After a 60-foot, 40-minute first dive with a one-hour surface interval, the second dive to 50 feet should last about 30 minutes

If a dive team went to 60 feet for 40 minutes and surfaced for one hour, how long can they dive if they want to do a second dive to 50 feet?

• A. 30 minutes
• B. 25 minutes
• C. 35 minutes
• D. 40 minutes

Answer: (A)

To determine the allowable duration for a second dive to 50 feet after previously diving to 60 feet for 40 minutes and surfacing for one hour, it’s important to consider residual nitrogen levels in the body and the use of a dive table or computer. After a dive, the body absorbs nitrogen, and it takes time for the body to off-gas that nitrogen, which occurs during the surface interval. Based on recreational diving guidelines, the more time spent at depth during the first dive and the deeper the dive, the more nitrogen is absorbed. The one-hour surface interval allows for some off-gassing but may not be sufficient for longer or deeper dives following a previous dive. When planning the second dive to 50 feet, specific recommendations often suggest limiting dive time based on the residual nitrogen from the first dive. The data shows that, typically, allowable dive times may be shorter after deeper or longer previous dives. Given that a dive to 60 feet can have a considerable residual effect, a more conservative dive limit would be expected for a dive to 50 feet. In this case, it is deduced that 30 minutes would be the maximum safe duration for the second dive when considering the previous nitrogen absorption and surface time. Therefore, the selection

Are you ever surprised by how quickly nitrogen builds up in your body after a long soak at depth? Here’s a practical way to think about a common two-part outing: you first go to 60 ft for 40 minutes, surface for a full hour, then consider a second leg down to 50 ft. How long can you stay on that second leg? The right answer, in most standard recreational scenarios, is 30 minutes. Let me explain why.

A quick reality check: nitrogen loading and off-gassing

When you spend time at depth, your body takes in nitrogen from the breathing gas. The deeper you go and the longer you stay, the more nitrogen the tissues absorb. On the surface, your body starts shedding that nitrogen—off-gassing—at a rate determined by your body tissues, the depth you’ve been at, and the surface interval (the time you spend on the surface between sessions). A longer or deeper first leg means more nitrogen to off-gas, which usually trims the amount of bottom time you can safely use on the next leg.

In our example, you heat up the first leg to 60 ft (roughly 18 meters) for 40 minutes. That’s a fairly substantial depth and duration, so the body does accumulate a meaningful amount of nitrogen. Then you give it a one-hour surface interval. The one hour helps, but it doesn’t erase all of the nitrogen loading—especially after a deeper, longer exposure. That means your second leg isn’t brand-new nitrogen-wise; you’re starting with residual nitrogen from the first outing.

How second-leg planning works, in plain terms

Think of the surface interval as a nitrogen “off-ramp.” The longer and deeper the first leg, the longer you’ll need to unload that nitrogen before safely pushing back down to depth. Some people picture this with the idea of “credit” for nitrogen off-gassing; others prefer to think in terms of remaining nitrogen in the tissues and no-decompression limits (NDLs) that apply to the next leg.

For a second leg to 50 ft after a first leg to 60 ft, most training guidelines push you toward a conservative bottom time. Why? Because the residual nitrogen from the first leg reduces the available NDL for the second leg. The goal is to stay well within safe no-decompression times and avoid increasing and extending decompression obligations later. In our scenario, the conservative, widely accepted answer is that 30 minutes is the maximum allowable bottom time for the second leg at 50 ft, given the 40-minute first leg and a 60 ft depth with a 60-minute surface interval.

Let’s connect the dots with a simple mental model

• First leg depth and time: 60 ft, 40 minutes. This isn’t a tiny loading; it’s enough to leave a footprint of residual nitrogen in the tissues.

• Surface interval: 60 minutes. This helps, but it doesn’t reset everything to a clean slate. It reduces the remaining nitrogen in your system, but not to zero, especially after the deeper exposure.

• Second leg depth: 50 ft. You’re not far from the first depth, so the nitrogen math is still playing a role.

• Resulting bottom time for the second leg: about 30 minutes. This is a conservative choice that keeps you well inside no-decompression limits and minimizes the risk of post-activity decompression requirements.

Think of it like cooking: if you bake a big cake (the first leg) and then you rest for a while (the surface interval), you can still bake a smaller cake (the second leg), but you shouldn’t load it with the same amount of batter (bottom time) as the first. The rest period doesn’t erase the batter from the first bake; it just gives you a chance to work with a cleaner slate, proportionate to what’s already been done.

