Diver Down - February Atlantis Divers SCUBA Show
February 15th, 2009
The February edition of Diver Down - Atlantis Divers SCUBA Show is available for your listening pleasure! Sit back, relax & enjoy as we take you to meet David M. Hay, Director of Dive Operations at Atlantis Divers. Hear about his adventures and what’s new on our online store.
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How to Become a Certified Diver
January 12th, 2009
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Scuba Pro? The Goliath has Been Brought to It’s Knees
January 12th, 2009
January 6, 2009
Johnson Outdoors to cut jobs
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RACINE, Wisconsin (5 Dec 2008) — Johnson Outdoors Inc. said Friday after it swung to a large loss in its fourth quarter that it plans to slash spending, cut jobs and suspend its quarterly dividend in response to the global economic slump, which is hurting sales of boats and other products.
The outdoor-recreation company, which makes everything from canoes and kayaks to scuba diving equipment, said it is looking to reduce operating costs and spending by more than $30 million, including $20 million in cost-saving projects and reducing capital expenditures.
The Milwaukee-based company said it will also eliminate about 7 percent of its jobs, or around 90 positions, freeze wages, and suspend its quarterly dividend. The company also is in breach of the net worth covenant in its debt agreements, and is working with its banks to amend the agreements.
Johnson Outdoors said it swung to a large loss in the fiscal fourth quarter, as the U.S. recession and accompanying economic weakness led the company to write down the value of its assets by $41 million, and sales in most of its product categories were weakened by “the rapid and steep economic downturn.”
It posted a loss of $74.6 million, or $8.18 per share, in the three months that ended Oct. 3. That compares with a year-ago profit of $942,000, or 10 cents per share. Revenue dipped 6 percent, to $81.8 million from $87.3 million, as sales in the marine-electronics business and watercraft business each fell 11 percent, and diving revenue also decreased.
The company reported $29.5 million in marine-electronics revenue, down from $33 million a year ago, and watercraft revenue slipped to $16.3 million from $18.4 million. Diving revenue declined 3 percent, to $26 million from $26.8 million, due to unfavorable foreign-currency exchange.
Faced with declining sales of its Scubapro diving equipment and other products, Johnson Outdoors Inc. announced plans to slash spending, cut jobs and suspend its quarterly dividend
Outdoor-equipment revenue edged up to $10 million from $9.4 million, as the company said it saw some growth in the military, commercial and consumer sectors.
Johnson Outdoors employs about 1,340 people, and announced the elimination of 60 jobs in fiscal 2008 after relocating a manufacturing facility and downsizing a Binghamton, N.Y., plant that made military tents.
The company’s third-quarter dividend was 5.5 cents for holders of Class A shares and 5 cents for holders of Class B shares.
Other cost-reducing moves include consolidating its dive-computer making and lowering data-management costs by 20 percent.
For the full year, the company lost $71 million, or $7.81 per share, compared with a profit of $9.2 million, or $1 per share, a year ago. Full-year revenue came to $420.8 million, down 2 percent from the previous year.
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Safety Equipment & Building on Your Gear
January 12th, 2009
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Diver Down - January Atlantis Divers SCUBA Show
January 11th, 2009
The January edition of Diver Down - Atlantis Divers SCUBA Show is available for your listening pleasure! Sit back, relax & enjoy as we take you to meet two Atlantis Divers staff members, we talk about diving in Costa Rica and we look at what’s new in our dive shop.
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Scuba Diving And The Irukandji Syndrome
January 10th, 2009
An Irukandji jellyfish causes a painful and life-threatening sting giving symptoms collectively called the Irukandji Syndrome. Little is known about these tiny, deadly jellyfish that has been found in waters popular for scuba diving, such as Australia’s Great Barrier Reef.
Irukandji Syndrome
About six different jellyfish are thought to produce the Irukandji Syndrome.
The term ‘Irukandji’ refers to an Australian Aboriginal tribe that inhabited the Palm Cove region of northern Queensland, Australia, where this illness is most common.
The Irukandji Syndrome has been observed throughout the Pacific basin and recently off Florida.
What Is An Irukandji Jellyfish
The first known jellyfish to cause the Irukandji Syndrome was discovered in 1964 by Jack Barnes, a doctor who thought the syndrome was caused by a tiny jellyfish. He spent hours in the water of Cairns in North Queensland, Australia until he found a tiny specimen. He used this jellyfish to sting himself, his son and a life saver. They all ended up in hospital. The jellyfish was named in his honour: Carukia Barnesi
The only other known Irukandji jellyfish is the Malo kingi, named after Robert King who died from a sting off Port Douglas in north Queensland, Australia in 2002.
Irukandji jellyfish are very small, in the shape of a cube with a single tentacle hanging off each corner of the square. The entire structure of body and tentacles is, on average, only around 50 millimetres long.
It is nearly transparent making it difficult to see in the water. Because they are so small, the stinger nets used to protect swimmers from the Box Jellyfish, or Sea Wasp, don’t keep out the Irukandji jellyfish.
Irukandji Jellyfish Venom
The venom from an Irukandji jellyfish takes a while to impact. It is usually felt as a slightly painful irritant like a rash. After about 30 minutes, the venom takes affect. The symptoms of the Irukandji Syndrome are:
- severe backache or headache
- shooting pains in the muscles, chest and abdomen
- nauseous
- anxious or restless
- vomiting
- sweating
- fast heart beat and high blood pressure
It is thought that the impact on the heart that leads to fatality.
Treatment Of Irukandji Stings
Because very little is known about the Irukandji jellyfish, a definitive first aid treatment has not been developed. If any of the above symptoms occur, and the scuba diver is in a known Irukandji area and season, flush the area of the sting with copious amounts of vinegar. The patient must be taken to a hospital immediately. (Source: reef.crc.org and University of Melbourne.)
Irukandji And Risks For Scuba Divers
Unlike the Box Jellyfish that are found close to the mainland, Irukandji jellyfish mainly inhabit the deeper waters of the reef. This can be a risk for scuba divers and snorkellers, as many divers believe they are safe diving on the reefs away from the mainland.
Irukandji jellyfish can be washed in shore on the prevailing winds and tides.
Preventing Irukandji Stings
Prevention from Irukandji stings is of the utmost importance. Full coverage is the best protection. However, a full length wetsuit is not always practical when scuba diving in the warm tropical waters of Australia’s Great Barrier Reef. In these situations, a full length lycra suit may suffice. This will also provide protection against other stingers such as Fire Coral.
A scuba diver should always have vinegar in their gear bag when diving in the Irukandji season and locations.
