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 decompression diving.
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.
What are the depths scuba divers can dive without the need to worry about decompression stops?
Many scuba divers get confused between ‘decompression‘ and ‘decompression stops.’ This leads to a number of beginner scuba divers asking ‘how deep can you dive without decompression.’ If you get decompression wrong when you’re scuba diving, this can result in injury.
That’s why its important to get it right between how deep you dive and how you decompress at the end of your dive along with the decompression stop . All dives are decompression dives no matter how deep you dive, so you must decompress on your ascent. You must ascend slowly from all dive depths using your dive computer as a guide and end your dive on a 5-6 metre safety stop. If you exceed the no-stop decompression limit decompression stops are needed.
As all diving is decompression diving, and with the answer “You can’t dive without needing to decompress,” the next question to ask is ‘how deep can you dive without decompression stops?’
Decompression stops are affected by how deep you dive and also by how much time you spend at your chosen depth. Which is is why it’s important to know how deep you can dive without decompression stops. The deeper and longer your dive the more chance you need decompression stops. Shallow dives of 6-10 metres (20-30 feet) you can spend over 200 minutes without a decompression stop. Dives to over 30 metres (100 feet) limit your dive time to around 20 minutes before a decompression stop is required.
What is decompression?
Firstly, let’s look at what decompression is with regards to scuba diving.
If we look at the definition of decompression this states that: “Decompression is to release from pressure or compression” or “to undergo release from pressure.” No decompression limit (NDL) is the maximum allowable dive time that you can remain at a specific depth.
When you scuba dive you enter an environment which is at higher pressure than it is on land. In fact water is nearly 800 times (784 times) more dense than air at sea level.
The deeper you go, the higher the pressure. The higher the pressure the more your body will be compressed. So by definition, as you ascend from ANY dive your body will be decompressing. That means that all dives are decompression dives.
When your body is under pressure the nitrogen element (i.e. 78%) of the air you breath is dissolved into your body. The longer you are at depth the more nitrogen is dissolved. The more nitrogen that’s been dissolved, the more time that’s required for it to release from your body as you ascend and decompress. Which is where dive tables help.
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 with a dive tables 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, dive tables are important for this.
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.