Your dive computer shows a number counting down: your no-decompression limit, or NDL. You plan your ascent to begin well before it ever reaches zero. The moment that matters is when you stay down past it. The countdown vanishes and a ceiling appears: a depth you are no longer allowed to ascend above until you have off-gassed enough to be safe. The dive has crossed from recreational into decompression territory. The computer does not stop working. It does not abandon you. It tells you exactly what to do next. The question is whether you understand what it is saying.
The NDL is a hard line. You plan your dive to stay well inside it, and you should never set out to cross it. But things happen underwater. A buddy low on air, an entanglement that takes minutes to clear, a current that pushes you deeper than you meant to go. Divers do occasionally find themselves staring at a zero where the countdown used to be, and many were never properly taught what to do in that moment. This article is for that moment, and it is the opposite of an excuse. Cross the line on purpose and you are gambling with a chamber ride, permanent injury, or your life. The point here is to protect yourself if it ever happens by accident, because what you do in the next 90 seconds matters more than anything else. Different dive computers handle it very differently, and if you have not read the manual for your specific device, you may be reading the display wrong when it counts most.
What the NDL actually is
Nitrogen dissolves into your tissues under pressure. Different tissues absorb and release it at different rates, modelled by your computer across anywhere from 8 to 16 theoretical compartments, each with its own half-time. The NDL is the point at which one or more of those compartments has absorbed enough nitrogen that a direct ascent to the surface would put you at elevated risk of decompression sickness (DCS). Cross it, and you have incurred a decompression obligation: the computer now requires you to stop at specific depths on the way up to allow controlled off-gassing.
The NDL is not the same as “safe depth.” You can get into decompression trouble at 18 metres if you stay long enough. You can dive to 30 metres and surface cleanly if your NDL allows it. The number on your screen is everything. Which is why it matters that you understand exactly what your specific computer shows when it changes from a countdown to a set of instructions.
In plain terms
Your computer gives you a time limit at depth, the NDL. Stay too long and it flips into deco mode. Now it shows a ceiling, a depth you are not allowed to go above, and a timer. You wait at or just below that ceiling until the timer clears, then the ceiling moves shallower and lets you up. Do every stop it gives you, ascend slowly, and add a safety stop at 5 metres. None of this is a game, but the rules are simple if you know them.
Three computers, three ways of telling you
Every modern dive computer follows the same underlying logic. While you are within your no-stop time, it shows you an NDL: the minutes you have left at this depth before a decompression obligation kicks in. Stay down past that and the computer enters deco mode. It stops showing a countdown and starts giving you instructions: a ceiling depth you cannot ascend above, and a timer that has to run out before that ceiling moves shallower. Clear each stop, follow the ceiling up, and it walks you back to the surface.
What changes from one computer to another is how that information is displayed, and how the device behaves if you ignore it. A number, an arrow, a colour, an alarm: the same obligation can look completely different depending on what is strapped to your wrist. The three systems below represent different generations and approaches. If you dive the Red Sea regularly, there is a good chance your computer is one of them or works on the same logic. Read the one that applies to you.
D4 · D5 · D6 · D9 series
Two numbers dominate the deco screen: Ceiling (the shallowest depth you must not exceed) and Ascent Time (total minutes to reach the surface safely). The D5 and later models use Fused 2 RGBM across 15 tissue compartments. The interface is minimal by modern standards but precise.
Algorithm lock is the worst outcome on the D series. If you ignore the red warnings and stay above the ceiling for over 3 minutes, the computer disables its decompression calculations entirely. It cannot recover mid-dive. You are on your own.
Descent Mk3 (and Mk2 series)
The Mk3 uses Bühlmann ZHL-16C, a widely respected algorithm with configurable Gradient Factors (GF Low and GF High) that let technical divers tune conservatism. In standard recreational mode, the display shows your current depth position relative to the ceiling, the ceiling depth itself, and a countdown stop timer. Cleared stops appear as empty segments.
The Mk3 does not have algorithm lock in the same way as older Suunto models, but if you repeatedly break the ceiling the stop times will increase as the algorithm recalculates. The longer you spend above your ceiling, the more time you will owe at the stop.
Ocean
The Ocean introduced a colour-coded arch display that runs around the watch face, giving a constant at-a-glance deco status. Green means you are within NDL. As your NDL shrinks, the arch shifts toward orange. When you cross into deco, the arch turns fully orange, a Deco badge appears on screen, and the ceiling value shows in the switch window. It is the most visually obvious deco alert Suunto has built.
The Ocean's colour system is intuitive but can create false comfort. An orange arch means you have deco obligations and are managing them, not that you are in danger. A red arrow is the danger signal. Know the difference before you get in the water.
What to actually do
The moment your computer switches to deco mode, the sequence is the same regardless of which device you are wearing.
The instinct to head for the surface is wrong. Hold your current depth. You have gas. The computer knows where you are.
Your computer now shows a ceiling depth. That is the shallowest point you may be at. Do not go above it.
If you are already above the ceiling, descend back through it now. The stop timer on most computers will not run until you are in the correct zone.
Do not skip stops because you are low on gas. If you are approaching a gas problem, signal your buddy and share. The stops are not negotiable.
Between stops, ascend slowly. Most computers include an ascent rate indicator. If yours shows you are ascending too fast, it will adjust your deco obligation upward.
Even after the computer clears your obligation, hold 5 metres for 3 minutes. It costs almost nothing and provides a margin against off-gassing errors.
Algorithm lock: the situation you cannot recover from underwater
On older Suunto computers (D4i, D6i and similar), if you ascend above your decompression ceiling and stay there for more than three consecutive minutes, the computer triggers algorithm lock. It stops calculating decompression entirely. The computer will still show depth and time but it cannot tell you what stops to make. It will display a warning to seek medical attention and will not return to normal dive mode for the rest of that dive.
If this happens, ascend slowly, stop for a minute at half the depth you were at, then continue to 5 metres and stay there for as long as your gas allows. Exit the water and do not dive for 48 hours. If your computer is in lock mode, you are also in lock mode. Monitor closely for DCS symptoms and contact DAN regardless of how you feel.
Garmin’s Bühlmann-based computers handle ceiling breaches more gracefully: they recalculate and extend stop times rather than locking up. The Suunto Ocean similarly recalculates. Algorithm lock is largely a legacy issue but it is still the reality on a significant number of computers still in active use on the Red Sea.
After the dive
Completing your deco stops does not mean you are guaranteed safe. It means you have done what the algorithm required. Algorithms are models, not certainties. For the 24 hours after any deco dive, regardless of whether it was planned or accidental: no flying, no altitude, no heavy exertion. Drink water. Rest.
Symptoms of DCS can appear immediately at the surface or up to 24 hours after the dive. The most common are joint pain (especially shoulders and elbows), fatigue, skin mottling, numbness or tingling, and in serious cases, vertigo and neurological symptoms. Any symptom that appears after a dive is DCS until proven otherwise.
Call DAN (Divers Alert Network) before you decide it is nothing. Their emergency line is available 24 hours a day. Give them the dive profile: depth, time, how long you exceeded the NDL, what stops you made. They will tell you whether to seek treatment. Do not wait to see if it gets better on its own.
DAN Emergency (Americas): +1 919 684 9111. DAN Europe: +39 06 4211 5685.
Atlas Position
Most accidental deco incidents on recreational dive boats happen within the first three dives of a trip, when divers are pushing depth and time simultaneously without fully accounting for residual nitrogen. The computer is not the problem in those situations. Reading it correctly is. Know your specific device before you enter the water, not when the display changes.
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