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Key takeaways
- A multilevel dive spends time at different depths. Computers calculate nitrogen loading continuously, so shallower sections load you more slowly than the deepest point would.
- Tables assume the whole dive was at maximum depth. That's why computers usually show more no-decompression time on a multilevel dive than tables would.
- Repetitive dives start with residual nitrogen from earlier dives. The computer carries that over, so no-deco times are shorter on later dives.
- Deepest part first, then work shallower, is the widely taught profile. Reverse and sawtooth profiles are discouraged in training.
- Use one computer per diver for the whole trip, and follow your training agency and manual for limits, surface intervals and flying after diving.
The short version
A dive computer samples your depth every few seconds and runs it through a decompression model. That lets it credit you for time spent shallower, instead of assuming the whole dive happened at the deepest point. It also remembers your nitrogen loading after you surface, and carries it into the next dive.
That's why computers are especially useful on dive trips with several dives a day. It's also why you need to understand what they're doing. A computer gives you information based on a model, and your training tells you how to use it.
How computers model nitrogen
Most recreational computers use a model based on the idea of theoretical tissue compartments. Each compartment represents a type of tissue that absorbs and releases nitrogen at a different rate, described by its half-time. Fast compartments load and unload quickly. Slow compartments load and unload slowly.
As you descend, nitrogen pressure in your breathing gas rises, and the compartments start absorbing nitrogen. As you ascend, pressure drops, and compartments that have absorbed more nitrogen than the surrounding pressure start releasing it. The model tracks each compartment and compares its loading with a limit. When one compartment gets close to its limit, your no-decompression time runs down.
Different algorithms use different numbers of compartments, different limits and different adjustments. Bühlmann ZHL-16C, RGBM variants and others all work on related principles. Our dive computer algorithms explained and Bühlmann vs RGBM vs VPM guides compare them.
Multilevel diving: how the credit works
Imagine two dives of the same length and the same maximum depth.
- Dive A: You descend to the maximum depth and stay there the whole time.
- Dive B: You spend a short time at the maximum depth, then work your way up a reef slope, spending most of the dive shallower.
A dive table treats both dives the same, because tables use maximum depth and total time. A dive computer sees that on Dive B, you spent most of your time at lower pressure, where nitrogen loads more slowly. As you move shallower, fast compartments may stop loading or even start releasing, and your no-decompression time usually increases.
That's the multilevel credit. It's not a loophole. It's a more accurate model of the dive you actually did.
| Profile | How a table sees it | How a computer sees it |
|---|---|---|
| Square profile (all at max depth) | Max depth for full time | Similar to table, since the dive really was at max depth |
| Multilevel (deep first, then shallower) | Max depth for full time | Credits shallower time, usually more no-deco time |
| Reverse profile (shallow first, then deep) | Max depth for full time | Tracks it, but training discourages this profile |
| Sawtooth (repeated up and down) | Max depth for full time | Tracks it, but training discourages this profile |
Why deepest first
The widely taught recommendation is to do the deepest part of a dive first, then work shallower, and to make the deepest dive of the day first. This profile is consistent with how recreational training and many decompression models are designed. Reverse profiles and repeated up-and-down sawtooth profiles are discouraged in training because of concerns about bubble formation. Your computer will still calculate them, but that doesn't make them a good idea.
Repetitive diving: tracking residual nitrogen
When you surface, your tissues still hold extra nitrogen. It releases over time, with fast compartments clearing first and slow compartments taking much longer. If you dive again before it's gone, you start the next dive with residual nitrogen.
Your computer tracks this continuously. On the surface, it keeps running the model, and many computers show:
- Surface interval: time since the last dive.
- Desaturation time: the computer's estimate of when your tissues return to surface equilibrium, based on its model.
- No-fly time: a countdown based on the manufacturer's flying-after-diving rules.
- Tissue graph: a bar chart showing loading in each compartment.
On your next dive, the starting loading is higher, so your no-decompression time runs down faster at the same depth. That's why second and third dives of the day usually have shorter no-deco times than the first.
