HOW LONG DOES THE AIR IN THE SCUBA TANK LAST?
How long does a scuba tank last? Good question! I once asked the same question and received a sigh of resignation from my scuba instructor before he launched into an explanation. Now, when a student asks me this very reasonable question, I, too, inwardly groan before answering.
Although the question is simple, the answer is complicated. But here’s an attempt at an answer.
An Average Diver, at an Average Depth, With an Average Tank
Based on personal experience, an average open-water certified diver using a standard aluminium 80-cubic-foot tank on a 40-foot dive will be able to stay down for about 45 to 60 minutes before surfacing with a safe reserve of air still in the tank.
Three Factors That Determine How Long a Diver’s Air Will Last
1. Tank Volume
One of the most common tanks in recreational diving is the aluminium 80, which holds 80 cubic feet of air compressed to 3000 pounds per-square-inch (PSI). However, scuba tanks are available in different sizes and materials for a variety of applications (learn more about the difference between steel and aluminum tanks). Divers who engage in very deep or long dives may prefer tanks with a greater internal volume. Petite divers who use very little air may choose to use smaller tanks for comfort. All other factors being equal, a tank that holds a higher volume of air will last longer underwater.
As a scuba diver descends, the pressure around him increases (learn how depth affects pressure in scuba diving). This increase in pressure does not affect the air inside the diver’s scuba tank because it is already compressed to a very high pressure and the scuba tank is a rigid container.
However, the water pressure does compress the air that exits the tank and flows through the scuba diver’s regulator hoses and second stages. For example, the quantity of air that fills 1 cubic foot of space at the surface will only fill ½ cubic foot of space at a depth of 33 feet due to the compression of water.
Similarly, a diver will consume twice the volume of air at 33 feet as he uses at the surface. In other words, the deeper a diver goes, the more quickly he will use up the air in his tank.
3. Air Consumption Rate
A diver’s air consumption rate will determine how long the air in his tank will last compared to the average diver. A diver with large lung volume (tall or large people) will require more air than a petite or short person with a smaller lung volume, and will usually have a higher air consumption rate. A variety of factors affects an individual’s air consumption rate, including stress, experience level, buoyancy control and the amount of exertion required for the dive. Relaxed, slow and deep breathing is usually the best way for a diver to reduce his air consumption rate.
Air Supply Is Not Always the Limiting Factor
In many cases, a diver must end his dive before reaching the limit of his air supply. Examples include reaching the no-decompression limit for a dive (in which case a diver may consider using enriched air nitrox) or ascending with a buddy who has reached the limits of his air supply.
Dive plans and dive sites vary. Just because a diver has air left in his tank doesn’t mean he should (or will even want to) stay underwater until it runs low.
In the end, several factors determine how long the air in a tank will last for a particular individual and a particular dive. This is the reason that the question is so difficult to answer. Predicting how long a tank will last underwater requires an understanding of the physics of water pressure, tank volumes and air consumption rates. However, I have one answer that applies to every diver who asks how long his tank will last underwater: Never long enough!
How long does it last? As long as it lasts.
By Natalie Gibb – updated 08 Sept 2017