Deep diving
Deep diving is underwater diving to a depth beyond the normal range accepted by the associated community. In some cases this is a prescribed limit established by an authority, while in others it is associated with a level of certification or training, and it may vary depending on whether the diving is recreational, technical or commercial. Nitrogen narcosis becomes a hazard below and hypoxic breathing gas is required below to lessen the risk of oxygen toxicity.
For some recreational diving agencies, "Deep diving", or "Deep diver" may be a certification awarded to divers that have been trained to dive to a specified depth range, generally deeper than. However, the Professional Association of Diving Instructors defines anything from as a "deep dive" in the context of recreational diving, and considers deep diving a form of technical diving. In technical diving, a depth below about where hypoxic breathing gas becomes necessary to avoid oxygen toxicity may be considered a deep dive. In professional diving, a depth that requires special equipment, procedures, or advanced training may be considered a deep dive.
Deep diving can mean something else in the commercial diving field. For instance early experiments carried out by COMEX using heliox and trimix attained far greater depths than any recreational technical diving. One example being its "Janus 4" open-sea dive to in 1977.
The open-sea diving depth record was achieved in 1988 by a team of COMEX and French Navy divers who performed pipeline connection exercises at a depth of in the Mediterranean Sea as part of the "Hydra 8" programme employing heliox and hydrox. The latter avoids the high-pressure nervous syndrome caused by helium and eases breathing due to its lower density. These divers needed to breathe special gas mixtures because they were exposed to very high ambient pressure.
An atmospheric diving suit allows very deep dives of up to. These suits are capable of withstanding the pressure at great depth permitting the diver to remain at normal atmospheric pressure. This eliminates the problems associated with breathing pressurised gases. In 2006 Chief Navy Diver Daniel Jackson set a record of in an ADS.
On 20 November 1992 COMEX's "Hydra 10" experiment simulated a dive in an onshore hyperbaric chamber with hydreliox. Théo Mavrostomos spent two hours at a simulated depth of.
Depth ranges in underwater diving
Assumed is the surface of the waterbody to be at or near sea level and underlies atmospheric pressure.Not included are the differing ranges of freediving – without breathing during a dive.
| Depth | Comments |
| Recreational diving limit for divers aged under 12 years old and EN 14153-1 / ISO 24801-1 level 1 standard. | |
| Recreational diving limit for Open Water Divers. | |
| Recreational diving limit for EN 14153-2 ISO 24801-2 level 2 "Autonomous Diver" standard. | |
| GUE Recreational Diver Level 1. | |
| Recommended recreational diving limit for PADI Advanced Open Water divers and GUE Recreational Diver Level 2. Average depth at which nitrogen narcosis symptoms begin to be noticeable in adults. | |
| Depth limit for divers specified by Recreational Scuba Training Council and GUE Recreational Diver Level 3. Depth limit for a French level 2 diver accompanied by an instructor, breathing air. | |
| Depth limit for divers breathing air specified by the British Sub-Aqua Club and Sub-Aqua Association. | |
| Depth limit for a group of 2 to 3 French Level 3 recreational divers, breathing air. | |
| Depth at which breathing compressed air exposes the diver to an oxygen partial pressure of. Greater depth is considered to expose the diver to an unacceptable risk of oxygen toxicity. | |
| One of the recommended technical diving limits. Maximum depth authorised for divers who have completed Trimix Diver certification with IANTD or Advanced Trimix Diver certification with TDI. | |
| Deepest scuba dive on compressed air. | |
| Limit for surface light penetration sufficient for plant growth in clear water, though some visibility may be possible farther down. | |
| First dive on a hydrox-rebreather. | |
| Deepest ocean dive on a rebreather. | |
| Deepest cave diving on a rebreather. | |
| Deepest dive on a rebreather. | |
| Deepest scuba dive, deepest dive on trimix. | |
| COMEX Hydra 8 dives on hydreliox. |
Particular problems associated with deep dives
Deep diving has more hazards and greater risk than basic open-water diving. Nitrogen narcosis, the "narks" or "rapture of the deep", starts with feelings of euphoria and over-confidence but then leads to numbness and memory impairment similar to alcohol intoxication. Decompression sickness, or the "bends", can happen if a diver ascends too rapidly, when excess inert gas leaves solution in the blood and tissues and forms bubbles. These bubbles produce mechanical and biochemical effects that lead to the condition. The onset of symptoms depends on the severity of the tissue gas loading and may develop during ascent in severe cases, but is frequently delayed until after reaching the surface. Bone degeneration is caused by the bubbles forming inside the bones; most commonly the upper arm and the thighs. Deep diving