A very common topic among divers is air consumption. Hang around dive boats and you’ll often hear people discuss their air consumption for any given dive, and a frequent question among new dive buddies is “how much air do you use?”
The topic is relevant because for many, air consumption determines dive time. Especially on relatively shallow dives, it’s often air-management considerations rather than nitrogen load that determine the end of the dive. And with modern dive computers and multi-level diving, the option to extend a dive by gradually moving to shallower water is that much easier, again meaning that a dive can often be extended until the tank pressure dictates the end of the dive.
Keeping track of your air consumption can be a great way to monitor your progress in stretching a tank. Typically, the more we dive, the more experience we have, and the more relaxed we are underwater, the less air we consume. Tracking your air consumption also greatly helps in planning dives, especially maximum dive time. Any dive should always be planned to have a maximum dive-time based on the expected air consumption of the diver who uses the most air (with a buffer for safety, of course).
But tracking your air consumption isn’t quite as easy as simply reading your pressure gauge when you return from your dive. Yes, that will tell you your air consumption on that particular dive, but unless every dive you do is to the same depth, you’ll find it difficult to compare one dive’s air consumption to another’s. The reason is pressure at depth. Pressure and volume are inversely related, meaning that the more pressure increases, the more the volume of air decreases. From your entry-level scuba class, you’ll remember that if you take a closed air container, like a balloon, from the surface and down to 33 feet, the pressure will increase by 100 percent (from 1 bar to 2 bar) and the volume of the air in the balloon will decrease by 50 percent. This also means that a breath of air will consume more air from your tank at 33 feet than at the surface, and the deeper you go, the quicker you’ll burn through your air.
Surface Air Consumption Rate (SAC)
In order to compare dives at different depths, we need to factor in the depth of the dive, which we do by calculating our Surface Air Consumption rate (SAC). Doing so is a little complex, but there are automated calculators online. However, if you calculate your SAC somewhere where there’s no Internet connection, the following formula allows you to do so manually. You can calculate either for an entire dive or for a segment of a dive, just make sure to note of the exact time, depth, and air consumption of the segment if you only calculate a portion of your dive.
First, take the amount of air you consumed during the dive, and divide that by the total number of minutes of the dive, to find your consumption per minute.
Next, take this number and multiply it by the total pressure at the depth of your dive (for multilevel dives, use average depth if your computer has this function). This would be the pressure at the depth of the dive, plus one atmosphere of pressure for the surface pressure.
Finally, divide your air consumed with the absolute pressure at depth to find your consumption per minute, adjusted for depth. The whole formula looks like this:
Air consumed during dive/dive time/total pressure at depth of dive = SAC per minute
So, as an example, we’ve done a dive to 10 meters/33 feet for 20 minutes, consuming 80 bars/1,200 psi of air. So the calculation would be:
Empirical: 1200psi/20 minutes/2 = 30 psi per minute
Metric: 80 bars/20 minutes/2 = 2 bars per minute
Taking It Further: RMV
The main problem with SAC is that it depends on tank volume. Two bars/30 psi is relative to the tank size: the bigger the tank, the more air is contained in those 2 bars. For this reason, many divers have started using Respiratory Minute Volume (RMV) rate. The main difference here is that this rate is independent of tank size. To turn your SAC into RMV, simply factor in the size of the tank you were using by adding a tank conversion factor.
Imperial: Divide the tank volume in cubic feet with the tank operating pressure. Then take this number and multiply your SAC with this factor.
Metric: Multiply your SAC with the tank volume in liters.
This final number is comparable across dives to different depths and with varying durations, as it is adjusted for both time and pressure. This makes it easier to compare and track progress over time.
Also, if you need to plan a dive to a given depth, you can simply take your SAC rate and multiply it by the total pressure at the new depth to find out your air consumption per minute at that depth. So if the diver in the example above wanted to do a dive to 30 meters/100 feet, she would simply multiply 2 (metric)/30 (empirical) with 4 (4 atmospheres being the total pressure at the new depth), to find that her air consumption would be 8 bars/120 psi per minute. She could then compare this number to the planned dive time to learn how large a tank would be