How much air can a scuba diving tank hold and how long will that last you?

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2 Responses to “How much air can a scuba diving tank hold and how long will that last you?”

  1. academicjoq says:
    September 11, 2011 at 6:03 am

    Tanks vary in capacity. The time they last underwater depends on the depth at which you dive. The deeper you go the less time you have. The more vigorously you move the less time you have. Typically a single tank will hold enough compressed air for you to stay underwater at depths of around 20 or 30 feet for about 20 to 30 minutes.

  2. moviebuff says:
    September 11, 2011 at 6:31 am

    As usual, the answer is “it depends”.

    There are many different sizes of scuba cylinders – some cylinders can hold as little as 3 cubic feet of air when filled while others can hold up to 130 cubic feet of air. The most commonly used scuba cylinder for recreational diving (at least in the western hemisphere) is the AL80 that holds a little over 77 cubic feet of air when filled to capacity. As a reference, that’s about as much air that would fit in a box that is 4 feet wide, 4 feet long, and just under 5 feet high.

    The amount of time that a full cylinder will last not only depends on the capacity of the tank but also on the air consumption rate of the diver. For example, if a diver is working hard they will be breathing faster and so will use air more quickly than if they were not exerting themselves so much. Also, breathing rates vary from individual to individual so some divers just naturally use less air than others. In general (but not always), newer divers tend to use more air than more experienced divers, larger divers tend to use more air than smaller divers, and men tend to use more air than women.

    Lastly, the depth at which the diver is breathing also has an effect on how quickly air in the cylinder will be used. Because water is so much heavier than air, pressure increases very rapidly as we descend underwater. For example, the pressure at 33 feet in the ocean is twice what it is at the surface, while the pressure at 100 feet is about 4 times what it is at the surface. This affects how quickly air is used because as you exert pressure on a gas it compresses into a smaller volume, which means the gas molecules get packed closer together. As you fill your lungs with gas that is under pressure the air space in your lungs will contain more gas molecules than it would if the gas was under less pressure. For example, at 100 feet where the pressure is about 4 times surface pressure, your lungs will contain 4 times as many gas molecules than they would at the surface. Since a scuba tank only contains a fixed number of gas molecules, the deeper you go the more quickly you will use up the limited amount of air in the tank.

    So the bottom line is that the answer to this question is highly variable. However, it is my experience that on an average ocean dive (say average depth of 50 feet) while using the standard AL80 tank the typical recreational diver will make a tank last somewhere on the order of 30-40 minutes. Heavy breathers (“air hogs”) may last only 20-30 minutes while more efficient breathers will make a tank last an hour or more.

    I hope this helps.

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