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Decompression Diving 101

Interested in decompression diving? Here’s what you need to know to get started and what you can expect once you try it.

As sidemount diving has gained popularity, many divers are becoming interested in technical diving overall. An introductory tec class can lead to more advanced training, which offers not only a challenge, but also an opportunity to become a more competent and safer diver in general. Tec training makes possible diving that’s off-limits to recreational divers. Following is an overview of one such type of technical diving, decompression diving.

What is decompression diving?

Decompression diving, or deco diving, means intentionally going beyond No Decompression Limits (NDLs) to achieve a longer bottom time at a given depth. This could mean spending an hour at 100 feet (30 m) or 20 minutes at 164 feet (50 m), depending on your dive’s objectives. There are, of course, limits to both depth and bottom time for tec diving. You won’t spend two hours at 330 feet (100 m) for physiological and logistical reasons, and even though you could probably spend three hours at 65 feet (20 m), you’d probably get cold, tired, or bored.

Going beyond your dive computer’s NDLs gives you freedom to do more on a dive. Perhaps you want to take your time exploring the engine room of a Japanese freighter at 88 feet (27 m). Maybe you want to do a wall dive to 164 feet (50 m) in Malapascua in the Philippines to hang out with the elusive thresher shark. Whatever the case, decompression diving can help you achieve both the bottom times and depths you seek.

Physiological aspects

Occasionally you’ll hear people say that “every dive is a decompression dive.” When you descend, you compress, and you decompress on the way up. Although that’s true, on a recreational no-stop dive, you directly ascend to the surface (save the safety stop). When decompression diving, you must periodically pause on the way up. If you don’t, you are at a much greater risk of decompression sickness (DCS). To explain, let’s investigate what’s happening to our bodies when we dive.

As you descend during a dive, the nitrogen you’re inhaling dissolves into your body tissues like sugar into a cup of tea. This is called on-gassing. When you spend time at depth, gas enters the bloodstream via the lungs. Here it dissolves and then diffuses into different tissues. When you ascend, nitrogen diffuses back from the tissues into the blood, where it is perfused back to the lungs and breathed out. This is known as off-gassing. How much and how fast you on- and off-gas depends on multiple factors. These include the percentage of nitrogen you breathe, how deep you go, how long you spend at depth, which type of body tissue, how hydrated, fit, and warm you are, how fast you descend and ascend, and more.

On-gassing nitrogen

It’s difficult to pinpoint how much nitrogen we have on-gassed, so we split the body into numerous theoretical tissues. Each tissue will on-gas and off-gas at a different rate. Modern dive computers use algorithms to keep track of up to 16 tissue “compartments.” Each compartment can contain a theoretical limit of gas at varying depths. NDLs are accordingly set to prevent you from staying too long at a given depth. Recreational diving aims to prevent your body from taking on so much gas that it cannot off-gas enough during a direct ascent to the surface.

Decompression diving involves on-gassing more nitrogen, which means a diver must make a series of stops during his ascent. Each stop allows time for gas to move out of the tissues and back to the lungs. The diver then continues to move closer to the surface between each decompression stop. The crude analogy would be the difference between opening a carbonated drink quickly and creating lots of bubbles, or opening it gradually and allowing gas to escape in a controlled way. 

Risk management and planning

So we are going deeper, and staying there longer. This means we need more gas to breathe. The deeper you go, the more atoms of gas you breathe per breath. This means you’ll go through your air faster. In addition to calculating how much gas we’ll need during a given dive, we must also consider what could go wrong. If going into decompression means that you cannot directly ascend to the surface, it also means that if you have any issues underwater, you’ll need to solve them underwater.

Tec divers think about probable risks, and all the different ways to manage those risks as safely as possible. What if your buddy is out of gas or you have a free-flow? What if you lose your buoyancy? Thusly, we don’t just carry the gas we need. We calculate a reserve for ourselves or our buddy and have contingency procedures should something go wrong. Divers may carry a back-up buoyancy device, or learn to “feather” a free-flowing regulator by turning it on and off to take a breath. Tec divers also practice emergency procedures until we can undertake them to a very high standard. We decide on our dive’s objective and then decide on a dive leader. Tasks are split between team members so that no one is overloaded.

Environmental conditions

We must also consider the environmental conditions. Choppy water can affect how easy it will be to do the shallow decompression stops and maintain buoyancy. Rough conditions also make it harder to get back on board at the end of the dive, and more difficult to spot a lost diver. Proper exposure protection is important too. Getting cold during the later stages of the dive can increase the risk of DCS. Strong currents likely mean we will breathe more, so we must account for that in our gas calculations as well. We use software to help calculate our decompression obligation and overall dive profile, gas requirements, contingency gas requirements, and exposure to narcosis and elevated oxygen levels.

We also need to think about using other gases than air. Air is quite inefficient as a decompression gas. We would need to carry way too much of it, and using it would mean very long decompression stops. When decompression diving, we use mixtures containing higher percentages of oxygen during deco stops to accelerate the off-gassing process. This means mixing, filling, analyzing, labeling, and carrying extra cylinders, which we switch to during the deco stops. There are very strict procedures for this, as oxygen can be poisonous if breathed at too great a depth. 


Without incremental training and practice, these considerations can seem a little overwhelming. But once you’re qualified and have had plenty of practice, the rewards can be amazing. Longer bottom times mean you never know what you might see or find. Popular dive spots can be a treasure trove of lost dive gear. You may enjoy spending a long time inside a wreck or cave. Or, you may get great satisfaction from undertaking a series of small, chronological tasks during the dive to achieve a safe outcome. Either way, decompression diving introduces a whole new world underwater.

For more information on decompression diving, visit the Technical Diving International website.