either exhale and sink, or submerge by raising his arms from his sides back over his head, thus forcing the body under water. Excessive movement may cause entanglement.

Kelp can easily be broken with a fingernail or pulled off. It is unwise to remove kelp from the regulator with a knife if the kelp is thick and the regulator hose cannot be seen or felt.

The diver who has run out of air and is forced to return to shore or a boat through a kelp bed has several choices. He may simply skin dive from opening to opening and if he needs a breath between openings he can come up through the main canopy. This is done easily with a little practice. He may also do a variation of the "dog paddle” in which the body is parallel to the surface and the swim fins used in a very close flutter kick. The arms reach across the kelp to an extended position then grasp the kelp and press down, with elbows close to the body to keep as streamlined a shape as possible. The regulator second stage should not trail behind the diver or it will become entangled in the kelp. Swim fins with adjustable heel straps may also pose a problem as the kelp fronds can become caught in the strap adjusting buckle and strap end. This may be eliminated by taping the strap end to the main strap. Knives worn on the belt or on the outside of the calf of the leg will also act as kelp catchers. Consequently, knives should be worn inside the calf. Inflating buoyancy compensators while under a kelp canopy may also increase the chance of entanglement

Egergia (ribbon kelp) grows from the intertidal to perhaps 45 feet in depth, and although quite thick it seldom forms the thick canopy associated with Macrocystis. “Elk kelp” (Pelagophycus porra), is a deeper-growing plant found usually in 45-60 feet to 150 feet of water. It consists of a single stipe, 1/2 inch across growing from a small holdfast. The stipe enlarges 3 to 6 feet from the float and is spherical and hollow in structure. The top of this ball gives rise to antler-like protrusions, each with several blades which may be 3 feet wide and 15 feet long. Elk kelp seldom reaches the surface in a healthy state but forms a bed 15 to 30 feet down. Often when the diver penetrates a large kelp bed in 60-90 feet of water, he will find a second canopy of elk kelp below the first, with a resulting drop in already low light levels.

All kelp beds are influenced by currents and surge and major beds may disappear in a swift current

as a result of being held down on perhaps a 45° angle. This has its advantages as the kelp streams in the direction of the current and may be used for navigation.

9.3.6 Wreck Diving

Diving around wrecks subjects the diver to many of the same hazards found in cave diving or diving under the ice.

Diving in or around wrecks exposes the diver to inherent dangers that must be considered both in planning the dive and in carrying it out. First, there may not be a direct route to the surface should an emergency arise; second, in many instances the diver will be venturing into an unknown area where risk of entanglement and reduced visibility due to stirred up silt are ever-present hazards. If the wreck is full of passageways, small entrances, chambers, etc., a line to the outside of the wreck should be used to insure a safe exit route. If warranted, because of depth and long duration dives, a spare air supply and regulator should be placed outside the wreck as a safety measure. Sharp objects inside a wreck are often hidden by algae, sea polyps or other marine growth. Decayed wood and corroded metal can collapse, and fishing nets, line, and hooks should be watched for. Underwater enclosures are havens for sea life of all types. Hazardous marine life should be considered at all times.

Remember, the only known way out is the way in.

9.3.7 Diving at High Elevations

An important factor to consider when diving at altitudes above sea level is the effect the increased elevation will have on various types of depth gauges. The bourdon tube and bellows type depth gauges will indicate a depth shallower than the actual depth, while the capillary tube type gauge will indicate a depth that is greater. Another factor that will affect the accuracy of the depth gauge is that the gauges are calibrated for use in seawater. Fresh water, being less dense than seawater, will result in the depth gauge reading slightly shallower than the same gauge would in seawater.

The bourdon tube and bellows depth gauges indicate the total ambient pressure (in air or water) minus 1 atmosphere, and are calibrated for sea level. As altitude increases, they will sense the decrease in atmospheric pressure, and, were it not

for the mechanical stop, actually would move backward. Consequently, this type of depth gauge will not indicate depth when submerged until the effect of the decreased atmospheric pressure is overcome, and, therefore, will always read shallower.

