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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!

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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. 

Photo by Derk Remmers.

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.

Additional Resources:

Counterculture and Counterdiffusion: Isobaric Counterdiffusion in the Real World  

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 BR, O’Leary T. “Ins and Outs of Mixed Gas Counter Diffusion.” Tech Corner. Sources 3Q 2004: 45-47

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 info@argentinadiving.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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Learning from Others’ Mistakes: The Power of Context-Rich “Second” Stories

Proper storytelling is a key to learning from the mistakes of others. Human Factors consultant and educator Gareth Lock explains the power of context-rich stories to inform and help us to develop the non-technical skills needed to make better decisions, communicate more clearly, and lead/teach more effectively.

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by Gareth Lock
Header image courtesy of Gareth Lock
. Divers from Red Sea Explorers’ examining a magnificent gorgonian coral.

Diving can be a fun, sociable, and peaceful activity; it can be challenging and technically difficult; and it can be a way of escaping the hustle and bustle of modern life. Sometimes new wrecks are discovered, caves have new line laid in them, new encounters with wildlife are experienced, and in many cases, courses are completed where both instructors and students have learned something new. 

However, it can also be scary, harrowing and frightening if things don’t go to plan or if the plan was flawed in the first place. 

Fortunately, the majority of dives which take place are the former and we consider the outcomes to be positive. If we think about it, the goal for every dive should be to surface, having had an enjoyable time, with gas reserves intact and no-one feeling physically or emotionally injured. But how do we achieve this goal considering the inherent risks we face while diving? 

The easy answer would be to have effective training, to have the correct equipment, and to have and apply the right mindset. These three things together then lead to safe diving practices. You could say that the majority of safe diving practices and safely designed and configured equipment comes from feedback following accidents, incidents, and near misses. You only have to look at the work which the late, famed cave explorer Sheck Exley did in terms of cave diving fatalities and his “Blueprint for Survival” to see how procedures and equipment have evolved.

What do we learn?

There are accident and incident reports available to us. What do we learn from them? Bearing in mind that the majority of reports which divers see are either in social media or summarised in reports like the Divers Alert Network Annual Incident Report or the BS-AC Annual Incident Report.

For example, the following incident reports are written in a style similar to those you would find on social media or in an organization’s incident report.   

An inexperienced diver entered the water to provide support for a guided dive to 24m. They got separated from their buddy, made a rapid ascent to the surface after nearly running out of gas. They were recovered on the boat without any symptoms of DCS being present.

A diver on the final dive of a rebreather training course entered the water from a dive boat. The diver swam to the side of the boat to receive their bailout cylinder to clip on. While sorting their gear out alongside the boat, they appeared to go unconscious and descend below the surface. The diver was recovered from 38 m/124 ft and despite CPR and first aid being applied, they were pronounced dead on arrival at the hospital ER. On inspection, the oxygen cylinder on their rebreather was found to be turned off and the controller logs showed that the pO2 had dropped to 0.05 while they were on the surface.

How much learning do you get from these reports? What emotions did you feel while reading them? What did you think was the primary cause of each of these events? If you were to choose two or three words to describe the causes, what would they be? 

Human error? Complacency? Inexperience? Rushing? Not paying attention? Overconfidence? Naivety? Arrogance? Stupidity? Who was it? Where was the instructor? Were they certified? Which agency? Were they qualified?

All of these are normal responses, and they make up the first story.

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Photo courtesy of 123rf Image Library

The First Story

The first story is the narrative we hear, and we start to make immediate judgments on. We can’t help making judgments, even when we try not to. We make judgments because we compare the stories we’ve just read or heard to our own previous experiences. We match patterns to what we ‘know’ and then fill in the gaps with what we think happened, all the time thinking about whether it was the ‘right thing’ to do based on our own experiences.

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Photo by Gareth Lock.

This ‘filling in gaps’ is normal human behavior. Because our brains are constantly trying to make sense of the situation when we don’t have enough information about a scene or a situation, we reflect on what we’ve seen, read, and heard in the past and then make a best guess or closest fit. During this process, we will be subject to a number of biases, and one of the strongest at this stage is called confirmation bias. This is where we think we know the answer to the question, then as we read or hear something in the story that aligns with our reasoning, we stop looking any further because we have confirmed our suspicions.

