Archive for November, 2005

Open-circuit scuba are generally made up of the following basic components: cylinders, air reserve mechanism, demand regulator, breathing tubes, check valves, mask or mouthpiece (or both), and exhaust valve.

Cylinders

The cylinders, sometimes called bottles or flasks, contain compressed air and are made of either galvanized steel or aluminum. They are specially constructed to withstand the high internal pressure of compressed air, usually 2,150 pounds per square inch for the galvanized cylinders, and 3,000 pounds per square inch for the aluminum cylinder. The volume of the cylinders varies with the different arrangements in use. The general type is a 2-cylinder arrangement, although some European scuba use a 3-cylinder arrangement. The cylinders are usually worn on the back and fastened to the diver by an arrangement of waist, shoulder and crotch straps. A means for quick release of these straps for emergency ditching of the cylinders is provided. The cylinders are fitted to permit easy, rapid replacement of cylinders on the surface or under water.

The Air-Reserve Mechanism

An essential part of the scuba is an air-reserve mechanism to provide a positive warning to the diver when his air supply is becoming critically low. The most commonly used mechanism is an air-reserve valve which permits free flow of air to the regulator until the cylinder pressure falls to a critical level-approximately 300 to 500 pounds per square inch-depending on the type of scuba used. At this pressure the valve restricts the air, causing increased breathing resistance. At this signal the diver opens the air-reserve mechanism, restoring a free flow of air from the reserve air supply. This reserve air supply should be sufficient to allow a safe return to the surface, provided that the dive is not prolonged and that decompression is not needed. The means of opening the air-reserve mechanism varies with different types of scuba.

The Demand Regulator

The demand regulator is a device which controls the flow of compressed air from the cylinders to the diver. An efficient scuba should have a demand mechanism allowing the release of compressed air at a pressure equal to that of the surrounding water and at the rate required by the diver.

The sketch illustrates the basic principle of the demand regulator. When inhalation reduces the pressure in the air chamber below the pressure in the surrounding water, the diaphragm deflects toward the air chamber, depressing the lever and opening the valve on the air supply. So long as inhalation continues, the valve remains open and admits air to the system. When inhalation stops, rising pressure in the air chamber returns the diaphragm to its original position, and the air-supply valve closes.

Demand regulators are of 2 general types, single stage and double stage. The single-stage regulator furnishes the air directly to the diver in a single reduction from cylinder

pressure to a pressure equal to that of the surrounding water. The double-stage regulator is merely a single-stage unit mounted upon another single-stage unit called the first-stage regulator. The first-stage regulator provides an intermediate reduction in the gas pressure through a high-pressure valve to approximately 100 pounds per square inch more than the depth pressure. The second-stage regulator then provides the final reduction through a low-pressure valve prior to the release of compressed air to the diver.

Demand regulators may be mounted either on the cylinders or in the face mask. Cylinder-mounted demand regulators have an advantage in that they minimize the need for medium-pressure tubing, but because of the difference in water pressure between the location of the demand regulator and the effortless breathing point of the diver (at the base of the throat), they make inhalation slightly difficult until the body adjusts. Mask-mounted regulators minimize this water-pressure difference, making breathing comparatively easy and eliminating the need for a mouthpiece.

Practically all of the scuba in sports use is of the open-circuit type. This type derives its name from the fact that the breathing medium is used only once; the expired gases are discharged into the water during exhalation. Normally compressed air is the breathing medium used in open-circuit scuba, but it is possible to used mixed gases for deep dives. However, oxygen alone must never be used in open-circuit scuba.

Once you are under water, your life depends on the operation of your scuba, and you should fully understand its operation before using it. There are two basic systems in open-circuit scuba, the continuous-flow system and the demand system. The demand system is the one currently favored, and is the only one which has approval of the United States Navy. The 3 types which are in Navy use are the United States Divers aqualung; the Scott Hydro-Pak, and the Northill air lung.

The continuous-flow system is somewhat simpler in construction. Basically, it is a self-contained substitute for the ordinary air hose with continuous flow. A simple example is a shallow-water face mask furnished with air from a cylinder carried by the diver rather than from a surface supply through an air hose. Since the flow has to meet the demands of inspiration, and since it continues during expiration, the cylinder must provide at least twice the volume of “breathable” air. This waste will soon exhaust a portable cylinder. In some such scuba, a reservoir bag is arranged to accumulate incoming air during the expiratory phase, to permit larger air flow and reduce waste, and it is necessary to adjust the flow to meet changes in the diver’s respiration.