Because Irukandji jellyfish are so rare, the chances of being stung are very slight. However, because the consequences of a sting are significant, scuba divers should take precautions against these deadly jellyfish.
http://scuba-diving.suite101.com/article.cfm/scuba_diving_and_the_irukandji_syndrome
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Almost a heart attack!!!! Or the importance of a full dive brief!
January 8th, 2009
| Comments from Atlantis Divers:
This is such a great blog post we found. Let this experience be a reminder to us all. These folks were very lucky everyone made it out ok. |
Hi
Just like to share a recent dive i had leading a dive on the zenobia.
I was diving with 5 guests, 2 of which are regular posters on this site. after a great first dive on the zen, exploring the outside of the wreck, we had a decent hour and a half surface interval before the second dive. After descending, we entered the canteen and my plan was to come out through a side window. unfortunately - and totally my fault - i hadn’t briefed the others to follow me through the same window. after exiting the wreck, and realising only 3 others had followed me through, i proceded to poo in my wetsuit. fortunately, i then realised that the other 2 had exited through another door, but only after 10 minutes of searching for them inside. The need for a good briefing is absolutely essential, especially when diving in an overhead environment.
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Diving with non-air gas mixtures: Nitrox, Heliox, Trimix
January 6th, 2009
WHAT IS NITROX?Nitrox is a gas mixture of oxygen and nitrogen, but with a higher oxygen percentage than found in ordinary air. As a result of its higher oxygen concentration, the percentage of nitrogen in nitrox is always lower than in air. There are two standard mixtures of Nitrox recognized by NOAA for diving: Nitrox I and Nitrox II (see Table 1).
| TABLE 1. COMPOSITION OF AIR, NITROX I AND NITROX II | ||
| % OXYGEN | % NITROGEN | |
| Air | 21 | 79 |
| Nitrox I | 32 | 68 |
| Nitrox II | 36 | 64 |
| * Included are trace inert gases such as argon, krypton, and neon which together make up less than one percent. |
Nitrox is usually prepared by mixing pure oxygen from one source (e.g., a tank of 100% oxygen) with air, until the desired oxygen concentration is reached (either 32% or 36%). Adding oxygen to air always lowers the percentage of nitrogen in the final nitrox mixture, because the sum of gas percentages cannot add up to more than 100%. The process requires quality control to assure the desired oxygen concentration is reached, and that the two gases are thoroughly mixed in whatever container holds the nitrox.
Synonyms of nitrox include “enriched air nitrogen” (EAN) and “oxygen-enriched air” (OEA). No matter what it is called, nitrox is not air and should not be called air. Also, when nitrox is discussed in relation to a specific dive profile it must always be qualified with the exact percentage of oxygen used; nitrox I and nitrox II have different risks.
WHY NITROX?Nitrox provides a lower percentage of nitrogen than ordinary air. In this way less nitrogen will enter the body at a given depth and decrease the risk of two nitrogen-related problems: decompression sickness (DCS) and nitrogen narcosis. Both DCS and nitrogen narcosis result from increased nitrogen pressure, the former from bubble formation on ascent and the latter from nerve inhibition at depth.
It cannot be over emphasized that nitrox does NOT allow one to go deeper than with air. Instead, the decreased nitrogen percentage provides two advantages, which define the two principal reasons nitrox is used:
1) as a time extender for dives to recreational depths, i.e., ability to dive longer at a given depth than allowed for by standard air tables, without increasing the risk of developing DCS;
2) to lessen the risk of developing DCS for dive profiles that adhere to the standard air tables.
Nitrox evolved from military and commercial diving, both considered professional activities. In professional settings the blending process is tightly controlled, divers are highly trained, and a hyperbaric chamber is likely to be available at the diving site. In non-professional use of nitrox, it has become clear that a principal potential hazard is oxygen toxicity.
Going to any depth increases the partial pressure of oxygen that is inhaled. The recreational diver who goes to 130 fsw on compressed air has a blood oxygen (and nitrogen) pressure almost five times that at sea level. While at depth, the principal hazard of excess nitrogen pressure is nitrogen narcosis. But there is also an increased risk of oxygen toxicity. The oxygen pressure at 130 feet when diving with ordinary air would lead to oxygen toxicity if the diver stayed long enough. Because the recreational dive tables allow for only a short time at 130 feet, oxygen toxicity is not a problem.
When the percentage of inhaled oxygen is increased the risk of oxygen toxicity increases, particularly with deeper dives (100 to 130 feet). With nitrox you can stay at a given depth longer because there is less nitrogen in your body (and hence less risk of narcosis and DCS), but stay too long and you risk oxygen toxicity. Since the first manifestation of oxygen toxicity can be seizures, diving with nitrox can lead to drowning.
Simply put, nitrox is a two-edged sword: less nitrogen good, more oxygen bad. Oxygen toxicity is the limiting factor when diving with nitrox. Thus, to the usual major concerns associated with scuba diving (DCS and AGE), nitrox adds the possibility of oxygen toxicity. Anyone diving with nitrox needs to be aware of this potential hazard. Table 2 compares the atmospheres of oxygen (Atm. O2) inhaled with air and with Nitrox I, when diving to 100 fsw and 130 fsw.
| TABLE 2. ATM. O2 WITH AIR AND NITROX I | ||
| depth | air (21 % O2) | Nitrox I (32 % O2) |
| 100 fsw | 0.80 | 1.29 |
| 130 fsw | 0.98 | 1.58 |
WHY ISN’T NITROX ROUTINELY USED BY RECREATIONAL DIVERS?For several reasons the use of nitrox by recreational divers is controversial. Nitrox has to be specially prepared; proper use requires special training; and there are differences in philosophy about what should be included in the purview of recreational diving.
As nitrox entered into non-professional diving a significant problem became apparent: poor quality control. The same quality control achievable in military, scientific and commercial applications was not always found in facilities promoting nitrox to the recreational diver.
Compressed air for scuba diving is universally available because, basically, air is air. The air compressor used to fill a tank to 3000 psi either works or it doesn’t. There is little to go wrong. Impurities can enter compressed air but that problem is preventable by good maintenance and proper positioning of the compressor with respect to environmental exhausts.
Nitrox is different. Nitrox has to be specially prepared, and without good quality control the resulting mixture may not contain the specified amount of oxygen. You can be sure that compressed air has 21% oxygen because that’s the composition of air, but you can’t be sure a tank of Nitrox I has 32% oxygen, and not 30% or 34%. A couple of percentage points one way or the other could make a difference in the safety of the dive. Nitrox experts recommend that the composition of tank gas be measured before each dive. Because of the need for special equipment (in preparing the mixture and then measuring its makeup), most dive shops are not equipped to offer nitrox.
