What Is NDL in Diving? No-Decompression Limit Table by Depth

What does NDL mean in diving?

NDL means no-decompression limit. It is the bottom time at which a dive table or computer stops allowing a direct ascent and starts listing decompression stops. Inside the limit you swim up at the set ascent rate and add a safety stop as extra margin. Past it, the stops become part of the dive, and leaving them out is missed decompression.

The limit goes by several names. DAN's Alert Diver treats NDL and no-stop limit as the same thing. The US Navy's Table 9-7 is titled No-Decompression Limits, yet its column header reads No-Stop Limit, and BSAC talks about no-stop time. Shearwater and Suunto computers label it NDL on screen. A no-stop or no-deco dive is one that stays inside the limit. Our DECO 2000 table and US Navy dive tables list it for every depth row.

Bottom time has a specific meaning here. For 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 keeps running until you reach the first stop. Every limit also rests on an ascent rate, 10 m/min (33 ft/min) for DECO 2000 and 30 ft/min (9 m/min) for the US Navy tables, and DECO 2000 assumes an altitude of 0-700 m. Once a dive breaks an assumption, the limit no longer describes it.

NDL table by depth: no-stop limits for 12-42 m

The table gives the no-stop limit on air for each DECO 2000 depth row, for the first dive of the day. The US Navy table works in feet of seawater and rounds up to the next deeper row, so its column shows the Table 9-7 row a diver at that depth would read. The last column comes from our scuba deco planner, which runs the Bühlmann ZHL-16C model with gradient factors 40/85, the default setting on the Shearwater Perdix 2.

• Read the row for your deepest point. If your depth falls between rows, use the deeper one: a 16 m dive uses the 18 m limit of 45 minutes, not the 72 listed for 15 m.
• The planner column is a model value for a square dive with an 18 m/min descent that counts as bottom time. A computer recalculates as your depth changes, so its number moves during the dive.
• Rows deeper than 30 m are there for training and reference. 30 m is the recreational limit on our DECO 2000 pages.

No-decompression limit table in feet: US Navy and NOAA

Divers who plan in feet mostly use tables built on the US Navy's. Here Table 9-7 of the US Navy Diving Manual, Revision 7, sits beside NOAA's no-decompression air table, which NOAA notes is based on the Navy's Revision 6 tables. They agree at most depths. At 60, 90 and 130 ft, Rev 7 allows 3, 3 and 2 minutes more.

Shallower than 40 ft, the Navy table allows 232 minutes at 35 ft, 371 at 30 ft and 1,102 at 25 ft, and sets no limit at 20 ft (6 m) or less. Deeper, it continues to 190 ft, where the limit is 5 minutes. NOAA's table stops at 130 ft.

Do not switch units between tables. A dive to exactly 60 ft is 18.3 m, which DECO 2000 reads in its 21 m row with a 31-minute limit, about half the Navy's 63. Use one table, in the unit it was printed in.

Why do dive tables give different NDLs?

Because each one is built around a different level of acceptable risk. At 18 m (60 ft) the tables on this page give 45 minutes (DECO 2000), 60 (NOAA) and 63 (US Navy Rev 7). PADI's Recreational Dive Planner sits in between: its instructions give 55 minutes at 60 ft and 80 at 50 ft.

Richard Vann, quoted by DAN, gave the US Navy's judgement as 2% acceptable for mild DCS and 0.1% for serious DCS, and added that each group of users has to decide what incidence it accepts. Commercial divers in the Gulf of Mexico set 0.1% and 0.025%. DAN's Project Dive Exploration measured a rate of 0.01-0.04% in recreational diving.

None of these numbers is wrong. DECO 2000 sits further from the edge: about 29% shorter than the Navy at 18 m and 40% shorter at 30 m, where the figures are 15 and 25 minutes.

How a dive computer shows your NDL

As a countdown of the minutes left at your current depth before stops become necessary. That is Shearwater's definition, and Suunto's guide for the Nautic S uses almost the same words. Both cap the display: Shearwater at 99 minutes, while Suunto shows anything longer as >99.

