Posted by Steve on August 09, 2001 at 17:59:35:
In Reply to: Re: what about price, Mike? posted by MHK on August 09, 2001 at 17:33:19:
You have taken a selective snip and extrapolated."
Okay, you're right I did selectively snip a paragraph from the following article. I've posted the article for your critiquing in it's entirety. The point is that noone should look at everything through rose colored glasses, for very gain there may be an increase risk or drawback. In the case of EAN it is a risk O2 tox even as low as .5 PPo2 very rare but it must have happened or Dr. Hamilton wouldn't mention it.
UnderWater Magazine Article reprint: March/April 2001
"Nitrox Saturation: The Road Not Taken"
By: Steven Maberry
The air we breathe is 78 percent nitrogen, 21 percent oxygen, and one percent trace gases (mostly argon and carbon dioxide). In diving, nitrox is a nitrogen/oxygen gas mixture different from natural air. Nitrox arises in two forms: one with more oxygen than air, which can decrease required decompression in shallow, surface-supplied diving, and one with less oxygen, which finds an application in saturation diving. Steven Maberry discusses nitrox saturation, including aspects of Hamilton Research's REPEX procedures and modifications to that research based on calculations applied to the fundamental REPEX models.
On August 14, 1986, at 5:17 p.m., four women stepped out of a saturation chamber and onto a live New York City television newscast. For these four, their seven-day saturation incarceration had become a long, delightful pajama party with serious connotations. They were such a pleasant group that the writers of normally staid research reports could not refrain from commenting on how these four had been "really special, the best ever." These women formed one of three test teams subjected to nitrox saturation chamber trials, which included excursions and decompression procedures developed by Hamilton Research for the National Oceanic and Atmospheric Administration (NOAA).
Those procedures, dubbed REPEX for Repetitive Excursions, have since become the standard for NOAA saturation operations. Yet, even the marine scientists who operate under NOAA rules make little use of much that REPEX procedures offer in operational flexibility.
Nitrox in Commercial Diving
For commercial divers, nitrox saturation has held little interest. Three main issues discouraged their participation in nitrox saturation development during the heady years of the world oil embargo. Comfort levels with heliox saturation were high, U.S. helium supplies were stable, and oil companies paid for consumed helium - plus a healthy profit tacked onto this cost.
Now, we are even more comfortable with heliox than we were in the 1970s, and helium supplies remain plentiful and stable. In contrast, the economics have changed dramatically. These days, price-conscious offshore construction buyers have caused saturation providers to install helium reclaim systems, cutting helium consumption by 80 to 90 percent, and greatly reducing this cost. Nitrox saturation, though, remains ignored, or even disdained.
There are good reasons for commercial diving companies to ignore nitrox saturation. Nitrogen narcosis and high gas density limit nitrox to relatively shallow waters. Heliox saturation works just as well in shallow as it does in deep waters. Decompression from nitrox takes longer than heliox at the same depth, and there have been several bad decompression experiences from deep nitrox saturations. Apparently, any cost or simplicity advantages available from nitrogen just do not overcome the disadvantages. Perhaps, though, we just have not given it adequate consideration.
Nitrox saturation requires consideration of three issues that do not trouble heliox saturation. The first of these is inert gas narcosis. Helium has very little, if any, narcotic potential. Therefore, narcosis is not an issue in heliox use. The second consideration is cumulative pulmonary oxygen toxicity in excursions from the saturation storage depth, which arises because it is common practice to use air on excursions. The third issue is gas density.
Inert gas narcosis
Those who have descended on air to great depths are familiar with nitrogen narcosis. It is a clean, crystal high from which a return to normal pressure provides an immediate recovery without adverse side effects. Intoxication and memory loss in the hostile submarine environment are, however, extremely dangerous.
The REPEX researchers wrote this about the four-woman team: "The Repex II divers exhibited all sorts of narcotic behavior but did not consider that they were narcotized; . . . They laughed and giggled so much all the time it was hard to see changes." This describes very well the difficulties inherent in diagnosing the presence of inert gas narcosis, and investigators have not developed any standard tests for scoring narcotic severity.
During the REPEX validation tests, four men stayed at 50 feet (15m), four women at 80 feet (25m), and four more men at 110 feet (34m). From these depths, the teams made excursions on air to as deep as 220 feet (68m) during the 50-foot saturation and 240-foot (74m) excursions during the other two. The saturation duration (from compression to start of decompression) was about five days for each of these experiments. The summary of their narcotic experiences was that they noted narcosis on excursions and there was some limited acclimation to the narcotic effects.
