How long will a 50 cu ft scuba tank last at 72 feet when breathing 1 cu ft per minute?

How long will a 50 cu ft tank last at a depth of 72' breathing 1 cu ft per minute?

• A. 30 minutes
• B. 50 minutes
• C. 60 minutes
• D. 40 minutes

Answer: (A)

To determine how long a 50 cu ft tank will last at a depth of 72 feet while breathing at a rate of 1 cu ft per minute, we must first recognize that the volume of gas available in the tank is 50 cubic feet. At a depth of 72 feet, the pressure increases due to the weight of the water above and is approximately 3.1 atmospheres (1 atmosphere at sea level plus 2.1 from the water depth). The effective volume of air that can be used from the tank is thus less than 50 cu ft because of the increased pressure. To find the effective volume of air, we need to adjust for the pressure: Effective volume = Tank volume / Absolute pressure. Calculating it gives us: Effective volume = 50 cu ft / 3.1 atm = approximately 16.13 cu ft. Now, if a diver breathes 1 cu ft per minute, we can find out how long the air will last: Time = Effective volume / Breathing rate = 16.13 cu ft / 1 cu ft per minute = approximately 16.13 minutes. Here, the answer of 30 minutes appears to reflect a misunderstanding, as it does not take into

Outline:

• Hook and context: a common math puzzle about a 50 cu ft cylinder at 72 feet with a 1 cu ft/min breathing rate.

• Core physics in plain language: how depth changes usable air (Boyle’s law, absolute pressure).

• Step-by-step calculation for the numbers most people actually use on the water.

• Common points of confusion and where the 30-minute idea comes from (and why it doesn’t fit this scenario).

• Practical takeaways: gas planning, SAC rate, and safe margins.

• Real-world tips and a friendly pull-through to IANTD Open Water standards.

Let’s clear up the math behind gas duration at depth

If you’re wearing a 50 cubic foot cylinder and you’re cruising at a depth of 72 feet, you’re dealing with pressure, not just volume. Water above you adds weight and squeezes the air in your cylinder into a smaller, denser form. That’s Boyle’s law in action: at greater pressure, the same amount of gas takes up less space. The key question isn’t “how big is the tank?” but “how much gas can you actually breathe per minute at that depth?”

First, a quick refresher you’ll hear echoed in training rooms and on the boat:

• Absolute pressure at depth = surface pressure (1 atm) plus the pressure from the water column.

• In seawater, every 10 meters (about 33 feet) adds roughly 1 atmosphere of pressure. At 72 feet, you’re looking at roughly 3.0 to 3.2 atmospheres of absolute pressure depending on exact water density and rounding.

• The amount of gas in the tank doesn’t vanish at depth; it’s just compressed into a smaller volume. If you started with 50 cu ft at the surface, that 50 cu ft of gas is still the same amount of gas, but the “breathing room” you have at depth is reduced because each minute you inhale gas at depth is drawn from a tank that’s effectively providing less volume per minute at surface conditions.

Let’s run the numbers the way many IANTD-style understandings approach this

• Tank size: 50 cu ft (at surface pressure, 1 atm)

• Depth: 72 feet (roughly 22 meters)

• Ambient pressure at depth: about 3.1 atmospheres (1 atm surface plus ~2.1 atm from the water column)

• Breathing rate: 1 cu ft per minute (this is often given as a surface-equivalent rate; the exact interpretation matters)

A clean way to think about it is this:

• The volume of air you can use at depth, in surface-equivalent terms, is Tank volume divided by the ambient pressure. In other words:

Effective volume at depth ≈ Tank volume / Absolute pressure

Here: 50 cu ft / 3.1 atm ≈ 16.1 cu ft of air in “surface terms.”

• If you breathe at 1 cu ft per minute (in surface terms), the time you have is:

Time ≈ Effective volume / Breathing rate

≈ 16.1 cu ft / 1 cu ft per minute ≈ 16 minutes.

That gives you roughly 16 minutes of usable air under those conditions, not 30. If someone says 30 minutes, that’s likely either a different interpretation of the breathing rate or a mistaken assumption about the depth and pressure, or perhaps they’re planning with a large safety margin or a different SAC value. The straightforward calculation for 72 feet with a 50 cu ft tank and a 1 cu ft/min surface-equivalent breathing rate lands around 16 minutes.

Why the 30-minute figure sometimes pops up—and what to watch for

• Misinterpreting breath rate: If instead the breathing rate was stated as 1 cu ft per minute at depth (not surface equivalent), you’d calculate differently. At depth, each minute would pull gas equivalent to 3.1 cu ft at surface pressure, so 50 cu ft would last about 16 minutes in surface terms, but only about 50 / 3.1 ≈ 16 minutes in that same sense. That’s the same result, just framed differently. The confusion usually happens when people mix up the reference pressure for the breathing rate.

