Last Updated: January 19, 2022
No decompression limit (NDL) refers to the time a diver can spend at a certain depth before needing to ascend so as to avoid decompression sickness.
In other words, it is the maximum time when a diver can still ascend directly to the surface without requiring decompression stops on the way up. This time can be calculated using a dive table or dive computer and added safety stops can help prevent the occurrence of decompression sickness.
In this article, I will delve deeper into what no decompression limit is, dive tables and how to calculate no decompression limit for the depths you plan to dive, what happens when you exceed no decompression limit, how deep you can dive without decompression, and what emergency decompression stops are.
What is No-Decompression Limit?
No decompression limit is defined as the amount of time a scuba diver can linger at a certain depth before risking decompression sickness by ascending nonstop to the surface. Our bodies are able to withstand being at depth for only a limited amount of time before there are consequences that can lead to nerve damage or even death.
Divers need to learn how to stay inside the no-decompression zone to avoid mandatory decompression stops. The time depends on the pressure at the given depth, which is determined by Boyle’s Law.
No Decompression Limit Dive Table & Calculation
Since decompression sickness is no joke, it is important to properly calculate how long you can stay at depth before risking the bends. Dive tables were created to help scuba divers figure out their no decompression limit.
Dive tables may seem extremely intimidating at first, but don’t worry I’ll walk you through it. PADI provides excellent, easy-to-read dive tables for every diver. This is presented as a three table system and if you are only performing one dive, then you just have to worry about Table 1 to figure out no decompression limits.
The depth at which you are planning your dive can be found along the top row of the table. As you trace down the column of your designated depth, the final number in the column is your no decompression limit time.
You should note that the no-decompression limit time is the sum of descent time, actual time spent at depth, and ascent time. Always round up to err on the side of caution.
If you plan to make multiple dives, your NDL changes because of the build up of nitrogen from your first dive. This is where Tables 2 and 3 come in handy. Use Table 2, to figure out your pressure group by taking the depth and total time of your first dive and comparing against the amount of time you spent at the surface between dives.
You then use the pressure group and planned depth for your second dive to figure out your adjusted no decompression limit.
For example, if your first dive was at 50 feet your original no decompression limit is 80 minutes. Let’s say you only dove for 30 minutes and your surface time before diving to 50 feet again was 10 minutes. According to the dive table, you would be in pressure group E.
For your second dive to 50 feet, according to the dive table, your no decompression limit is now 59 minutes. It’s important to consult these tables especially for multiple dives in order to avoid risking decompression sickness which becomes more likely the more dives you perform in a day.
What is No Decompression Limit for 60 Feet?
The no decompression limit for a dive at 60 feet according to the dive table is 55 minutes. However, if your dive exceeds 49 minutes, you will have to add an additional safety stop at fifteen feet. If your dive is between 49 and 55 minutes, your safety stop should last for about three minutes.
What is No Decompression Limit for 100 Feet?
The no decompression limit for a depth dive of 100 feet is 20 minutes, which means you only have that amount of time to descend, spend at the bottom, and ascend in order to hit the no decompression limit. You will be required to perform a safety stop no matter how long your dive takes if you plan to go to 100 feet.
If your dive last less than the 20 minute no decompression limit, then your safety stop should be taken at fifteen feet depth for three minutes.
What Happens If You Exceed No Decompression Limit?
If you exceed the NDL, you must complete a safety stop at a depth of fifteen feet before fully ascending to the surface. If your dive time was less than five minutes over the no decompression limit, your safety stop should last eight minutes.
Additionally, once you surface, you should wait six hours before attempting another dive. If you exceeded your no decompression limit by more than five minutes, your safety stop should last fifteen minutes. You should also wait at least twenty-four hours before attempting another dive.
By exceeding your NDL, you put yourself at risk of developing decompression sickness. Also known as the bends, this condition occurs when nitrogen forms bubbles in your blood due to the change in pressure.
If you have exceeded your no decompression limit, be aware of the symptoms of decompression sickness which can begin to occur up to 48 hours after surfacing. The most common symptoms include arm, leg, and joint pain.
Rarer symptoms also include paralysis, extreme fatigue, shortness of breath, unconsciousness, and dizziness. If you are feeling sick after your dive and experience any of these, you should notify your doctor immediately as untreated decompression sickness can be fatal.
How Deep Can You Dive Without Decompression?
You can generally dive up to 40 meters (130 feet) without having to perform a decompression stop. But obviously the time spent at that kind of depth would be very limited.
The no decompression limits for depths of 40 feet or less are longer than most divers would plan to stay at depth.
Additionally, most scuba tanks only have about 45 to 60 minutes of air and the no decompression limit for 40 feet is more than twice that amount at 140 minutes.
Emergency Decompression Stop
An emergency decompression stop is required if you exceed your no decompression limit. This is basically a safety stop with a longer time at the fifteen foot depth. For decompression dives that last less than five minutes over the no decompression limit, the emergency stop must be made for eight minutes.
For dives that last more than five minutes over the NDL, the emergency stop must be made for fifteen minutes. These emergency stops occur for longer than the three minute safety stops in order to allow the nitrogen in the blood to normalize to the pressure of the depth.
If you change depths too quickly after being at the bottom for longer than the NDL, you risk the nitrogen forming bubbles in your blood causing decompression sickness.
No decompression limits are extremely important for scuba divers because they allow divers to understand how long they can stay at depth before risking decompression sickness. Dive tables are essential to help calculate no decompression limits, safety stops, and adjusted no decompression limits for multiple dives.
The consequences of exceeding NDLs can be extremely severe so emergency stops are very important to perform. If decompression sickness sets in, the symptoms can range from joint pain and dizziness to paralysis and even death. Its essential to notify your doctor of any symptoms experienced within 48 hours of diving.
My unbounded love for the oceans and everything it has to offer motivated me to pursue my passion and become a professional scuba diving instructor.
I keep reading, exploring, and learning more about scuba diving and the underwater world all the time, so I’m excited to share my knowledge with fellow scuba enthusiasts and hopefully contribute a little to your development as a diver. I want people to fall in love with the oceans with as much passion as I have. Read more about me here.