The more widely used demand system provides for periodic release of the compressed air as it is needed for the inspiration of the diver, and shuts off the flow during expiration. A demand regulator controls the release automatically. This is a special low-pressure regulator which maintains the breathing system at surrounding depth pressure, opening to slight negative pressure at the start of inspiration and remaining open only until the end of inspiration.

At Diving Depth

When you reach the diving depth, level off and orient yourself. Use any natural aids that are available such as sunlight, current, bottom, channel, etc. Check with a compass if available. Avoid underwater exertion and keep your activity to a practical minimum. Breathe continuously, as slowly and deeply as possible. At the first sign of breathlessness, slow down, or stop if possible. Catch your breath before starting any activity. If fighting a current does not let you slow down, break off the dive.

Be especially careful to watch out for entanglement around wreckage, lines or vegetation. When swimming with poor visibility, keep your hands extended ahead. For free diving wear a comfortable, satisfactory pair of swim fins. Use an efficient kick and maintain a steady pace geared to the ability of your buddy. Watch the depth and time carefully. Keep your buddy in sight and look at him frequently. Signal him before any change in direction. Be sure that he understands the signal and watch that he follows the maneuver.

“Line” diving calls for some cautions. When using a float line, keep it taut, but do not pull the float under the surface. Keep in mind that it can snag objects above you, so watch for entanglements. In any area where there is any possibility of boat traffic, make certain that a diver’s flag is firmly set on the float. If you are using a line held by another person, be sure the tender keeps the line taut. Signal him to slacken or tauten as necessary. Keep in communication with him by line pulls. Remember the possibility of entanglement when using a line. Avoid going through any small passages or near snags, and keep your knife in mind for emergency disentanglement.

The Ascent

At the end of time at any deep dive, signal your buddy or the line tender and start for the surface. Breathe continuously and naturally during the entire ascent. Never hold your breath.

Do not exceed the rate of ascent specified in the decompression table for the type of dive and equipment. If decompression is necessary, follow the table, using the proper techniques.

The generally considered safe rate of ascent from a dive is 60 feet per minute, and usually decompression should present no problems unless you go deeper than 40 feet. For greater depth diving, you should provide yourself with a United States Navy Decompression Table and follow its directions to the letter. One safe way of handling the decompression problem is to provide yourself with decompression depth markers. A weighted line with knots every 10 feet should work. Weight the line heavily enough to keep it completely vertical in a strong current. An extra air cylinder may be hung at the first decompression stop. If it becomes necessary to surface before getting full decompression, you can complete the decompression by returning to the water.

Diving Technique

Before starting a dive, carry out all the preliminary preparations required for the type of scuba you are using. The following is the minimum equipment for safe scuba diving: swim trunks or protective suit (with distinctive band of coloring), life preserver, belt and knife, swim fins, face mask, scuba. Two highly desirable pieces of equipment are a wristwatch and a depth gage. Other accessories that may be necessary are a weight belt, wrist compass and a buddy line.

Enter the water feetfirst, climbing down a ladder or easing over the side of the boat. Do not jump in. Stop at the surface and make the proper surface check for the type of scuba you are using. Check the scuba for satisfactory operation. Check your buddy’s scuba for leaks of any sort, and have him check yours. Check your face mask for proper seal to minimize flooding. Adjust your buoyancy.

Orient yourself with available natural aids such as sunlight, current, or landmarks. If you are planning to swim to a specific point, check the compass bearing of that point. When you are sure that you and your buddy are ready, check the time and start the dive. Make a slow, orderly descent.

Do not try to race your buddy down or outrun him. Swim or pull yourself headfirst down the descending line. Be sure that the pressure is equalizing in the ears and sinuses. Stop the descent if pain develops; level off or ascend slightly until the pressure equalizes. Discontinue the dive if the pressure does not equalize after several tries.

Numerical Signals

In underwater signaling, it is often necessary to be able to convey numerical information clearly and rapidly. The following signs are the digits from 0 through 9:

ZERO-Bend all of the fingers into a half circle, holding them together. Complete the circle by touching the tip of the thumb to the tip of the middle and index fingers. (Fig. 7.)

ONE-Extend the index finger. Fold the other 3 fingers lightly into the palm and lay the thumb across the middle finger. (Fig. 8.)

