Technical diving

Definition

There is some professional disagreement as to what exactly technical diving encompasses. Nitrox diving and rebreather diving were originally considered technical, but this is no longer universally the case as several certification agencies now offer Recreational Nitrox and recreational rebreather training and certification. Some training agencies classify penetration diving in wrecks and caves as technical diving. Even those who agree on the broad definitions of technical diving may disagree on the precise boundaries between technical and recreational diving.

The European diving agencies tend to draw the line between recreational and technical diving at 50 metres (160 ft) and many teach staged decompression diving as an integral part of recreational training, rather than as a "bolt-on" extra.

Scope

The following table gives an overview of the activities that various agencies suggest to differentiate between technical and recreational diving:

Hazards and risk

One of the perceived differences between technical and other forms of recreational diving is the associated hazards, of which there are more associated with technical diving, and risk, which is often, but not always greater in technical diving. Hazards are the circumstances that may cause harm, and risk is the likelihood of the harm actually occurring. The hazards are partly due to the extended scope of technical diving, and partly associated with the equipment used. In some cases the equipment used presents a secondary risk while mitigating a primary risk, such as the complexity of gas management needed to reduce the risk of a fatal gas supply failure, or the use of gases potentially unbreathable for some parts of a dive profile to reduce the risk of harm caused by oxygen toxicity, nitrogen narcosis or decompression sickness for the whole operation. Reduction of secondary risks may also affect equipment choice, but is largely skill-based. Training of technical divers includes procedures which are known from experience to be effective in handling the most common contingencies. Divers proficient in these emergency drills are less likely to be overwhelmed by the circumstances when things do not go according to plan, and are less likely to panic.

Depth

Technical dives may be defined as being dives deeper than about 130 feet (40 m) or dives in an overhead environment with no direct access to the surface or natural light. Such environments may include fresh and saltwater caves and the interiors of shipwrecks. In many cases, technical dives also include planned decompression carried out over a number of stages during a controlled ascent to the surface at the end of the dive. The depth-based definition is based on risk caused by the progressive impairment of mental competence with increasing partial pressure of respired nitrogen. Breathing air under pressure causes nitrogen narcosis that usually starts to become a problem at depths of 100 feet (30 m) or greater, but this differs between divers. Increased depth also increases the partial pressure of oxygen and so increases the risk of oxygen toxicity. Technical diving often includes the use of breathing mixtures other than air to reduce these risks, and the additional complexity of managing a variety of breathing mixtures introduces other risks and is managed by equipment configuration and procedural training. To reduce nitrogen narcosis, it is common to use trimix which uses helium to replace some of the nitrogen in the diver's breathing mixture, or heliox, in which there is no nitrogen.

Inability to ascend directly

Technical dives may alternatively be defined as dives where the diver cannot safely ascend directly to the surface either due to a mandatory decompression stop or a physical ceiling. This form of diving implies a much larger reliance on redundancy of critical equipment and procedural training since the diver must stay underwater until it is safe to ascend or the diver has successfully exited the overhead environment.

Decompression stops

A diver at the end of a long or deep dive may need to do decompression stops to avoid decompression sickness, also known as "the bends". Metabolically inert gases in the diver's breathing gas, such as nitrogen and helium, are absorbed into body tissues when breathed under high pressure, mainly during the deep phase of the dive. These dissolved gases must be released slowly from body tissues by controlling the ascent rate to restrict formation and growth of bubbles. This is usually done by pausing or "doing stops" at various depths during the ascent to the surface. Most technical divers breathe oxygen enriched breathing gas mixtures such as nitrox and pure oxygen during long duration decompression, as this increases the rate of inert gas elimination. Elimination of inert gases continues during the surface intervals (time spent on the surface between dives), which must be considered when planning subsequent dives. A decompression obligation is also referred to as a "soft", or "physiological" ceiling.

Physical ceiling

These types of physical overhead, or "hard" or "environmental" ceiling can prevent the diver surfacing directly:

In all three of these situations, a guide line or lifeline from the exit to the diver is the standard method of reducing the risk of being unable to find the way out. A lifeline fixed to the diver is more reliable as it is not easy to lose, and is often used when diving under ice, where the line is unlikely to snag and the distance is reasonably short, and can be tended by a person at the surface. Static guidelines are more suitable when a lifeline is likely to snag on the environment or on other divers in the group, and may be left in situ to be used for other dives, or recovered on the way out by winding back onto the reel. Guide lines may be very much longer than lifelines, and may be branched and marked. They are used as standard practice for cave diving and wreck penetration.