involves a much greater danger of all of these, and presents the additional risk of oxygen toxicity, which may lead to convulsions underwater. Very deep diving using a helium-oxygen mixture or a hydrogen-helium-oxygen mixture carries the risk of high-pressure nervous syndrome and hydrogen narcosis. Coping with the physical and physiological stresses of deep diving requires good physical conditioning.Using open-circuit scuba equipment, consumption of breathing gas is proportional to ambient pressure – so at, where the pressure is, a diver breathes six times as much as on the surface. Heavy physical exertion makes the diver breathe even more gas, and gas becomes denser requiring increased effort to breathe with depth, leading to increased risk of hypercapnia – an excess of carbon dioxide in the blood. The need to do decompression stops increases with depth. A diver at may be able to dive for many hours without needing to do decompression stops. At depths greater than, a diver may have only a few minutes at the deepest part of the dive before decompression stops are needed. In the event of an emergency, the diver cannot make an immediate ascent to the surface without risking decompression sickness. All of these considerations result in the amount of breathing gas required for deep diving being much greater than for shallow open water diving. The diver needs a disciplined approach to planning and conducting dives to minimise these additional risks.
Many of these problems are avoided by the use of surface supplied breathing gas, closed diving bells, and saturation diving, at the cost of logistical complexity, reduced maneuverability of the diver, and greater expense.
Limiting factors
In ambient pressure diving the work of breathing is a major limitation. Carbon dioxide elimination is limited by the capacity of the diver to cycle breathing gas through the lungs, and when this reaches the maximum, carbon dioxide will build up in the tissues and the diver will succumb to acute hypercapnia. Work of breathing is affected by breathing gas density, which is a function of the gas mixture and the pressure due to depth.In atmospheric pressure diving the limitations include the ability of the diver to bend the joints of the suit under pressure, and for the joints to remain watertight while bending.
Dealing with depth
Both equipment and procedures can be adapted to deal with the problems of greater depth. Usually the two are combined, as the procedures must be adapted to suit the equipment, and in some cases the equipment is needed to facilitate the procedures.Equipment adaptations for deeper diving
The equipment used for deep diving depends on both the depth and the type of diving. Scuba is limited to equipment that can be carried by the diver or is easily deployed by the dive team, while surface-supplied diving equipment can be more extensive, and much of it stays above the water where it is operated by the diving support team.- Scuba divers carry larger volumes of breathing gas to compensate for the increased gas consumption and decompression stops.
- Rebreathers, though more complex, manage gas much more efficiently than open-circuit scuba.
- Use of helium-based breathing gases such as trimix reduces nitrogen narcosis and reduces the toxic effects of oxygen at depth.
- A diving shot, a decompression trapeze, or a decompression buoy can help divers control their ascent and return to the surface at a position that can be monitored by their surface support team at the end of a dive.
- Decompression can be accelerated by using specially blended breathing gas mixtures containing lower proportions of inert gas.
- Surface supply of breathing gases reduces the risk of running out of gas.
- In-water decompression can be minimized by using dry bells and decompression chambers.
- Hot-water suits can prevent hypothermia due to the high heat loss when using helium-based breathing gases.
- Diving bells and submersibles expose the diver to the direct underwater environment for less time, and provide a relatively safe shelter that does not require decompression, with a dry environment where the diver can rest, take refreshment, and if necessary, receive first aid in an emergency.
- Breathing gas s reduce the cost of using helium-based breathing gases, by recovering and recycling exhaled surface supplied gas, analogous to rebreathers for scuba diving.
- The most radical equipment adaptation for deep diving is to isolate the diver from the direct pressure of the environment, using armoured atmospheric diving suits that allow diving to depths beyond those currently possible at ambient pressure. These rigid, articulated exoskeleton suits are sealed against water and withstand external pressure while providing life support to the diver for several hours at an internal pressure of approximately normal surface atmospheric pressure. This avoids the problems of inert gas narcosis, decompression sickness, barotrauma, oxygen toxicity, high work of breathing, compression arthralgia, high-pressure nervous syndrome and hypothermia, but at the cost of reduced mobility and dexterity, logistical problems due to the bulk and mass of the suits, and high equipment costs.