Fig. 1. Hold everything

Fig. 2. I am having trouble with my ear

Fig. 3. All right or All right?

Fig. 4. Let’s go up

Fig. 5. Pick me up

Fig. 6. Pick me up now

Fig. 7. Sign for 0

Fig. 8. Sign for 1

Fig. 9. Sign for 2

Fig. 10. Sign for 3

Fig. 11. Sign for 4

Fig. 12. Sign for 5

Fig. 13. Sign for 6

Fig. 14. Sign for 7

Fig. 15. Sign for 8

Fig. 16. Sign for 9

Fig. 17. How deep?

Fig. 18. What direction? or The time is

Fig. 19. What time? or Compass course?degrees

TWO-Extend the index and middle fingers, separating them somewhat. Fold the other 2 fingers lightly into the palm and lay the thumb across the ring finger. (Fig. 9.)

THREE-Extend the index and middle fingers and the thumb, separating them somewhat. Fold the other 2 fingers down at the knuckle. (Fig. 10.)

FOUR-Extend all 4 fingers, separating them somewhat. Lay the thumb across the palm to touch the base of the little finger. (Fig. 11.)

FIVE-Extend all 4 fingers and the thumb, separating them slightly. (Fig. 12.)

six-Hold the nail of the little finger with the thumb. Extend the other fingers, separating them slightly. (Fig. 13.)

SEVEN-Hold the nail of the ring finger with the thumb. Extend the other fingers, separating them slightly. (Fig. 14.)

EIGHT-Hold the nail of the middle finger with the thumb. Extend the other fingers, separating them slightly. (Fig. 15.)

NINE-Hold the nail of the index finger with the thumb. Extend the other fingers, separating them slightly. (Fig. 16.)

To indicate a number larger than 9, give the individual digits of the number in the order that you would write them from left to right. For example to say “The time is 1435″ (use the military time system), you would tap your wrist and give the signs for 1, 4, 3, and 5 in that order.

Other Signals

Certain other signs are valuable for conveying basic information:
HOW DEEP? or DEPTH??FEET.-Extend one arm to the side,
holding the hand palm down. Swing the forearm horizontally back and forth about 60 degrees. (Fig. 17.)
WHAT DIRECTION? or COMPASS COURSE??DEGREES.
-Close the hand and extend the thumb. Twist the hand about the wrist to the right and left several times. When signaling the compass course, always give it as a 3-digit number. For example, to say “Compass course 045,” make the sign for compass course, followed by the digits 0, 4, and 5. (Fig. 18.)
WHAT TIME? or THE TIME is??.-Crook the index finger of
one hand and tap it several times on the back of the other hand at the wrist. When signaling the time of day, always give it as a 4-digit number. (Fig. 19.)

The buddy system is possibly the biggest single safety factor in scuba diving, and the lone wolf in this sport is gambling with his life. Each of the pair of buddies is responsible for the other’s safety throughout a dive. The system calls for you to keep continuous contact with your buddy. Where visibility is good, keep him in sight at short range. Where visibility is poor, use a short buddy line to link each other together. Know the standard diving signals and any special signals. Watch for any signal from your buddy. Acknowledge it promptly. Be as alert to help him as he is to help you! If he shows any sign of distress, whether he signals or not, get to him at once. Find out what the trouble is and take action as necessary.

Never separate from your buddy unless he is hopelessly entangled and you must leave him in order to get help.

Visual Signals

Communication between buddies and among scuba divers, is by a series of visual signals, although an underwater walky-talky is being developed that will provide diver-to-diver voice communication under water. The following are the standard visual signals for scuba divers (see drawings on pages 80 and 81) :

Carbon-monoxide poisoning happens infrequently in scuba diving and its only common cause is contaminated compressed air in open-circuit scuba. Symptoms-the diver’s becoming disoriented or unconscious, especially during ascent-may not appear until the diver approaches the surface. Bring the victim to the surface and give him fresh air. Get him out of the water if possible and give him oxygen to breathe. If he is not breathing, give him artificial respiration. To prevent carbon-monoxide poisoning, you must be sure of the purity of the breathing gas. Keep compressor air intakes away from engine exhausts. The maximum allowable concentration of carbon monoxide in compressed air for scuba use is 20 parts per million, or 0.002 per cent.