Extremely limited visibility

Technical dives in waters where the diver's vision is severely impeded by low-visibility conditions, caused by turbidity or silt out and low light conditions due to depth or enclosure, require greater competence. The combination of low visibility and strong current can make dives in these conditions extremely hazardous, particularly in an overhead environment, and greater skill and reliable and familiar equipment are needed to manage this risk. Limited visibility diving can cause disorientation, potentially leading to loss of sense of direction, loss of effective buoyancy control, etc. Divers in extremely limited visibility situations depend on their instruments such as dive lights, pressure gauges, compass, depth gauge, bottom timer, dive computer, etc., and guidelines for orientation and information. Training for cave and wreck diving includes techniques for managing extreme low visibility, as finding the way out of an overhead environment before running out of gas is a safety-critical skill.

Equipment

Technical divers may use diving equipment other than the usual single cylinder open circuit scuba equipment used by recreational divers. Typically, technical dives take longer than average recreational scuba dives. Because a decompression obligation prevents a diver in difficulty from surfacing immediately, there is a need for redundancy of breathing equipment. Technical divers usually carry at least two independent breathing gas sources, each with its own gas delivery system. In the event of a failure of one set, the second set is available as a back-up system. The backup system should allow the diver to safely return to the surface from any point of the planned dive, but may involve the intervention of other divers in the team. Stage cylinders may be dropped along the guideline for later use during the exit or for another dive.

Equipment configuration

The usual configurations used for increased primary gas supply are manifolded or independent twin back mounted cylinders, multiple side mounted cylinders, or rebreathers. Bailout and decompression gas may be included in these arrangements, or carried separately as side-mounted stage and decompression cylinders. Cylinders may carry a variety of gases depending on when and where they will be used, and as some may not support life if used at the wrong depth, they are marked for positive identification of the contents. Managing the larger number of cylinders is an additional task loading on the diver. Cylinders are usually labeled with the gas mixture and may also be marked with the maximum operating depth and if applicable, minimum operating depth.

Gas mixes

Technical diving can be done using air as a breathing gas, but other breathing gas mixtures are commonly used to manage specific problems. Some additional knowledge is required to understand the effects of these gases on the body during a dive and additional skills are needed to safely manage their use.

Deep air/extended range diving

One of the more divisive subjects in technical diving concerns using compressed air as a breathing gas on dives below 130 feet (40 m). Some training agencies still promote and teach courses using air up to depths of 60m. These include TDI, IANTD and DSAT/PADI. Others, including NAUI Tec, GUE, and UTD consider that diving deeper than 100–130 feet (30–40 m), depending upon agency, on air is unacceptably risky. They promote the use of mixtures containing helium to limit the apparent narcotic depth to their agency specified limit should be used for dives beyond a certain limit.

Such courses used to be referred to as "deep air" courses, but are now commonly called "extended range" courses. The 130 ft limit entered the recreation and technical communities in the USA from the military diving community where it was the depth at which the US Navy recommended shifting from scuba to surface supplied air. The scientific diving community has never specified a 130-foot limit in its protocols and has never experienced any accidents or injuries during air dives between 130 feet and the deepest air dives that the scientific diving community permits, 190 feet, where the U.S. Navy Standard Air Tables shifts to the Exceptional Exposure Tables. In Europe some countries set the recreational diving limit at 50 metres (160 ft), and that corresponds with the limit also imposed in some professional fields, such as police divers in the UK. The major French agencies all teach diving on air to 60 metres (200 ft) as part of their standard recreational certifications.

Deep air proponents base the depth limit of air diving upon the risk of oxygen toxicity. Accordingly, they view the limit as being the depth at which partial pressure of oxygen reaches 1.4 ATA, which occurs at about 186 feet (57 m). Both sides of the community tend to present self-supporting data. Divers trained and experienced in deep air diving report fewer problems with narcosis than those trained and experienced in mixed gas diving trimix/heliox, although scientific evidence does not show that a diver can train to overcome any measure of narcosis at a given depth, or become tolerant of it.

The Divers Alert Network does not endorse or reject deep air diving, but indicates the additional risks involved.

Mixtures to reduce decompression time

Nitrox is a popular diving gas mix, and while it is not used for deep diving, it reduces the buildup of nitrogen in the diver's tissues by increasing the percentage of oxygen in the breathing gas as a substitute for part of the nitrogen. The depth limit of a nitrox mixture is governed by the partial pressure of oxygen, which is generally limited to 1.4 to 1.6 bar depending on the activity of the diver and duration of exposure.

Nitrox and pure oxygen are also used for accelerated decompression.

