Many entry level, but also more experienced divers, are unfamiliar with SAC: many times, when speaking to students and divers about this topic, they look back at me with the eyes wide open!
Not all teaching systems include SAC calculation in entry level courses material. Anyway in my opinion is essential that divers get acquainted to its calculation in order to be independent and be in the condition of planning their dives and knowing their personal limits, for their own safety.
Even when dive planning plays a secondary importance, like for example in the case of diving tourism, knowing your own SAC can simplify the activity of your divemasters and facilitate the choice of the right cylinder size, making the dive more comfortable out and in the water.
Let us have a look at the SAC formula and at the data necessary for its calculation [metric system].
Air Consumpton (AC).
This is our total air consumption during the dive, indicated in bar. This data can be calculated by subtracting from the initial pressure of the full cylinder the pressure left when we exit the water (e.g. 200 bar - 50 bar = 150 bar).
Volume of the cylinder (V).
Easy to understand, this is the volume of the cylinder in use, expressed in liters (e.g. 10l, 15l…).
Time (t).
Total time of the dive, in minutes.
Absolute pressure at depth (P).
This is the absolute pressure, indicated in bar, corresponding to the average depth of the dive. To obtain this value we simply need recall from your dive computer the average depth of the dive in meters and convert it in bar. Make sure that the value indicates the absolute pressure!
These data can be put toghether in the following formula to calculate the SAC:
SAC = AC x V / ( t x P)
Now a concrete example, assuming that we have been 50 minutes underwater, at an average depth of 12 meters (corresponding to 2,2 bar absolute pressure), and we used a total of 150 bar from a 12 liters cylinder:
150 bar x 12l / (50min x 2,2 bar) = 16,36 l/m
Now that we have our SAC we can easily plan our next dive (depth, duration and gas supply needed) or check if it will be within our personal limits!
It is important that we record our SAC in our logbooks on a larger number of dives, so that we dispose of a significant average value. In fact, SAC rate may vary by many factors such as water temperature, level of fitness, ambient conditions, buoyancy and trim, underwater activity and air leaks.
Moreover, by monitoring the SAC you will be able to track any improvement of your gas consumption over time and experience!
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