Worked example (conceptual, no limits)
A diver makes a morning dive to a reef wall, spending the first part deep and the rest on a shallower reef top. After surfacing, the computer shows a surface interval clock and a desaturation countdown. The diver plans a second dive after lunch at a shallower site.
When the diver starts the second dive, the computer still carries residual nitrogen from the morning, so the displayed no-deco time at a given depth is shorter than it would be on a fresh first dive at the same depth. By the third dive, the effect builds further, especially in slower compartments.
The practical takeaway: plan later dives shallower, follow your computer's displayed no-deco time, and use surface intervals the way your training teaches. See surface interval planning for multi-dive days for how to structure a dive day.
How to use your computer on multi-dive days
- Wear the same computer for every dive. It needs your complete history. Don't swap computers mid-trip or borrow one.
- Plan the deepest dive first. Make each later dive the same depth or shallower.
- Watch no-deco time, not just depth. Start your ascent while you still have a comfortable margin.
- Take the safety stop your training teaches. See safety stops vs deco stops.
- Respect surface intervals. Longer intervals let more nitrogen release before the next dive.
- Check no-fly time before booking flights. Follow your computer and your agency's flying-after-diving guidance. See flying after diving.
- Consider more conservative settings on long trips with many dives, if your training supports it.
Using the tissue graph and planner
Many computers include a tissue loading graph and a dive planner. The tissue graph shows how loaded each compartment is. After a dive, fast compartments drop quickly while slow ones stay high. That picture helps explain why repetitive dives get shorter.
The planner shows available no-decompression time at different depths, based on your current loading. Use it during surface intervals to see what your next dive can look like. Our how to plan a dive with your computer guide walks through planner screens, and how to read NDL on a dive computer explains the no-deco display.
Buddy teams with different computers
Buddies often wear different computers with different algorithms or conservatism settings. On multilevel and repetitive dives, those differences grow. One computer might show several more minutes of no-deco time than the other.
Agree on a rule before the dive. Many teams follow the most conservative computer in the group. Also agree on turn points and ascent plans, since your remaining times will differ.
What happens if you exceed a limit
If you exceed your no-decompression limit, most computers switch to showing required decompression stops. What to do then depends on your training and manual. Some computers lock you out or restrict modes afterward. See dive computer lockout modes and dive computer alarms explained. Learn the signs of decompression illness and the first-response steps in DCS basics.
Backup computers on multi-dive trips
If your computer fails partway through a trip, you lose your nitrogen history. Many divers carry a second computer and wear both from the first dive. Our what to do if your dive computer fails during a trip guide covers the options, including the waiting periods training agencies commonly teach before resuming diving.
Mistakes to avoid
- Diving someone else's computer. It carries their nitrogen history, not yours.
- Reverse profiles. Going deeper on later dives, or later in a dive, is discouraged in training.
- Sawtooth profiles. Repeated ascents and descents aren't what recreational models are designed around.
- Pushing no-deco time to zero. Leave a margin.
- Ignoring surface intervals. Short intervals mean more residual nitrogen.
- Forgetting no-fly time. Check it before your flight home.
Frequently asked questions
What is multilevel diving?
A multilevel dive spends time at different depths instead of staying at one depth. Computers track depth continuously, so time spent shallower loads nitrogen more slowly than time at the deepest point.
Why does my computer give me more time than the dive tables?
Tables assume you spent the whole dive at your maximum depth. A computer calculates your actual profile, so it credits shallower time on multilevel dives.
Why is my no-deco time shorter on my second dive?
Your tissues still hold residual nitrogen from earlier dives. The computer carries that loading into the next dive, so no-decompression time runs down faster.
What is a reverse dive profile?
A reverse profile goes deeper later in a dive, or makes a later dive deeper than an earlier one. Training agencies generally teach deepest dive first and deepest part of a dive first.
Can I share a dive computer with my buddy?
No. A computer tracks one diver's nitrogen history, so it can't be shared between divers, even between dives.
How long should my surface interval be?
Follow your training, your computer's planner and your dive operator. Longer surface intervals let more nitrogen release before the next dive.