The no decompression limit (NDL) is the maximum allowable dive time that you can remain at a specific depth and ascend directly to the surface without requiring staged decompression stops on the way up. Remember the higher the partial pressure of nitrogen (ppN2), the shorter the dive time (NDL).
You can use air dive tables to determine your NDL, but the best way is to consult your PDC and plan the depth for your dive. It is important to know how to manually calculate NDL in case the PDC is not working or has a dead battery before the dive.
No-decompression limits vary from dive to dive, depending upon depth and previous recent dive profiles. 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.
Partial Pressures in Practice
On the surface in a 1 bar environment, the respective partial pressures are 0.21 O2 and 0.79 N2. Prior to a dive a diver’s body is saturated to this environment meaning all their body tissues such as bone, blood and flesh are saturated to 1 bar of air. Adding 0.21 and 0.79 equals 1.
In diving we refer to Oxygen as the active gas because it is metabolized in the body and required for survival. Nitrogen on the other hand is referred to as an inert gas that we do not require and simple expel with our breath. Our body’s tissues, however, absorb both active and inert gases.
Physics determines how fast gas pressures reach equilibrium. The higher the pressure difference the faster they attempt to equalize initially. So if we dive to 30 meters, we are saturated to 0.79 bar of Nitrogen and breathing 3.16 bar (4 * 0.79) of Nitrogen.
It does not harm us to fully saturate to 3.16 bar of Nitrogen. In fact, the human body can saturate to much higher pressures. What does harm us however is the rate at which we release the surrounding pressure. In diving this is equivalent to ascending to the surface.
It is precisely due to this reason that scuba divers learn safe ascent rates during their training. Safe ascent rates however only protect us up to a certain amount of absorbed nitrogen in our bodies. The no decompression limit determines the time needed to reach this limit.
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.
What is the No Decompression Limit for 60 feet?
The NDL or No-Stop time for 60 feet / 18 meters is 56 minutes according to the Recreational Dive Planner.
On a Suunto dive computer using their algorithm, the NDL is 51 minutes for your first dive.
Regardless of which branded computer or dive table, you are using, always go with the most conservative and lowest number.
What is the No-Decompression Limit for 100 feet?
The NDL or No-Stop time for 100 feet / 30 meters is 20 minutes according to the Recreational Dive Planner table.
On a Suunto dive computer using their algorithm, the NDL is 17 minutes for your first dive.
Exceeding No Decompression Limits
This is a common question among new scuba divers. It is the diver’s responsibility to monitor their NDLs during a dive and determine whether they have enough time to conduct consecutive dives.
Today, most divers rely on dive computers. This makes repetitive diving a lot easier. Divers can even set underwater alarms to warn them prior to reaching a no decompression limit. If a diver were to accidentally exceed their NDL, they need to do the required mandatory decompression stops dictated by either their dive computer or RDP.