How to Calculate How Long Your Air Will Last Underwater

Scuba diving is an exciting hobby that allows you to explore an underwater world. Successful scuba diving requires the proper training and equipment as well as a healthy dose of respect for the dangers underwater. Being cognizant of your air supply is a constant consideration while diving. By finding your surface consumption rate, or SCR, you can calculate the rate at which you consume air underwater, allowing you to predict how long your air supply will last.

Things You'll Need

  • Scuba equipment
  • Calculator
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Instructions

    • 1

      Dive to a depth of 33 feet and swim steadily for 10 minutes using a pressure gauge, a depth gauge and a watch. Record your starting pressure and ending pressure.

    • 2

      Calculate air used by dividing your ending pressure by your starting pressure and multiplying by the size of your tank. This provides information on the volume of air you used:

      (Ending pressure in psi / Starting pressure in psi) x Size of tank in cubic feet.

      For example, (2,500 psi / 3,000 psi) x 80 cubic foot tank = 67 cubic feet of air remaining.

    • 3

      Subtract cubic feet of air remaining from the tank volume to find the number of cubic feet you used.

      For example, if 67 cubic feet of air remain and the starting volume was 80 cubic feet, then you used 13 cubic feet of air.

    • 4

      Find your SCR. Calculate the amount of cubic feet you breathe per minute, taking into account the pressure at your depth. With increased depth comes increased pressure. This pressure reduces the volume of air in your tank. Calculating the amount of air you breathe as a function of time and depth will allow you to calculate how long your air supply will last at any depth.

      Cubic feet of air used in test dive / 2 atmospheres of pressure at 33 feet for 10 minutes = Surface consumption rate.

    • 5

      Calculate how long your air will last by using your surface consumption rate and this equation:

      (Cubic feet of air in tank / SCR) / [(Dive depth + 33) / 33] = Length of time your air will last underwater.

      Example: A tank with 80 cubic feet of air being used by a diver with an SCR of 0.70 at a depth of 70 feet will have approximately 37 minutes of air.

      (80 / 0.70) / [(70 + 33) / 33] = 36.6 minutes