How a two-hour surface interval affects nitrogen loading after an 80-foot, 20-minute submersion followed by a 60-foot, 25-minute second session

What is the letter group designation after a diver has been to 80' for 20 minutes, surfaced for two hours, and then did a repetitive dive to 60' for 25 minutes?

• A. A
• B. B
• C. C
• D. D

Answer: (B)

To determine the letter group designation after the described dives, it’s essential to consider the dive profiles and the effect of surface intervals on residual nitrogen levels. In this scenario, the diver first descends to 80 feet for 20 minutes. After surfacing for two hours, the diver performs a repetitive dive to 60 feet for 25 minutes. The initial dive would contribute to nitrogen absorption, and the two-hour surface interval allows some of that absorbed nitrogen to off-gas, helping to reduce the diver’s nitrogen saturation. When calculating the letter group designation, dive tables or dive computers are used to assess both the depth and duration of the dives as well as the time spent at the surface. After the first dive, the diver's initial group would be based on the time and depth. After the two-hour surface interval, the diver may move to a new group, and when they complete the second dive, their nitrogen loading will be evaluated again based on the parameters of that dive. The correct answer of letter group designation 'B' indicates that after the two dives and surface interval, the diver's nitrogen absorption is within the range of that specific group according to the dive tables or algorithms. Thus, recognizing how nitrogen loading works cumulatively from the

Understanding how those letter groups work after a two-part underwater session

If you’ve ever peeked at the logbook while planning an open-water course, you’ve probably seen those letter group designations—A, B, C, D—tied to nitrogen loading. They’re not some mysterious code; they’re a straightforward way to gauge how much nitrogen a person has absorbed and how ready they are for the next underwater session. Here’s the thing: depth, time, and what you do when you surface all matter. Let me walk you through a concrete example and show how the math (well, the tables and the computer’s logic behind them) leads to a final letter group.

The quick setup: why surface intervals matter

Nitrogen gets absorbed into your tissues when you submerge, especially at greater depths and for longer times. When you surface, your body off-gasses some of that nitrogen. The longer you wait at the surface, the more off-gassing happens. That’s the surface interval. It’s the reason two people can do the same bottom-time at similar depths but end up in different letter groups if one person has a longer surface interval than the other.

The legwork behind the scenes is where the learning happens. Dive tables or dive computers crunch depth, time, and surface intervals to estimate how much nitrogen remains in your tissues after each stop. Then they set your next steps so you stay within safe limits. It’s not magic. It’s a disciplined way to keep you safe while you’re exploring the underwater world.

A concrete scenario to connect the dots

Imagine a course participant who completes a two-part underwater session. First, they descend to 80 feet and stay there for 20 minutes. After that, they surface and rest for two hours. Then they take a second submersion to 60 feet for 25 minutes.

Here’s what’s happening in plain terms:

• First submersion: The 80-foot depth and 20-minute duration add nitrogen into the tissues. The deeper and longer you stay, the more nitrogen you accumulate.

• Surface interval: The two-hour break lets some of that absorbed nitrogen off-gas. The goal is to reduce the overall nitrogen load before the next underwater spell.

• Second submersion: The 60-foot return for 25 minutes adds more nitrogen, but starting from the reduced loading after the surface interval changes the calculation. The second submersion can push your overall nitrogen level into a different category than if you had shorter or no surface time.

So what’s the final letter group after these two steps? The conventional answer, based on standard open-water tables or the matching computer algorithm, is B. Why is that the case? It comes down to how cumulative nitrogen loading, depth, and surface time interact. The first deep spell adds a certain amount of nitrogen. The two-hour surface interval doesn’t erase that loading, but it does reduce it enough that the second, shallower submersion doesn’t push the total loading into the next higher category. In other words, you land in a lower or equal category after the second submersion than you would with less surface time, and in this particular profile that lands you in Group B.

Let’s unpack the logic a bit more, without getting lost in numbers

Think of letter groups as “risk bands” for nitrogen saturation. The bands are defined by:

• Depth: How far below the surface you go.

• Bottom time: How long you stay there.

• Surface interval: How long you rest on the surface between spells.

