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 inhaled under high pressure during the deep phase of the dive. These dissolved gases must be released slowly from body tissues by pausing or “doing deco stops” at various depths during the ascent to the surface.

In recent years, most technical divers have greatly increased the depth of the first stops to reduce the risk of bubble formation before the more traditional, long, shallow stops. Most technical divers breathe enriched oxygen breathing gas mixtures such as nitrox during the beginning and ending portion of the dive. To avoid nitrogen narcosis while at maximum depth, it is common to use trimix which adds helium to replace nitrogen in the diver’s breathing mixture. Pure oxygen is then used during shallow decompression stops (max depth 6 meters) to reduce the time needed by divers to rid themselves of most of the remaining excess inert gas in their body tissues, reducing the risk of “the bends.”

Surface intervals (time spent on the surface between dives) are usually required to prevent the residual nitrogen from building up to dangerous levels on subsequent dives, but this is no different from conventional scuba diving.

4) Different Gas mixes

Breathing normal air (with 21 percent oxygen and 79% Nitrogen) at depths greater than 180 feet (55 m) creates a high risk of oxygen toxicity. Technical dives may also be characterised by the use of hypoxic breathing gas mixtures other than air, such as trimix, heliox, and heliair. The first sign of oxygen toxicity is usually a convulsion without warning which usually results in death, as the breathing regulator 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. Increasing pressure due to depth also causes nitrogen to become narcotic, resulting in a reduced ability to react or think clearly (see nitrogen narcosis).

HELIUM / TRIMIX

By adding helium (He) to the breathing mix, divers can cut these effects significantly, as helium does not have the same narcotic properties at depth. 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 18 percent the mix is known as a hypoxic mix as it does not contain enough oxygen to be used safely at the surface. Trimix divers only start and stop using the trimix mixes at certain depths, this is why they need to carry different cylinders with different gasses on the dives.

NITROX EAN

Nitrox (EAN) is another common gas mix, and while it is not used for deep diving, it decreases the build up of nitrogen within the diver’s body by increasing the percentage of oxygen. This reduces the nitrogen percentage, as well as allowing for a greater number of multiple dives compared to standard air. The depth limit of nitrox is governed by the percentage of oxygen used, as there are multiple oxygen percentages available in nitrox. (such as a EAN 50 can only be used from 0 – 21 meters) . Please note that further training and knowledge is required to use safely and understand the effects of these gases on the body during a dive.

Deep air extended range diving

One of the more divisive subjects in technical diving concerns using compressed air (21% O2 and 79% Nitrogen) as a breathing gas on dives below 40m. While some diving training agencies still promote and teach such courses (TDI, IART and PADI), a minority (NAUI Tec, GUE & UTD) argue that diving deeper on air is unacceptably risky, saying that helium mixes should be used for dives beyond a certain limit (30–40 m), depending upon agency.

Such courses used to be referred to as “deep air” courses, but are now commonly called “extended range” courses.  This 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 incorporated the 40 meter limit into its protocols and has never experienced any accidents or injuries during air dives between 40 meters and the deepest air dives that the scientific diving community permits,58 meters, where the U.S. Navy Standard Air Tables shifts to the Exceptional Exposure Tables.

DIFFERENT DEPTHS IN DIFFERENT COUNTRIES

In Europe some countries set the recreational diving limit at 50 meters, 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 meters as part of their standard recreational certifications.

OXYGEN AND DEEP DIVING

Deep air proponents base the proper 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 (PPO2) reaches 1.4 ATA, which occurs at about 57 m. Helitrox/triox proponents argue that the defining risk should be nitrogen narcosis, and suggest that when the partial pressure of nitrogen reaches about 4.0 ATA, which occurs at about 40 m, helium is necessary to offset the effects of the narcosis. Both sides of the community tend to present self-supporting data.

Divers trained and experienced in deep air diving report less 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. Some people just choose “deep air” diving as Helium becomes more scares thus more expensive.

The Divers Alert Network (DAN) does not endorse or reject deep air diving, but indicates the extra risks involved and always recommend proper training.