Deep-sea diving is an activity in which a person can descend beyond the recreational scuba diving limit of 40 m (130 ft). Beyond this depth, safer and more complex equipment must be used. Due to the logistical support and expenses required, deep-sea diving is normally done for science or profit.

Underwater Observation and Work
Deep-sea diving allows divers to perform underwater observation, surveys, or work. Diving scientists geologists, biologists, ecologists, physiologists, and archaeologists use deep-sea diving techniques and equipment both for profit and to further humanity's knowledge about the planet Earth. Commercial divers perform wreck surveys; submarine rescue operations; welding and cutting operations on pipelines, bridges, and platforms; and inspections of piers, platforms, breakwaters, dams, nuclear power plants, and sewer discharge lines. They also salvage valuable or polluting cargo from sunken ships. In all such diving, one factor must be overcome: exposure of the diver to cold water. Breathing-gas heaters, body heaters, bell-system insulation and heaters, dry suits with layered underwear, and warm-water suits are all used.

The Breathing Medium
Breathing gases are delivered using sophisticated systems similar to those worn by astronauts. Delivery may be by means of hoses connected to surface compressors or banks of gas bottles, or by means of a surface-supplied bell system, or from bottom-stationed habitats.

The use of compressed air is limited to depths of less than 76 m (250 ft). The toxic effects of oxygen and the narcotic effects of nitrogen become the depth-limiting factors. To extend the range of divers, various mixes of other gases with the atmospheric gases have been tried, including hydrogen, helium, argon, and neon. The difficulty of breathing dense gases at depth, along with dangerous physiological side effects of decompression sickness, or bends, when returning to surface atmospheric pressures, become depth-limiting factors as well.

Helium-oxygen mixes extend the range of divers to working depths of more than 660 m (2,165 ft) before the density of the helium becomes a problem. Hydrogen offers the potential for divers to go beyond the limits set by helium, but accurately controlling the minute percentages of oxygen required at such depths is both difficult and dangerous. Breathing gases may be dumped or else recirculated, filtered, and recharged with oxygen. Some systems are designed to aid divers in inhaling and exhaling the dense gases.

Diving Techniques and Equipment
Surface-tethered diving requires surface support systems and uses lightweight full-face masks, fiberglass helmets with neck seals, or heavy metal helmets attached to dry-suits. These also allow for diver-to-diver and diver-to-surface communications. Special systems protect divers from contaminated waters. To limit decompression times, one-atmosphere suits or submarines with manipulator arms are used if these systems can do the job.

For deep dives involving long-duration decompression schedules, so-called saturation diving systems are used. On some such jobs, divers are kept at dive-depth pressures in surface chambers for up to 60 days, then are delivered to the dive site by means of a diving bell that limits their exposure to the water and provides a measure of comfort and protection during the long descent and ascent. Two divers would rotate duties for up to 8 hours. Divers saturated on helium-oxygen can anticipate approxiately 24 hours of decompression for each 33 m (100 ft) of saturation depth. That is, 198 m (600 ft) of saturation depth equals 6 days of decompression.

One-atmosphere diving systems enable the diver to ascend directly to the surface without concern about the bends or delays for decompression. The systems provide life support by removing carbon dioxide and adding oxygen. The one-atmosphere "JIM" suit is named for Jim Jarrett, who made the first experimental dives with it in the 1920s. It and its successors are extremely heavy and require surface tethering, thereby limiting their mobility. Soft ocean bottoms can also limit their usefulness.

Submersibles with manipulator arms are also used for deep-ocean work, as are remote-operated vehicles (ROVs) equipped with television cameras, thrusters, and manipulator arms. In all surface-tethered systems, surface conditions and ocean currents play a major role in whether or not a dive can be made. Extensive surface support is necessary in all deep-ocean exploration techniques.

Lance Rennka

Bibliography: Gordon, Bernard, ed., Man and the Sea (1980; repr. 1994); National Oceanic and Atmospheric Administration, The NOAA Diving Manual, 4th ed. (2000); Sisman, David, ed., The Professional Diver's Handbook (1986); Miller, Jim, and Koblick, Ian, Living and Working in the Sea, 2d ed. (1995).

See also:
bathyscaphe; bathysphere; oceanography.