The Editor says: No-Decompression Limit (NDL) is one of the technical terms we learn when we first get into diving. The numbers on our dive computer constantly pop up to guide us toward safer dives — yet the Editor often encounters divers who can't quite tell the difference between a safety stop and a decompression stop. That kind of knowledge gap can quietly increase our diving risk. We'd like to thank PADI (certification agency) dive instructor Chen Zheng-Wei for explaining these terms so clearly. We hope everyone can dive more safely! Reprinted in full with the author's permission. Original article.
One of the biggest concerns for scuba divers is how long they can stay underwater. After all the effort of traveling abroad for a dive trip, with breathtaking scenery all around and marine life in full display — who wouldn't want to linger a little longer?
That said, every diver knows that recreational diving is a form of "No Decompression Dive." No matter how carefully you manage your air consumption, your dive time must never exceed the No-Decompression Limit (NDL). During dive briefings abroad — especially at deeper dive sites — it's common to hear the dive guide emphasize "No Deco Dive," warning that they reserve the right to deny entry to any diver who exceeds it. This shows just how critical it is to stay within the NDL — it's a matter of safety.
So what exactly does "No-Decompression Limit" (NDL) mean? In the PADI (certification agency) Open Water Diver (PADI/SSI cert) course materials, there are several easily confused but critically important terms: no-stop, no-decompression, and emergency decompression. Let's work through each concept one by one.
Why Do We Perform a Safety Stop?
The main reason divers can't spend unlimited time on the bottom comes down to two risks: residual nitrogen and water pressure. When a diver enters the water with a scuba tank / cylinder and begins breathing compressed air, the body metabolizes the oxygen, but nitrogen accumulates in the blood and tissues. The human body is adapted to atmospheric pressure on land, so under normal breathing conditions, nitrogen maintains a stable partial pressure. However, as depth increases underwater, water pressure rises accordingly, and nitrogen becomes "compressed" into the body — causing nitrogen saturation in the blood and tissues to climb. This is what creates residual nitrogen, and as it builds up, the physiological strain on the body increases.
A 10-liter cylinder is typically filled to 200 bar (200 times the atmospheric pressure at sea level) — in other words, the diver carries 2,000 liters of air underwater. Air is composed of approximately 21% oxygen and 78% nitrogen. So if a diver uses 150 bar of air, their total air consumption in volume is 1,500 liters — including 315 liters of oxygen and 1,170 liters of nitrogen. During the breathing cycle, oxygen is metabolized by the body. Nitrogen, however, is not — while most of it is expelled through exhalation, a small portion crosses into the body through gas exchange. The greater the depth and the longer the dive, the more nitrogen water pressure "compresses" into the blood and tissues until saturation is reached. The more residual nitrogen that accumulates, the greater the danger.
As nitrogen saturation in the diver's body increases, ascending rapidly to the surface becomes extremely dangerous. As depth decreases, water pressure drops and gases expand quickly. When the water pressure can no longer hold the nitrogen in check, it releases rapidly — forming bubbles.
Decompression Sickness
At that point, the body behaves like a shaken, pressurized can of soda cracked open all at once — bubbles surge out rapidly. These bubbles circulating through the body can cause decompression sickness (DCS): mild cases may involve skin tingling, joint pain, or limb numbness; severe cases can lead to pulmonary emphysema, pulmonary embolism, or cerebral embolism.
To prevent decompression sickness (DCS), divers must ascend slowly — no faster than 18 m per minute (or no faster than their own exhaled bubbles). The recommended maximum depth for Open Water Diver (PADI/SSI cert) divers is 18 m, so when ascending from maximum depth, the ascent should take no less than one minute. For added safety, most dive computers are more conservative, halving the ascent rate to approximately 9 m per minute — exceed that, and the computer will sound an alarm.
Before returning to the surface, divers typically perform a three-minute safety stop at 5 m. Because water pressure increases by 1 atmosphere for every 10 m of depth in seawater, there is still 1.5 atmospheres of pressure at 5 m. Under that pressure, nitrogen remains suppressed — but can be released safely and relatively quickly within the body's tolerance. As a result, the safety stop dramatically reduces the risk of decompression sickness (DCS). Of course, this requires the diver to maintain good neutral buoyancy to stay steadily at 5 m without shooting up or sinking down. If a diver cannot hold the depth, the dive computer will extend the safety stop time — what divers often call "penalty time." Too little time in the safety stop raises the risk of decompression sickness (DCS); too much means everyone else is already back on the surface while you're still waiting underwater. Maintaining a stable depth is therefore key to completing the safety stop efficiently and correctly.
If a dive is shallow enough or short enough that the residual nitrogen accumulated poses no real danger even without a safety stop, that dive is called a no-stop dive (a dive that requires no safety stop). On the PADI (certification agency) Recreational Dive Planner (RDP), any portion not shaded in gray represents a no-stop dive. At 10 m, the no-stop time extends to 145 minutes; at 18 m, it is 45 minutes. So after a single dive at 10 m, a recreational diver can ascend directly to the surface — slowly, of course — without needing to perform a safety stop.
