IANTD Open Water Diver Practice Exam

To determine how long an 80-cubic foot tank will last at a depth of 66 feet when breathing at a rate of 0.5 cubic feet per minute, it's important to consider both the tank's capacity and the effects of pressure on the gas volume.

At a depth of 66 feet, the pressure is approximately 3.0 atmospheres (1 atmosphere at the surface plus 2 additional atmospheres for the water column). The volume of air supplied from the tank is effectively reduced due to the pressure. While the tank contains 80 cubic feet of air at surface pressure, the equation for gas volume under pressure indicates that at depth, you must consider the effective volume:

80 cu ft (tank volume) multiplied by the pressure factor (3.0 atm) gives a total effective volume of 240 cubic feet of air available at the surface pressure equivalency.

Next, you divide this effective volume by the breathing rate:

240 cu ft (effective volume) divided by 0.5 cu ft per minute equals 480 minutes of breathing time.

However, the question specifically asks how long at the depth of 66 feet the tank will last, looking for a direct evaluation at the breathing rate under the current pressure conditions. Understanding that