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by Jarrod Jablonski
Header Photo by JP Bresser
Technical diving means different things to different people, opening legitimate arguments about different time periods and personalities involved in shaping this activity. Our purpose is more confined as I wish to focus mainly upon the development of decompression procedures. Leading figures like Jacques Cousteau (1910–1997) performed reasonably significant “technical” dives including both deep and overhead exposures. Yet, his dives were relatively short, often on air, and absent a community of fellow tech divers with whom to evolve varying strategies. Meanwhile, military or commercial activities reached significant depth with notable exposures but used different technologies and procedures. Hopefully, the following overview will be independently interesting—although I also hope to illustrate what I consider an intriguing wrinkle currently lacking in most debates over the best ascent schedule for a decompression dive.
For our purposes, I mark the 1980s as the period in which technical diving truly started to become a globally recognized activity. Of course, numerous significant projects occurred before this period, but the 1980s established a growing global awareness that amazing diving feats were possible by enthusiasts. The 1990s and the developing internet age brought these ideas progressively into the mainstream. Most importantly, these global communities could now easily communicate information and, of course, disagree about the best way to do one thing or another.
The 1980s and 1990s were also an interesting period of development because people were doing more aggressive dives but still lacked important support tools that would take another decade to materialize. For example, most early tech dives were using some form of military, commercial, or scientific tables, such as the U.S. Navy, Oceaneering International, Inc. or the National Oceanic and Atmospheric Administration (NOAA), which were often not well-suited to the dive at hand. This is because there were no decompression programs available. Divers were unable to calculate their own profiles and would trade whatever tables they could gather. In many cases, they were forced to choose from tables that were calculated at a different depth, time, and/or breathing mix to the planned dive.
Over time, the physiologist Dr. R.W. “Bill” Hamilton (1930–2011) began creating custom tables, but the lack of flexibility, as well as the cost, discouraged frequent use for most divers, especially when considering a range of depth profiles and their various time adjustments. This problem was especially prominent in deep cave diving where variable profiles and long bottom times created numerous complications . This landscape encouraged, if not demanded, that exploration divers be creative with their decompression practices. For example, Woodville Karst Plain Project (WKPP) explorers George Irvine and I began to explore ways to extrapolate from existing tables. The birth of what is now called ratio decompression originated with this practice.
By exploring the way decompression schedules develop, we began outlining simple ratios that allowed divers to adjust their decompression based upon a ratio of time spent at depth. Then, in 1998, I went on to form Global Underwater Explorers (GUE), and such practices became part of the process for helping divers appreciate the structure of decompression while supporting adjustments to profiles when depth or time varied from that expected. Regrettably, some individuals took this too far and began promoting complex adjustments and marketed them as superior to the underlying algorithms from which they had been derived.
We should leave a more in-depth review of ratio deco for another time. For now, I intend to illustrate that “rules” such as ratio deco had their roots in limited availability of decompression tables and evolved as a useful tool for understanding and estimating decompression obligation. The fact that early tech divers had limited ability to calculate profiles encouraged a “test-and-see” philosophy, further fueling the early popularity of ad hoc adjustments to decompression profiles. This was most notable in the DIR and GUE communities through ratio-style adjustments and also prominent with non-DIR advocates who used a modification known as Pyle-stops, originating from ichthyologist Richard Pyle’s early deep dives and his attempts to refine efficient ascent protocols. One common adjustment in these approaches involved a notable reduction in ascent speed, adding additional stops known as “ deep stops ”
Driven by self-discovery, the migration toward deep stops resulted in rare agreement among technical divers, and the practice developed a life of its own in conference symposiums, being advocated far and wide by a healthy share of technical divers. The earliest phases of these modifications were driven in part by limited access to decompression tables, though by the end of the 1990s there were a variety of decompression programs available. This was a big improvement, although it was still true that divers had a limited baseline of successful dives while planning mixed-gas dives in the 50 to 100 meter range and beyond. Today, most divers take for granted that technical dives planned with available resources and within today’s common use (a few hours around 90m/300 ft) are relatively safe in terms of decompression risk. When these programs first came to market, there remained many question marks about their efficacy.
