On the surface this would seem like a simple question, but it is quite complicated. I’m going to take you through all of the factors that determine how long a tank will last on your dive.
For simplicity sake, if you want the short answer, at moderate depth (60 feet or less) a tank should last you from 40 to 60 minutes.
If you want to understand all of the variables that go in to determining how long a tank will last, keep reading.
The Factors
The main factors that determine how long a tank is going to last during a dive are; depth, activity, physiology, stress, efficiency, and tank size.
I have been on more than 2000 dives, taught hundreds of people how to dive, and taken thousands of novices and experienced people diving. The information that follows is both science based as well as what I have observed through a lifetime of observation.
Depth
This is the single most important factor to determine how long a tank is going to last, and the easiest to calculate.
Let me give you an example. Let’s say you are sitting on the back of a boat breathing out of your regulator and it takes you two hours to go through a tank. You are doing no physical activity, not swimming through the water, just sitting there on the back of the boat. At 100 feet deep, sitting on the bottom doing no physical activity that same tank will last you 30 minutes.
Every 33 feet deep you are in the water column your air consumption will double. So, at the surface you are at one atmosphere, every 33 feet is another atmosphere of pressure, which is why at 100 feet (99 feet in reality) our test subject is breathing 4 times the air as someone on the surface.
Activity
As I specifically mentioned while discussing depth the person sitting on the back of the boat was doing nothing, just breathing. In a normal diving situation you are using your main muscle groups, your legs, to propel you under water. To operate these muscle groups takes oxygen, found in the air you are breathing. So, the more activity you are doing underwater, like swimming against current, the more air you need to use.
Physiology
This is pretty obvious, but let’s take two people, the first is a man that is 6 feet 8 inches tall and weighs 300 lbs, and the second is a man that is 5 feet tall and weighs 135 lbs. The larger person has larger lungs, more muscle tissue and is just a much larger person. That muscle tissue is going to require more oxygen at a resting metabolic rate.
More simply said, the larger you are the more air you are going to use.
Stress
Now I have never seen a white shark underwater, and I’m not sure I want to unless I am in a shark cage, but I’m going to assume my breathing rate is going to go up if I do. This is stress. Stress comes in many forms when diving, disorientation, lack of visibility, trouble with navigation, search and rescue, assembling something underwater, seeing white sharks, all cause stress underwater.
The more stressful of a situation the more air you are going to use. It’s that simple.
Efficiency
The more efficient you are the less air you are going to use underwater.
The best example I have of this is me as compared to my brother. I have more than 2000 dives, my brother probably has around 25 dives. I am just a lot more comfortable underwater with my level of experience and training. We are about the same weight, I am a little taller and he is a little more stocky, but we’re roughly the same size. So, when we are diving together we should use roughly the same amount of air.
Every time we go diving my brother uses nearly double the amount of air I use. He’s a lot less comfortable in the water than I am, I see him using his hands and arms to propel himself, this is inefficient. I’m not bragging, I’m just a more efficient diver. I should be.
So, the less body movement you have, and as a result the more efficient you are with your propulsion through the water, the less you are using those large muscle groups the less air you are going to need.
Tank Size
The vast majority of scuba tanks out there in circulation, especially at recreational diver operations are called Aluminum 80’s. They are aluminum cylinders that hold 80 cubic feet of air at 3000 psi. There are a lot of different sizes of scuba tanks out there. A popular one is a steel 100. The steel 100 holds 25% more air than the aluminum 80, so you need to be aware of your tank size.
I often take a 50 cubic foot tank on dives when I am with a bunch of novices and we are doing a relatively shallow dive. I know my air consumption is much less than a novice diver so I know I will need less air.
Conclusion
As you can see, there are a lot of variables that determine how much air you use while scuba diving, and as a result how long a tank is going to last you on a dive. The more experience you have the more you will know how much air you will likely use for a certain type of dive and the easier it will be for you to plan out your dive.