The most useful snippets from our authors, all in one place. DEPTH discusses topics of diving, equipment and environment, physics and physiology, technique and technology, and hyperbarics.

Salvaging Lives Through Dive Training with Fred Johnson

Salvaging Lives Through Dive Training with Fred Johnson

What do the California Prison Industry Authority (CALPIA), commercial dive training, and the NOAA Diving Manual 5th Edition textbook have in common? We found the answer when we were recently introduced to Fred Johnson of CALPIA by Dan Orr, author of Scuba Diving Safety and former President and CEO of Divers Alert Network.

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Drysuits 101

Drysuits 101

Drysuits provide the greatest form of passive thermal protection for the diver. They are designed as one piece suits with a waterproof zipper for entering and exiting and have attached boots and seals at the diver’s wrists and neck to provide a dry internal environment. The suits are normally designed so a wide variety of insulating undergarments may be worn beneath them. These undergarments trap a layer of air providing the primary protection against cold. Too much air trapped in the drysuit can create buoyancy problems because the air forms a “bubble” that will move inside the suit, but some air is needed in order prevent the suit material from compressing and catching skin in the folds and causing a suit squeeze. A suit “squeeze” can be uncomfortable but is avoidable by adding a small amount of air to remove any suit wrinkles.

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Sinuses and Diving

Sinuses and Diving

The term “sinus” can mean any channel, hollow space, or cavity in a bone, or a dilated area in a blood vessel or soft tissue; most often sinus refers to the four, paired, mucus-lined air cavities in the facial bones of the head. The same kind of membrane lines the sinuses and nose, so nasal infections spread easily to the sinuses. In sinusitis, mucous membranes inflame and swell, closing sinus openings and preventing infected material from draining. If nasal inflammation, congestion, deformities, or masses block sinus openings, the sinus lining swells and inflames, absorbing pre-existing gas that forms negative pressure. When blockage occurs during descent, the relative vacuum in the sinus increases the risk of damage. Hemorrhage into the sinus and then into the divers mask may occur.

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Basic Underwater Navigation

Basic Underwater Navigation

Basic underwater navigation by means of simple observation or use of a compass and depth gauge remains a fundamental and essential skill for all divers. For most short excursions, these are the only instruments needed. Even when using advanced navigation instruments, basic navigation skills provide an important backup.

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Five Basic Rules for Cave Diving Safety

Five Basic Rules for Cave Diving Safety

Cave diving is a specialized form of diving that can be performed in both inland freshwater caves and oceanic “blue holes.” To scientists, caves offer new laboratories for research. In cave diving, the emphasis should be placed on developing the proper psychological attitude, training in specialized techniques and life-support systems, dive planning, and the selection of an appropriately trained buddy diver.

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The Rescue of a Convulsing Diver

The Rescue of a Convulsing Diver
Convulsions in the Water: Dive Accident Management and Emergency Procedures

A convulsion in itself rarely causes injury, but the secondary consequences for a scuba diver can be disastrous. First, the intense muscle contraction of the neck and jaw can cause the diver to spit out the mouthpiece, which is difficult to reinsert. Consequently, the diver is likely to drown unless rescued quickly. There is a risk of pulmonary barotrauma leading to AGE (arterial gas embolism) if a diver ascends too rapidly or out of control; however, the threat of drowning outweighs that of AGE.

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Safety Considerations for Diving Around Sharks

Safety Considerations for Diving Around Sharks

The danger from sharks to humans is a combination of size, aggression, and dentition. Any shark over 3 ft (0.9 m) long should be regarded cautiously, and if over 8 ft (2.4 m) long, should be avoided even if this requires that the diver leave the water. For example, grey reef sharks (Carcharhinus amblyrhynchos) that range between 3–7 ft (0.9–2.1 m) in length are numerous in shallow tropical waters, and diving operations often cannot be performed unless the presence of sharks in the area is tolerated. When such sharks are in the vicinity, divers should avoid making sudden or erratic movements. Common sense dictates that no injured or distressed animals should be in the water because these are known to precipitate shark attacks. When operations are conducted in the presence of sharks, each group of divers should include one diver who keeps the sharks in view and is alert for changes in their behavior. The chances of trouble are minimal as long as the sharks swim slowly and move naturally. The situation may become dangerous, however, if the sharks assume agitated postures, such as pointing their pectoral fins downward, arching their backs, or elevating their heads. If feeding in a group, sharks may become highly agitated and bite at anything and everything, including each other. Most victims are attacked violently and without warning by single sharks. The first contact may be a “bumping” or an attempt by the shark to wound the victim prior to the definitive strike. Severe skin abrasions and lacerations can be caused in this manner due to the abrasiveness of shark skin, which is covered with denticles, small tooth-like projections which are modified placoid scales.

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Nitrox Breathing Mixtures

Nitrox Breathing Mixtures

Divers have used air as a breathing gas since the beginning of diving. Its principal advantages are that it is readily available and inexpensive to compress into cylinders or use directly from compressors with surface-supplied diving equipment. Air is not the “ideal” breathing mixture for diving because of the decompression liability it imposes. Since decompression obligation is dependent on exposure to inspired PN2 (nitrogen partial pressure), this obligation can be reduced by replacing a portion of the nitrogen content of the diver’s breathing gas with oxygen, which is metabolized by the body. This is the fundamental benefit of enriched air nitrox (Nx) diving (Wells 1989). Historically, the two most commonly used nitrox mixtures in NOAA have been 32% and 36% oxygen. Once called NOAA Nitrox 32 (NN32) and NOAA Nitrox 36 (NN36), such mixtures are now identified using a more general nomenclature as Nx32 and Nx36. The remaining gas in nitrox mixes is considered to be nitrogen, even though it may contain other inert gases like argon. “Nitrox” is a generic term that can be used for any gaseous mixture of nitrogen and oxygen, but in the context of this chapter, the implication is that nitrox is a mixture with a higher concentration of oxygen than that of air. Using such oxygen enhanced mixtures can significantly increase the amount of time a diver can spend at depth without incurring additional decompression when compared to air diving.

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Contingency Planning

Contingency Planning

Performing repetitive dives requires a the use of a dive plan. The diver must know what the no-stop dive time limits will be for the dives prior to descending so as not to incur additional decompression obligations. A planned dive schedule will work assuming the diver adheres to the maximum depth and time parameters defined before descending; however, this does not always occur. There are many reasons why divers may find themselves deeper than planned. Some of these might include: higher than normal tides while working on a specific site, down-welling currents, the need to descend deeper to pick up tools or experimental apparatus that may have been dropped, the unexpected need to provide assistance to divers who are working at deeper depths (either on a routine or emergency basis), or perhaps just plain inattention of the divers.

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Thermal Stress Irrespective of Ambient Temperature

Thermal Stress Irrespective of Ambient Temperature

Thermal Stress Irrespective of Ambient TemperatureHypothermia is not a problem exclusive to frigid environments—it can occur irrespective of ambient temperature. Similarly, divers may also suffer extremes of hot and cold thermal stress simultaneously during the same dive. There have been documented cases of severe heat exhaustion in arctic waters by commercial divers as a result of wearing thick, occlusive drysuits, aggravated by dehydration from breathing dry compressed gas and perspiring from prolonged underwater swimming or heavy underwater work. Perspiration from excessive or from pre-dive overheating can also cause the diver’s drysuit underwear to lose insulation, thus predisposing him to hypothermia.

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