The availability of decompression programs was a big advantage for technical divers, as they now had more sophisticated tools at their disposal. Yet, the output from many of these programs produced tables that were sometimes different to profiles thought to be successful by some groups. Typically, the difference related to an increase in the total decompression time and/or a distribution of stops that varied from developing consensus. These divers were keenly interested in the developing tools but reluctant to change from what appeared successful, especially when that change required additional hours in the water. A debate developed around the reason for these differences, bringing the already brewing interest in bubbles well into the mainstream.
At the time, all tables were based upon dissolved gas models like Buhlmann, and thus not directly modeling bubble development during an ascent. Dissolved gas models preference ascents that maintain a reasonably high gradient between the gas in tissues and the gas being breathed, which should be supportive of efficient elimination from tissues. Dissolved gas models manage—but do not explicitly control for—bubbles and were thus labeled (probably unfairly) as “bend and mend” tables. In fact, dissolved gas models are designed to limit supersaturation (explicitly), because this is supposed to limit bubble formation. Haldane references this control of supersaturation in his pioneering publication .
Those advocating for different ascent protocols imagine that a lack of deep stops creates more bubbles, which then need to be managed during a longer series of shallow decompression stops. In this scenario, a slower ascent from depth, including “deep stops,” would reduce the formation and growth of bubbles while reducing time that might otherwise be needed to manage previously formed bubbles. Support for these ideas gained momentum in diving and professional communities, although some of these individuals were arguably conflicted by a vested interest in promoting deep stop models. Eventually, the practice received more critical consideration but during the intervening years deep stops were largely considered as common knowledge.
The idea of controlling bubbles became extremely popular through the 1990s, encouraging deeper stops designed to limit bubble formation and growth. The “test-and-see” approach developed by early tech divers appears to have fueled the promulgation of deep stops though the determining characteristics remained poorly defined. Early tech divers embraced the uncertainty of their activity, realized that nobody had the answers, and decided to “experiment” in search of their own answers. In fact, divers were experimenting with a lot more than deep stops, altering gases breathed, the placement of various decompression stops, and the total amount of decompression time utilized. This meant that divers were sometimes aggressively adjusting multiple factors simultaneously.
For example, divers using a decompression program might input significantly less helium than was present in their breathing mixture. This was done because the algorithm increased decompression time with elevated helium percentage, sometimes known as the helium penalty . In other cases, divers would completely change the structure of stop times. For example, divers would invert the way a profile should be conducted by doing more time near a gas switch and less time prior to the next gas switch, believing the higher-oxygen gas only 3m/10ft away was time better utilized.
I do not intend an exhaustive or detailed review but only to assert the many, sometimes radical approaches being taken by tech divers who often perceived success with these various strategies. In some cases, it appears these practices may have been leading the way toward improvements (eliminating the Helium penalty) while other adjustments might prove disadvantageous. In both cases, it is important to note that divers understood—or should have understood—these actions to be potentially dangerous, accepting risk as a natural part of pushing into uncertain territory where definitive guidance and clear borders are rarely available.
None of this is to argue that divers should engage in aggressive decompression or challenge convention or place themselves at risk to unknown complications. I merely wish to clarify the atmosphere under which these adjustments were conducted, while highlighting that conventional ideas of risk mitigation are inherently complicated against a backdrop of novel exploration. A sense of relative risk dominated most of these trials since there are also risks associated with lengthy, in-water exposure. Eliminating what might be an avoidable decompression obligation could reduce risk from other obvious factors, including changing weather, dangerous marine life, being lost at sea, hypothermia, and oxygen exposure. Decompression experimentation was but one of many attempts to establish new protocols during extensive exploration projects.
