The differences between recreational and technical diving are sometimes not terribly clear, but it is generally understood that technical diving requires a high level of expertise in order to execute a safe and successful dive. The latter expertise is not only a matter of training, but also includes a significant element of experience. A general rule of thumb that distinguishes the line between recreational and technical diving is the forty metre depth mark. Under forty metres, the dive is still considered as being recreational, albeit that someone diving at around this depth must have a significant amount of training and experience. This owes to the fact that at 40m, oxygen toxicity can occur, and that decompression sickness can become a significant risk factor involved in the dive.
Oxygen toxicity is potentially deadly as it results in convulsions during which the mouthpiece can fall out of the mouth of the diver, starving them of air. The convulsions are known to erupt without any warning whatsoever, and unless the mouthpiece is replaced, the severely debilitated diver will drown. When symptoms of oxygen toxicity do occur, these include intense mood swings, uncontrollable tremors in the hands, face twitching and other muscle spasms, and hallucinations. To avoid oxygen toxicity, deep dives are often conducted with gaseous mixtures where the oxygen content of the mixture is dramatically increased (up to 38% of from the usual 21%). Owing to the fact that gaseous mixtures that are more “complex” or deviate from the standard require more complex decompression techniques, the type of dive conducted with modified mixtures is considered technical.
Attention to the correct ascent from a dive is vital in a gambit to avoid the bends. Incorrect surfacing, especially from deep dives, can result in joint pain, nausea, confusion, marked general discomfort and a loss of consciousness. In extreme cases, the bends can result in death. Beginner divers are advised to ascend at a rate of 10m per minute, at a gradual and slow pace. Technical dives schedule decompression stops which must be adhered to strictly in order to avoid difficulties.
As noted elsewhere on this site, decompression sickness, or the bends, occurs when gas entered into the body and its respective tissue expands due to a sudden drop in ambient pressure. The immediate result is that the expanding gas forms bubbles in the blood and in organ tissue. The list of unpleasant symptoms that results from the bends can be remedied by placing the diver’s body back under pressure: this can be done in either a hyperbaric chamber or by the diver descending back to adequate depth and re-surfacing at a slow pace, giving the body a chance to dispel pressurised gas. In highly technical dives, divers are often required to change between two types of mixtures as the pressure of the water changes; this is to say that at higher pressures, more oxygen is required, whereas at lesser depths compressed atmospheric gas can be used with complications. In addition to oxygen toxicity and the bends, one last disorder that divers have to keep an eye on is nitrogen narcosis.
Another factor that acts to decide the technical difficultly of a dive is whether a diver can ascend directly or if they have to travel a certain horizontal distance before surfacing can be achieved. Diving under ice and in caves are prime examples of technical dives where a diver can only surface at certain places. Lastly, a few more features used to judge the technical difficulty of a dive include the levels of visibility, the quality of natural daylight, the complexity and specialisation of the equipment needed, and the level of training required for the dive to be legal.