This article is part of the Beginners Guide to Scuba Diving
The term decompression diving is sometimes confusing for beginning divers. It brings forth images of complicated formulas and in-depth planning. They are told that it is “just” for technical divers who have a vast amount of training and it is not for mere recreational divers.
They are right mostly, however, that does not mean that the recreational diver should not be aware of the topic and what it involves. In a broad definition, we can say that a decompression dive is one where the diver must make one or more mandatory stops prior to reaching the surface. The purpose of the stops is to allow the reduction of inert gases in the body. We refer to these mandatory stops as Deco stops.
In our Open Water Diver training, we learned the importance of the dive tables. These tables gave us a set of guidelines we could use to safely dive to certain depths and how long we could stay there. This time limit is called the No-decompression limit (NDL).
Staying within the NDL time and making a proper assent means there are no mandatory decompression stops. The second part of the table helps us calculate the effects of nitrogen still in our body before our next dive. The surface internal and previous dive gave us a new NDL for our next dive at the depth we planned. In our training, we learn that these tables assumed a maximum ascent rate. We generally see 30 feet per minute at the rate we wish to stay below.
This slow-controlled assent allows the body to continue to expel nitrogen from our tissue at a rate that balances its growth in size due to less pressure. It also helps prevent expanding air to be caught in the ears and chest causing ruptures. We are also taught that we should pause at 15ft/5m for 3 minutes as a safety stop. This is the same as a deco stop as far as its effect on the body, allowing more time to let the body expel gasses.
For the recreational diver, the question of should we avoid a decompression dive, the answer is a simple yes. The explanation might not be as you expect, however. Let’s start by saying an Open Water Diver following the dive tables and staying within the limits of their certification will not likely encroach onto the decompression dive profile. On your first dive, you can stay at 60ft/18m for about 55 minutes.
Most divers will find that the amount of air remaining will require them to surface before reaching the NDL. Depending on the surface internal, the same may be true for the second dive. Advanced Open Water and Deep Divers, however, can exceed NDL before reaching the air consumption limit.
As an example let’s say a dive plan using computers calls for a descent to 100ft/30m. The divers are to start to ascent when the first diver reaches 70 bar of gas remaining or 3 minutes NDL time. There will be a 3-minute safety stop at 15ft/5m. Let’s assume a diver exceeds his NDL and breathes down to the 70 bar. His dive computer will go into “deco” mode and give him one or more “deco” stops.
The more time passes over the NDL, the more time and maybe more stops will be added. While a slow return to the surface with 70 bars would have left a safe air margin within NDL, the additional time required to ascend may exceed the remaining air. Very few divers, especially a new diver, can do an extended “Deco” dive on a single tank.
My Dive Computer says “Deco” What Do I Do?
The first thing to do is not to panic. If you just went into deco mode, the safety stop can still be short enough not to affect your ability to return to the surface. Ascend slightly and signal your dive buddy it is time to go up, let your buddy know you are in “deco”. Your dive computer will give you a depth where you need to make a safety stop. Ascend to that depth or slightly below it and level off. The computer will show you how long you need to remain at that depth, often with a countdown timer.
When you complete that stop, the dive computer will let you know if an additional stop is needed. If you were just slightly over the limit the next stop may be the 5 meter stop with a minute or two added. Longer time over the NDL may require an additional stop. If you miss a “deco” stop, your dive computer will go into violation mode. It will still provide recommended stops, however, it will not start a new dive for 24 hours. Both you and your dive buddy must monitor your air consumption.
Practice a “Fake” Decompression Dive.
Reading and fully understanding your dive computer manual and applying it to your computer is very important. Advance and deep divers have to pay close attention to the NDL on their computers. Knowing what to do and how your computer will react is critically important. Depending on your computer, you might be able to “fake” a decompression dive for training purposes. This is best done under the supervision of an instructor.
While there are many models of dive computers on the market, they all use a very small number of mathematical models. Even between different mathematical models, the results are very similar. To accommodate divers with different risk factors, many computers will let you set your computer to be more conservative than the normal model, having a normal mode, conservative mode, and a very conservative mode. There may be as much as 5 minutes difference between the different settings at 30 meters.
To experience a training decompression dive requires you to use two dive computers. Set your computer to the very conservative setting, and use the other as your control computer with the setting you normally would use. This exercise will not work if you normally dive in a very conservative setting. Dive at a depth within your certification level and let the very conservative set dive computer go into “deco”, however, do not exceed the NDL on the control computer. Follow the decompression dive instructions from your computer as you surface. This will give you the experience of using your computer if you accidentally go into a decompression situation.
Deep Diving Safety Stop
The addition of the safety stop at 15ft/5m for recreational divers was designed to provide a larger margin of safety. It is in fact the same thing that a deco stop does. Many deep dive recreational divers are adding a deep safety stop. This stop is at half the depth of the dive. So if you dived to 30m, the diver will do a deep stop at 15m. The stop is only for a minute or two, however, it will add a safety margin in addition to the 5m stop.
Training for Decompression Diving
As you become a more confident and efficient diver, you may reach a point where you would want to learn decompression diving. The training agencies offer an introduction to technical diving programs that include decompression diving. PADI’s Tec 40 course and SSI’s Extended Range Nitrox are two examples. These programs do require other certifications and are for experienced divers, however, they are a worthy goal to strive for.
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