A second reason nitrox is not routinely used is that it requires additional training. Air-trained divers are not trained to dive with nitrox. Nitrox diving requires more education about the risks of oxygen toxicity and the importance of measuring the gas composition before diving. No air-certified diver should use nitrox unless accompanied by a qualified nitrox instructor (or has otherwise completed an accredited training course).
The third reason has to do with differences in philosophy. Some people in the dive industry believe nitrox complicates the recreational experience, and that it should be used only by the “technical” diver, never by the casual diver who puts on scuba gear once or twice a year. People of this philosophy readily concede the potential benefits of nitrox, but believe the potential hazards far outweigh those benefits for recreational divers and the recreational diving industry. They are openly afraid the casual diver will abuse the mixture, that the typical dive shop will not be able to maintain adequate quality control, and that the overall result will be an increase in diving accidents and bad publicity for the industry. These people don’t want to see nitrox promoted to the recreational diver.
On the opposite side are many scuba enthusiasts who see this attitude as regressive, as inhibiting growth of the industry and enrichment of the scuba experience. They are passionate about the benefits of nitrox, and feel anyone who is certified for open water can be properly trained in use of nitrox, and that it should be widely available. They point out that exceeding the limits can (and does) happen with compressed air, and feel nitrox is no more dangerous than air if used properly. They also point out how scuba has changed dramatically in the last three decades, and see nitrox as just one more evolutionary advancement whose “time has come.”
In truth, both groups have good arguments. Nitrox would complicate the scuba experience, and not just for the occasional diver, but for the dive operator who must make it available. On the other hand, nitrox does offer advantages if properly used. It is probably just a matter of time before nitrox becomes an option to compressed air at many dive resorts. Like other aspects of recreational diving (for example, the dive computer), nitrox can be a great benefit if divers learn to use it wisely and do so.
For all the reasons indicated, nitrox diving comes under the general heading of technical diving. It is a semantic argument whether or not nitrox should instead be part of “recreational diving.” Anyone who chooses to dive with nitrox should just be aware of its benefits and potential hazards, and obtain proper training. As long as those goals are accomplished, it is not particularly relevant what label the activity goes by.
BUT IS NITROX DANGEROUS?Nitrox has actually received “bad press” from much of the recreational dive community, for reasons explained above. The bad press is largely undeserved because nitrox, per se, is not dangerous. In fact, its purpose is to make diving safer than with compressed air, by lowering the risk of decompression sickness. Problems occur when nitrox is abused, such as staying longer on deep dives or going deeper than allowed. The hazards include:
- using an improperly mixed gas composition
- going deeper than allowed
- staying longer than allowed at a given depth
If all nitrox divers limited themselves to 130 feet and used the standard air tables, nitrox would be safer than diving with air, as the risk of decompression sickness and nitrogen narcosis should be lower. This is a big “if.” After all, one of the two reasons divers use nitrox is because it allows them to stay down longer than with compressed air (since less nitrogen is taken up for a given time at depth).
| 1. You are diving with nitrox I to a depth of 60 feet. Your computer, designed for air use only, states you can stay at this depth for 40 minutes. Since you are using nitrox, you can: a. stay an extra 5 minutes b. stay an extra 10 minutes c. stay no longer than your computer indicates d. do what you want |
| 2. For nitrox I at recreational depths and times, compared to compressed air, all the following are true except one a. less risk of nitrogen narcosis b. more risk of oxygen toxicity c. less risk of decompression sickness. d. less risk of arterial gas embolism e. more risk of improper air mix |
| 3. Valid reasons for objecting to use nitrox in recreational diving include all of the following except one: a. a greater cost than air b. more difficult to prepare c. special knowledge and training required d. greater risk of Type II decompression sickness e. greater risk of oxygen toxicity |
TEST YOUR UNDERSTANDING Answers
WHO CERTIFIES FOR NITROX DIVING?In the past decade many reliable Nitrox training facilities have opened up around the country. Many of these are regular dive shops that have decided to enter the field of nitrox diving; they train divers under the auspices of a national certification agency.
For a while there were only two agencies which sanctioned nitrox diving and arranged for standards of use: ANDI (American Nitrox Divers International) and IANTD (International Association of Nitrox and Technical Divers). (See Appendix B.) Most recently NAUI and PADI have developed training programs for nitrox. No doubt by the end of the decade there will be many nitrox certifying agencies, probably as many as currently certify for basic open water diving.
WHAT IS HELIOX?Heliox is a mixture of helium and oxygen used for very deep diving, usually to greater than 200 feet. Helium’s great advantage is that it does not lead to nitrogen narcosis. Helium diving requires as much or more decompression time as nitrogen, so there is no saving there. Beyond 300 feet heliox may cause the ‘high pressure nervous syndrome’, a shaking sensation that can be incapacitating. Another disadvantage of helium is that it conducts heat about six times faster than nitrogen, so divers get colder than with air diving. A third problem is caused by the fact that helium is much less dense than nitrogen or air; as a result, the vocal cords vibrate much faster and divers sound like Donald Duck. Professional divers can use voice unscramblers to make their speech intelligible.
Overall, helium offers no advantage for recreational divers. Diving with heliox is strictly for technical and professional divers.
WHAT IS TRIMIX?Trimix is a mixture of oxygen, helium and nitrogen. Nitrogen, usually in a small percentage (e.g., 15%), is added back to heliox to create trimix, in order to lessen the risk of the high pressure nervous syndrome seen with helium breathing. Nitrogen slows down nerve conduction.
Trimix is used for the deepest scuba dives, usually greater than 400 feet. Like Heliox, Trimix is strictly for non-recreational use: military, scientific, commercial, and advanced technical diving.
ARE THERE OTHER GAS MIXTURES USED FOR SCUBA DIVING?Several other gas mixtures have been used, such as hydrogen-oxygen, argon-oxygen, and neon-oxygen. These mixtures are all in the realm of technical and experimental diving. The goal with any non-air mixture, of course, is to dive deeper or longer than can safely be accomplished with compressed air. It is apparent that, for a long time to come, recreational diving as we know it will be done only with mixtures of oxygen and nitrogen.
1. c. Unless your computer is designed to incorporate nitrox diving, you must follow the standards for air diving.
2. d. Less risk of arterial gas embolism is not afforded by nitrox.
3. d. There is no greater risk of Type II DCS with nitrox.