When you pair a deeper bottom time with a long surface interval, you’ve given your body a chance to off-gas, which shifts you into a lower risk category than you’d land in with a shorter surface interval or with a deeper second submersion. In our example, the two-hour surface interval buys you time to off-gas substantially, so the follow-up 60-foot, 25-minute spell doesn’t push you into a higher category than B.

A few notes to keep this grounded

• The exact letter you end up in depends on the specific edition of the open-water tables or the computer you’re using. Different table sets and algorithms can nudge the final letter a notch, even with the same numbers.

• The scenario here is designed to illustrate the principle: deeper first exposure, a healthy surface interval to off-gas, then a second, shallower exposure. That combination often lands you in a conservative group designation like B, instead of drifting into a more nitrogen-loaded category.

• In practice, you’ll see this concept come up with repetitive diving plans, surface intervals of varying lengths, and a mix of depths. The pattern to watch for is: longer surface intervals generally reduce the final nitrogen load compared to no rest, especially when the subsequent bottom time isn’t deeper than the first.

Connecting it to real-world, non-test talk

If you’re new to all this, you might wonder why we bother with these letter groups at all. The short answer: safety and confidence. Nitrogen is not the enemy—mismanaging it is. By understanding how your body off-gasses and how it loads up again with each underwater spell, you gain the confidence to plan responsibly. You’ll know when the schedule allows for a second submersion and when you should pause longer on shore before heading back in.

For many divers, this is where the wonder of the sport reveals itself as something practical rather than abstract. It’s one thing to read about 80 feet and 60 feet in a textbook; it’s another to realize that a two-hour break on the surface is a real tool that helps you stay within safe limits while still enjoying two meaningful underwater experiences in one day. And yes, you can still savor the thrill of the second leg—the bottom line is keeping you safe while you chase those underwater moments that feel almost cinematic.

A few tangible takeaways you can carry forward

• Surface intervals matter: They’re not just time resting; they’re active nitrogen management.

• Depth and time matter together: The deeper and longer you go, the more nitrogen you accumulate, and the surface interval must be considered in light of the next exposure.

• Final group designations are practical labels: They guide what you can do next and help you decide whether to continue the day or rest longer.

• Your exact letter may vary with the table or computer you use: The core idea stays the same, but the numbers shift slightly depending on the tools in your kit.

If you like analogies, think about it like road-tripping with a full tank of fuel. You burn fuel on a big leg (the first submersion), you stop for a rest (surface interval) and let the engine cool (nitrogen off-gassing), then you take another leg (the second submersion). Depending on how long you rest and how far you go the second time, you end up in a “fuel efficiency” category that tells you how much you can push on the next leg without risking a run dry or a crash.

Wrapping it all up

The core idea behind letter group designations is simple in spirit, even if the math behind it can feel a touch academic. Depth, time, and the surface break combine to shape how much nitrogen you hold at any moment, and that in turn determines your group after the session. In the scenario we walked through—a first submersion to 80 feet for 20 minutes, a two-hour surface interval, then a second submersion to 60 feet for 25 minutes—the final grouping lands in B. That result reflects a conservative balance: enough time on the surface to off-gas, yet a second spell that’s not so deep or long that the total loading climbs into an upper category.

If this topic intrigues you, you’re not alone. It’s a practical, real-world piece of the open-water journey that turns what could feel like abstract numbers into a clear, actionable safety framework. And as you gain more experience, you’ll start seeing patterns: how different surface intervals interact with various bottom times, how your own physiology influences off-gassing, and how smart planning keeps your underwater adventures rewarding—and safe.

Key takeaways in a nutshell

• Two hours on the surface can significantly reduce nitrogen loading before a second exposure.

• A second, shallower exposure after a longer surface interval often lands you in a lower or same letter group, depending on the exact numbers.

• Final letter groups come from tables or algorithms that blend depth, time, and surface intervals; the exact result can vary with the tool you’re using.

• Understanding this helps you plan more confidently and keeps your underwater excursions enjoyable and safe.

If you’d like, I can walk through another example with different depths and times to show how the final letter group shifts. It’s a helpful way to see the logic in action and build that intuitive feel for how these safety guidelines translate to real-world planning.