During this time, deep stops and similar adjustments were part of the “norm” for aggressive technical explorers who were sometimes notably reducing in-water time as compared to available tables. It is interesting to note that individual differences in susceptibility seemed among the most prominent variables across the range of tested adjustments, and we will return to this in a later discussion. For now, we should acknowledge that the “success” being achieved (or imagined) is greatly complicated by a small sample size of self-selected individuals who were simultaneously experimenting with a range of variables. On the other side, I do not want to entirely discount the results being seen by these divers. We should remember that development of safe decompression ascents for the general diving community was not the goal of most tech divers. These divers were interested in maximizing their personal and team efficiency during decompression. These strategies may or may not have been objectively successful or broadly applicable, but many teams imagined them so, at least within the narrow scope being considered. Later, we will return to these important distinctions with careful consideration for the potentially different interests being pursued by divers evaluating different decompression profiles.
Fortunately, a desire to create broadly useful tools was high on the list of priorities for some individuals in the technical diving community, leading to a relevant and important contribution. This inventive approach would introduce a new way to think about decompression, remaining to this day at the center of the debate about deep stops.
Creating a New Baseline
Despite the popularity of deep stops and other modifications presented previously, technical divers lacked a common language for comparing the results, especially across different profiles. Compounding this problem was the variable way decompression programs considered a dive to be more or less “safe.” For example, some programs developed “safety factors” which increased total decompression time by an arbitrary factor, i.e., made them 10% longer. Other programs used different strategies, though it was not clear whether any of the various safety factors actually made the decompression safer. Whether safer or not, these factors were typically inconsistent and added complications when comparing various profiles. Just as these debates were reaching a fervor, help arrived from an unlikely place. An engineer by trade and decompression enthusiast by choice, Erik Baker had developed a novel solution.
Baker was seeking a way to establish consistency in considering the safety or lack of safety between various profiles. The term Baker applied was Gradient Factor (GF). I will not invest considerable time exploring the science behind GF, as many useful resources are widely available. For our purposes, it is sufficient to say that GFs allows a user to establish a lower threshold than the maximum recommended by a dissolved gas algorithm. This maximum pressure or M-value is assigned to “compartments” having an assumed amount of tolerance relative to the flow of blood they receive. Decompression theory is complicated by many factors but when the pressure of a gas in a compartment nears the M-value, it is thought that the risk of decompression sickness becomes higher. By adjusting a profile through the use of GF, one presumably reduces the risk. However, this also means there is less gradient between the gas in tissues and the gas in blood, reducing the driving force for the removal of gas and probably requiring additional decompression time.
Gradient factors took an important step toward using a consistent language when talking about a variety of adjustments divers might make to their decompression. As part of his work on GF, Baker had developed a keen interest in the decompression adjustments by leading technical divers. Baker and I began working together while evaluating some of our most extreme diving profiles. This collaboration led to a number of productive developments including GUE’s DecoPlanner, released in 1997, and among the first to utilize GF methodology. These collaborations further highlighted what appeared to be a discrepancy between the decompression expected by dissolved gas algorithms and the decompressions being conducted by many technical divers.
Decompression experimentation tended to cluster in small groups whose size was likely affected by self-selection with members who would stop or reduce aggressive dives when experiencing decompression sickness. Yet, it seemed possible that there was more to the story. These divers were doing several things that should have exposed them to notable risk and yet were repeatedly completing decompressions of more than 15 hours. Deep stops were only part of the story as these divers were ignoring conventional wisdom in several areas while notably reducing decompression time. What was behind this discrepancy? Were deep stops right or wrong? Was the conservative approach to helium right or wrong? Were these individuals lucky? Were they unusually resistant to decompression sickness, or were there other factors lurking in the background?
Note: I will outline many of these developments in the upcoming Part Three, where we more directly consider the modern challenges to deep stops and most especially the assertion they are dangerous. In the interim, I hope to hear from our readers. Do you have different experiences from this period? Do you think such experimentation is reckless or inadvisable? Please let us know your thoughts.
Jarrod is an avid explorer, researcher, author, and instructor who teaches and dives in oceans and caves around the world. Trained as a geologist, Jarrod is the founder and president of GUE and CEO of Halcyon and Extreme Exposure while remaining active in conservation, exploration, and filming projects worldwide. His explorations regularly place him in the most remote locations in the world, including numerous world record cave dives with total immersions near 30 hours. Jarrod is also an author with dozens of publications, including three books.