REFERENCES AND BIBLIOGRAPHYSee references for SECTIONS B-E, plus the following.
Mount, T, Gilliam B: Mixed Gas Diving. Watersport Publishing Co., San Diego, 1993.
Betts EA. Introduction to Enriched Air Diving. American Nitrox Divers Association, 74 Woodcleft Avenue, Freeport, N.Y. 11520; 1994.
http://www.lakesidepress.com/pulmonary/books/scuba/sectionl.htm
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Diving Medicine Articles
January 6th, 2009
Diving Medicine Articles OXTOX: If You Dive Nitrox You Should Know About OXTOX
DAN discusses the dangers of oxygen toxicity when using nitrox as a breathing gas
By Dr. E.D. Thalmann, DAN Assistant Medical Director; Captain, Medical Corps, U.S. Navy (retired)
It’s a fact: we need oxygen to live. It’s because of the way our cells use oxygen that we are able to breathe, exercise, and even think. In each of our cells, structures called mitochondria take the oxygen which diffuses in from our blood, disassemble it into its two component atoms (remember, oxygen - O2 - is composed of two oxygen atoms), and then hook some available hydrogen nuclei to them to form water.
The process releases energy, which is used for all functions of life. The problem is that in disassembling the oxygen molecule, it involves a step in which an extra electron is hooked on. This forms an intermediate called a superoxide anion, and this is a bad actor. It is highly reactive, and it will make mincemeat out of most other molecules it comes in contact with.
These anions are like coals in a furnace: as long as they are contained, we get lots of safe chemical energy; if they get out we get a great deal of damage. The mitochondria are designed to contain these superoxide anions, but just in case some get loose, there are a host of protective chemical reactions designed to sop them up and prevent them from doing any damage.
Besides producing excessive amounts of the superoxide anion, elevated tissue oxygen levels also affect a variety of other biochemical reactions which may affect oxygen toxicity in ways that are only beginning to be understood. Tissue-protective mechanisms and biochemical reactions are tuned to life in an atmosphere containing 21 percent oxygen, or 0.21 atmospheres absolute (ata) oxygen partial pressure. (See sidebar: “Remember Partial Pressure?”, page 34.) As the partial pressure increases above this comfortable 0.21 ata, protective mechanisms are slowly overwhelmed and biochemical reactions are affected. This may eventually result in “oxtox,” or oxygen toxicity.
Oxtox - What Is It?
Oxygen toxicity is a time duration phenomenon: that is, both time and partial pressure play a role. If an oxygen partial pressure of 2 ata is breathed for a few minutes, there would probably not be any problem. But, breathing it for an hour, might cause problems. This is why oxygen exposure limits are given as partial pressure/time limits. As the partial pressure gets higher, the recommended exposure time gets shorter.
What kind of problems might breathing a high oxygen partial pressure cause? It is the lungs and the brain which are the target organs of major concern in diving oxygen toxicity. Oxygen toxicity in the lungs (pulmonary oxygen toxicity) is like getting a bad case of the flu, but it will rarely cause permanent damage. The most common situation in which pulmonary oxygen toxicity might occur is during very long recompression treatments.
Oxygen toxicity of the brain, commonly referred to as central nervous system (CNS) oxygen toxicity, is different. It can occur during actual diving, and when it does, it can ruin your day - and possibly more. Some symptoms of CNS oxygen toxicity include flashing lights in front of the eyes, tunnel vision, loud ringing or roaring in the ear (tinnitus), confusion, lethargy, a feeling of nausea or vertigo, areas of numbness or tingling, and muscular twitching, especially of the lips.
These CNS symptoms are inconvenient, and a warning to change to a breathing gas with a lower oxygen partial pressure as soon as possible, but do not put the diver at risk of injury at this point. The big daddy of CNS symptoms does, however. It is the full-blown grand mal convulsion. During a convulsion, a diver will thrash about, perhaps bang his head into something hard, or if underwater, may lose his mouthpiece. The result can be trauma or drowning.
The good news is that convulsions are rare; the bad news is that all the inconvenient CNS symptoms noted above do not always provide warning of an impending convulsion. In some cases, a convulsion may occur without any warning at all. One more piece of good news: the convulsion in and of itself is not harmful, so if you don’t crack your head or drown, you should have no permanent damage.
By now you’re probably asking where these dire descriptions are leading.
To a better understanding, we hope, of diving on nitrox. As air-breathing sport divers need to know about decompression sickness (DCS), divers using high oxygen in nitrogen mixtures (nitrox) need to know about oxygen toxicity. (To read more about nitrox, see Alert Diver, January/February 1996, p.32.)
Both decompression sickness and oxygen toxicity are rare occurrences; they can be made rarer with good diving practices. With DCS, it’s using your table or computer conservatively and keeping the ascent rate down. With oxtox, it’s paying attention to the partial pressure and the amount of exposure time.
The main thing we’re discussing here is CNS oxygen toxicity, because this is the most dangerous kind. Lung oxygen toxicity is unlikely to be a problem for recreational divers, so it will be mentioned only in passing.
Remember Partial Pressure?
The partial pressure of a gas is a measure of the number of molecules in a given volume - the molecular concentration. The physiological effects of a gas are due mainly to its partial pressure, no matter what the total pressure is.
If a gas has only one component, say 100-percent oxygen, the partial pressure and the pressure are the same. If there is a gas mix, then the partial pressure is the gas fraction times the total pressure. A 50 percent oxygen-in-nitrogen mix has an oxygen partial pressure (pO2) of 1.0 atmosphere absolute (ata) at a depth of 33 feet / 10 meters where the total pressure is 2 ata.
At this depth the 50 percent oxygen would have the same physiological effect as 100 percent oxygen at the surface. Breathing a 100 percent oxygen mix at a depth of 33 feet / 10 meters (2 ata total pressure) would be equivalent to breathing the 50 percent mix at 132 feet / 40 meters (5 ata total pressure).
Royal Navy Studies
The grand old man of CNS oxygen toxicity is Professor Kenneth Donald, who cut his teeth on the problem during World War II in Great Britain. (Want to know more? Read Reference 1, page 40.) At that time the Royal Navy was under pressure to develop the technology used by the Italians to severely damage the battleships HMS Queen Elizabeth and HMS Valiant in the harbor of the port city of Alexandria, Egypt, in 1941.
Italian divers wearing 100 percent oxygen rebreathers, drove a torpedo close into a ship. While submerged to avoid detection, they detached its warhead under the ship’s hull, and beat a hasty retreat after a timer was set.