Excess carbon dioxide may accumulate in the breathing system of closed-circuit scuba if the absorption system fails, or if the diver exceeds the capacity of the absorption system by overexertion. The accumulation may be gradual. The usual symptoms are awareness of increased breathing, light-headedness, sleepiness, dizziness, faintness, blurring of vision, and difficulty in hearing. In some instances, rapid increase of carbon dioxide may cause the diver to lose consciousness without becoming aware of these usual warning signals. The treatment is simple, provided that there is no complication such as drowning. If your buddy develops symptoms at depth, flush his breathing bag to wash out any retained carbon dioxide. Bring him to the surface and let him breathe air. Exposure to the air is all that is required if the victim is breathing. If he is not breathing, give him artificial respiration and administer oxygen. The aftereffects rarely include more than headache, nausea and fatigue.

Another possible problem with closed-circuit scuba is oxygen deficiency. This may occur because the system was not properly purged, or because the supply gas is not pure oxygen. In semiclosed-circuit scuba, oxygen deficiency may be a considerable hazard. The supply contains inert gas, and the failure of the injector system can allow the oxygen to fall to a critically low point. Here again, the symptoms may only appear during ascent. The diver may lose consciousness, stop breathing, and die if he does not have oxygen in short order.

Under water, give the victim more oxygen in his breathing medium. In closed-circuit scuba, flush his bag with oxygen. In semiclosed-circuit scuba, flush his bag with mixed gas and continue to flush it periodically. He will recover rapidly if he is still breathing. If the victim is not breathing, get him out of the water, remove his scuba, and start artificial respiration immediately. Provide oxygen if available.

Proper maintenance of any equipment in which it can occur will minimize the hazards of oxygen deficiency. Bends, or decompression sickness, are the result of formation of gas bubbles in the blood or tissue. They will not occur unless a diver comes up too rapidly from depth. The best prevention is to observe all precautions to avoid a too rapid ascent from any deep dive. The symptoms are pain, twinging, numbness, or nervous system disturbances, and the treatment is by recompression in a recompression chamber.

There are certain inherent medical hazards in scuba diving. But knowing them can make it safer, and most of them can be avoided by carefully observing the safe limits of duration and by limiting the depth of dives to 100 feet.

Drowning is perhaps the most frequent cause of death in self-contained diving. It can happen for many reasons. The most common cause is physical exhaustion resulting from swimming after surfacing. Other common causes are exhaustion of gas supply, entanglement, flooding of the apparatus, and loss of mask or mouthpiece.

The best measures to prevent drowning are thorough training in emergency procedures, ability to swim well, and the avoidance of panic.

Air embolism-a condition resulting from excess pressure in the lungs-is probably the second most common cause of scuba fatalities. When a man loses his air supply under water, he has an overwhelming instinct to hold his breath and surface immediately. The lack of adequate exhalation during ascent in panic creates excessive pressure in the lungs. This condition has produced air embolism in less than 15 feet of water. Increased lung pressure may also occur in a normal ascent if the diver fails to breathe continuously.

Air embolism may usually be prevented by thorough training in scuba. Learn to avoid hazardous situations and to handle emergencies without panic. Breathe continuously during ascent from depth so that overpressurization of the lungs will not occur. Treatment calls for recompression in a recompression chamber, or the use of oxygen tanks and immediate return to the water so that a gradual return to the surface can be made. However, treatment must be immediate, and in the absence of a recompression chamber, treatment in the water presents many difficulties and risks.

Overexertion is another serious hazard to the scuba diver. Muscular exercise increases the breathing rate and can eventually increase it enough to cause a sensation of inadequate lung ventilation. This sensation can occur even in the free air, and is unpleasant under any conditions. If the scuba restricts breathing, the sensation occurs more readily and also becomes terrifying. The breathing response to a burst of activity may not occur immediately. If the response is delayed, it does not adequately warn a man when he is exceeding his powers. When the response finally does occur, he must stop and pant for some minutes. A period of rest is the best way to relieve the condition. If complete rest is not possible, slacken your activities as much as possible. Remember that overexertion can readily make breathing difficult and that muscular fatigue may not occur before shortness of breath. Untrained divers, especially inefficient swimmers, tend to panic under these conditions and try to surface.

