No-Decompression Limits

What the limit means

The deeper you dive, the faster your body absorbs nitrogen, as explained in decompression sickness. A no-decompression limit (NDL) is the time at which, according to a decompression model, you would have absorbed so much nitrogen that you would need to stop on the way up to release it safely. Recreational diving is planned to stay inside these limits, so you can always ascend directly to the surface, with only an optional safety stop.

Typical limits on air

Limits vary between tables and computers, because they use different models. As an example, these are the published no-decompression limits for a first dive on air from the widely used PADI Recreational Dive Planner (metric):

These figures are for illustration. Never plan a dive from this table: use the tables or dive computer you are trained on, and stay well inside the limit rather than diving right up to it.

What shortens the limits

• Repetitive dives. Nitrogen from earlier dives is still in your body, so later dives get shorter limits. A longer surface interval lets you release more.
• Depth. The deeper you go, the shorter the limit, as the table shows.
• Conservatism. Many divers set their computer to a more conservative level, which shortens the limits.
• Altitude. Diving in mountain lakes needs adjusted limits.

What lengthens them

Breathing enriched air nitrox reduces the nitrogen you absorb, so the limits are longer for the same depth, within nitrox's own maximum depth. A multilevel profile, deep first and gradually shallower, also earns more time on a computer than a square profile at one depth.

Planning inside the limits

1. Make the deepest dive of the day first, and the deepest part of each dive first.
2. Agree a maximum depth and time with your buddy, well inside the limit.
3. Watch your computer and start up with a few minutes of NDL left, not at zero.
4. Ascend slowly and make a safety stop.
5. Leave a good surface interval between dives, often at least an hour.
6. Plan your last dive with flying after diving in mind.

If you exceed a limit

If you go past a no-decompression limit, your computer will show required decompression stops. Ascend slowly to the depth shown and stay there until each stop clears, then make a safety stop if you have gas to spare. Afterwards, stay out of the water for the rest of the day and watch for symptoms. Planned decompression diving is a separate, technical form of diving with its own training.

Frequently asked questions