How Two Tech Agencies Address Isobaric Counterdiffusion
NAUI is one of the few training agencies that offers specific protocols to address Isobaric Counterdiffusion (ICD). Here NAUITEC Instructor Trainer and Examiner Daniel Millikovsky explains their approach to minimize ICD risks based on their Reduced Gradient Bubble Model (RGBM). GUE Instructor Trainer and Evaluator Richard Lundgren then explains GUE’s position on the subject. The gas diffuses both ways!
Header image by Derk Remmers
Not A Theory — A Fact! How NAUITEC Manages Isobaric Counter Diffusion
by Daniel Millikovsky
There is some confusion in the technical diving community as to whether we should pay attention to the physical law while planning gas switches, particularly on ascent. Here are some of the basics of this topic and how NAUI’s technical division, NAUITEC, has addressed this matter in training and diving operations since 1997.
Fact: Isobaric counterdiffusion is a real gas transport mechanism. We need to pay attention to it in mixed gas diving.
Fiction: Isobaric counterdiffusion is a theoretical laboratory concept and doesn’t affect divers at all.
From NAUI Technical Diver (textbook):
Isobaric counterdiffusion (ICD) describes a real gas transport mechanism in the blood and tissues of divers using helium and nitrogen. It’s not just some theoretical concoction, and it has important impacts for tech diving. It was first observed in the laboratory by Kunkle and Strauss in bubble experiments, is a basic physical law, was first studied by Lambertsen and Idicula in divers, has been extensively reported in medical and physiology journals, and is accepted by the deco science community worldwide.
Isobaric means “equal pressure.” Counterdiffusion means two or more gases diffusing in opposite directions. For divers, the gases concerned are the inert gases nitrogen and helium and not metabolic gases like oxygen, carbon dioxide, water vapor, or trace gases in the atmosphere. Specifically, ICD during mixed gas diving operations concerns the two inert gases moving in opposite directions under equal ambient pressure in tissues and blood. In order to understand this, we have to consider their relative diffusion speeds. Lighter gases diffuse faster than heavier gases. In fact, helium (He) is seven times lighter than nitrogen (N2) and diffuses 2.65 times faster.
If a diver has nitrogen-loaded tissue, and if their blood is loaded with helium, this will result in greater total gas loading because helium will diffuse into tissue and blood faster than nitrogen diffuses out, resulting in increased inert gas tensions. Conversely, if a diver has helium-loaded tissues, and their blood is loaded with nitrogen, this will produce the opposite effect: Helium will off-gas faster than nitrogen on-gases, and total inert gas tensions will be lower. This last case is what we can call in decompression planning a “Good ICD,” but we need to choose the fractions of N2 wisely on ascent.
Also, Doolette and Mitchell’s study of Inner Ear Decompression Sickness (IEDCS) shows that the inner ear may not be well-modelled by common (e.g. Bühlmann) algorithms. Doolette and Mitchell propose that a switch from a helium-rich mix to a nitrogen-rich mix, as is common in technical diving when switching from trimix to nitrox on ascent, may cause a transient supersaturation of inert gas within the inner ear and result in IEDCS. They suggest that breathing-gas switches from helium-rich to nitrogen-rich mixtures should be carefully scheduled either deep (with due consideration to nitrogen narcosis) or shallow to avoid the period of maximum supersaturation resulting from the decompression. Switches should also be made during breathing of the largest inspired oxygen partial pressure that can be safely tolerated with due consideration to oxygen toxicity.
In the case of dry suits filled with light gases while breathing heavier gases, the skin lesions resulting are a surface effect, and the symptomatology is termed “subcutaneous ICD.” Bubbles resulting from heavy-to-light breathing gas switches are called “deep-tissue ICD,” obviously not a surface-skin phenomenon. The bottom line is simple: don’t fill your exposure suits with a lighter gas than you are breathing and avoid heavy-to-light gas switches on a deco line. In both cases, the risk of bubbling increases with exposure time.
More simply, light to heavy gas procedures reduces gas loading, while heavy to light procedures increases gas loading. Note, however, that none of these counter transport issues come into play when diving a closed circuit rebreather.
