The three letters SAC are an acronym for the phrase Surface Air Consumption and hence SAC Rate is a calculation based on the average depth of your dive to determine your breathing rate while underwater. SAC Rate is measured in litres per minute and equates as a figure that a diver may breathe on the surface. Yes, the surface.
Why Is It Important?
I’m sure you’ve been on a dive when the guide or instructor has asked you to inform him/her when you’re at 100 bar? You don’t want to be that first one, do you? Neither did I.
I came up the PADI education route and nobody told me anything about SAC Rate until I stumbled upon the phrase on my dive computer. This seems quite bizarre as PADI mandates planning a dive and diving the plan. To me, SAC Rate is a very important calculation to make and once out of the PADI education system, I calculated my SAC Rate after each dive. Why? So I could understand how each dive affected my SAC rate and hence understand myself better and improve. It also gave me the basis to plan my dives more effectively with my buddy. As we know, the three things that determine our bottom time is breathing gas used, non decompression limits (NDL) and coldness. So, surely you will need to know how much breathing gas you will need for a dive and that is why you need to calculate your SAC rate.
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.
What Is A Good SAC Rate?
There is no such thing as a good or bad SAC rate as this will differ between diver to diver. On average a new diver may equate a 25 or 27 litres per minute but a relaxed and experienced diver may equate 15 to 18 litres per minute.
What Affects SAC Rate?
- Stress
- Exertion
- Streamlining
- Buoyancy
- Coldness
- Kit Maintenance
You are likely to breathe and consume more gas if you are stressed or anxious. Scuba diving is supposed to be a relaxed passtime, so relax and enjoy the dive. Finning against a current can be exhausting by example. Such exercise will increase your breathing rate and hence your SAC Rate. Streamlining and buoyancy entwine as one here. An overweighted diver will have to inflate his/her BCD with more gas, which not only reduces streamlining but the gas used for inflation could be used for breathing. Poor buoyancy, inflating and deflating your BCD is a waste of good air. Being cold can be linked to stress and exertion too, but the colder you are, the more air you will breathe as your body warms your core. Lastly kit maintenance. Bubbles leaking from poorly maintained kit will have a negative effect as you are simply losing gas.
How Do I Calculate My SAC Rate
Firstly, you need to know some information. These are:
- Cylinder Size
- Start Air Pressure
- End Air Pressure
- Average Depth
- Dive Duration
Let’s assume a dive to a maximum 27 metres for 42 minutes on a 15 litre cylinder with a start pressure of 212 bar and an end pressure of 53 bar. The average depth for the dive is 21.56 metres.
First we need to calculate the amount of gas used. This simple calculation is the start pressure less the end pressure in bar. In this instance, 212 bar less 53 bar equals 159 bar used. Second, we multiply this 159 bar by the size of the cylinder in that 159 bar multiplied 15 litres equates as 2,385 litres of gas consumed. Next, divide these 2,385 litres by the dive duration of 42 minutes to equate as 56.79 litres per minute. Finally, we need to compensate for the average depth. You will recall from your initial training that you have to add the surface pressure of one atmosphere to obtain the water pressure. So, the average depth of 21.56 metres now becomes 3.156 atmospheres for this calculation. Namely 56.79 litres divided by 3.156 average depth equals 17.99 litres per minute SAC Rate. It does not matter what the maximum depth is.
How Can I Decrease My SAC Rate?
Together with addressing the aforementioned issues that may affect SAC Rate, then managing your breathing rate is key. There’s a lot of talk in the industry at the moment about being fit to dive and fitness is paramount. No, it’s not mandated you have to be an athlete, but a good cardiovascular system will aid and reduce your SAC Rate. Breathing slowly and deeply rather than short shallow breaths saves air by promoting the most complete exchange of oxygen and carbon dioxide, mitigating dead air. Pause after inhaling and use your diaphragm to hold air in your lungs a few extra seconds while keeping your throat open. This allows even more time for gas exchange. We’re not advocating holding your breath here. I know some divers who will inhale and exhale as a ratio of 1:3, counting inhale on one and exhale on one, two and three. Oh and, no smoking.
