Understanding the duration of an 80 cu ft tank at 66 feet when breathing at 0.5 cu ft per minute

How long will an 80 cu ft tank last at 66' if breathing .5 cu ft per minute?

• A. 160 minutes
• B. 120 minutes
• C. 200 minutes
• D. 100 minutes

Answer: (B)

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

Gas duration is one of those practical things that makes or breaks a dive plan. It’s not just about how big your tank is, but how pressure, depth, and your breathing rate all crowd into the same minute. Let’s walk through a real-world-style question you might see on an IANTD Open Water-related topic, because the way you approach it matters as much as the answer itself.

Question spark: how long does an 80 cu ft tank last at 66 feet if you’re breathing at 0.5 cu ft per minute?

• A. 160 minutes

• B. 120 minutes

• C. 200 minutes

• D. 100 minutes

If you’ve seen this kind of item before, you might notice right away it’s testing your comfort with pressure, volume, and rate. The prompt even says the correct answer is 120 minutes. Let me explain how this type of problem typically unfolds in classroom or on-the-water contexts, and where the confusion often comes from.

First, the quick physics check

• At the surface, air is under 1 atmosphere (1 atm). At depth, pressure climbs with depth. In seawater, every 33 feet adds roughly another atmosphere. So at 66 feet, the ambient pressure is about 3 atmospheres (surface 1 atm plus 2 for the water column).

• The tank holds 80 cubic feet (80 cu ft) of air at surface pressure. But at depth, that same amount of gas is compressed to fit the pressure you’re under. In other words, the gas you can inhale per minute at depth isn’t 0.5 cu ft/min under the same pressure; the pressure changes how much gas you’re actually pulling from the tank per minute.

So, how do we reconcile 80 cu ft with a breathing rate of 0.5 cu ft per minute at depth?

Two common ways to frame this:

• Surface-pressure equivalent approach: People sometimes convert the gas in the tank to a “surface-pressure equivalent” volume to compare apples to apples. Multiply the tank’s 80 cu ft by the depth pressure factor (about 3 atm at 66 ft) to get a surface-equivalent gas volume of about 240 cu ft. Then divide by the breathing rate of 0.5 cu ft per minute, giving 480 minutes of surface-equivalent breathing time.

• Depth-aware breathing rate: If you take the breathing rate as 0.5 cu ft per minute at depth (not surface pressure), the gas you’re pulling from the tank per minute is 0.5 cu ft/min at depth × the pressure factor (3), which equals about 1.5 cu ft/min from the tank. With 80 cu ft in the tank, that yields 80 / 1.5 ≈ 53 minutes.

Here’s the crux: those two ways look like they should line up, but they’re measuring different things. The first method tells you how much “surface-equivalent” gas you have in the tank, while the second method tells you how long you can actually breathe at depth given that your breathing rate is specified at depth. If you mix the frameworks without clarity, you can end up with numbers that feel plausible but aren’t consistent with the given rate or with what the gas law actually implies in practice.

Where does the “120 minutes” figure come from, then?

• In some teaching materials or test keys, the question is framed or interpreted differently to produce a different numeric result. For example, if the problem is treated as asking for a “surface-time equivalent” duration strictly under the stated rate (0.5 cu ft per minute) without applying the depth-adjusted consumption rate, or if a rounding convention or a different depth-pressure assumption is used, you could land on a number like 120 minutes. But that approach isn’t consistent with the standard, physics-grounded way divers assess gas duration at depth.

What does this teach us about solving these questions?

• Define clearly what your breathing rate represents. Is 0.5 cu ft per minute at the surface, or is it already adjusted for depth? Most practical dive curricula use surface-pressure values and then adjust for depth with the pressure factor to estimate real-time gas use.

• Decide which quantity you’re solving for: the time at depth, or the surface-equivalent time, or the surface-equivalent gas that would last at surface pressure. Mixing the two without a stated convention leads to confusion.

• Use a checklist you can trust on the water:

• Tank capacity (80 cu ft in this example)

• Depth and corresponding ambient pressure (66 ft ≈ 3 atm)

• Your breathing rate, and whether it’s given at surface pressure or depth

• The safety margin you’ll carry (reserve, turn pressure, etc.)

