The Truth about Nitrox

Subject: WEST WITS SUB AQUA CLUB - Article and info on Nitrox

The Truth About Nitrox

Our ScubaLab team investigates the myths and discovers some new facts.

by Jon Hardy

The Testers and the Thanks For these tests, the following 10 divers made more than 130 dives between 33 and 132 feet to test gas consumption, narcosis effect, thermal balance and the feel-good effect. Vicki Durst, Jon Hardy, Bruce Haveri, Mike Jones, Jason Manix, Bill Mercadante, Ron Moore, Pete Pehl, Susie regeimbal, Lorraine Sadler Catalina Scuba LUV (800-262-3483) supplied all the nitrox used for the tests conducted on Santa Catalina Island, Calif.

Since 1996, nitrox certifications have soared. Nearly a quarter-million nitrox certifications have been issued to divers, and more than 33,000 qualifications in nitrox for instructors. But as the number of nitrox divers has soared, so have the misconceptions, myths and pseudo-science, but not the accidents. The purpose of this article is to present the current best thinking on nitrox, from its advantages and disadvantages, to the surprising results of ScubaLab's in-water investigation of the breathing gas that ends with the mysterious "x".

Expanding Horizons, Defining Limits

The hype about nitrox-sometimes called "enriched air nitrox" or EAN-usually invokes the remarkable gains in no-decompression time you get. The truth is, significant increases in no-decompression limits are dependent upon depth and the mixture used, and must be offset by strict adherence to a maximum operating depth (MOD). Tables I and II, below, provide this information for air, EAN 32 (32 percent oxygen) and EAN 36 (36 percent oxygen).

Table I shows that the most advantageous and useful increase in the available no-decompression time is between 60 and 110 feet, where nitrox is at its best. Table II underscores the dangers of too much oxygen. The partial pressure of oxygen (PPO2) is not a real concern on air dives until the 187- to 218-foot range. However, with EAN 32, oxygen toxicity becomes a concern at 111 to 132 feet, and 95 to 114 feet with EAN 36. As a result, divers are well advised to observe a 130-foot MOD with EAN 32, and a 110-foot limit with EAN 36.

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The Advantages of Nitrox

Clearly the most important reason for using nitrox is the decrease in nitrogen content. But the possibility of longer bottom times as a result is far from the only advantage. By using nitrox instead of air at the same depth, you can also:

· Have shorter surface intervals between dives.

· Decrease time before flying.

· Shorten decompression stops.

· Reduce the risk of decompression sickness (DCS) when nitrox is used with air tables or with an air dive computer.

· Help reduce the symptoms of subclinical DCS.

· Due to the depth limits of nitrox, help prevent a variety of other diver problems, such as narcosis, deep-water blackout and carbon dioxide excess.

· Either decrease the DCS risk or shorten deco stops because nitrox is useful as a decompression gas after air dives.

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Disadvantages of Nitrox

We live in a world of trade-offs, and nitrox is no exception. The disadvantages include:

· A significantly increased risk of oxygen toxicity with mixtures containing increased oxygen percentages.

· Need for greater care and skill in observing depth limits due to clearly defined maximum operating depths for each mixture.

· Special requirements associated with the use of nitrox, including the need for additional training, and possible equipment modifications.

· Need to analyze and sign off on the blend.

· Additional dive planning steps, particularly if using dive tables.

· An increased risk of fire or explosion as oxygen percentages are increased.

· Impracticality of the most common nitrox mixtures in water less than 50 feet and greater than 130 feet deep.

· Increased cost (compared to air).

· Lack of wide availability.

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ScubaLab Research and Testing

There are a number of unsubstantiated beliefs about nitrox, strongly held by some and dismissed by others, that have taken on the quality of urban myths. ScubaLab set out to see if it could find any proof or run any tests that would prove or disprove that:

· Divers breathing nitrox consume gas more efficiently than divers breathing air.

· Divers breathing nitrox lose less heat.

· Divers using nitrox are less susceptible to nitrogen narcosis.

· Divers feel better after nitrox dives.

Predictably, an extensive search of the internet, current periodicals and textbooks, plus a review of courses and workshops on nitrox, provided plenty of opinions, but no proof of these assertions. So it was time to get wet.

