No-Decompression Limits Explained: What Your NDL Means at Each Depth

What is a no-decompression limit?

It is the point where your table or computer stops allowing a direct ascent and starts listing decompression stops. Under it, you go straight up at the set ascent rate and add an optional safety stop. Over it, the stops are part of the dive, and leaving early means missed decompression. Our DECO 2000 table lists the limit for every depth in the 12-42 m range.

Bottom time has a specific meaning here. In DECO 2000 it runs from leaving the surface to the start of the ascent, so the descent counts. The VDST's training notes for the table add a catch: if you ascend slower than the permitted rate, bottom time runs until you reach the first stop. A computer's NDL is a different number. Shearwater defines it as the time left at your current depth before stops become necessary, and it is recalculated continuously, so it grows as you move shallower.

Every NDL rests on assumptions. DECO 2000 assumes a maximum ascent rate of 10 m/min (33 ft/min) and an altitude of 0-700 m. The US Navy tables assume 30 ft/min (9 m/min). Shearwater's computers assume 10 m/min and state that ascending much faster or slower changes the obligation. Once a dive breaks an assumption, the limit no longer describes it.

No-stop limits at common depths: DECO 2000 vs US Navy

The table sets the DECO 2000 limits used in CMAS training beside the air no-stop limits in Table 9-7 of the US Navy Diving Manual, Revision 7. The Navy works in feet of seawater, so each Navy value is for the nearest depth in feet, which is 0.2-0.7 m deeper than the DECO 2000 row.

Why the two tables give different numbers

DECO 2000 is stricter at every depth. At 18 m it allows 45 minutes against the Navy's 63, about 29% less. At 30 m the figures are 15 and 25. Neither table is wrong. They sit at different points on the risk curve: Richard Vann, quoted by DAN, gave the US Navy's judgement as 2% acceptable for mild DCS and 0.1% for serious DCS, while DAN's Project Dive Exploration measured a rate of 0.01-0.04% in recreational diving.

DECO 2000 also rounds against you. A depth or time that falls between rows is read at the next deeper depth and the next longer time, so a 16 m dive uses the 18 m limit of 45 minutes, not the 72 minutes listed for 15 m. The rows deeper than 30 m are there for training; 30 m is the recreational limit on our DECO 2000 pages.

An NDL is not the line between safe and unsafe

It is a point on a curve. DAN's Alert Diver describes decompression sickness as probabilistic, with injury risk rising the closer you get to the NDL, and DAN's guide to dive computers notes that divers are often surprised when DCS follows a dive inside their computer's limits. The same Alert Diver article says that staying within 50% of the allowable bottom time carries a negligible risk with a direct ascent.

Doppler work showed this decades ago. In 1976 M. P. Spencer measured venous gas bubbles in divers after chamber and open-water air exposures. No diver developed bends without bubbles being detected first, and his data did not support the belief that unlimited time at 30 fsw (9 m) was free of problems. Open-ocean dives produced more bubbles and more bends than dry chamber exposures.

Your own condition moves the curve. DAN lists breathing gas, dive profile, decompression time, thermal stress, exertion, hydration, age, sex, fitness, body composition, acclimatisation and circulatory health. The VDST's rules for DECO 2000 build in two of them. After short heavy exertion or in very cold water you read the next longer time row, and after long heavy exertion you add 50% to the bottom time.

Why a dive computer and a table give different NDLs

Mostly because a table assumes you spent the whole bottom time at your deepest point, while a computer tracks the dive you actually made. On a dive that starts at 25 m and works up a reef, the computer credits the time spent shallower. The table charges all of it at 25 m, read as 27 m.

On a square profile the difference can go the other way. Sayer and colleagues held 43 computer models at fixed depths in a chamber and logged when each first demanded a stop. Shallower than 30 m the computers were generally more conservative than the standard air tables they were compared with. Between 30 and 50 m they were less conservative.

The model and its settings matter too. Shearwater's recreational computers run Bühlmann ZHL-16C with gradient factors, 40/85 by default, with presets of 45/95 (least conservative) and 35/75 (most conservative). The second number sets how close to the model's limit your tissues may be when you surface, and on a no-stop dive that is what ends the NDL.

Why two dive computers disagree on the same dive

Because there is no standard algorithm. In the same chamber study, Oceanic units used a modified Haldanean model built on the DSAT database, Suunto used its RGBM, Uwatec used ZH-L8 ADT and Mares its own RGBM variant. For most depth and decompression combinations, the gap between the most and least permissive model was 20-40% of the longest time recorded.

Manufacturers rarely say what they changed. Neal Pollock wrote in Alert Diver that many designers have modified the base algorithms, frequently without publishing details. Repetitive dives widen the spread: standard Bühlmann adds no penalty for consecutive dives, while the RGBM versions from Suunto and Mares do.

Settings shift the numbers again. When the same group tested user settings on seven models, some computers on default settings were already stricter than others that had been switched to a more conservative option. A buddy pair with different computers should plan to the stricter one and treat the other's longer NDL as information, not permission.

Worked example: five extra minutes at 20 m

Plan a dive to 20 m for 30 minutes in the DECO 2000 planner. There is no 20 m row, so the dive is read at 21 m, where the limit is 31 minutes. Thirty minutes rounds up to the 31-minute row: no stops, group E.

Stay 5 minutes longer and the dive changes category. Thirty-five minutes rounds up to the 36-minute row, which requires 2 minutes at 3 m and leaves you in group F. Past that, each row adds more time at 3 m, as the table below shows.

On a real dive to 20 m a computer would also count the minutes you spent shallower, so its numbers would differ. The table figure is the one to use when you have no computer, and a useful cross-check when you do.

What to do if you go past the NDL

Do the stops your table or computer now requires, at the depth it shows, and do not ascend above that depth. When a Shearwater's NDL reaches zero, the safety stop display is replaced by the shallowest depth you may ascend to and how long to stay there. Suunto computers running the Fused RGBM 2 algorithm warn when you rise more than 0.6 m (2 ft) above the ceiling and lock the algorithm for 48 hours if you stay above it for more than 3 minutes. Shearwater has no lockout, only a MISSED STOP warning, so the decision stays with you.

Tables have their own rules. DECO 2000 lists stops for times past the limit, as in the 21 m example. The PADI Recreational Dive Planner treats an overshoot as an emergency. Exceed the limit by 5 minutes or less and you make an 8-minute stop at 5 m (15 ft), then stay out of the water for at least 6 hours. Exceed it by more than 5 minutes and the stop at 5 m lasts at least 15 minutes, followed by at least 24 hours out of the water.

If you surfaced without finishing the stops, remember that symptoms can start late. In one DAN case summary, a diver who interrupted a mandatory stop and tried to make it up afterwards developed arm pain about 12 hours after the dive. It progressed to paralysis within 6 hours and needed recompression treatment. The steps are:

• Do not go back down to finish them. BSAC says no attempt should be made to re-enter the water to complete missed stops.
• Treat it as possible DCI even without symptoms: give oxygen and fluids, and keep the diver under observation for the following hours.
• Call local emergency services first, then the DAN emergency hotline on +1-919-684-9111, which is staffed 24 hours a day.
• Plan no more diving until the waiting period has passed. The RDP sets 6 or 24 hours, and a locked Suunto gives no decompression information for 48 hours.