Scuba Diving

The self-contained underwater breathing apparatus or scuba diving system, as we know it today, is the result of technological developments and innovations that began almost 300 years ago. Scuba diving is the most extensively used system for breathing underwater by recreational divers throughout the world, and in various forms is also widely used to perform underwater work for military, scientific and commercial purposes.

Advantages and Disadvantages

Scuba diving has many advantages over free diving, mixed gas, helmeted, saturation, and other forms of “technical” diving. Scuba divers have great freedom of movement under water because they swim with fins and without heavy equipment; The gear is relatively inexpensive, simple to operate and maintain, and requires a small support crew, or none at all.

Despite all of these apparent advantages, recreational scuba also has its drawbacks. These include no direct link between the diver and the surface; no method of communicating with the diver or monitoring his activities; limited dive time (since the diver must carry all of his air in a tank); and limited depth (since decompression diving is normally avoided due to the limited quantity of air in the tanks).

Essential Equipment

In addition to a mask and fins, basic recreational scuba equipment consists of a cylinder of compressed air attached to a two-stage "demand regulator." The regulator lowers the air pressure in “steps” from the cylinder and dispenses it to the diver as needed.

Cylinders for scuba diving are made of steel or aluminum alloy, and are designed to operate safely at pressures ranging from 2,250 to 3,500 psi (pounds per square inch). As a means of comparison, air pressure at sea level is only about 15 psi. One of the most commonly used types of diving cylinders is made of aluminum alloy, and has a capacity (the quantity of gas that can be compressed into the cylinder) of 80 cubic ft. The amount of time that it takes a diver to use up all of the air in the dive cylinder is dependent on several factors, including, the diver’s breathing rate, and the depth to which the diver descends (the deeper the dive, the greater the amount of air used). All cylinders used by scuba divers should be inspected internally at least once a year for damage and corrosion.

The primary function of the “demand regulator” attached to the diving cylinder is to reduce the high-pressure gas supplied by the scuba cylinder to the ambient pressure surrounding the diver at depth. If the diver were to breath compressed air directly from the cylinder, it could easily rupture his lungs. The reduction of air pressure from the diving cylinder to the diver is accomplished in two steps. The first stage of the regulator, which attaches to the cylinder valve, reduces the high pressure in the cylinder to an intermediate pressure approximately 140 psi over ambient pressure. This intermediate pressure fills a low-pressure hose that connects the first stage of the regulator to the second stage. The second stage, contained in the diver’s mouthpiece, reduces the intermediate pressure to the ambient pressure. The regulator is known as a “demand regulator” because it only supplies air when the diver “demands” it; that is, gas flows through the regulator only when the diver inhales.

Critical for Safety

Other devices, while not directly involved in the breathing circuit of the recreational scuba diver, are nonetheless critical for safety. These include a pressure gauge, depth gauge, and dive timer. These instruments inform the diver about the amount of air left in the cylinder, their depth in the water, and how much time has been spent underwater. A diver who exceeds the prescribed depth or time spent underwater, may become susceptible to nitrogen narcosis, and/or decompression sickness, which can be fatal.

Two additional items generally considered essential for the scuba diver are a “BC” or “BCD” (buoyancy compensator device) and a dive knife. Almost all BCs are worn like a vest, and include a band for mounting the air cylinder. The BC contains an air bladder that the diver inflates or deflates to maintain control over buoyancy, thereby avoiding uncontrolled ascents and descents.

In addition to the BC, few divers enter the water without a dive knife. Most divers carry them in the event that they become entangled in a line, net, or some other gear, and need to cut themselves free. A dive knife can also be used as a signaling device by banging it against a dive cylinder.

Technical Diving

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 (other 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 ft, technical divers may work in the range of 170 ft to 350 ft, 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.

Nitrox

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% 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.

Essential Equipment

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% and 36% oxygen by volume. This differs significantly from compressed air, which contains approximately 21% oxygen by volume. While an increase of 12 to 16% 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.

Mixed-Gas Diving

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 (helium 79% and oxygen 21%) 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

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; SSD operations require a large support crew and a great deal of equipment.