Early work in air and nitrox saturation suggested that there was a remarkable saturation-induced adaptation to nitrogen narcosis. Studies in 1972 and 1973 (SHAD by the Navy Submarine Medical Research Laboratory, NOAA OPS I and II by Union Carbide, and Puerto Rico Undersea Excursion PRUNE, to name three) yielded a conclusion that there is a significant improvement over making deep dives from the surface. The investigators were especially impressed with performance improvements in the 200-foot to 250-foot ranges (61m to 77m). This led them to conclude we could develop safe and effective nitrox operations for depths to 250 feet, and perhaps beyond. However, these researchers were predisposed to make findings of narcotic accommodation. The common claim that divers subjected to daily deep air exposures develop a tolerance to nitrogen narcosis foreshadowed, and influenced, such a finding.
Other investigations have not been so optimistic. Duke University's Scientific Cooperative Operational Research Expedition (SCORE) concluded that there was little or no adaptation to saturation excursions when compared to dives to the same depth from the surface.
A French investigation by Comex called NERIEDE used psychometric tests that suggested mild narcosis at saturation depths of 137 feet and 195 feet (42m and 60m), but a 12 percent decrement on excursions to 244 feet (75m). Test outcomes from repeated excursions to this depth improved, but the NERIEDE researchers concluded that the potential for harm was too great, and considered such excursions in open water would be dangerous.
In 1982, the Swedish Naval Centre conducted a 200-foot (61m) saturation on nitrox. Living at this depth caused immediate narcosis, but most participants returned to near normal in four to six days. Lingering narcotic effects remained until decompression. These investigators, too, were not pleased with the outcome for excursions to 245 feet and 325 feet (75m and 100m).
About the same time, or shortly afterward (1982 or 1983), Oceaneering International, NOAA, and Duke also studied deep saturation to 165 feet (51m). Although they never published a report on their narcosis data, individuals involved relate that the results were disappointing.
Although less in vogue than during the optimistic 1970s, saturation-induced nitrogen narcosis adaptation is not fully resolved; it may yet exist. Typically, any information gathered about nitrogen narcosis during saturation research was incidental. The primary purposes for conducting the various tests were, like in REPEX, for other reasons, therefore narcosis data was usually gathered ad hoc, not as an integral part of the test plan. Variables that could affect the narcotic mechanisms, or the reliability of the data, were often not controlled or recorded. These variables include specific gas partial pressures, temperature, rate of compression, age and experience of the divers, and effects of motivation and learning. Further, the poor standardization among the test criteria for identifying the presence or severity of narcosis makes it impossible to correlate results among various investigations.
For those who still conclude that remarkable saturation-induced accommodation to nitrogen narcosis exists, the belief is that this adaptation becomes notable only after five days or more of saturation. In 1971, sensitive tests conducted on short-term memory decrements at 100 feet (31m) suggested that significant adaptation began at five days, and memory did not return to normal until eight or nine days. Several nitrox saturations that concluded there was little or no adaptation were shorter than this five-day-or-more notion.
Concluding that divers can make safe, useful dives to 200 feet (61m) from nitrox saturation is defensible. Between 200 feet and 250 feet (61m and 77m), such a conclusion may yet be possible, but is certainly more difficult. Beyond 250 feet, the likelihood of adequate nitrogen narcosis adaptation is remote.
Chronic Oxygen Toxicity
Any dose of oxygen that exceeds 0.5 atmospheres absolute (ata), partial pressure, carries some toxicity risk. At exposures below 1.5 ata, the greater risk is pulmonary, or chronic, toxicity, not central nervous system, or acute, toxicity. Oxygen toxicity is why we put up with inert gases at all. We have to dilute oxygen in the diving atmosphere.
To avoid excessive oxygen exposure, nitrox or heliox replaces air as an atmosphere in saturation living quarters deeper than 45 feet (14m). In nitrox saturation, common practice is to use air on diving excursions. For excursions deeper than 45 feet, this exposes divers to potentially toxic oxygen concentrations.
By 1972, researchers had established methods for predicting oxygen toxic risk. This predictor is in the form of an empirical mathematical model. First called Unit Pulmonary Toxic Dose (UPTD), it is now more commonly called Chronic Pulmonary Toxic Dose (CPTD). While CPTD is widely accepted for calculating the oxygen dose, acceptable CPTD limits are not as universally established. One standard is a limit of 615 CPTD for dive plus decompression and for mild decompression sickness, and 1425 or less for medical treatment (as in serious decompression illness). However, the data justifying these limits are of questionable application for repeated daily compressions.
REPEX investigators at Hamilton Research recognized this flaw and did their best, with limited data, to establish better limits suited to daily cyclic exposure to elevated oxygen concentrations. Since 1988, when REPEX procedures were reported, experience with the REPEX CPTD chronic exposure limits appears to confirm their validity. They concluded that, for short duration operations, larger daily doses are acceptable. Table One reflects the REPEX recommendations. Note that for eleven days or more, the average oxygen dose should be 300 CPTDs per day.
