No Decompression Limit (NDL) – The Easy Answer

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Scuba divers diving under a boat. What is a no decompression limit in scuba diving?
Scuba diving involves checking your No Decompression Limits and diving according to your plan. Photo by Jim Beaudoin / Unsplash

As a scuba diver, you would have come across the term, No Decompression Limit or NDL. It is also referred to as no-stop time or zero time. So, what exactly are these limits or times? In layman’s terms, a no decompression limit is the maximum time divers are able to spend at certain depths before absorbing too much nitrogen in order to return to the surface without completing decompression stops.

So how do we know what our No Decompression Limit is, and what happens when we exceed these limits?

Let’s take a look.

No Decompression Limit

The No Decompression Limit is the time in minutes we can spend at a certain depth before we absorb too much nitrogen to simply return to the surface without stopping. In recreational diving, safety stops are non-essential stops to err on the side caution. Recreational divers learn how to stay inside the no-decompression zone to avoid mandatory decompression stops. The time in minutes depends on the pressure at the given depth. This is determined by Boyle’s Law, which states that the pressure of the gas increases as the volume decreases.

In short, the deeper we dive the faster we absorb the nitrogen. Also, the longer we stay at a certain depth the more nitrogen we absorb over time.

Here is an example table of Boyle’s Law.

Depth

Surface

10m

20m

30m

40m

Pressure

1 bar

2 bar

3 bar

4 bar

5 bar

Volume

1

1/2

1/3

1/4

1/5

Density

1x

2x

3x

4x

5x

A standard scuba tank is filled with normal air, which consists of 21% Oxygen and 79% Nitrogen (ignoring the less than half a percentile of trace gases for simplicity). The pressures of these gases change the deeper we dive. This is referred to as a gas’ partial pressure.

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.

No-Decompression Limit Table

Now we know that it is absolutely crucial to know how much nitrogen we absorb on a dive, we need to know where to find this information. This is where the No-Decompression Limit tables or Recreational Dive Planner (RDP) table come in.

These tables were created to tell us our time limits and how long we can stay at a certain depth before we are not able to simply ascend at the safe ascent rate.

Padi RDP Table No Decompression Limits
An example of the PADI Recreational Dive Planner in imperial measurements.

When divers dive repetitively they still hold a higher pressure of nitrogen in their body than they did before their first dive. This is known as residual nitrogen and can be converted to residual nitrogen time. This will determine shorter no decompression times on consecutive dives unless a diver waits long enough to be fully desaturated, which takes about 12-18 hours.

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.

Read More: The Smallest Dive Computers for Everyday Use.

What is a Decompression Stop?

When we adhere to no-decompression limits, we are conducting no-stop diving. This means at any point during our dive, we could in theory swim up to the surface without stopping. Yes, that is without safety stop too. It is imperative, however, that we remain within our given ascent rate defined by our dive computer or RDP.

However, if they were to exceed their limits, they would need to complete their decompression stops. They would do this by stopping at around 5 meters to allow our body to off-gas slowly under 1.5 bar of pressure. Recreational divers do not account for the extra gas required to do these stops and therefore refer to this procedure as emergency decompression.

Skipping a mandatory decompression stop means exposing yourself to a much higher risk of decompression sickness. When the body fails to expel excess nitrogen slowly enough, small bubbles form in a diver’s body, namely decompression sickness.

Decompression Sickness Explained

So we know that we should follow our No-Decompression Limits, and we know what to do if we accidentally exceed our limits. But what is decompression sickness and why is it so bad for our bodies?

Decompression sickness is when the absorbed nitrogen in our bodies forms little gas bubbles that can get stuck somewhere in our system. Post dive, a diver’s body contains silent bubbles. They are so small that they are undetectable by ultrasound. If we remember Boyle’s Law, ascending means expansion of volume. That means that some of these silent bubbles could expand to form a physical and observable bubble. This can cause a lot of destruction in someone’s body.

SIMPLIFIED: Imagine your body is a can of fizzy drink. If you shake it and open it, then the pressure will explode and your drink will be all over you. But if you shake it and open it slowly, release that pressure bit by bit, then you will be safe from a mess.

Read More: Tips to Ascend Safely in Scuba Diving

Symptoms of decompression sickness range from tingling in your arms and legs, pain in your joints, and even death. This sickness is also known as The Bends.

Decompression Diving

A Decompression dive is the opposite of a No-Decompression dive. Technical divers plan their dive including their expected decompression stops. They account for the additional time needed underwater in order to decompress and take more breathing gas to accommodate this. They do not exceed NDLs, they plan a decompression dive. We refer to exceeding NDLs in recreational diving since these divers do not plan for the required decompression stops. In fact, for a recreational diver, a decompression stop is an emergency procedure. This is why they refer to it as an emergency decompression stop. Technical divers prepare to handle a series of decompression stops during their ascent.

In order to keep decompression stops shorter, many technical divers learn to complete accelerated decompression stops using up to 100% O2 as their breathing gas.

Technical or decompression diving is different from commercial or saturation diving. Saturation divers saturate their entire bodies to the breathing gas and pressure their work environment dictates. Often this is around 100 meters depth breathing Heliox, a mixture of Helium and Oxygen.

Learn more about commercial diving in our interview with Aiden, a Commercial Diver from Canada.

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