Both warn as the limit gets close. Shearwater's low NDL alert defaults to 5 minutes and turns the number yellow, and its manual says that is the point to begin the ascent. Suunto's NDL arch turns orange between 0 and 5 minutes. At zero the display changes. Shearwater replaces the safety stop counter with the shallowest depth you may ascend to and how long to stay there. Suunto swaps NDL for time to surface, adds a Deco badge and shows the ceiling.

A computer's NDL grows when you swim shallower, because it is recalculated from the dive you actually made. A table charges every minute at your deepest point, so on a multi-level reef dive the computer usually allows more time. On a square profile the gap can go the other way. Sayer and colleagues held 43 computer models at fixed depths in a chamber and found them generally more conservative than standard air tables shallower than 30 m, and less conservative between 30 and 50 m.

Settings move the number as well. The Perdix 2 offers three presets: 45/95, 40/85 (the default) and 35/75. Set our planner to each and an 18 m air dive gets 57, 47 and 37 minutes. GF High, the second number, is what ends a no-stop dive. Two computers on different algorithms or settings will disagree on the same dive, so a buddy pair plans to the stricter one.

Why the NDL is shorter on a repetitive dive

Nitrogen from the first dive is still in your tissues when the second one starts, so the limit runs out sooner. A computer carries that loading over and shows a shorter NDL. A table turns it into a group letter at the end of the dive and a residual nitrogen time that you add to the next dive's bottom time.

Take a first dive of 35 minutes at 18 m on DECO 2000, which ends in group E. After 1 hour 45 minutes on the surface, a second dive to 18 m starts with 26 minutes of residual nitrogen, so only 19 of its 45 minutes are left. The same pair on the US Navy tables, 60 ft for 35 minutes, ends in group G, drops to group E during the surface interval and carries 29 minutes of residual nitrogen time against the 63-minute limit: 34 minutes left. You can check the Navy figures in our US Navy planner.

DAN's Alert Diver names staying close to the NDL across a series of dives as a likely contributor to many DCS cases, and suggests longer surface intervals and fewer dives per day when dives push the limits. Surface intervals and multi-day diving are covered in planning repetitive dives.

Does nitrox change the NDL?

Yes, it makes it longer. Tables and computers set the limit from the nitrogen you breathe, and EAN32 carries 68% nitrogen against 79% in air. Our planner at GF 40/85 gives 47 minutes at 18 m on air and 82 on EAN32. DECO 2000 is an air table, so the usual method is to read it at the equivalent air depth (EAD). EAN32 at 18 m works out to 14.2 m and is read in the 15 m row, with 72 minutes.

The EAD route gives fewer minutes than the planner at every depth, partly because the EAD is rounded up again to the next table row. The table stops at 33 m because the maximum operating depth of EAN32 at a ppO₂ of 1.4 bar is 33.7 m. DAN gives the same limit as 112 ft.

Is a dive inside the NDL free of DCS risk?

No. An NDL is a point on a curve, not a line between safe and unsafe. DAN's Alert Diver calls decompression sickness a probabilistic condition in which injury risk increases with proximity 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.

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.

Your own condition moves the curve. DAN lists breathing gas, dive profile, thermal stress, exertion, hydration, age, fitness and circulatory health among the factors. The VDST's rules for DECO 2000 build two of them into the table: 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.

What happens if you go past the NDL?

The dive becomes a decompression dive, and the stops your table or computer now shows are mandatory. On DECO 2000, 30 minutes at 20 m is read in the 21 m row's 31-minute line: no stops, group E. Five minutes more moves it to the 36-minute line, which requires 2 minutes at 3 m. On the US Navy table, 64 minutes at 60 ft is read on the 65-minute schedule: 2 minutes at 20 ft on air, ending in group L.

Computers handle it differently. 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.

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 lasts at least 15 minutes, followed by at least 24 hours out of the water.

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

• Do not go back down to finish the stops. BSAC says no attempt should be made to re-enter the water to complete missed decompression.
• 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.