This approach to daily toxic dose recognizes that divers enter the saturation system with no oxygen-induced damage. Exposure then gradually affects the diver. Fortunately, it takes long exposures to produce circumstances from which the diver's lungs and other involved systems will not readily recover. When a diver returns to lower oxygen concentrations, recovery begins and, over time, reverses damage. For many days of consecutive exposure, minor damage can accumulate. Hence, repeated daily exposures over a long period justify lower limits.
At 91 feet (28m), air produces 303 CPTD in eight hours. Hence, for commercial divers who make daily eight-hour excursions to depth during a saturation that lasts 11 days or more, using air deeper than 90 feet could result in unacceptable levels of pulmonary symptoms. It is therefore likely that commercial nitrox saturation would want to consider supplying nitrox, instead of air, on excursion dives.
If a diving company wanted to continue using air, it could design procedures so that the bell atmosphere was nitrox. The divers would then make excursions from the bell on air for four hours each. For four-hour exposures, it takes 149 feet (46m) on air to exceed 300 CPTDs. This way, air could remain the excursion breathing gas to 148 feet (45m).
Helium is one-seventh the density of nitrogen. With greater depth and pressure, gases compress and become more dense. Helium reaches the density of one-atmosphere nitrogen at 199 feet (61m). At this depth, nitrogen would be seven times denser than at the surface. With each breathing cycle, the lungs pull in the atmosphere. Then, they stop the gas flow and turn it around, pushing the breathing gases back out. Denser gases take greater effort to pull in, stop, and then push back out in each cycle. The work required to inhale and to exhale increases markedly as gas density increases. At some point, increased pulmonary work causes problems.
The complete path for metabolic oxygen uptake and carbon dioxide removal includes lung function and heart function. At normal atmospheric gas densities, heart function limits a person's maximum aerobic capacity. As gas density increases, lungs work harder and harder. At some depth-induced density, pulmonary respiratory function takes over as the weak link that limits aerobic capacity.
Excursions from Nitrox Saturation Storage Depth
The mission for Hamilton Research in developing the REPEX procedures included developing operational nitrox saturation procedures for excursions to greater depths from the saturated storage depth. They developed a complete set of "no-decompression" and very limited decompression dive tables for each saturation storage depth. It was operations typical to research scientists that shaped their work. Therefore, procedures used in REPEX do not spell out any use of nitrox as an excursion breathing gas. All of the REPEX excursions assume air in excursion dives. REPEX does not include procedures for in-water nitrox. Commercial divers who might spend up to eight hours at excursion depth need to include nitrox as an excursion gas.
Although REPEX does not investigate decompression while diving on nitrox, they used a Modified Haldane decompression model to compute their excursion diving procedures. One can use the some model and extend it to include using nitrox on long excursions. Table Two presents REPEX eight-hour and four-hour no decompression limits where available, and it presents results of extending the same approach to in-water nitrox. REPEX also limits the saturation storage depth to 120 feet (37m). Table Two shows deeper saturation storage depths to illustrate the possibilities for eight-hour, no-decompression diving to deeper depths.
The excursion mix chosen in the accompanying illustration is 10 percent oxygen and 90 percent nitrogen. This allows diving to as deep as 220 feet (68m) while maintaining the oxygen toxic dose to less than 300 CPTD. Also, divers can safely use this nitrox mix as shallow as 36 feet (11m). Note there is a significant reduction in allowed excursion depth differential when the diving mix switches to the greater nitrogen-containing nitrox.
While the REPEX procedures have been subjected to some test validation and field use, the extrapolations presented here go beyond REPEX and have not been exposed to peer review, testing, or field experience. Hence, they are not now suitable for placing into practice. Further, while REPEX has provided thoughtful decompression from saturation depths up to 120 feet (36m), several deeper nitrox saturation tests experienced trouble during or after decompression. Finally, the deepest excursion depths in Table Two are probably well into areas where nitrogen narcosis could become a problem, perhaps even if adaptation occurs.
With the continued gains by work class ROV's, it is possible that saturation divers will find their deepest work shifted to these machines. Perhaps shallow saturation will become the rule rather than the exception. If so, nitrox might merit a revisit. UW
Steven Maberry will complete his time as a Stanford Graduate Fellow in the next year, earning his doctorate. Before Stanford, Steven was a consulting engineer and adjunct professor at the University of New Mexico. He owned the Red Owl Marine diving company and spent time with Taylor Diving and Salvage and American Oilfield Divers. E-mail him at email@example.com.
Post a Followup