When you engage in any underwater work be prepared for the subsequent shortness of breath. At depths beyond 100 feet, the scuba diver faces the problem of possible nitrogen narcosis. Symptoms of this condition resemble drunkenness-an almost complete loss of judgment and skill. At the greater depths, fatigue, exertion, and carbon-dioxide build-up increase the susceptibility to nitrogen narcosis. There is no treatment for nitrogen narcosis. The effect diminishes as the diver leaves the diving depth and vanishes before he reaches the surface. If it is desired to make deep dives, the diver may lessen the effect of possible nitrogen narcosis by exerting strong will power and self-control and by slowing down his activity.

Many scuba divers have compared the “thrill” of their sport to that of piloting a plane. The scuba diver is exposed directly to the underwater environment. He has no contact with the surface and depends entirely on his breathing apparatus and its limited air supply. Even though he is diving with a buddy (a basic rule of scuba diving), he must face most of his problems alone. These conditions demand an ability to adjust mentally to diving. The mobility of scuba diving is perhaps its greatest appeal. The diver has no bulky equipment to hamper his actions. At neutral buoyancy he can swim under water in any direction. He can cover considerable distances unaided, and with the use of any of a number of propulsive devices he can greatly increase his operating range. Depth control is another major advantage of scuba. There is little buoyancy in the equipment. This eliminates the need for carrying heavy weights. As a result, the scuba diver can maintain or change his depth at will. He can cruise under water at safe depth, can search deep areas from shallow depths, can explore underwater caves and travel under ice floes. The water is his domain.

Limitations of Scuba

Any person who undertakes scuba diving should be aware of its limitation of duration. The most important factor in this limitation is the gas supply in the apparatus. In open-circuit scuba the gas supply seldom lasts over 3 hours at the surface, and exhausts in much shorter time at any depth. In closed-circuit or semiclosed-circuit scuba, the gas supply may last 4 hours or more, but the absorptive capacitiy of the canister rarely exceeds 3 hours under any exertion. These durations are the outside limits. Under normal conditions, the safe duration of a scuba is 2 hours or less.

The second important limitation is that of depth. The United States Navy has found that in open-circuit scuba the increase of air consumption with depth limits the apparatus to 130 feet for reasonable working dives. Nitrogen narcosis and decompression limit the open-circuit apparatus to 200 feet, even for short dives. In closed-circuit or semiclosed-circuit scuba, oxygen tolerance imposes very restrictive limits on depth. Also, scuba limits exertion to some extent. In open-circuit scuba, the main limitation is breathing resistance. In closed-circuit or semiclosed-circuit scuba it is usually canister capacity.

The next development was the patenting of devices for open-circuit scuba, providing a supply of air for the use of the diver, rather than treating the oxygen to allow rebreathing of the same supply. In 1925, Commander Le Prieur of the French Navy developed a self-contained unit with cylinders of compressed air rather than oxygen. The apparatus was basically an open-circuit scuba. However, its defect lay in the fact that the flow of air was regulated manually by the diver. This feature resulted in excessive use of the limited air supply.

The Aqualung

The final step toward a scuba outfit that would meet the needs of water sportsmen came in 1943, when Commander Cousteau, another French naval officer, brought out the Cousteau-Cagan aqualung. This device also utilized cylinders of compressed air, but was equipped with a demand regulator which adjusted the air pressure automatically and supplied air to the diver as needed. Basically this equipment was identical to Rouquayrol’s except that it had a much larger air supply. The cylinder of the aqualung held high pressure air (2,000 pounds per square inch) rather than the lower pressure air (500 pounds per square inch) available to Rouquayrol. The greater air supply naturally gives the diver a much longer time beneath the surface. Since 1943 many individuals and companies have developed demand regulators based on Rouquayrol’s principles, and with minor changes this type of open-circuit equipment is in wide use today.

Scuba diving is one form of water sport that has commercial possibilities. Propeller and ship bottom inspections are natural scuba functions. Recovery of equipment and almost any form of underwater work are possible with scuba at depths down to 200 feet, although at that depth the underwater time is extremely short. In some areas scuba divers have worked with oyster and clam fishermen. In addition to the sheer enjoyment of underwater swimming and exploration, scuba has added new dimension to spearfishing, underwater photography, salvage and wreck exploration, and the collection of specimens for marine biology study.

The establishment in 1954 of the United States Naval School, Underwater Swimmers, in Key West, Florida, has done much to increase the safety of scuba diving. Many alumni of the school now in civilian life are instructing others in safe scuba diving, and the Y.M.C.A. has based much of its scuba training on the work done at that school.