps you in constant depth. Once you reached the bottom, take note of how much air you have in your tank and start the stopwatch.
Try as much as possible to have a constant level of physical activity for this one. As we try to establish a baseline, the less variation we have in depth, stress level and physical activity, the better.
Of course, you can repeat this experiment later on by increasing the intensity to simulate how you’d perform under stress and in a more sportive setting like when facing the current, for example.
2) Now let's do the math
To illustrate, we will use the following example numbers:
- Dive Duration = 30 min
- Dive Depth = 20 m (66 ft)
- Starting Pressure in the Tank = 200 Bars (2900 PSI)
- End of Dive Pressure = 100 Bars (1450 PSI)
- Size of the tank = 10 Liters (80 Cubic Ft)
The first step will be to understand how much air you used during the dive in your desired unit.
In Bars:
200 Bars – 100 Bars = 100 Bars
To find how many air litres are packed in 10 Liters tank inflated at 100 Bars, we need to multiply the number of Bars by the tank size in Liters.
100 Bars * 10 Liters = 1 000 Liters
In PSI:
With imperial units, we can use the same calculation because of the way Cubic Ft is used. The best way is to multiply the total capacity (80 Cubic Ft) by the ratio between starting pressure and end pressure.
80 Cubic Ft * (1450 PSI/2900 PSI) = 40 Cubic Ft
Once we know how much air we used, we need to bring that divide this number by the duration of the dive to have a consumption per minute.
In Liters/Minute:
1 000 Liters / 30 minutes = 33 Liters/Minute
In Cubic Ft Per Minute:
40 Cubic Ft / 30 Minutes = 1.33 Cubic Ft / Minute
But as we saw above, these numbers are dependent on the depth of the dive. To take it out of the equation and have a number that can be compared for every depth, we need to divide our number by the amount of pressure (in atm) equivalent to our depth.
The current amount of pressure is equal to 1 atm (atmospheric pressure) + 1 atm for every 10 m or 33 ft of water above you.
So finally, your SAC is:
In Liters/Minute:
33 Liters per Minute/ 1 atm + 2 atm = 11 Liter per Minute
In Cubic Ft/Minute:
1.33 Cubic Ft per Minute / 1 atm + 2 atm = 0.44 Cubic Ft per Minute
Feet can become tricky to convert in atm, so I would recommend converting them into meters (feet / 3.2808 = meters) and dividing the result by 10.
Ex: 75 feet / 3.2808 = 22.8 m => 22.86 m / 10 = 2.28 atm
Why is knowing your SAC useful?
This is all great and sound, but “Why to bother calculating my SAC?” you might say. Well, you should, for several reasons.
It can shine light on skills you need to improve
It’s not a coincidence that experienced divers will generally have a way lower Surface Air Consumption rate than beginners; much of it is based on skills and confidence.
Scuba Diving is a particular sport in which you learn to reduce the amount of effort you make. A good scuba diver is efficient with every move and avoids spending excessive energy and air.
If your SAC is high, it probably means that you can improve in many areas of your scuba diving skills, such as your control of buoyancy or your swimming technique.
Ultimately, you should always aim to control how much air you are consuming and extend as much as possible the amount of time you can spend safely underwater.
It will help you stay safe and keep other people safe
This might be the most obvious, but knowing the limits of your air consumption and increasing it with experience is one of the best ways to improve your safety and the safety of the people diving with you.
This actually becomes mandatory when you start to dive deep or are in charge of others’ safety.
Knowing how much air you consume in the different scenario will help you and your buddies be confident that there is enough air to come back safely, even with stress and/or difficult conditions in case of trouble.
It will allow you to plan dives in the best condition
If you are a certified autonomous diver, you will definitely need to take your air consumption rate and your group’s air consumption into consideration.
Having to cut a dive short because one of you is running out of air is no fun at all… and this alone should justify that you take into consideration your SAC when planning your dive depth.
More importantly, even knowing how much air you consume is becomes mandatory when planning dives in more stressful and risky conditions such as cave diving, wreck diving or deep dive with decompression stops.