The capillary tube gauge utilizes the compression effect of water pressure on an entrapped quantity of air to indicate depth. If the air in the tube is of a lesser pressure than sea level when the gauge is submerged, such as when diving in the mountains, the entrapped air, while of the same volume, will be less dense. Consequently, it will take less water pressure to compress the entrapped air, and the depth gauge will indicate a depth greater than the actual depth.

Unless altitude and fresh water corrections are made to depth gauges, as discussed in the previous paragraphs, a descent line should be used to measure the depth of dive and decompression stops.

One of the criteria used for the computation and validation of the U.S. Navy Standard Air Decompres

sion Tables, was that they would be used at sea level where standard atmospheric pressure is 14.7 psi. Because atmospheric pressure varies significantly with altitude, decompression tables, depth of stops, rate of ascent and repetitive dive planning may have to be altered for safe diving at altitudes above 1000 feet. Sea level equivalent depths and rate of ascent may have to be altered based on dive site atmospheric pressure and density of water. Variations in pressure for a given altitude can be as much as 0.5 psi due to latitude and seasonal changes (U.S. Standard Atmospheric Supplements 1966). This is equivalent to 1000 feet of altitude or more based on standard atmospheric pressure. Variations in pressure for a given altitude can be as much as 1.0 psi due to daily weather conditions (Weatherwise Feb. 1971). This is equivalent to 2000 feet of altitude or more based on standard atmospheric pressure.

Upon surfacing, the lowered oxygen partial pressure may cause air hunger and pulmonary difficulties.

10.0 GENERAL

Compressed air is the most commonly used breathing medium. The medical and decompression aspects of air diving are well known, have been thoroughly tested, and indicate a margin of safety not yet achieved with mixed gas.

Several definitions are important to an understanding of various aspects of air diving. The most important of these are:

No-Decompression Diving. In no-decompression diving, a diver can return directly to the surface at a rate of 60 feet per minute without spending time at shallower depths to allow inert gas to be eliminated from the body (U.S. Navy Diving Manual 1973).

Decompression Diving. In decompression diving, a diver must return to the surface according to a decompression schedule, utilizing timed stops at shallower depths, which allows inert gas to be safely eliminated from the body.

Single Dive. A single dive is the first dive of the day conducted more than 12 hours after the completion of a previous dive.

Repetitive Dives. A repetitive dive is a dive performed within 12 hours of a previous dive. A repetitive dive may be a no-decompression repetitive dive or a decompression repetitive dive, depending on the depth and time of the repetitive dive and the previous dive.

Bottom Time. Bottom time is the total elapsed time beginning when a diver leaves the surface and ending when he begins ascent back to the surface. Surface Interval. The surface interval is the elapsed time between surfacing from the previous

dive and the time when the diver leaves the surface on the repetitive dive.

Decompression Stop. The decompression stop is the required time the diver must stop and hold his depth relatively constant, during ascent following a decompression dive. Decompression stops are tabulated in decompression tables.

Diving with air as the breathing medium can be conducted using a variety of life support equipment. The most widely used is the open-circuit scuba, where compressed air is carried by the diver. A diver may also use air supplied through an umbilical utilizing a full face mask, demand regulator, a lightweight diving helmet, or deep-sea diving equipment.

When entering the water on an air dive a diver can descend at any comfortable rate. However, there are several physiological reasons for limiting the rate of descent, among them the possibility of a squeeze; the inability to equalize pressure on both sides of the eardrum; pains in the sinuses; and a tendency toward dizziness, or narcosis which may occur when rapidly descending. Practical reasons for limiting the descent rate are a matter of prudent evaluation of the effects of current and a careful approach to an unknown bottom.

The 1973 edition of the U.S. Navy Diving Manual has incorporated changes in the standard air decompression tables. Due to confusion which resulted from table designation changes associated with frequent revisions, all table numbers have been eliminated. The tables are now designated by their name only.

10.1 NO-DECOMPRESSION DIVING

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