The NAUITEC Way
ICD is not scientific theory, it is fact. Understanding and avoiding ICD is the way to reduce bubble formation and an increased risk of DCS, and to allow for a more efficient decompression practice in the long term.
Deep trimix dives require a high helium and low nitrogen mix [Note that NAUITEC mandates an equivalent narcotic depth (END) of 30 m, similar to Global Underwater Explorers (GUE)]. NAUITEC takes a hierarchical approach to trimix decompression based on risk reduction.
In its preferred “Zero Order Rule” (zero risk from ICD), NAUITEC recommends that divers not switch from helium to nitrogen (nitrox) breathing mixtures upon ascent. Instead, divers decompress on their bottom gas (trimix) until reaching their 6 m/20 ft stop, and then decompress on pure oxygen (O2). This reduces task loading and minimizes switch changes.
If the diver wants to reduce their deco obligation and/or add a deep deco gas, they would switch to an intermediate deco mix, specifically a “hyperoxic” trimix, also called helitrox or triox, with an oxygen fraction greater than 23.5%. In practice this is accomplished by replacing the helium with oxygen and keeping the fraction of N2 the same, or ideally less. This avoids a N2 slam from ICD. Note that it is recommended that NAUI divers always maintain an equivalent air depth (END) of no more than 30 m/100 f.
This is what we recommend and practice, and we believe it offers less risk than switching from a trimix bottom gas to an enriched air nitrox (EAN) 50, (i.e. 50% O2, 50% N2) at 21 m/70 ft, which is a common community practice. The bottom line here is that in-gassing gradients for nitrogen have been minimized by avoiding isobaric switch. THERE MUST BE A HIGH BENEFIT TO RISK RATIO to deviate from Zero Order Rule!
The additional rules present increased risk. The First Order Rule: No switches from helium to nitrogen breathing mixes deeper than 30 m/100 ft. The Second Order Rule mandates no switches from helium to nitrogen mixes deeper than 21 m/70 ft.
The last rule seems to be common in technical diving, but it has certainly not been formally tested. Just say no when the risks outweigh the benefits. Many times, the benefit of a gas switch does not outweigh the risk. Risk reduction is always the primary goal.
GUE On Isobaric Counterdiffusion
By Richard Lundgren
GUE does not dispute Isobaric Counterdiffusion (ICD) as it’s a natural part of how we achieve decompression efficiency, i.e. maximizing the gradient between the different inert gases in a diver’s tissues and what is being respired. This is sometimes referred to as the positive ICD effect.
The flip side of the coin, the negative ICD effect, involves a potential increased risk for decompression illness (DCI), most commonly subclinical manifestations affecting the inner ear and causing inner ear decompression sickness (IEDCS).
Although the exact mechanics are not known, one potential aggravating factor could well be ICD when the gradient resulting from a switch from a helium to a nitrox mix is too large. This is sometimes called a “nitrogen slam.” This occurs when a gas with slow diffusivity is transported into a tissue more rapidly than a higher-diffusing gas is transported out, like when switching from bottom gas, for example a Trimix 15/55 (15% O2, 55% helium, balance N2) to a nitrox decompression gas like Nitrox 50 (50% O2, 50% N2) at 21m/70 ft. This can result in supersaturating of some tissues and consequently, bubble formation.
Based on ICD theory alone, one could draw the conclusion that any gas switch not containing helium after a trimix/heliox dive would be provocative and increase the decompression stress. This is where academics need to be tuned to the application and empirical evidence.
The practice of “getting off the mix early and deep,” which led some divers to switch to air at great depth in order to maximize the off gassing of helium, was a common early practice in the tech community. It was a practice that most likely resulted in elevated risk of not only DCI, but also inert gas narcosis and the problems it can engender. This practice, as most people likely know, was not subscribed to by GUE.
On the contrary, GUE was the first organization to call for helium-enriched gases when diving deeper than 30 m/100 ft, both for bottom gas and decompression gas. We were also early advocates for switching from helium-based bottom gas to nitrox 50 under special circumstances.