The Royal Navy soon began developing its own band of underwater divers called “Charioteers” to carry out similar missions. Dr. Donald was assigned as a Surgeon Lieutenant to provide medical care during training of the divers using the British 100 percent oxygen rebreathers. The accepted safe limits for breathing 100 percent oxygen at the time (2 hours at 50 feet / 15 meters, 30 minutes at 90 feet / 27 meters) produced enough convulsions that the British Admiralty decided some sort of studies were needed to define the scope of the problem and, hopefully, find a solution.
About to be transferred to the Shetland Islands, Dr. Donald had a change of fortune and proceeded instead to a facility just outside of London, where he found himself heading up a major research effort to get a handle on the problem of CNS oxygen toxicity.
Royal Navy Discoveries
Over the next three years, Dr. Donald’s team conducted literally hundreds of exposures on human volunteers (remember, there was a war on). This series of studies formed the basis of what we know about CNS oxygen toxicity, namely:
As a result of these studies, the Royal Navy considered it unsafe to breathe 100 percent oxygen below a depth of 25 feet / 7.6 meters (an oxygen partial pressure of 1.76 ata). In fact 25 feet / 7.6 meters was the shallowest depth tested. No particular time limit was given for this exposure, but the longest time tested was two hours. The carbon dioxide absorbent canisters of the diving rigs of the day rarely lasted more than 90 minutes.
The Royal Navy made deeper dives by using nitrogen-oxygen mixtures in the newly developed semi-closed circuit rebreathers. This was the beginning of so-called “mixed-gas diving,” where the breathing gas is mixed from oxygen and nitrogen rather than simply being compressed from atmospheric air.
U.S. Navy Studies
In the 1950s, Dr. E.H. Lanphier, then a Lieutenant in the U.S. Navy Medical Corps, undertook a series of studies at the Navy Experimental Diving Unit (NEDU), located at that time in Washington, D.C., to investigate whether oxygen exposure limits could be developed for 100 percent oxygen dives deeper than 25 feet / 7.6 meters. Table 1 (below) shows the limits that he recommended. The 100 percent oxygen exposure limits in Table 1 remained in use up to 1970 and with only slight modifications were used through 1991 when they were again changed.
Dr. Lanphier was also charged with investigating how these limits should be applied to the oxygen partial pressures encountered in mixed-gas nitrox diving. During nitrox diving, oxygen partial pressures similar to those used in 100 percent oxygen diving may be encountered, but since nitrogen has been added, these partial pressures are reached at a greater depth and, therefore, at a greater breathing gas density.
U.S. Findings
From his studies, Dr. Lanphier concluded that the increased gas density encountered during mixed-gas nitrox diving required the exposure times at a given oxygen partial pressure to be shorter than for 100 percent oxygen rebreathers, which can be used only at shallow depths, and which result in a lower gas density. The reason for this decreased tolerance during nitrox diving was thought to be due to decreased carbon dioxide elimination at the greater depths, resulting in higher blood carbon dioxide levels. This would make the diver more sensitive to oxygen toxicity.
These U.S. Navy nitrox mixed-gas nitrogen-oxygen exposure limits are shown in Table 2 (page 36). Notice that compared to those for 100 percent oxygen breathing in Table 1, these are quite a bit shorter for the same partial pressure. With the advent of closed-circuit oxygen rebreathers, the U.S. Navy no longer uses nitrox scuba and no longer publishes nitrox exposure limits in their official diving manual.
The Conflict - and Some Good Advice
The British disagreed with Dr. Lanphier’s findings, and the Royal Navy set exposure limits for nitrox diving that were no different than for 100 percent oxygen diving. This area remains controversial - Dr. Donald’s case for keeping the exposure limits the same for both 100 percent oxygen and nitrox diving has weaknesses and should not be accepted as proven.
Dr. Lanphier’s work is certainly compelling enough that divers should be very cautious before extrapolating oxygen exposure limits based on 100 percent oxygen rebreathing directly to nitrox diving at higher gas densities. Ideally, nitrox limits should be tested at the maximum gas density anticipated for their use.
CO2 Retention
Why would carbon dioxide (CO2) retention become a problem at increased gas densities? There have been many studies showing that as depth increases while breathing air, the high oxygen and increased gas density will normally slow the rate at which we breathe and thereby the rate at which we eliminate carbon dioxide. This will raise the blood levels of carbon dioxide. On top of this, however, is the fact that, because of individual variations, not all divers will slow their breathing in the same amounts.
Dr. Lanphier investigated the problem of divers who tended to breathe more slowly during diving than would normally be expected - so-called “carbon dioxide retainers.” He felt that these individuals would be at an especially high risk of CNS oxygen toxicity when breathing high oxygen in nitrogen gas mixtures. Should a nitrox diver be concerned about whether he is a carbon dioxide retainer? Unfortunately, there is no good test to reliably identify carbon dioxide retainers. The best strategy at present is to use conservative oxygen exposure limits.
More U.S. Studies - Oxygen Exposure Limits
In the late 1970s and early ?s, the Navy Experimental Diving Unit (NEDU) - now moved to Panama City, Fla.- conducted a series of studies to look at longer exposure times breathing 100 percent oxygen at shallow depths while exercising at levels typically encountered by combat swimmers while swimming long distances underwater. (Remember, exposure times developed using divers at rest may well cause problems for exercising divers, since exercise decreases oxygen tolerance.)
The conclusion of the study was that four-hour exposures at 25 feet / 7.6 meters (1.76 ata) had a low probability of causing CNS symptoms but were not without hazard since a convulsion was reported at this depth after 72 minutes of exercise. Because of this hazard, it was recommended that routine exposures be carried no deeper than 20 feet / 6.1 meters (1.6 ata) for up to four hours, with a single excursion between 21 and 40 feet / 6.4 and 12 meters for 15 minutes, or between 41 and 50 feet / 12 and 15 meters for five minutes.
Even this recommendation does not completely eliminate the possibility of a convulsion. One diver had a convulsion at 20 feet / 6.1 meters approximately 48 minutes after making a 15-minute excursion to 40 feet / 12 meters at the beginning of the dive. These studies had their share of oxygen convulsions and verified their unpredictability as observed by Dr. Donald some 40 years earlier. One feature of these convulsions that deserves mentioning is that they usually occurred with little or no warning.
With the advent of nitrox diving it is wise to consider these studies. Dr. Andrea Harabin, a scientist at the Naval Medical Research Institute (NMRI) in Bethesda, Md., analyzed the human oxygen exposures from the NEDU studies and used a mathematical model to predict the probability of CNS oxygen toxicity symptoms occurring. (See Reference 2, page 40 for details.)
