How Long Does an Average Tank Last for an Average Diver at an Average Depth?:
Based on personal experience, an average open water certified diver using a standard aluminum 80-cubic-foot tank on a 40-foot dive will be able to stay down for about 45 minutes before surfacing with a safe reserve of air.
What Factors Determine How Long the Air in a Scuba Tank Will Last?:
• Tank Volume
One of the most common tanks in recreational diving is the aluminum 80-cubic-foot tank, which holds 80 cubic feet of air compressed to 3000 pounds per a square inch. 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 effects pressure in scuba diving). This increase in pressure does not effect the air inside the diver's scuba tank because it is already compressed to a very high pressure and he scuba tank is a rigid container. However, the water pressure does compress air that exits the tank and flows through the scuba diver's regulator hoses and second stages. The quantity of air that fills 1 cubic foot of space at the surface will only fill ½ a cubic foot of space at a depth of 33 due to the compression of water. Therefore, the quantity of air that fills a diver's lungs at the surface will only half fill his lungs at 33 feet. The deeper a diver goes, the greater the quantity of air required to fill his lungs with each breath, and the more quickly he will use up the air in his tank.
• Air Consumption Rate All other factors being equal, a diver's air consumption rate, also called his Surface Air Consumption Rate (SAC rate) or Respiratory Minute Volume (RMV), 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 effect an individual's air consumption rate, including stress, experience level, buoyancy control, and the amount of exercise the diver does on a dive. Relaxed, slow, and deep breathing is usually the best way for a diver to reduce his air consumption rate.