Between a 70-foot dive for 35 minutes and a 50-foot dive for 35 minutes, a 60-minute surface interval helps keep nitrogen off-gassing safe.

What surface interval is required in order to do a dive to 70' for 35 minutes followed by a dive to 50' for 35 minutes?

• A. 30 minutes
• B. 60 minutes
• C. 45 minutes
• D. 20 minutes

Answer: (B)

The requirement for a surface interval between dives is determined by the need to allow the body to off-gas nitrogen absorbed during the previous dive. The deeper and longer the previous dive, the more nitrogen is absorbed, and consequently, more time is necessary for the body to safely eliminate this nitrogen before embarking on another dive. In this scenario, the first dive is to a depth of 70 feet for 35 minutes. This dive generates a significant amount of nitrogen absorption due to its depth and duration. Following this, the second dive is to a depth of 50 feet for 35 minutes. Although the second dive is shallower, it still requires consideration of the nitrogen taken on from the first dive. The appropriate surface interval would ideally be based on tables or dive computer data which correspond to the specific depths and times of both dives. A surface interval of 60 minutes allows adequate time for off-gassing, as this exceeds the minimum requirements provided by no-decompression limits for both dives concerned. This extended interval helps to ensure diver safety by reducing the risk of decompression sickness, which is caused by too much nitrogen remaining in the tissues when ascending from subsequent dives without sufficient time to eliminate it.

Surface intervals are the quiet guardian of safe, enjoyable sessions underwater. They don’t make the headlines, but they matter more than you might think. If you’ve ever wondered how long you should wait between back-to-back dives, you’re in good company. Let me explain why a 60-minute surface interval is the map most divers follow when one leg of the day involves a deep, longer dive followed by a second, somewhat shallower one.

The nitrogen story behind the wait

Think of nitrogen as a guest that sticks around after a party. The deeper you go and the longer you stay, the more nitrogen your body absorbs. When you rise, your body starts the slow process of letting that nitrogen go back out of your tissues. If you blast off into another dive too soon, you risk having too much nitrogen hanging around—enough to cause decompression sickness, or the bends, if the ascent is rushed or the next dive is too aggressive.

The math isn’t just about “how deep.” It’s about “how much” nitrogen is in the tissues and “how fast” your body can off-gas. A 70-foot dive that lasts 35 minutes is a deeper, more nitrogen-rich scenario than a shallow, short one. A second dive to 50 feet for 35 minutes, while shallower, still taps into the nitrogen you’ve already picked up. The surface interval is the body’s cooldown period, a chance to shed a portion of that nitrogen before you begin another exposure.

Why 60 minutes, specifically?

In this particular sequence—first a long, deep outing (70 ft for 35 min) and then a second dive (50 ft for 35 min)—experts and dive planning tools tend to converge on a 60-minute surface interval as a prudent middle ground. Here’s the practical logic:

• The first dive loads your tissues with nitrogen. The deeper and longer that exposure, the more loading occurs.

• The second dive re-exposes you to nitrogen, starting the clock on absorption again. If you go too soon after the first exposure, you’re no longer simply reaping the benefits of off-gassing; you’re adding another layer of tissue nitrogen before this off-gassing can complete.

• A 60-minute interval gives your body a significant window to shed nitrogen accumulated during the 70-foot, 35-minute exposure, while still allowing you to complete the second dive within a safe no-decompression framework, according to standard tables or a dive computer’s model.

Of course, you’ll often hear divers talk about “no-decompression limits” (NDLs). Those are the ceilings that tell you how long you can stay at a given depth without needing staged decompression stops. When you add a surface interval into the plan, you’re intentionally letting the nitrogen load drop enough so that your next dive can stay within those NDLs. In everyday terms: the 60-minute wait is about giving your body a comfortable margin so you can explore again without pushing the limits.

What this looks like in real life planning

If you’re looking at the day’s plan, you’ll probably rely on one of two trusted tools: dive tables designed for recreational profiles (such as those used in IANTD guidelines) or a capable dive computer. Both approaches aim to estimate how much nitrogen remains in your tissues after the first dive and how that affects your safe bottom time on a second exposure.

