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What Is a No-Decompression Limit?


Photo of a scuba diver swimming near the anchor of a shipwreck

A scuba diver must be careful not to exceed his no-decompression limit without advanced training.

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A no-decompression limit (NDL) is a time limit. No-decompression limits vary from dive to dive. A diver who stays underwater longer than the no-decompression limit for his dive can not ascend directly to the surface, but must pause periodically as he ascends to avoid a high risk of decompression sickness. A diver should never exceed a no-decompression limit without specialized training in decompression procedures.

What Determines the No-Decompression Limit for a Dive?:

Nitrogen. Underwater, a diver's body absorbs compressed nitrogen from his breathing gas. (Gasses compress underwater according to Boyle's Law). This compressed nitrogen is trapped in his tissues. As the diver ascends, this trapped nitrogen slowly expands (or de-compresses). The diver's body must eliminate the nitrogen before it expands to the point that it forms bubbles and causes decompression sickness.

If a diver absorbs too much nitrogen, he can not make a normal ascent because his body will not be able to eliminate the expanding nitrogen quickly enough to prevent decompression sickness. Instead, the diver must pause periodically during his ascent (make decompression stops) to allow his body time to eliminate the excess of nitrogen. A no-decompression limit is the maximum time that a diver can spend underwater and still ascend directly to the surface without the need for decompression stops.

What Factors Determine How Much Nitrogen a Diver Absorbs?:

The amount of nitrogen in a diver's body (and therefore his no-decompression limit) depends upon several factors:

1. Time: The longer a diver stays underwater, the more compressed nitrogen gas he absorbs.

2. Depth: The deeper the dive, the more rapidly a diver will absorb nitrogen and the shorter his no-decompression limit will be.

3. Breathing Gas Mixture: Air has a higher percentage of nitrogen than many other breathing gas mixtures, such as enriched air nitrox. A diver who uses a breathing gas with a low percentage of nitrogen will absorb less nitrogen per a minute than a diver using air. This allows him to stay underwater longer before reaching his no-decompression limit.

4. Previous Dives: Nitrogen remains in a diver's body after surfacing from a dive. The no-decompression limit for a repetitive dive (a second, third, or fourth dive within last 6 hours) will be shorter because he still has nitrogen in his body from the previous dives.

When Should a Diver Calculate His No-Decompression Limit?:

A diver must calculate his no-decompression limit before every dive and carry a method of monitoring his dive time and depth to ensure that he does not exceed it. Following a dive guide's (or buddy's) no-decompression limit is unsafe. Each diver must be responsible for calculating and observing his own no-decompression limit because an individual diver's no-decompression limit will vary with small depth fluctuations and previous dive profiles.

Have a Contingency Plan:

A diver should have a plan in case he accidentally descends beyond the planned maximum depth or exceeds the no-decompression limit for his dive. He can make a contingency plan by calculating the no-decompression limit for a slightly deeper dive than the anticipated one. For example, if the planned dive depth is 60 feet, the diver should calculate the no-decompression limit for a dive to 60 feet and calculate a contingency no-decompression limit for a dive to 70 feet. If he accidentally exceeds the planned maximum depth, he simply follows his contingency no-decompression limit. A diver should also be familiar with the rules for emergency decompression so that he knows how to proceed if he accidentally exceeds his no-decompression time.

Don't Push No-Decompression Limits:

Observing the no-decompression limit for a dive only reduces the chances of decompression sickness. No-decompression limits are based on experimental data and on mathematical algorithms. Are you a mathematical algorithm? No. These limits can only estimate how much nitrogen an average diver will absorb during a dive; every diver's body is different. Never dive right up to a no-decompression limit.

A diver should reduce his maximum dive time if he is exhausted, sick, stressed or dehydrated. He should also shorten his maximum dive time if he has dived many days in a row, is diving in cold water or will be physically exerting himself underwater. These factors may increase nitrogen absorption or decrease the body's ability to eliminate nitrogen elimination on ascent.

The Take-Home Message Regarding No-Decompression Limits:

No-decompression limits provide useful guidelines to help a diver reduce the chance of decompression sickness. However, a no-decompression limit is not infallible. A diver should know his decompression limit for every dive, and dive conservatively.
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