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Gas Consumption

Do breathing rates improve when a diver uses nitrox rather than air? According to our tests, the answer is a pretty clear "yes." On average, a diver breathing a nitrox mix will use gas more efficiently than a similar diver who breathes regular air.

In-water tests were conducted between 33 and 99 feet, comparing air (21 percent oxygen) and EAN 36 (36 percent oxygen). Tests were done "in the blind"-all tanks were labeled nitrox, but the oxygen percentage was not marked. The tanks were carefully analyzed, but only the dive supervisor knew which contained air and which contained EAN 36. The tanks were issued in random order to the divers, who swam at a steady rate (one mph), or sat at rest on the bottom. All tests were carried out in the ocean with divers in full scuba gear.

The at-rest tests (done at 99 feet) revealed no difference between the basic breathing rates of divers on air and divers on nitrox. However, in the controlled course runs, swimming at one mph, nitrox divers averaged 2 psi per minute better gas consumption than air divers. This result was based on 54 trials.

If this nitrox advantage held true over an entire dive of 60 minutes at 66 feet, there would be 360 psi more gas available to the nitrox user, a 12 percent improvement when using a standard aluminum 80.

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Thermal Balance

We could find no data or evidence in our searches that thermal balance is better with nitrox. Because of the significant number of uncontrolled variables that could affect diver warmth during our tests, no conclusion could be drawn on thermal balance. In fact, in our research, we could not even find what concept this belief might have been based on.

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Nitrogen Narcosis

For this test we selected a child's puzzle for the test divers to complete. Again, the divers were issued cylinders of air or EAN 36 in random order by the dive supervisor so that the divers did not know which gas they were breathing. The evaluation criteria included time to complete the test and video analysis of errors during performance.

The puzzle we used is made for children 5 years of age and up, with a goal for children to complete in 30 seconds or less. The completion time for adult divers ranged from 54 seconds to 3 minutes and 23 seconds, with an overall average of 1 minute and 42 seconds. Of course, the divers were doing this in full scuba gear, including gloves, at 99 feet.

Results: There was no significant difference between the divers' performance on air and on nitrox. Predictably, the divers became more adept at the puzzle on repeated tests.

The results are not surprising: Scientists tell us there should be no difference in the narcotic effect of oxygen and nitrogen because they are so close in physical characteristics, which this test certainly substantiates. Also, most experienced divers' ability to think and execute manipulative skills is not measurably affected by narcosis when they remain in the best range of nitrox use, 60 to 110 feet. Narcosis in this range is likely to be affected more by diver fitness, experience, training, equipment performance and environmental conditions than by a switch of gases.

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Nitrox: The Feel-Good Gas

Ample anecdotal evidence suggests that divers "feel better" after diving with nitrox than they do after diving with air. Although we have yet to devise an effective objective test of this hypothesis, our review of the medical research in this area does provide some insight.

One piece of evidence from the literature is that many air divers likely suffer subclinical DCS on a regular basis. This includes fatigue, soreness, headaches, or aches and pains that do not rise to a level requiring medical treatment. But divers using nitrox, particularly those who dive it with air tables or air computers, reduce their decompression stress and, therefore, reduce the occurrence of subclinical DCS.

As a result of our research, we believe that the decompression stress of air diving should be added to the list of primary physical stresses of scuba diving.

Another insight also comes from the medical field: When people are suffering from respiratory distress and many other injuries, pure oxygen is used to help ease the distress and promote healing. If increasing the oxygen in the inhaled gas helps in these cases, may it not also help the uninjured diver? A number of medical studies indicate various body functions are improved after breathing pure oxygen, although these improvements are short-lived.

While it seems there is credence for this notion that nitrox is a "feel-good gas," the variables affecting it must be kept in mind:

· Percentage of oxygen in the nitrox

· Depth of the dive

· Proximity to the no-decompression limits

· The impact of breathing, swimming and temperature.

Given these, we believe a diver's education and training should emphasize all four basic physical stresses-breathing resistance, resistance to movement, maintaining thermal balance and decompression stress-and that nitrox should be presented as a primary way of reducing the latter.

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Nitrox Equipment: Computers

The best way to dive nitrox is with a nitrox-compatible computer The trend among manufacturers is to add nitrox functionality to all new computers.

