No-Decompression Limits and Safety Stops

A no-decompression limit looks like a fact about the water. It is really the output of a mathematical model of gas moving through tissue — which is why two computers on the same dive can disagree, and why staying inside the number is a margin rather than a guarantee.

What an NDL represents

Breathing gas under pressure drives inert gas into your tissues. On ascent that gas comes back out, and if it comes out faster than the body can carry it away, bubbles form. A decompression model divides the body into hypothetical tissue compartments that take up and release gas at different rates, and defines how much supersaturation each compartment tolerates.

The no-decompression limit is the point at which the model says a direct ascent — at the prescribed rate — is still acceptable. One minute beyond it, the model may require stops on the way up. The limit depends on the model, its settings, and your prior exposure.

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 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.

The practical rules are simple. 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.

Ascent rate is part of the limit

A table or computer result is tied to its own ascent-rate assumptions. 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.