Technical diving is a term used to describe all diving methods that exceed the limits imposed on depth and/or immersion time for recreational scuba diving. Technical diving often involves the use of special gas mixtures (rather than compressed air) for breathing. The type of gas mixture used is determined either by the maximum depth planned for the dive or by the length of time that the diver intends to spend underwater. While the recommended maximum depth for conventional scuba diving is 130 feet, technical divers may work in the range of 170 feet to 350 feet, sometimes even deeper.
Technical diving almost always requires one or more mandatory decompression "stops" upon ascent, during which the diver may change breathing gas mixes at least once. Decompression stops are necessary to allow gases that have accumulated in the diver's tissues (primarily nitrogen) to be released in a slow and controlled manner.
If an individual exceeds the limits of time and/or depth for recreational diving, and/or ascends too quickly, large bubbles can form in the tissues, joints, and bloodstream. The formation of these bubbles leads to an extremely painful condition known as Decompression Sickness (DCS), more commonly known as the "bends," which can cause paralysis and even death.
People have used compressed air as their breathing medium since the advent of diving in the 1950s. Its main advantage is that it is readily available and relatively inexpensive to compress into cylinders. Nevertheless, air is not the "ideal" breathing mixture for diving. With a concentration of approximately 79 percent nitrogen, compressed air poses two potential problems for all divers: susceptibility to nitrogen narcosis (a condition resembling alcoholic intoxication) at deeper depths; and decompression sickness (DCS). Both of these can prove fatal to a diver. In an effort to reduce the ill effects of nitrogen on divers, nitrox was developed.
Nitrox is a generic term that can be used to describe any gaseous mixture of nitrogen and oxygen. In the context of technical diving, nitox is a mixture containing more oxygen than air. The two most commonly used nitrogen-oxygen mixtures contain 32 percent and 36 percent oxygen by volume. This differs significantly from compressed air, which contains approximately 21 percent oxygen by volume. While an increase of 12 to 16 percent oxygen by volume may not seem drastic, it allows divers to significantly extend their bottom time, and decreases their risk of developing DCS.
While diving with nitrox has definite benefits, it also has clearly associated risks. The major hazard is oxygen toxicity. This comes about when oxygen is inhaled in high concentrations for an extended period of time; this occurs primarily when a diver exceeds the recreational limits for depth. Under these circumstances, a diver can experience an epileptic-like seizure, which may lead to drowning. Due to this potentially fatal hazard, divers using nitrox must adhere to special dive tables. These tables list the maximum safe amount of time that a diver can stay underwater at a certain depth.
The term "mixed-gas diving" refers to any activity in which the diver breathes a mixture other than air or nitrox. The main incentive to dive with "non-air" gas mixtures is to avoid nitrogen narcosis. Mixed-gas diving can also be beneficial in improving decompression and avoiding oxygen toxicity. Mixed-gas diving operations require detailed planning, sophisticated equipment, and, at times, extensive support personnel and facilities. The fact that such dives are often conducted at great depths and for extended periods of time increases the risks associated with them. It is extremely important for the breathing mixture to be properly identified, because breathing the wrong mix can lead to a fatal accident.
One type of mixed gas diving involves the use of heliox. This (79 percent helium and 21 percent oxygen) mixture is often used for very deep diving. Unlike nitrogen, helium is not known to have an intoxicating effect at any depth; it has a lower density than nitrogen, making it easier to breathe; and in cases of extended submersion, it improves decompression.
Still, heliox has its drawbacks. It is expensive, has a limited availability, and its thermal conductivity is six times greater than that of nitrogen. This means that a diver breathing heliox will lose body heat six times faster than someone breathing compressed air or nitrox, making them susceptible to hypothermia. To prevent this, divers often wear special suits filled with hot water that is pumped down from the surface. Heating the heliox before the diver inhales it is another strategy used to combat hypothermia. Either of these procedures require specialized equipment and highly trained personnel.
Surface-supplied diving (SSD) is an alternative to self-contained equipment. This method consists of lowering divers into the water on a support platform, or stage, and supplying them with breathing gas (air or another gas mixture) through a flexible hose attached to a diving helmet. Since the diver does not need to be concerned with a limited supply of breathing gas, SSD gives divers the flexibility they need to perform a variety of underwater tasks. The diver's helmet is connected to an "umbilical" that supplies breathing gas, two-way communications, a depth measurement tube and, optionally, hot water to warm the dive suit. In many cases, a camera and lights are mounted on the diver's helmet. Video from the diver's cameras, as well as audio communication, allow a dive supervisor to monitor activity throughout the dive, and provide recommendations if any difficulties arise. SSD is particularly effective on deep or extended operations when divers are working in a relatively restricted area.
Surface-supplied diving provides several advantages over scuba. These include a direct physical link between the diver and the surface; a continual, unlimited supply of breathing gas; a means of controlling the diver's depth and location; and a means of providing video and audio links to the surface.
Surface-supplied diving does, however, have disadvantages. Among them, a diver's mobility and range are limited by the length of the umbilical; in strong currents, the pull on the umbilical can be severe; divers must walk along the bottom on weighted boots, and are unable to swim effectively; and SSD operations require a large support crew and a great deal of equipment.