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Nitrox Equipment: Regulators

General practice in the diving community is not to clean or modify regulators and associated hoses or instruments for nitrox use unless the nitrox has an oxygen content greater than 40 percent. There are those who say that cleaning should be required for all mixtures beyond 23.5 percent oxygen. As the DAN workshop concluded, there is no evidence to support a need to clean equipment for nitrox with less than 40 percent oxygen.

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Nitrox Equipment: Cylinders

Scuba cylinders and their valves must be oxygen clean and use the O-rings and lubricants compatible with 100 percent oxygen. Dive stores actually become manufacturers when they supply you with nitrox. Depending on the method they use to produce nitrox, the greatest risk of fire or explosion when handling nitrox is at the dive store.

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Nitrox Equipment: Oxygen Analyzers

The DAN workshop recommended that oxygen analyzers use a controlled-flow sampling device. There may be a problem with terminology in this recommendation, as all such devices must control the gas (on/off), but not all use continuous flow. Some use a brief flow and then take a static reading.

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Oxygen Exposure Safety

The most dangerous aspect of exposure to the increased oxygen content in nitrox is that by going too deep oxygen toxicity of the central nervous system (a CNS hit) is possible, leading to convulsions, loss of the mouthpiece and drowning. To prevent this, most divers limit their oxygen exposure to 1.4 atms for general diving and 1.6 atms for contingencies or short periods of time. Of course, more conservative divers can use even lower partial pressures.

Oxygen tolerance units (OTUs) are a measure of your whole body exposure to oxygen over an extended period of time. As recreational divers, we usually do not exceed 36 percent oxygen, have relatively short exposures (30 to 60 minutes) and have substantial air breaks called surface intervals, and therefore our risk of oxygen exposure problems in this area is extremely small. There are tables to calculate this exposure, and most dive computers figure OTUs while also calculating PPO2 and display both or the more important of the two. All in all, whole body exposure is far less of a concern than a CNS hit.

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DAN Nitrox Workshop Consensus Recommendations

In November 2000, DAN brought together three dozen leaders in nitrox use and training from recreational and technical training associations, manufacturers, researchers, medical and legal experts, and educational institutions, under the leadership of Michael Lang, Diving Officer of the Smithsonian Institution.

The two-day workshop covered operational data, physiology, risk management, training and equipment. Although significant and worthwhile debate occurred, a higher level of consensus was reached than has been reached in similar workshops. For entry-level, recreational open-circuit nitrox diving, the consensus was:

· No evidence was presented that showed an increased risk of DCS from the use of oxygen-enriched air (nitrox) versus compressed air.

· A maximum PPO₂ of 1.6 atms was accepted, based on its history of use and scientific studies.

· Routine carbon dioxide retention screening is not necessary.

· Oxygen analyzers should use a controlled-flow sampling device.

· Oxygen analysis of the breathing gas should be performed by the blender or dispenser and verified by the end user.

· Training agencies recognize the effectiveness of dive computers.

· For recreational diving, there is no need to track whole body exposure to oxygen (OTU/UPTD).

· Use of the CNS Oxygen Clock concept, based on NOAA oxygen exposure limits, should be taught. However, it should be noted that CNS oxygen toxicity could occur suddenly and unexpectedly.

· No evidence was presented, based on history of use, to show an unreasonable risk of fire or ignition when using up to 40 percent nitrox with standard scuba equipment. The level of risk is related to specific equipment configurations and the user should rely on the manufacturer's recommendations.

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Learning More About Nitrox

· Books: Nitrox: A Guide to Diving with Oxygen Enriched Air. Hamilton and Silverstein (NAUI at www.naui.org); DAN Nitrox Workshop Proceedings. Edited by Lang. (DAN, 919-684-2948).

· Video: "Enriched Air Nitrox" (Scuba Schools International, 970-482-0883).

· Web: http://diver.ocean.washington.edu/nitroxpage.html

http://home.earthlink.net/~rottner/karntrox.htm

http://scubacentral.com/links/ead.html

http://icon.co.za/~jan/dive/andi.html

http://www.iro.umontreal.ca/~legua/rec/Nitrox/n2o2_m00.htm

http://www.webcorp.ca/IANTD_canada/