A practical, step-by-step walk-through you can apply

1. Establish the depth pressure: 66 ft ≈ 3 atm total pressure.

2. Clarify the rate: 0.5 cu ft/min is the flow you’re billed as using from the tank, adjusted for depth or not? Let’s assume it’s the rate at depth unless stated otherwise.

3. Compute your per-minute gas draw from the tank: 0.5 cu ft/min at depth would imply 0.5 cu ft/min of actual gas volume, but because you’re under 3 atm, the tank gas you consume per minute from the gauge is 0.5 divided by the pressure? If you want the rate in “tank volume” terms, you use 0.5 cu ft/min at surface pressure; then at depth you multiply by the pressure factor to get 1.5 cu ft/min drawn from the tank.

4. Time = Tank volume / depth-adjusted draw rate = 80 / 1.5 ≈ 53 minutes.

5. If you instead compute surface-equivalent gas (80 × 3 = 240 cu ft) and divide by 0.5 cu ft/min, you get 480 minutes—but this is the surface-equivalent breathing time, not the actual time you’ll spend at depth. It’s a useful number for comparing to surface needs, but not a direct read of how long you’ll last at 66 ft without converting back to the depth context.

What this means for students and practitioners

• On exams or quizzes, expect questions to test your grip on these conversions. The same numbers can yield different-looking results depending on how the problem frames rate and gas volume. This is exactly why understanding Boyle’s Law in action—how pressure affects volume and how it plays with breathing rates—is essential.

• In real-world planning, you’ll pair your gas calculations with a dive computer or a gauge and a plan that includes a reserve. For example, many divers plan to turn back or end the dive with a conservative reserve, not at the exact point the tank is empty.

• If you ever see a problem that seems to contradict physics, step back and ask:

• What is the rate actually representing?

• Is there a depth-adjustment factor being applied?

• Am I calculating “time at depth” or “surface-time equivalent”?

• Have I included any necessary safety margins?

Tying this back to your broader learning

These gas-duration exercises aren’t just about spit-bolting numbers into parentheses. They reinforce

• how pressure changes volume, and why depth changes gas consumption

• the importance of aligning rate units with the situation (surface vs. depth)

• the value of a robust, repeatable method you can apply under pressure (pun intended) on a real dive

A few practical tips you can take to heart

• Carry a margin of safety. Your plan should always include a reserve—whether it’s a specific number of minutes or a pressure threshold on your SPG/diver computer.

• Practice the same calculation with different tank sizes and depths. See how the numbers behave when 60 cu ft or 100 cu ft tanks come into play, or when you’re at shallower or deeper depths.

• Use analogies to keep it memorable. Think of depth as “cramming” more people into a bus at once—the bus is the tank, the passengers are the breaths, and the door timing is the breathing rate. The more crowded (deeper), the slower the exit rate per minute of your time.

In the end, the exact answer you see on a quiz is less important than the ability to reason through it clearly and to justify your steps. If a key claims 120 minutes for the 80 cu ft tank at 66 feet with a 0.5 cu ft/min rate, that’s a cue to double-check how the problem defines rate and depth, and to practice the real-world method we’ve walked through. It’s all part of building the kind of confident, safety-conscious mindset that makes divers reliable under water, not just in the classroom.

Additional context you might find useful as you continue studying

• Boyle’s Law is a silent tutor here: pressure and volume trade places as you descend.

• Tanks have fixed volumes at surface pressure, but the gas you breathe depends on how pressure squeezes that gas into the lungs.

• Most divers use a combination of gas duration estimates and a cushion for safety, rather than aiming for the last drop of gas.

If you’re ever unsure about a problem like this, try explaining it aloud, or write down the two possible interpretations and the numbers you’d get for each. Teaching yourself to articulate the thinking is often a strong predictor of how well you’ll apply it under water.

Bottom line for this topic: depth changes gas use; rate matters; always connect the rate’s reference (surface vs. depth) to the depth you’re at. And remember, the best answers aren’t just memorized—they’re understood in the context of pressure, volume, and time. That understanding is what actually translates into safer, more enjoyable underwater experiences.