What is Decompression Diving?
Technical divers hover near an ascent line during a decompression stop.
Decompression diving is when a diver is required to make one or more stops during their ascent to give their body time to safely release the nitrogen (or other gas, such as helium) that dissolved into their tissues during the dive.
The pressure you’re under as you descend through water causes nitrogen to dissolve into your body tissues. The deeper the dive, the more quickly gas dissolves into your tissues. When you ascend, nitrogen dissolves from your tissues into your lungs and leaves the body through normal breathing. This is known as offgassing. If the amount of dissolved gas is within certain limits, you can ascend to the surface without any required stops – though a safety stop is standard recommended practice. This is called "no stop" or "no decompression" diving. Standard recreational diving is always planned as no stop diving, but you make a safety stop — hanging out at 15 feet for 3 to 5 minutes — as a conservative practice to further reduce risk.
If you exceed the no stop time limits given by your dive computer, you move into "decompression dive" territory. This means you can't swim directly to the surface without unreasonable risk of getting the bends. You now have to instead ascend in stages, making progressively shallower and longer stops to give dissolved gas time to leave your tissues.
In recreational diving, a dive with a required decompression stop is considered an emergency situation caused by accidentally exceeding a no stop limit, or being forced to do so by circumstances (which should be very rare). In commercial, tec, scientific and military diving, however, dives with required decompression may be planned. This type of diving requires additional training and specialized gear.
Decompression Diving FAQs:
What is deco diving used for?
Decompression diving is appropriate when there's no other way to reasonably accomplish the dive. This is most commonly due to depth because no stop time limits become very short below 100 feet. Shallow dives can require a decompression when they are long, however. A two-hour cave exploration dive may not exceed 60 to 100 feet, but the dive is well beyond the no stop limits. Commercial divers may also make a shallow decompression dive simply because logistically it is more time and cost effective to do a single deco dive to carry out a task than to make multiple no stop dives.
How deep can you dive without decompression?
Practically speaking, you can make no stop dives to 130 feet. While you can, in theory, go deeper than that and stay within no stop limits, the no stop times are so short that "well within" limits is essentially impossible.
Are diving decompression tables the same as recreational dive tables?
Decompression dive tables differ from recreational dive tables because they list times, depths, durations and required stops well beyond the exposures recreational divers experience. While commercial and military divers often use tables, tec divers primarily use dive computers for planning and executing decompression dives.
How do you calculate decompression stops when planning a decompression dive?
Depending upon the depth, duration and equipment used, planning a decompression dive may take only a bit longer than a recreational dive, or may take hours over several days as the team considers and investigates alternatives and options.
In tec diving and much scientific diving, planning decompression dives is typically done using software and/or a dive computer. Decompression dives typically involve different mixes of breathing gases, which are selected based on the dive depth and duration. To best consider all these variables, today computers are used to determine and plan the best gases and the dive schedule, plus emergency alternatives to handle reasonably possible problem situations.
Trimix (helium/nitrogen/oxygen) is used on deeper dives to reduce gas narcosis to acceptable levels. Enriched air nitrox and pure oxygen are used during decompression because they accelerate how fast dissolved gas leaves the body. The diver switches between these during ascent, or, when using a closed circuit rebreather, changes the gas ratios during ascent. Each gas mix has a limit on how deep and how shallow it can be breathed safely, and decompression time increases disproportionately with depth, so that for dives deeper than 200 feet, often decompression is longer than the time spent at depth. The dive plan must therefore include how much of each gas is needed, where and when it's used, backup gas and equipment for emergencies, and how much of all this the diver can reasonably manage.
What happens if you don’t decompress when scuba diving?
If you exceed a no stop limit and surface without making the required stop or stops, your risk of decompression sickness is considered unacceptably high. How high? It depends. Any dive has some risk of decompression sickness because people vary in their physiologies and susceptibility. No computer or table can guarantee decompression sickness will never occur, even within its limits.
Support tech divers hover near a decompression ladder.
What’s a good deco dive computer?
Choose a dive computer intended for technical decompression diving. It should be capable of using several different gas mixes on the same dive. Fortunately, these are not difficult to find – ask you PADI Instructor and PADI Dive Center or Resort for guidance.
An important point is that you need not one, but two compatible dive computers for this type of diving. While dive computers are highly reliable, you would not want to be stuck without your decompression info if there were a malfunction, so standard practice is to dive with two, staying within the limits of the most conservative (even identical computers will vary slightly throughout a dive).
What should I do if my dive computer says “deco,” but I didn’t mean to do a decompression dive? What should I do if I miss the stop?
Assuming this happens by accident on a no stop recreational dive, ascend to 15 feet (or deeper if specified by your computer) and stay there until it "clears," meaning you have stayed the required stop time. Most computers show you the time as it counts down. For a recreational "oops" situation, the time would typically be short with only one stop required. However, you may be low on gas, so do not run out of gas underwater. Stay as long as you can, but if you don't have enough air to do the stop (or you miss it altogether), surface with enough air to ascend at a proper rate and exit the water. Then stay calm, alert the divemaster and your buddy, breathe oxygen if available and monitor yourself for DCS signs and symptoms. Do not dive for at least 24 hours or as specified by your computer. If you have or suspect DCS, contact the Divers Alert Network and emergency medical care.
How can I learn decompression diving?
You can start into tec diving with either PADI’s Tec 40 course (open-circuit tec diving) or the PADI Tec 40 Closed Circuit Rebreather Diver course. These courses begin the transition from recreational diving to technical diving, which includes planning and making tec dives. After completing these, you continue into the Tec 45, Tec 50, and Tec 65 courses, in which train you to make deeper, multi-stop decompression dives.