Learn how to record depth on tables for repetitive underwater sessions when the deepest point is under 40 feet

When conducting a repetitive dive to less than 40 feet, which depth should be used on the dive tables?

• A. 30 feet
• B. 40 feet
• C. 50 feet
• D. 20 feet

Answer: (B)

In the context of repetitive diving, the depth that should be recorded on the dive tables is the maximum depth reached during any of the dives in a specific series. If the maximum depth of the dive is less than or equal to 40 feet, you would use 40 feet for the calculations on your dive tables. This is important because the calculations for no-decompression limits, surface intervals, and residual nitrogen in your body are all based on the maximum depth of the dive. Using 40 feet allows you to correctly assess the necessary safety measures and limits for subsequent dives. If the previous dives were at shallower depths, the data would still be based on the deeper limit to ensure a safe ascent and avoid the risks associated with decompression sickness. The dive tables are specifically designed to account for the decompression status resulting from the deepest depth reached, thus providing a comprehensive safety guideline for divers engaged in repetitive diving.

Understanding depth in repetitive bottom-time days isn’t the flashiest topic, but it’s the kind of detail that keeps you from sticky situations after multiple exposures. If you’ve ever logged a few underwater sessions in a row, you’ve probably wondered how to keep the math honest when people call it a “repetitive day.” Here’s the practical bit you’ll want to carry with you: the deepest point you reach in a sequence sets the baseline for the calculations you’ll use on your tables. And in this particular rule set, if that deepest point is 40 feet or less, you still use 40 feet for the table figures. Let me explain what that means for safety, planning, and peace of mind.

The core idea: why depth matters across a series

Think of it this way: your body doesn’t forget the deepest moment from an underwater day the moment you surface. The nitrogen you’ve absorbed at that deeper point lingers and affects what’s safe to do next. The table-based planning system—whether you’re looking at no-decompression limits, surface intervals, or residual nitrogen—uses a single depth marker to gauge risk. In a repetitive day, that marker isn’t the shallowest moment; it’s the deepest moment you reached during the sequence.

What the 40-foot rule actually says

Here’s the practical takeaway, stated plainly: when you have a sequence of underwater sessions, record the maximum depth reached in that sequence. If that maximum depth is 40 feet or shallower, you use 40 feet as the depth for calculating your table values for the remainder of the day.

• If you peak at 40 feet exactly, you’re in the same boat as deeper sequences for table calculations—40 feet is your baseline.

• If the deepest moment is shallower than 40 feet, say 35 or 25 feet, the rule still calls for using 40 feet on the calculations.

This approach sounds a little counterintuitive at first, especially if you’ve spent time among more intuitive rule sets that tie calculations to the exact deepest point. But there’s a reason behind it: it provides a safety cushion for surface intervals and the residual nitrogen your body carries forward. By anchoring the plan to 40 feet, you’re ensuring you don’t underestimate the time you’ve effectively spent under pressure, so to speak, during that day.

A simple way to picture it

Imagine you rode through a shallow corridor on your underwater day, with one moment hitting about 30 feet and another closer to 40 feet. If that deepest moment is 40 or less, you treat the entire day’s plan as if 40 feet had been the deepest. Your surface intervals, no-decompression limits, and subsequent considerations are then calculated against that 40-foot baseline. It’s not about punishing yourself for the shallow legs—it’s about keeping the nitrogen loading and the schedule on a safe, predictable track.

Why this matters for safety and planning

• No-decompression limits remain a central idea. Those limits shrink as depth increases. By using 40 feet as the working depth when the deepest moment didn’t exceed it, you’re keeping the planning conservative enough to avoid unexpected decompression needs on later legs of the day.

• Surface intervals are better controlled. If you’ve spent time at depth before the surface, the “clock” for safety resets differently than if you stayed shallow. The 40-foot baseline helps standardize those intervals.

• Residual nitrogen is considered more carefully. Even if you lingered at a shallower depth, the deepest exposure lingers in the tissues. The rule helps ensure you don’t underestimate what’s left to vent before the next exposure.

How to apply this in real life without overthinking

• Log the sequence. Between sessions, jot down the maximum depth reached in the set. If it’s 40 feet or less, prepare to use 40 feet for the table-based calculations for the remainder of that sequence.

• Treat the day as a single planning window. Don’t treat each shallow moment in isolation. The safety math looks at the deepest point reached, not the average or the shallowest moment.

• Check your gear and options. If you have access to a no-decompression table, a planning log, or a simple depth-time card, make sure you’re applying the 40-foot rule consistently. If you’re using a planning tool, verify that it’s aligning with the deepest-depth approach for repetitive sequences.

A couple of practical scenarios to ground the idea

• Scenario A: You have three underwater sessions on one day. The deepest point you reach is 38 feet. Under this rule, you would use 40 feet for the calculations on your tables for the subsequent parts of that day. The exact times and surface intervals depend on the specific table you’re using, but the depth anchor remains 40 feet.

• Scenario B: Your deepest moment is exactly 40 feet in one of the sessions, and none go deeper. You still base the remaining table calculations on 40 feet.

• Scenario C: The deepest moment hits 42 feet. In this case, you’d use 42 feet as the working depth for the later portions of that day, since the maximum exceeded 40 feet. The rule about 40 feet is specifically for the case where the deepest point is at or below 40 feet.

A quick note on tools and real-world practice

Some people pair table work with a reassuringly reliable planning sheet, and others lean on underwater computers that continuously check no-decompression limits and surface intervals. If you’re using a table-based approach, the 40-foot rule is a reminder to stay conservative and explicit about what you’re logging for the day. If your gear includes a modern planning tool, make sure it’s set up to reflect the same depth-based logic when you’re handling repetitive sequences.

Common questions that come up in the chatter

• Do I always treat the day as deeper than it feels? Not always. The rule is about the deepest point you hit during the sequence. If that deepest point sits at or below 40 feet, the day’s calculations proceed with 40 feet as the baseline.

• What about variations during the day? If you have a couple of deep stints and then a long shallower period, the maximum depth across the day still governs the baseline. The intent is to preserve a safety margin across the entire set of sessions.

• Is a back-up plan necessary? Absolutely. A backup plan—like noting the sequence in a log and double-checking with a planning chart—reduces errors, especially when you’re tired or weather conditions change.

A few words on staying mindful

Any time you’re planning multiple underwater moments, the goal is clear: keep the math honest, keep the plan conservative, and keep you and your team safe. The 40-foot rule for repetitive sequences is a straightforward way to anchor yourself to the deeper exposures you encountered, even if you’re spending a lot of time in shallower zones. It’s one of those rules that makes more sense once you see how the nitrogen loading and surface intervals intertwine across a day on the water.

A final thought

If you’re curious about why the system uses that depth anchor, it boils down to a simple tension: you want to respect the reality of how your body handles gas in tissues while still letting you enjoy a day’s exploration. The conservative shift to 40 feet in such sequences is a practical safeguard that keeps the planning process honest and reliable. And when you carry that mindset—log accurately, plan thoughtfully, and stay conservative—you’re building a solid foundation for safer, more enjoyable underwater days.

If you’d like, we can walk through a concrete example using your own sequence of moments and sketch out the table values step by step. It’s often easier to see the logic when you map it to a real day you’ve had, even if you’re just recalling it from memory. The bottom line remains the same: the deepest moment in a repetitive day guides the safety math, and when that deepest moment stays at 40 feet or less, the charting assumes 40 feet as the working depth for the rest of the day. Safe planning, steady progress, and calm confidence—that’s the aim.