However, it should be made very clear though, that among the very active GUE dive community, we have seen no indications or significant statistics implying that the DCI risk or occurrence is elevated when switching to Nitrox 50 as the first deco gas after a 72m/250ft dive breathing Trimix 15/55. For deeper dives, additional deco gases are used. All of these contain helium.
Another possible issue could occur when divers switch to their helium-based back gas briefly after decompressing on Nitrox 50 but before switching to pure oxygen, and/or taking an oxygen break during their 6 m/20 ft O2 decompression. However, based on thousands of decompression dives in the GUE community, these gas breaks have not been reported to cause problems. Note that these switches occur at shallow depths, and therefore reduced pressure gradients.
Superficial ICD, i.e. when the body is surrounded by a less dense gas compared to what’s being respired is more of a theoretical problem for divers, as we don’t use helium mixes to inflate our dry suits for the obvious reasons of thermal conductivity.
Interestingly, the concerns over ICD may at first glance seem irrelevant to rebreather (CCR) divers, assuming that their diluent remains the same throughout the dive. But remember most CCR divers rely on open circuit bailout, which may require gas switches.
Note: The British Sub Aqua Club (BSAC) recommends that divers allow for a maximum of 0.5 bar difference in PN2 at the point of the gas switch. According to former BSAC Tech lead Mike Rowley, “The recommendation isn’t an absolute, but a flexible advisory value so a 0.7 bar differential isn’t going to bring the Sword of Damocles down on you.”
Not A Theory — A Fact! References:
NAUI Technical Diver, National Association of Underwater Instructors, 2000.
Wienke B.R. & O’Leary T.R. Isobaric Counterdiffusion, Fact And Fiction. Advanced Diver Magazine
Technical Diving in Depth, B.R. Wienke
Lambertsen C. J., Bornmann R. C., Kent M. B. (eds). Isobaric Inert Gas Counterdiffusion. 22nd Undersea and Hyperbaric Medical Society Workshop. UHMS Publication Number 54WS(IC)1-11-82. Bethesda: Undersea and Hyperbaric Medical Society; 1979; 182 pages.
Doolette, David J., Mitchell, Simon J. (June 2003). “Biophysical basis for inner ear decompression sickness.” Journal of Applied Physiology, 94(6): 2145–50.
Daniel Millikovsky is a lifetime NAUI member (NAUI# 30750). He’s been a NAUI instructor exclusively for 22 years, a Course Director for 20 years, and in 2016, became a Course Director Trainer and Representative in Argentina. Daniel is a very active NAUI Technical Instructor Examiner (#30750L) for several courses including OC and CCR mixed gas diving. He has also been a member of the NAUI Training Committee since 2020. He owns Argentina Diving, a NAUI Premier, Pro Development, and Technical Training Center based in Buenos Aires, Argentina.
Daniel began diving in 1993 as a CMAS diver and then continued with his NAUI career, becoming an instructor in 1998. He opened his first NAUI Pro Scuba Center (DIVECOR) in Cordoba, Argentina. Daniel is enthusiastic about teaching and training and is a sought after presenter at numerous international dive conferences and shows. He can be reached at email@example.com, website: www.argentinadiving.com.
Richard Lundgren is the founder of Scandinavia’s Baltic Sea Divers and Ocean Discovery diving groups, and is a GUE Instructor Trainer, an Instructor Examiner, and a member of its Board of Directors. He has participated in numerous underwater expeditions worldwide and is one of Europe’s most experienced trimix divers. With more than 4000 dives to his credit, Richard Lundgren was a member of the GUE expeditions to dive the Britannic (sister ship of the ill-fated Titanic) in 1997 and 1999, and has been involved in numerous projects to explore mines and caves in Sweden, Norway, and Finland. In 1997, in arctic conditions, he performed the longest cave dive ever carried out in Scandinavia. Richard’s other exploration work has included the 1999 filming of the famous submarine, M1, for the BBC; the side scan sonar surveys of the Spanish gold galleons off Florida’s Key West in 2000; and the search for the Admiral’s Fleet, an ongoing project that has already led to the discovery of more than 40 virgin wrecks perfectly preserved in the cold waters of the Swedish Baltic Sea.
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