When she considered all symptoms which resulted in the diver stopping his dive, she found that the model had a threshold at 1.3 ata; that is, the probability of a CNS symptom occurring at or below this level should be essentially zero.
Some of the CNS symptoms that caused dives to be halted could have been due to many other reasons besides oxygen toxicity and were classified as “Probable.” In contrast, with “Convulsions” and “Definite Symptoms” (see Table 3, page 37), there is usually no question that oxygen toxicity is the culprit. When Dr. Harabin considered just the convulsions and definite symptoms, she found the thresholds to be 1.7 ata. This analysis again reflects the large degree of uncertainty inherent in these types of human exposures.
USN 100 Percent Oxygen Rebreather Exposure Limits (1954)
TABLE 1
| Normal Operations | |
| Depth (feet) 10 15 20 25 | Time (min) 240 120 90 65 |
| Exceptional Exposure Operations | |
| Depth (feet) 30 35 40 45 | Time (min) 45 34 25 15 |
USN Oxygen Exposure Limits for Nitrogen-Oxygen Mixed-Gas Diving (1956)
TABLE 2
| Normal Exposures | |
| Oxygen Partial Pressure (ata) 1.6 1.5 1.4 1.3 1.2 1.1 1.0 | Time (min) 30 40 50 60 80 120 240 |
| Exceptional Exposures | |
| Oxygen Partial Pressure (ata) 2.0 1.9 1.8 1.7 1.6 1.5 1.4 1.3 | Time (min) 30 40 50 60 80 120 240 |
What Oxygen Level Is Safe?
So, what levels of oxygen can be breathed safely? Currently, the U.S. Navy is using 1.3 ata as the maximum limit in its closed-circuit rebreathers - the more conservative threshold found by Dr. Harabin for exercising divers. Using these closed-circuit rigs, exposures exceeding eight hours are possible, and at the 1.3 ata level the chance of CNS oxygen toxicity should be very rare.
Very long exposures, however, may put the diver at risk for some lung toxicity symptoms. The National Oceanic and Atmospheric Administration (NOAA) takes a slightly more conservative approach, recommending 180 minutes at 1.3 ata for normal exposures and 240 minutes only for exceptional exposures (see Table 4). This additional conservatism, according to NOAA, “take(s) operational safety considerations into consideration and are sufficient in duration for anticipated NOAA dives.”
The NOAA limits shown in Table 4 are based on the results of the NEDU oxygen exposure limit studies done in the ?s, taking the increased gas densities encountered in nitrox diving into account. The “normal exposure limits” are longer than the nitrox limits proposed by Dr. Lanphier in Table 2 (page 36) but are quite a bit shorter than the 240 minutes, 1.6 ata exposure, currently allowed by the U.S. Navy for 100 percent oxygen diving. However, the “exceptional exposure limits” are virtually the same as originally recommended by Dr. Lanphier, showing that there has not been much change in opinion as to what is safe at these higher partial pressures.
PADI, the Professional Association of Diving Instructors, has proposed a limit of 1.4 ata for open-circuit nitrox scuba diving. Because open-circuit scuba diving would not expose divers to this level continuously, in practice it should be as safe, or safer, than the 1.3 ata U.S. Navy limit for continuous exposures. (See sidebar “Continuous vs. Intermittent Exposures,” page 40.) In fact, the shallow exposure times in the 1.3- to 1.4-ata range are mainly to avoid lung oxygen toxicity; the likelihood of CNS toxicity at these levels is very low and probably not much different over this range.
Is it possible to breathe oxygen at a higher oxygen partial pressure (pO2)?
The answer is yes, but! Dr. Harabin’s analysis gave a threshold limit of 1.7 ata (23 feet / 7 meters) for an exercising diver when considering only “convulsions” and “definite” symptoms. This is uncomfortably close to the 25-foot / 7.6-meter (1.76 ata) depth where a convulsion was reported, so backing off to 20 feet / 6.1 meters(1.6 ata) gives a little more breathing room.
Currently the U.S. Navy would allow an exercising exposure at this partial pressure for up to four hours, but that assumes breathing 100 percent oxygen at 25 feet / 7.6 meters by trained combat swimmers. A depth excursion of only 5 feet / 1.5 meters would put the diver in an area where convulsions have been reported, and divers who tend to retain carbon dioxide during exercise may be at increased risk.
The NOAA limit for nitrox diving at 1.6 ata is 45 minutes for normal diving and 120 minutes for exceptional exposure diving. Again, some conservatism is built into these limits and consideration given to the fact that this partial pressure may be breathed at higher gas densities than would be encountered by the divers using 100 percent oxygen.
During a nitrox dive done at Duke University’s F.G. Hall Hypo/Hyperbaric Center at 100 feet / 30 meters, breathing 1.6 ata pO2 (oxygen partial pressure) during heavy exercise, a convulsion occurred after 40 minutes. Perhaps this would not have occurred had there been a lower level of exercise, but it does seem to indicate that the NOAA limit of 45 minutes for 1.6 ata nitrox diving is not overly conservative.
Breathing 100 percent oxygen during the 20-foot / 6.1-meter decompression stop is common practice, and at this depth, the partial pressure will be about 1.6 ata. At this shallow depth, under conditions of rest, the chance of CNS oxygen toxicity should be very low. But, like most things in life, this is not certain, as evidenced by a recently reported oxygen convulsion at 20 feet / 6.1 meters during decompression by a technical diver after completing a dive on the Lusitania.
TABLE 3
Symptoms of CNS Oxygen Toxicity Encountered in NEDU Studies
Convulsions: the most serious symptom and the one to avoid at all cost.
Definite: muscle twitching, tinnitus (ringing in the ears), blurred or tunnel vision, disorientation, aphasia (inability to express oneself by speaking), nystagmus (rapid side-to-side motions of the eye), or incoordination.
Probable: more equivocal signs which could be due to oxygen toxicity as well as other causes: light headdress apprehension, dysphoria (”just didn’t feel right”), lethargy, and transient nausea.
Recommendations
One thing you should be impressed with by now is that oxygen toxicity is fickle; convulsions have occurred at shallow depths under conditions where most experts would not have expected them to occur.
So, as an air sport diver, how should you view nitrox diving? The answer is: carefully.
Experts rationalizing why particular oxygen exposure limits do or do not cause oxygen toxicity are like investment analysts rationalizing movements in the stock market - everyone has a reason, but know one really knows why!
