The complete guide to decompression stops
Scuba diving is a hobby that involves every year, new followers that are going to observe the seabed, discover the myriad of creatures that populate the oceans, explore ships or caves. This is an program rich in emotions! However, scuba diving is still considered a risky activity because you have to rigorously follow certain rules and behaviors to protect yourself against all danger. Decompression stops are among the most important rules that govern the practice of diving.
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What is a decompression stop?
A decompression stop is a procedure that always takes place at the end of a dive. This is the time we spend at a given depth to reduce the amount of nitrogen or helium remaining in human tissue before we rise to the surface. The decompression stops differ according to each dive profile depending on the maximum depth reached during the dive as well as the time spent underwater at this depth. Depth and time are given by the decompression tables or by a dive computer.
What is the difference between a safety stop and a decompression stop?
In France, as part of recreational diving, we avoid dives with decompression stops. However, for experienced divers, some remarkable dive sites require you to cross the 40-meter-deep bar. If the dive lasts a certain time beyond the threshold of 40 meters, you must imperatively carry out a decompression stop at the ascent. For very deep dives, deep and long decompression stops are REQUIRED first 12m then 9m then 6m then 3m. The divers then use gases trimix, nitrox, pure oxygen which proportions have been calculated, beforehand, to reduce the duration of these levels.
This step is not mandatory but strongly recommended. Most instructors have the habit of marking this three-minute safety step between three and five meters deep. Historically, this landing was used to correct the ascents too fast. During an immersion, in most cases, diving instructors arrange for safety stops during the ride so as not to interfere with the diving experience. This level of safety becomes mandatory if you dive beyond 30 meters.
Who are the decompression stops reserved for?
Decompression stops can be used by all divers above the non-decompression limits indicated by their dive table or dive computer. The duration of the decompression stops is determined by the depth and duration of exploration. Most dive computers indicate 7 minutes of stops then this number rises rapidly.
Why is it necessary to mark a decompression stop?
In scuba diving, the diver breathes a gas that is at a pressure greater than the surface pressure. If it is air, the mixture will consist of 21% oxygen and 79% nitrogen. On earth, this last gas is useless for our organism, it is inhaled then rejected; it is an inert gas. In scuba diving, things are a little different, the nitrogen passes from the lungs to the blood and blood to the tissues. The greater the depth, the greater the pressure and consequently the more the nitrogen absorption is. The body of the diver quickly becomes saturated with nitrogen. At the ascent, the pressure of the surrounding water decreases and the nitrogen accumulated during the dive will return to the blood in the form of microbubbles and then be rejected by the lungs at each expiration. Without gradual recovery that makes the decompression stops necessary, these small bubbles, if not properly eliminated, can cause decompression sickness.
Decompression stops are therefore used to prevent decompression sickness.
Example of no-stop dive times as a function of depth see: PADI tables
10m – 3h40
16m – 1h12
20m – 45min
25m – 29 min
30m – 20min
35m – 14min
40m – 9min
42m – 8min
How is a good decompression stop done?
The ideal way to make a good decompression stop is to stay in the horizontal position for as long as possible. In this position, your body slows down the water column and helps you stay at the right depth. The extended position also allows the entire body to be subjected to the same pressure. It is also essential to breathe slowly and deeply so that the blood circulates better and promotes the return of nitrogen to the lungs.
Should we avoid certain behaviors during the stops?
Certain behaviors are to be avoided during a diving stage like to make apneas, to make the hyperventilation and not to respect the depth of the decompression stop.
How do I know which decompression stop need to be done?
To answer this crucial question, you will have the choice between two proven methods:
The most tedious but the most economical is to use decompression tables of recognized federations like those of the FFESSM or the one used at PADI. In any case, you will be required to use them during your dive training.
The dive computer
The other option is to get a dive computer. There are models adapted to each budget and each practice.
What to choose ? Dive computers are more used for recreational diving because their understanding is almost immediate. The computer algorithm calculates for you, in real time, your non-decompression limit and your bearings according to your dive profile. During your dives, make sure that your partner has one also in case yours fails underwater.
Decompression stops should not be seen as an unnecessary constraint. It is a must for anyone who wants to prevent decompression sickness.