Mixtures to reduce nitrogen narcosis

Increased pressure due to depth causes nitrogen to become narcotic, resulting in a reduced ability to react or think clearly. By adding helium to the breathing mix, these effects can be reduced, as helium does not have the same narcotic properties at depth. Helitrox/triox proponents argue that the defining risk for air and nitrox diving depth should be nitrogen narcosis, and suggest that when the partial pressure of nitrogen reaches approximately 4.0 ATA, which occurs at about 130 feet (40 m) for air, helium is necessary to limit the effects of the narcosis.

Mixtures to reduce oxygen toxicity

Technical dives may also be characterised by the use of hypoxic breathing gas mixtures, including hypoxic trimix, heliox, and heliair. A diver breathing normal air (with 21% oxygen) will be exposed to increasing risk of central nervous system oxygen toxicity at depths greater than about 180 feet (55 m) The first sign of oxygen toxicity is usually a convulsion without warning which usually results in death when the demand valve mouthpiece falls out and the victim drowns. Sometimes the diver may get warning symptoms prior to the convulsion. These can include visual and auditory hallucinations, nausea, twitching (especially in the face and hands), irritability and mood swings, and dizziness.

These gas mixes can also lower the level of oxygen in the mix to reduce the danger of oxygen toxicity. Once the oxygen is reduced below about 18% the mix is known as a hypoxic mix as it does not contain enough oxygen to be used safely at the surface.

Safety

Technical diving encompasses multiple aspects of diving, that typically share lack of direct access to surface, which may be caused by physical constraints, like an overhead environment, or physiological, like decompression obligation. In case of emergency, therefore, the diver or diving team must be able to troubleshoot and solve the problem underwater. This requires planning, situational awareness, and redundancy in critical equipment, and is facilitated by skill and experience in appropriate procedures for managing reasonably foreseeable contingencies.

Accident modes

Several factors are identifiable as predispositions to accidents in technical diving. The techniques and equipment are complex, which increases the risk of errors or omissions - the task loading for a CCR diver during critical phases of a dive is greater than for open circuit scuba equipment, The circumstances of technical diving generally mean that errors or omissions are likely to have more serious consequences than in normal recreational diving, and there is a tendency towards competitiveness and risk taking among many technical divers which appears to have contributed to some well publicized accidents.

Some errors and failures that have repeatedly been implicated in technical diving accidents include:

Accident statistics

There is very little reliable data describing the demographics, activities and accidents of the technical diving population, and conclusions about accident rates must be considered tentative. The 2003 DAN report on decompression illness and dive fatalities indicates that 9.8% of all cases of decompression illness and 20% of diving fatalities in the USA happened to technical divers. It is not known how many technical dives this was spread over, but it was considered likely that technical divers are at greater risk.

The techniques and associated equipment that have been developed to overcome the limitations of conventional single cylinder, open circuit scuba diving are necessarily, more complex and subject to error, and technical dives are often done in more dangerous environments, so the consequences of an error or malfunction are greater. Although skill levels and training of technical divers are generally significantly higher then those of recreational divers, there are indications that technical divers in general are at higher risk, and that closed circuit rebreather diving may be particularly dangerous.

Operations

Relatively complex technical diving operations may be planned and run like an expedition, or professional diving operation, with surface and in-water support personnel providing direct assistance or on stand-by to assist the expedition divers. Surface support might include surface stand-by divers, boat crew, porters, emergency medical personnel, and gas blenders. In-water support may provide supplementary breathing gas, monitor divers during long decompression stops, and provide communications services between the surface team and the expedition divers. In an emergency, the support team would provide rescue and if necessary search and recovery assistance.

Training

Technical diving requires specialised equipment and training. There are many technical training organisations: see the Technical Diving section in the list of diver certification organizations. Technical Diving International (TDI), Global Underwater Explorers (GUE), Professional Scuba Association International (PSAI), International Association of Nitrox and Technical Divers (IANTD) and National Association of Underwater Instructors (NAUI) were popular as of 2009. Recent entries into the market include Unified Team Diving (UTD), and Diving Science and Technology (DSAT), the technical arm of Professional Association of Diving Instructors (PADI). The Scuba Schools International (SSI) Technical Diving Program (TechXR – Technical eXtended Range) was launched in 2005.

British Sub-Aqua Club (BSAC) training has always had a technical element to its higher qualifications, however, it has recently begun to introduce more technical level Skill Development Courses into all its training schemes by introducing technical awareness into its lowest level qualification of Ocean Diver, for example, and nitrox training will become mandatory. It has also recently introduced trimix qualifications and continues to develop closed circuit training.