Practical takeaways you can actually use

• Use a computer or a trusted table: If you carry a nitrogen computer, it will factor in depth, time, and surface interval to give you a current no-decompression limit for the next leg. If you’re using a table, you’ll look up the remaining bottom time at 50 ft after spending time at 60 ft and applying the surface interval.

• Be conservative with second-leg decisions: Even if a quick mental calculation suggests you could push a bit longer, lean toward the safer side. The goal isn’t to squeeze in every minute of buoyant time; it’s to stay within safe margins and keep decompression obligations off the table.

• Plan the surface interval deliberately: If you know you’ll be doing a second leg, a longer surface interval is often a smarter choice. A few more minutes above the surface can noticeably improve your bottom-time options later on.

• Consider alternatives: If the schedule or conditions push you toward longer second legs, you can opt for shallower depths or shorter first-leg times next time. Flexibility is part of smart planning, not a sign of weakness.

A few common questions people ask (and plain-language answers)

• Do nitrogen levels reset to zero after any surface time? Not exactly. They drop, but residual nitrogen can linger, especially after deeper exposures. A full reset usually requires a substantial amount of surface time at the surface, and even then, you’ll want to verify with a computer or table before planning a long second leg.

• What if the surface interval were longer, say two hours? A longer surface interval would reduce residual nitrogen further, which could allow for a longer second-leg bottom time at 50 ft, possibly approaching the first-leg numbers under some conditions. Still, you’d check with your planned plan tool to be sure.

• Can I skip the second leg altogether if I’m not confident about the numbers? Absolutely. The whole point is to stay safe and enjoy the experience without unnecessary risk. You can choose to rest, stay at a shallower depth, or shorten the second-leg bottom time.

Real-world flavor: not every outing plays out the same

No two days at the water’s edge are identical. Water temperature, current, and your own physical condition all nudge the numbers a bit. Some days your body off-gasses nitrogen more quickly; other days you might feel more tired, which can affect your safety margins even if the math says you’re fine. That’s why many divers rely on a computer more than a paper table, especially when planning back-to-back legs. The computer’s algorithms are built on a lot of data and experience, and they help you stay within safe limits even when conditions shift.

A quick note on the language we use

You’ll hear terms like bottom time, surface interval, and no-decompression limit tossed around in training and on the boat. They’re not just jargon; they’re practical ways to describe what your body is actually doing. Bottom time is the clock you’re counting down during the underwater leg. The surface interval is the break on the surface where your tissues off-gas. No-decompression limit is the maximum time you can stay shallower than a depth where you’d have to decompress on the way up. Understanding how these pieces fit together helps you plan smarter outings and keeps the focus on safety and enjoyment.

Putting it all together

So, when you’ve already spent 40 minutes at 60 ft and you've given yourself a one-hour surface interval, planning a second leg at 50 ft with a practical cap around 30 minutes is a sensible, conservative approach. It’s not about maximizing time; it’s about staying within safe nitrogen levels while still letting you experience the wonder of the underwater world. If you’re curious, you can run the exact numbers on a computer or a table used by divers. The goal is clear: keep the experience enjoyable and safe, season after season.

A few closing reflections

If you’re new to multi-leg outings, this kind of planning can feel a little tedious at first. But once you get the hang of how depth, time, and surface intervals interact, it becomes almost second nature. And here’s a small encouragement: the more you learn to balance these variables, the more you’ll trust your instincts when you’re on the boat, checking gauges, listening to the swell, and deciding whether to push a little more or call it a day.

In the end, the exact number—30 minutes for the second leg at 50 ft after a 60 ft, 40-minute first leg and a 60-minute surface interval—reflects a prudent approach that many divers use in real-world settings. It’s a reminder that smart planning and respect for the physiology of nitrogen loading aren’t just rules on a page; they’re tools that help you keep exploring with confidence and calm.

If you’re curious to explore more scenarios like this, you’ll find that the same principles apply: depth, time, surface interval, and the ever-present need to respect your limits. And as you gain experience, you’ll start to notice patterns—how certain combinations of depth and duration translate into different safe second-leg plans. That familiarity is the quiet payoff of deliberate practice and thoughtful planning, the kind of understanding that makes every outing a step forward in your journey beneath the waves.