No-Decompression Limits and Safety Stops

A no-decompression limit is a model's answer, not a property of the water or a promise that decompression sickness cannot occur. Plan against the algorithm and settings you will actually dive, and leave time rather than treating zero remaining minutes as a target.

What an NDL represents

Breathing gas under pressure drives inert gas into your tissues. On ascent that gas comes back out, and excessive supersaturation can promote bubble formation and growth. A decompression model divides the body into hypothetical tissue compartments that take up and release gas at different rates, then defines how much supersaturation each compartment tolerates.

The no-decompression limit is the point at which the model still permits a direct ascent under that model's ascent rules. One minute beyond it, the model may require stops on the way up. The limit depends on the model, its settings, the profile, and your prior exposure.

The repetitive dive model makes those dependencies visible with one specific set of assumptions: Bühlmann ZHL-16C nitrogen compartments, gradient factors 100/100, sea-level pressure, and instantaneous transitions for a chronological sequence of square profiles. Because it omits the ascent and uses no personal conservatism, its rounded-down allowance is an educational comparison, not a replacement for a computer's live NDL.

Why two computers disagree

Different manufacturers implement different algorithms, and most let you select a conservatism setting on top. Buddies with identical profiles routinely surface with materially different remaining no-stop times, and neither device is necessarily broken.

Common families include Bühlmann ZHL-16, often adjusted with gradient factors; DSAT, used in some recreational computers and derived from Doppler research; and proprietary or modified bubble-aware models. A label alone is not enough: two Bühlmann computers with different gradient factors can disagree substantially.

Plan the dive on the more conservative computer and ascend on it. Never average two devices, and never treat the more generous one as permission. Your own history matters too: recent dives, altitude, and the profile you have just swum all feed into the number, which is why a printed table entry and a computer reading are not interchangeable.

How Nitrox changes the modelled limit

At the same depth, Nitrox contains less nitrogen than air, so every tissue compartment takes up nitrogen more slowly. That usually moves the modelled no-stop boundary later. The size of the change is not a fixed multiplier: it depends on depth, oxygen fraction, prior loading, and the decompression model.

The comparison below uses one named, reproducible model rather than copying rows from a proprietary dive table. Its assumptions are:

• Decompression model: Bühlmann ZHL-16C nitrogen coefficients at gradient factors 100/100, validated against the pinned MIT-licensed GasPlanner implementation. Like that reference, it uses the 5-minute compartment 1b as its fastest compartment. The repetitive dive model linked above instead uses Bühlmann's original 4-minute compartment 1, and the two also differ slightly in two other coefficients, in their pressure constants, and in whether descent time is counted. Their results can therefore differ by a minute or so: at 33 m110 ft on EAN32, the repetitive dive model reports 21 minutes20 minutes where this comparison reports 2221.
• Starting state: a rested first dive at sea level, with tissues initially equilibrated on 20.9% oxygen air. The model uses 1.01325 bar surface pressure and 0.0627 bar water-vapour pressure.
• Profile: fresh water using 0.0980665 bar per meter and a linear 20 m/min descentthe equivalent model pressure gradient and a linear descent at the model's 20 m/min, about 66 ft/min, then a square-profile exposure advanced in whole-minute steps. The result is elapsed time from the start of descent, rounded down to the last minute without a surface ceiling.
• Depth rows: 18–42 m in 3 m stepsindependently calculated 60–140 ft rows in 10 ft steps. Rows deeper than the selected mix's MOD are removed.
• Planning equations: MOD and EAD retain DiveCalc's sea-level 10 m33 ft per ATA approximation. Those equations are separate from the more exact pressure constants used by the tissue model.

The calculation does not simulate ascent, a safety stop, a repetitive or multilevel profile, altitude, salt water, oxygen exposure, or a particular computer's conservatism. It detects the first whole-minute exposure whose GF100 ceiling has risen above surface pressure; that makes it useful for a controlled comparison, not for generating an operational dive plan.

Worked example: EAN32 near its MOD

Inputs. Select EAN32 and a maximum PPO₂ of 1.4 ATA. The 10 m33 ft planning equation places MOD at:

Model result. The deepest listed row inside that MOD is 33 m110 ft. At that row the model returns 14 minutes on air and 22 minutes on EAN3213 minutes on air and 21 minutes on EAN32. Those are already whole-minute model outputs; the unrounded percentage comparison is:

So the comparison shows eight additional minutes at the deepest available row, while the EAD is 27.0 m90.1 ft. That gain describes this model and these assumptions only; using all eight minutes spends the nitrogen-loading margin that EAN32 created.

Ascent rate is part of the limit

A table or computer result is tied to its own ascent-rate assumptions. The interactive comparison above deliberately stops before simulating ascent, so it cannot replace those instructions. Published tables and computer manufacturers do not all specify the same rate. Many current computers use roughly 9 to 10 meters per minute30 feet per minute as a maximum through much of the ascent and may require slower movement near the surface, while some training tables specify a different rate. Follow the active device or table rather than importing a number from another model.

This is the part divers most often get wrong while believing they complied. Staying within your no-stop time and then rushing the last 10 meters33 feet is not a dive inside the limits. Ascent rate is a condition of the limit, not a separate courtesy.

If you overrun the NDL

Do not race to the surface and do not invent a stop from memory. Stop descending, signal your buddy, follow the missed-NDL or decompression procedure displayed by your own computer, and maintain the prescribed ascent rate. Preserve enough gas to complete every required stop.

After surfacing, note what the computer reports and follow your training and the manufacturer's instructions before considering any further diving; many missed-stop procedures end the diving day. If a required stop was omitted, or if any symptom consistent with decompression illness appears, provide emergency oxygen if trained, contact local emergency services and diving-medicine support, and do not re-enter the water to recompress.

A normal safety stop remains a useful precaution on a no-stop dive, but it is not repayment for an overrun. Plan gas for the ascent and stop; that is what minimum gas planning exists to prevent.

Staying well inside the limit

Prudent practice treats the limit as a boundary rather than a target. Surface with meaningful no-stop time remaining. Add conservatism for cold, exertion, dehydration, poor sleep, repeated days of diving, and age.

Nitrox interacts with all of this. A richer mix lengthens the no-stop time because you absorb less nitrogen — see equivalent air depth for the mechanism — but the extra time spends the margin the mix bought rather than adding safety on top of it. Confirm the oxygen ceiling with the MOD calculator. If you are diving again the same day, how the limit shrinks is covered in surface intervals.