In these conditions, you often can’t afford surprises and realise too late that someone in the team is sucking on its air like a 1969 Mustang on fuel.
What is a good SAC rate?
First of all, you should remember that while your SAC can be drastically improved by mastering skills and becoming more confident underwater, it would be wrong to try and compete to reduce it to the maximum.
Without wanting to state the obvious, everybody is different, and your air consumption depends greatly on physiologic traits. Ultimately, a 6-foot man with big lungs will always use more air than a 5-foot woman with the same amount of training and experience.
However, we can still set some standards that most people can easily achieve.
For example, we can agree that a good diver should comfortably spend 40 to 45 min at an average depth of 20 m without having to pump on its reserves.
According to our calculator, this would give us a Surface Air Consumption Rate of around 15 to 17 Liters/Minute or 0.45 to 0.50 Cubic Ft/Minute.
These rates are common for most experienced divers and should allow you plenty of room to engage in every type of dives with confidence and safety.
How to improve your air consumption rate?
Finally, I wanted to give you a few tips on how to improve your Surface Air Consumption Rate.
Learn How To Control Your Buoyancy
It would be best if you become a master at “feeling” your buoyancy at all time. Being in a constant state of compensating a positive or negative buoyancy will drastically increase your air consumption by forcing you to fight against your own buoyancy. You will increase the number of unnecessary movements to maintain yourself at a particular depth.
It will distract your attention from your breathing and trick you into taking deeper breaths with a greater amplitude, which will make it increasingly difficult to control your buoyancy.
By being overly heavy or too light, you will also have trouble adopting the optimal position underwater. This will lead to your movement being less efficient, requiring more air.
Work On Your Breathing
Another dimension which I feel is not being talked about enough is your actual breathing.
Stop for a second and think of how you are breathing in your daily life. You are not always taking BIG breath through your mouth at a regular cadence. You are actually just taking a small amount of air, pausing for 1 or 2 seconds and then breathing out naturally and this without needing to think about it.
Breathing through a regulator isn’t something our body is accustomed to. Your breathing becomes continuous and forced to some degree.
Take a step back and consciously reduce the amplitude of your breath, then take a small conscious pause before and after every breath.
Of course, you should never stop breathing entirely! Especially in ascending phases
But by consciously taking control of your breathing and its rhythm, you can train your mind to consider breathing underwater a normal activity, and in turn, develop a natural breathing pattern that mimics the one on the surface.
Protect Yourself From The Cold
Outside of your own abilities, a number of external factors can impact your air consumption rate.
Actually, everything that increases your physical activity is a source of higher consumption of air and in this category one of the champions is temperature.
When feeling cold, your body reacts by trying it’s best to warm itself up. He is burning fuel like crazy in order to maintain its temperature.
This phenomenon involves your heart and lungs pumping faster to bring more oxygen in and increasing the blood flow inside your body. Of course, a direct consequence of this is your air consumption increasing drastically.
Always make sure you are always equipped with the appropriate wetsuits, gloves, hoods, etc.. or potentially go full dry suit when needed.
When planning dives, you should also remember about the potential thermoclines and adjust your gear accordingly.
You don’t want to reach hypothermia after spending most of your dive at high depth!
Reduce Stress by Building Experience
There is a reason why experienced divers and instructors seem to always come back on the boat with much more air than anyone else.
They are simply more experience and have faced more challenging or exciting situations than others.
They have trained their minds to be at ease with the idea of breathing underwater and they have better control over their emotions.
This can only be built with experience, after facing many different situations of stress or excitement in a variety of environment.
Improve Your Position And Movement Efficiency
This dimension is very intertwined with your mastery of buoyancy your breathing and your experience.
In a nutshell, you when in perfect control of your buoyancy and breathing, it will become a lot easier for you to maintain an optimal horizontal posture.
This will make you more stable, improve the efficiency of each fin stroke and reduce the impact of currents on your body.
Moving less will also allow you to reduce the flow of water between the exterior and your wetsuit, helping you keep warm.
You can see that everything is linked together, a good position will make it easier to control your buoyancy, making it easier to control your breathing which improves your buoyancy, which improves your posture, etc…