PADI RDP Table
The Conditions for a "No-Decompression" Dive
The portions of the RDP shaded in gray fall under no-decompression dives. This means that within this time range, performing a three-minute safety stop at 5 m is sufficient to significantly reduce the risk of decompression sickness (DCS) — without the need for the specific decompression procedures required in technical diving, and without needing to enter a recompression chamber for decompression treatment after surfacing. Recreational diving is fundamentally no-decompression diving. For example, a single dive at 10 m lasting 160–199 minutes, or a single dive at 18 m lasting 51–55 minutes, both fall within the no-decompression category.
If a dive runs too long and too much nitrogen accumulates in the body, even a three-minute safety stop at 5 m may not be sufficient to safely off-gas. To prevent residual nitrogen from exceeding what a safety stop can handle, recreational diving imposes a maximum allowable dive time — this is the No-Decompression Limit (NDL), indicated by solid black shading on the RDP.
In short, the NDL is the maximum amount of time a recreational diver may spend at a given depth. At 10 m, the NDL is 219 minutes; at 18 m, it is 56 minutes. In other words, a recreational diver's single dive at 10 m may not exceed 219 minutes, while at 18 m, the maximum is 56 minutes. Because most divers will only get 90–100 minutes from a single cylinder at 10 m — and even with sidemount doubles it would be very difficult to push past the NDL — the dive computer typically displays "–" at that depth, meaning "you can safely stay until your air runs out."
Once a diver exceeds the NDL, the risk of decompression sickness (DCS) rises sharply. If the NDL is only exceeded by a short margin, the diver enters an emergency decompression procedure — meaning the safety stop must be extended to ensure a safe ascent. According to the RDP, if the NDL is exceeded by five minutes or less, the diver must perform an 8-minute decompression stop at 5 m (rather than the standard 3 minutes). After surfacing, a surface interval of at least 6 hours is required before the next dive. If the NDL is exceeded by more than 5 minutes, a 15-minute decompression stop at 5 m is required, and no diving is permitted until a 24-hour surface interval has elapsed. When using a dive computer, however, the moment you exceed the NDL, the computer will adopt more conservative safety measures and instruct you to stop at a specific depth for a decompression stop — in that case, follow the dive computer's instructions exactly.
Understanding Enriched Air Nitrox (EANx) and Oxygen Toxicity
To extend the NDL and reduce residual nitrogen in the body, divers can choose to breathe enriched air nitrox (EANx) — also known as Nitrox or EAN. This is not pure oxygen: under water pressure, pure oxygen becomes toxic. At just 4 m depth, a diver faces the risk of oxygen toxicity (ambient pressure 1.4 bar × oxygen concentration 100% = oxygen partial pressure 1.4); at 6 m, oxygen toxicity is nearly certain (1.6 bar × 100% = partial pressure of 1.6).
Enriched air nitrox (EANx) is a breathing gas mixture with a higher-than-normal oxygen concentration — typically between 28% and 36%. By increasing oxygen content, nitrogen content is proportionally reduced, meaning the body absorbs less nitrogen and residual nitrogen accumulates more slowly. A 10-liter cylinder filled to 2,000 liters of standard air contains 1,560 liters of nitrogen; switch to EANx30 (30% oxygen), and the nitrogen content drops to 1,400 liters. Less nitrogen absorbed means a significantly extended NDL.
Take 18 m as an example: the NDL for a single air dive is 56 minutes, but with EANx32 it extends to 95 minutes, and with EANx36 it reaches 125 minutes — truly getting the most out of every dive! Nitrox is designed to let divers stay underwater longer, not to dive deeper. In recreational diving, the depth limit is 40 m — regardless of certification level or breathing gas mixture, that ceiling should never be exceeded for safety. This is why many divers say that earning Advanced Open Water Diver (PADI cert) certification plus a nitrox certification is essentially all you need to dive recreationally anywhere in the world!
Finally, it's worth noting that the no-stop and no-decompression times on the RDP apply to single dives at a fixed depth. When divers make multiple dives at multiple depths, the nitrogen loading in the body becomes considerably more complex to calculate. For multi-dive planning, a dive computer makes the job much easier. A good dive computer not only provides essential dive information — depth, temperature, dive time, NDL, and more — but also assists with the three-minute safety stop at 5 m and stores your dive log. Some dive computers even feature a built-in electronic compass and tank pressure gauge (SPG) integration (requiring a first stage regulator transmitter), giving divers a comprehensive picture of all critical data. This is precisely why the dive computer is the first line of defense in dive safety — and why it is the first piece of equipment most divers purchase.
Cover photo by Marek Okon on Unsplash
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