Have you ever wondered how to compare your gas consumption in different dives? If you want to be able to plan your gas needs for a certain dive, you need to know how much gas you breath per minute.
And that is actually an easy calculation:
bar.l = divers x minutes x Ødepth (in atm.) x SAC (surface air consumption)
So let’s do an example:
After a 40 minutes dive, you surface with 70 bar left in your 12l tank (you started with 200 bar). The dive led you to a maximum depth of 16m whilst your average depth was 12m.
bar.l = 200 – 70 = 130 bar used per 12 (12 liter tank) = 1560
divers = assuming you didn’t share gas, it was only you breathing from the tank, so 1 diver
minutes = 40
Ødepth = 12m so 2,2 atm / bar
SAC = what we would like to calculate, so our ?
Now let’s fill in the numbers:
1560 = 1 x 40 x 2,2 x ?
Re-ajusting the calculation this would be:
? = 1560 / 1 x 40 x 2,2
doing the maths:
? = 17,73
So in this case, our Surface air consumption rate would be 17,73 liters per minute.
So what does this tell you? Not much, I guess. It really starts to become interesting once you start to compare your SAC from different dives. Looking at my SAC from a dive I can tell a lot about the dive itself. Did I feel comfortable? Was I relaxed? Was there any current? Whom did I dive with? All those factors play a role in my SAC. Also, I noticed my average air consumption rate has been going down about 1 liter per year. Again, those things are interesting to know for geeks like me, but it starts to really become interesting when you want to plan a dive and calculate the gas you need.
Also, most divers remember from their Open Water Diver class they have to start their ascent with 50 bar left in their tank. But they don’t know the reason behind that. Because of course there is a reason! Understanding and knowing that reason, you will see that 50 bar of gas might actually not be enough to safely get back to the surface.
If you are interested in knowing more about gas consumption, gas needs and dive planning, feel free to send me a message so we can schedule a Mini class.