First, whenever a gas is breathed with an oxygen fraction above 21 percent, you should assume that oxygen toxicity is a possibility and have appropriate training. This not only means having a buddy clearly visible at all times but also knowing what action to take should oxygen toxicity occur. (See sidebar: “What do you do if oxygen toxicity or a convulsion happens?” )
Second, using equipment designed to compress high oxygen mixtures can be hazardous in itself and requires special training.
Third, what you get in your tank may not be what you expect. A method of analyzing the amount of oxygen in the tank independent of the filling station must be available.
Fourth, if you are attracted to rebreathers, remember that they are complex pieces of life-support gear, requiring much more care and feeding than the good old scuba regulator. If you get into rebreathers, expect to get hit with good-sized training and maintenance costs.
Finally, there is the matter of keeping the possibility of oxygen toxicity to a minimum.
Moving Ahead
For open-circuit scuba diving, consider the “green light” region any oxygen partial pressure of 1.4 ata or less (this is about 82 feet / 25 meters on a 40-percent oxygen mix.) As long as this level is never exceeded, other limitations of open-circuit scuba diving will limit the exposure time to lengths where CNS oxygen toxicity is unlikely to be encountered, even for exposures approaching four hours.
Proceeding With Caution
Between 1.4 ata and 1.6 ata (this is 99 feet / 30 meters on a 40-percent mix) is the “yellow light” region. The possibility of oxygen toxicity at 1.6 ata is low, but the margin of error is very slim compared to 1.4 ata. Individual variation, the likelihood of an unplanned depth excursion causing an increase in oxygen partial pressure, and the possibility of having to perform heavy exercise in an emergency put the possibility of oxygen toxicity at levels where caution should be exercised. Thus, levels of 1.5 to 1.6 ata should be reserved for conditions where the diver is completely at rest, such as during decompression. Again, as noted previously, the dive team must still be prepared for the possibility of an oxygen convulsion at these levels.
Stop!
Above 1.6 ata is the “red light” area. Just don’t do it. Yes, there is evidence that short exposures at higher levels of pO2 (oxygen partial pressure) are possible but so are convulsions. At these levels, oxygen exposure depth/time limits must be adhered to. Even mild exercise may put divers breathing high-density nitrox mixes at increased risk; and even open-circuit scuba divers can achieve durations likely to get them into trouble at these levels. Diving using these high partial pressures of oxygen should be left to the trained professionals who can weigh the risks and benefits and who have the necessary training and support structure in place, if an oxygen convulsion occurs.
Finally…
Nitrox diving may extend bottom times or decrease the possibility of decompression sickness, depending on how it’s used, but it adds to the risk of oxygen toxicity. Decompression sickness rarely occurs in the water and is rarely life-threatening. When it happens underwater, however, life support is usually not an issue - instead, attention is focused on getting to a treatment chamber. If an oxygen convulsion occurs, it almost always occurs underwater, greatly complicating treatment. So while the probability of a convulsion may be low, the possibility of severe injury or death is high if it does occur. Taken together this makes it a risky occurrence, and each diver needs to consider that risk whenever nitrox is used. Experience and good training are essential. This is an area that requires team diving, with the whole team full trained in nitrox diving.
What do you do if oxygen toxicity or a convulsion happens?
Editor’s note: After reading the article on nitrox in the January/February 1996 Alert Diver, a DAN member asked what the recommended procedure was in the event of an underwater oxygen convulsion. An oxygen convulsion in the water is rare but potentially life-threatening. Like learning CPR, practicing the proper handling of an oxygen convulsion is maintaining a skill you hope you’ll never use. The organization with the most experience with 100 percent oxygen diving is the United States Navy. Its recommendations for managing oxygen toxicity is as follows:
According to the USN Dive Manual sections 14.9.1.1 and 14.9.1.2 the suggested procedure for dealing with seizures is:
Management of Nonconvulsive Symptoms. The stricken diver should alert his dive buddy and make a controlled ascent to the surface. The victim’s life preserver should be inflated (if necessary) with the dive buddy watching him closely for progression of symptoms.
Management of Underwater Convulsion. The following steps should be taken when treating a convulsing diver:
a. Assume a position behind the convulsing diver. Release the victim’s weight belt unless he is wearing a drysuit, in which case the weight belt should be left in place to prevent the diver from assuming a face-down position on the surface.
b. Leave the victim’s mouthpiece in his mouth. If it is not in his mouth, do not attempt to replace it; however, if time permits, ensure that the mouthpiece is switched to the surface position.
c. Grasp the victim around his chest above the underwater breathing apparatus (UBA) or between the UBA and his body. If difficulty is encountered in gaining control of the victim in this manner, the rescuer should use the best method possible to obtain control. The UBA waist or neck strap may be grasped if necessary.
d. Make a controlled ascent to the surface, maintaining a slight pressure on the diver’s chest to assist exhalation.(see commentary below)
e. If additional buoyancy is required, activate the victim’s life jacket. The rescuer should not release his own weight belt or inflate his own life jacket.
f. Upon reaching the surface, inflate the victim’s life jacket if not previously done.
g. Remove the victim’s mouthpiece and switch the valve to SURFACE to prevent the possibility of the rig flooding and weighing down the victim.
h. Signal for emergency pick-up.
i. Once the convulsion has subsided, open the victim’s airway by tilting his head back slightly.
j. Ensure the victim is breathing. Mouth-to-mouth breathing may be initiated if necessary.
k. If an upward excursion occurred during the actual convulsion, transport to the nearest chamber and have the victim evaluated by an individual trained to recognize and treat diving-related illness.
Deciding whether to ascend with a diver who is convulsing can be tricky. In section 8-2.4 of Volume 1 of the U.S. Navy diving manual it states:
“If a diver convulses, the UBA should be ventilated immediately with a gas of lower oxygen content, if possible. If depth control is possible and gas supply is secure (helmet or full face mask), the diver’s depth should be kept constant until the convulsion subsides. If an ascent must take place, it should be done as slowly as possible. If a diver surfaces unconscious because of an oxygen convulsion or to avoid drowning, the diver must be treated as if suffering from arterial gas embolism.”
Obviously, a full face mask is the best way to perform diving with high oxygen mixes because the diver can be kept at depth until the convulsion subsides. If the diver is breathing from a mouthpiece and it comes out of his mouth, there is no option but to surface the diver, since when the convulsion stops he will try to take a breath. Training and practice are the only ways to ensure that divers will know how to bring a convulsing diver to the surface, using a slow, controlled ascent, if that becomes necessary.
In the section on the management of underwater convulsions, the reference to switching the mouthpiece to the surface position would refer only to rebreathers where an open mouthpiece which inadvertently becomes submerged can flood the UBA.