• Rounding and depth estimates: Depths aren’t always remembered perfectly. If someone uses 2.0 atm instead of 3.1–3.2 atm, you get a longer duration (50 / (1 × 2.0) = 25 minutes). If you push the numbers toward 3.0, you get ~16–17 minutes. The exact rounding can change the number noticeably.

• Safety margins and reserves: Some crews plan to keep a gas cushion for emergencies. If you keep a reserve, your “runtime” under normal conditions will be shorter than the nominal calculation. That’s not a wrong calculation; it’s intentional planning for safety.

A practical, human way to look at it

Think of gas planning like packing a suitcase for a road trip. The tank is your luggage, depth is the weather, and your breathing rate is how aggressively you use your gear. You know the climate (the pressure) gets harsher as you go deeper, so you can’t treat the tank’s 50 cu ft as if you’re at the surface. You have to account for the extra pressure squeezing that air into a tighter space.

In real-world terms, the math isn’t just about plugging numbers into a formula; it’s about understanding how depth changes air usage. It’s one of those “aha” moments you get when you first start working with gas planning in open-water environments. The better you understand it, the more confident you’ll be in making smart, safe decisions out in the water.

What this means for gas planning under IANTD Open Water standards

• Start with a clear gas plan. Figure out your expected SAC rate at the planned workload (moving, talking, using equipment). Then translate that into a depth-adjusted gas burn rate.

• Use ambient pressure to estimate duration. The simple, robust formula is:

Runtime ≈ TankVolume / (SAC × P_ambient)

Where TankVolume is the cylinder’s volume at surface pressure, SAC is your surface air consumption rate per minute, and P_ambient is the absolute pressure at your planned depth.

• Factor in safety margins. Decide on a reserve gas amount you won’t touch except in an emergency. That reserve becomes part of your plan and helps keep you out of a risky situation.

• Check your devices and references. Computer screens, depth gauges, and tank regulators are your partners in this, not competitors. They’ll give you real-time cues about remaining air and ascent planning.

From theory to practice: a few grounded tips you can use

• Know your SAC. It’s different for everyone and varies with activity level, gear, and stress. If you haven’t measured it lately, do a calm, measured excursion with a buddy and estimate your typical burn rate over a set period.

• Plan for the worst, not the best. Always leave breathing gas in reserve. A conservative approach beats chasing a number you calculated on land.

• Talk through the plan. With your partner, agree on your minimum gas when you start your ascent, and establish a clear signal if either of you hits that limit.

• Use reliable references. Your cylinder, regulator, and computer should provide consistent feedback. If something reads off, pause and verify before continuing.

• Be mindful of profiles. Longer systems or heavier workloads at depth require more careful gas management. Short, shallow segments are kinder on gas.

A few notes on language and tone you’ll hear in IANTD-style training

You’ll hear phrases like “gas management” and “air consumption rate” discussed frequently. The underlying idea is simple: the deeper you go, the more air you use per minute, because pressure compresses the air you breathe. The practical takeaway is not to memorize a single number, but to internalize a process: estimate, plan, monitor, and adjust.

A quick mental model for the curious reader

• If you picture the cylinder as a fixed bucket of air you’re squeezing into a smaller space, the math clicks faster. At depth, you’re squeezing more gas per breath, so you reach the end of the tank sooner than you’d expect from the surface.

• If you picture the water as a pressure blanket, you’ll understand why a small change in depth can dramatically shift your gas burn rate.

Bringing it back to the water-loving you

At the end of the day, the math isn’t just about numbers. It’s about staying safe, confident, and relaxed while you’re exploring the underwater world. The scenario of a 50 cu ft cylinder at 72 feet, with a breathing rate of 1 cu ft per minute, serves as a useful reminder: depth changes everything. The gas isn’t “larger” at depth; it’s simply more pressurized, and your breathing rate in surface terms translates into a shorter window of time than you might expect.

If you’re enthusiastic about the topic, you’ll appreciate how these ideas weave into broader IANTD Open Water standards. Gas management, buoyancy control, buddy procedures, and dive planning all hinge on thoughtful, precise thinking about pressure, volume, and time. And that’s a skill you’ll carry with you beyond any single outing.

Final takeaway

In this particular setup, the straightforward calculation points to roughly 16 minutes of usable air under the stated conditions, not 30. That 30-minute figure is a reminder that numbers can be slippery when reference points aren’t crystal clear. The better approach is to anchor your plan in pressure-adjusted gas usage, keep a healthy reserve, and stay attuned to your instruments and your partner. With that mindset, you’ll navigate depth with clarity, confidence, and safety—even when the math gets a bit tricky.