• With tables: You’d locate the first dive’s depth and duration, find the corresponding nitrogen-loading category, and then check the recommended surface interval before the second dive. The deeper and longer the first leg, the longer the interval you’ll see recommended.

• With a computer: The device can model off-gassing more dynamically, taking into account your actual depth profile, surface interval duration, and even your breathing gas mix. It’s a more fluid way to gauge safety margins.

In the scenario at hand, a 60-minute surface interval is commonly preferred because it comfortably accommodates the nitrogen off-gassing from the deep, 35-minute first dive, while still keeping the second 35-minute, 50-foot exposure within safe margins. The important takeaway is that the interval isn’t arbitrary—it’s rooted in tissue loading, off-gassing kinetics, and the practical realities of staying within known safety boundaries.

What to do if life gets in the way

Reality check time: sometimes a 60-minute wait isn’t possible. Maybe you’re on a tight schedule, or you’re a group with differing dive goals. Here are a few practical moves, always keeping safety first:

• If you can extend the stop beyond 60 minutes, great. Even 15–20 minutes more can reduce nitrogen levels further and add a bigger cushion for the second dive.

• If you must shorten the interval, consider shortening the second dive or opting for a shallower second depth. The key is not to stack nitrogen loading without giving the body time to off-gas.

• Use your computer’s warnings seriously. If your device signals a higher risk level or a “no-fly” or “no-deco” status after a shorter interval, prioritize safety and adjust on the fly.

• Debrief with your buddy. A quick check-in on how you each feel can be as important as the numbers on your screen. Fatigue, headache, or unusual numbness aren’t something to ignore.

Tips that make surface intervals feel less like math and more like common sense

• Plan ahead. If you’re on a multi-dive day, map out your target surface intervals ahead of time and write them into your day’s plan. A little foresight saves a lot of guesswork later.

• Keep a simple log. Jot down depth, time, and interval. It’s amazing how quickly numbers add up and how easy it is to lose track in the moment.

• Respect the signs. If you feel off after a session—tingling, unusual fatigue, or joint pain—never downplay it. Sit the next few hours out and seek professional guidance if anything seems out of the ordinary.

• Stay refreshed. Hydration and a light snack can help your body handle the post-dive load more smoothly, but don’t plan a heavy meal right before the next dive.

Common-sense takeaways you can carry on the boat

• Deeper, longer first dives demand more thoughtful spacing before a follow-up exposure.

• A 60-minute surface interval is a standard, prudent rule of thumb for many back-to-back dive scenarios in recreational planning.

• Always pair a surface interval with smart second-dive planning based on your table data or dive computer readouts.

• If the schedule or conditions force you to shorten the interval, adapt by reducing second-dive depth or duration, rather than rushing through the second leg.

A quick mental model you can apply

• Picture your first dive as loading a sponge with nitrogen. The deeper the dive, the more the sponge fills.

• The surface interval is the squeeze—the longer you wait, the more water drips out of the sponge.

• Your second dive then uses less “nitrogen in the sponge,” but you still want to avoid refilling too quickly. The goal is not to refill the sponge faster than it can drain.

Final thoughts: safety circled with planning

If you’re going to string together two dives in a day, the surface interval becomes less about a rule of thumb and more about a safety habit. The 60-minute interval for a 70 ft/35 min followed by 50 ft/35 min session is a thoughtful balance between maximizing your underwater time and keeping risk in check. It respects the physics of nitrogen in the tissues and the practical realities of a day on the water.

In the end, what matters most is your comfort, your buddy’s awareness, and the tools you trust. Whether you’re leaning on tables, a computer, or a hybrid approach, the aim stays the same: keep your next ascent as smooth and safe as possible.

If you’re curious about how these principles show up in day-to-day dive planning, you’ll notice a familiar pattern: you’re building a show that respects the tide, the depth, and the time, all while keeping a steady eye on how your body feels. And that, more than anything, makes the ocean feel like a welcoming place to explore—one thoughtful decision at a time.