Also, step g should be modified if the victim is breathing nitrox using open-circuit scuba. If someone is convulsing, you won’t be able to remove the mouthpiece; and this should never be done by force. Once the convulsion subsides, if the mouthpiece is secure (or if the diver is wearing a full face mask) and if the diver is still in the water and breathing, then leave everything in place until you can get the injured diver out of the water. If he is not breathing, then remove the mouthpiece once on the surface and begin rescue breathing.
The main goal while the injured diver is in the water is to keep him from drowning. Next is to ensure that his airway is open after the convulsion stops by keeping the neck extended.
Finally, be on the lookout for foreign bodies in the trachea. It is possible to bite off the parts of the mouthpiece between the teeth during a convulsion, which can find their way into the trachea, blocking the airway. In these cases, the injured diver will begin coughing as he returns to consciousness, or he may try to breathe but not get any air into his lungs. Here you need to institute the standard procedures taught in CPR classes for foreign body obstruction of the trachea.
Continuous vs. Intermittent Oxygen Exposures
Remember that CNS oxygen toxicity symptoms are a time-duration phenomenon. They will not suddenly occur the minute a particular partial pressure is exceeded - it takes time. As you can see from the exposure limits in the tables (Table 4), as the inspired oxygen partial pressure increases, the exposure time decreases.
The U.S. Navy limit of 1.3 ata for continuous exposures reflects their desire to keep the risk of CNS symptoms essentially zero, no matter how long the dive.
In nitrox diving, however, divers breathe from open-circuit scuba with a fixed fraction of oxygen in the breathing mix. PADI has chosen 1.4 ata as the maximum open-circuit scuba limit; the limitations placed on duration by open-circuit scuba will ensure that the likelihood of CNS oxygen toxicity is no greater than would be experienced by the U.S. Navy closed-circuit divers.
When using open-circuit scuba, the 1.4 ata maximum oxygen partial pressure is reached only at the maximum depth, and for the vast majority of recreation divers, the time spent at this maximum depth will be limited to times where CNS oxygen toxicity is unlikely to be encountered. At all shallower depths, the oxygen partial pressure will be lower, and the overall exposure during the entire dive is unlikely to have physiological effects significantly different than a continuous 1.3 ata exposure. Be careful when extending this analogy to higher partial pressures, however. Formulas are available for integrating the exposures at various depths to predict overall exposure times when looking only at lung oxygen toxicity. This concept does have some support research done at Dr. C.J. Lambertsen’s laboratory at the Institute of Environmental Medicine in Philadelphia, Pa.
The case for CNS oxygen toxicity is much more complicated. Research done at the Navy Experimental Diving Unit (NEDU) in 1986 specifically looked at how brief exposures to oxygen partial pressures of 2.0 ata or greater would impact the overall exposure time at 20 feet / 6.1 meters of sea water (fsw). The results were not clear, and it was obvious that no formula could be developed which would allow integration of oxygen exposures at various depths into a single indicator which would help the diver avoid CNS oxygen toxicity. The best that could be said is that a single 15-minute excursion to 40 fsw/12 msw, or for five minutes at 50 fsw/15 msw, probably had no significant effect. This formed the basis of the current U.S. Navy recommendations. No such research has yet been carried out for high oxygen nitrox diving, to my knowledge.
Dr. E.D. Thalmann
Oxygen Partial Pressure and Exposure Time Limits for Nitrogen-Oxygen Mixed-Gas Working Dives (from NOAA 1991 Diving Manual)
| Normal Operations | ||
| Oxygen Partial Pressure (ata) 1.6 1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 | Maximum Duration for a Single Exposure (min.) 45 120 150 180 210 240 300 360 450 570 720 | Maximum Total Duration for any 24-Hour Day (min.) 150 180 180 210 240 270 300 360 450 570 720 |
| Exceptional Exposures | |
| Oxygen Partial Pressure (ata) 2.0 1.9 1.8 1.7 1.6 1.5 1.4 1.3 | Time (min) 30 45 60 75 120 150 180 240 |
TABLE 4
REFERENCES
Donald KM. Oxygen and the Diver. England: Images, 1993. Available through Best Publishing Co., Flagstaff, Ariz. (This reference also covers all of the NEDU studies mentioned and gives full citations for them.)
Harabin AL, Survanshi SS. A statistical analysis of recent Navy Experimental Diving Unit (NEDU) single-depth human exposures to 100-percent oxygen at pressure. Bethesda, M.D. Naval Medical Research Institute Report NMRI 93-59, 1993.
Note: Both NEDU and NMRI Reports are available through: National Technical Information Service, 5385 Port Royal Road, Springfield VA 22161.
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Starting Underwater Photography
January 6th, 2009
Underwater photography is very different to every other kind of photography. In fact, it’s the most difficult kind of photography there is.
There are the environmental pressures and, compared to land photography, there is a whole new set of variables to contend with (which are many and difficult to assess). You don’t have to be a masochist to be an underwater photographer but it helps!
Not put off yet? Good! Underwater photography can also be FUN. In fact it can be as much fun as you’ll ever have with your dive gear on! But, it does cost money, it does involve a load of effort, and it does mean that you have to take on board some knowledge to get the most out of it.
Who should do u/w photography?
People who take up underwater photography generally do so from one of two angles; either as a photographer who would like to specialize, or a diver who would like a second interest in the sport.
For me, it started when I got my dive qualification my instructor said “Fine! now what are you going to do?”. I pondered on what she meant. I think it was that no matter how wonderful the diving experience is it does take something to motivate you on a cold day! Enhancement of your diving is just one valid reason to take up underwater photography. So is wanting to share the experience of the good dives with others (who may not even be divers).
The pre requisite is not just learning to dive but also reaching the stage where you are totally at ease in the water. Once you have done that learning underwater photography is no harder than learning to dive, although there are fewer sources of instruction! However, by picking up this book, you have tapped in to a mine of information and, providing you get over the first few hurdles, you will soon be snappin’ with the best of them!
A word of warning: beware, when the bug bites, it bites hard. If you’re already a diver it may change you diving habits forever. Don’t worry though, you won’t automatically become an outcast on dive boats - and you will learn to appreciate the underwater environment even more than you do already!
To reiterate: a lot of underwater photographers begin as land photographers. It is a logical progression to graduate to underwater photography when you start diving. If this is you, and you are already familiar with the basics of photography, then you could skip this chapter and move on to the next. If you are not already a photographer then read this chapter as many times as it takes to understand it fully.