In many cases, we carry on and don’t think anything of the learning opportunities presented because we know what happened, we know that ‘we wouldn’t do that’ because we would have spotted the issue before it became critical. We often make use of counterfactuals (could have, should have, and would have) to describe how the incident could have been prevented.



Unfortunately, this means that often we don’t learn. There is a difference between a lesson identified and a lesson learned—a lesson learned is where we make a conscious decision to accept how we do things based on the conditions and outcomes, or we actually put something in place which is different than what was there before and see how effective it is to resolve the problem encountered. 

If we are to make improvements, we need to look at the errors, mistakes, and deviations that were made. However, we must recognize that errors are outcomes, not causes of adverse events. If we want to stop an adverse event from occurring, we need to look closer at the conditions which led to the error occurring i.e., the error-producing conditions. 

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Extracted from INPO/DOE Human Performance Improvement Handbook Vol 1 – The Human Diver.

The easiest way to look for error-producing conditions in an event that has already happened is to get those involved to tell context-rich stories. This becomes the second story.

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Photo courtesy of 123rf Image Library.

The Second Story

Second stories look much deeper than what we first hear. They look at the context, the local rationality, the conditions, especially those conditions which might lead to errors. Ultimately, they expose the inherent weakness and gaps in any system, where the system includes people, paperwork, equipment, relationships, the environment and their interactions. 

Second stories also highlight how divers and instructors are constantly adapting and changing their behaviors/actions to deal with the dynamic nature of diving. They describe ‘normal work’. This adaptation could be moving dive sites, increasing or reducing the time for a course, the order in which skills are taught or the amount of gas used/planned for a dive. Second stories describe the difference between ‘Work as Imagined’, which is what is written down, what is expected to happen, and against which compliance is assessed, and ‘Work as Done’ which is what actually happens in the real world and takes into account the pressures, drivers, and constraints which are faced by those on the dive or the course.

The easiest way to see what a second story looks like is to tell it, and the following account is the same recreational event as above but told as a second story. 

An Advanced Open Water (AOW) diver with around 50 dives was acting as an ‘assistant’ to the instructor and dive-centre owner on a guided dive with five Open Water (OW) divers and recent graduates from the school they themselves had learned at. The AOW diver felt a social obligation to help the Open Water Scuba Instructor (OWSI) who was leading the dive, because the OWSI had done so much to help her conquer her fear of mask-clearing during her own training. However, she was also wary that, over time, her role had moved from being a diver on the trip to being almost the divemaster by helping other divers out, which she wasn’t trained to do. In addition, the instructor regularly asked her, at the last minute, to help out and change teams to ensure the ‘experience’ dives happened.

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Photo by Gareth Lock.

On this particular occasion, the AOW diver was buddied with a low-skilled OW diver who acted arrogantly and did not communicate well. In fact, she didn’t believe that three of the five on this trip should have received their OW certificates, given their poor in-water skills. As they approached the dive site, the visibility could be seen to be poor from the boat and the surface conditions weren’t great. The instructor said to the AOW diver, “Don’t lose the divers. I want you at the back shepherding them.”

They entered the water and descended to 24 m/78 ft and made their way in the poor visibility. On two occasions, the OW buddy had to be brought back down by the AOW diver as they ascended out of control. At one point, the OW diver turned around quickly and accidently knocked the AOW diver into the reef. Unfortunately, the AOW diver became entangled in some line there, and the OW diver swam off oblivious to the entanglement. When the five divers and instructor reached the shot-line ready to ascend, the instructor realized the AOW diver was missing. The instructor couldn’t trust the five divers to ascend on their own and didn’t have enough time to wait at the bottom and conduct a search, so the six ascended. On the surface, the buddied OW diver said that the AOW diver had swum off looking at fish in a certain area.

In the meantime, the AOW diver had managed to free herself; but in her panic, while stuck on the bottom, she breathed her gas down to almost zero and had to do a rapid ascent. She surfaced, feeling very scared and sick with panic, just as the instructor was speaking to the other six on the surface. On seeing the AOW diver break the surface, the instructor swam to her but turned and shouted at the other divers, admonishing them for abandoning their buddy on the bottom. The AOW diver felt very alone and wanted to give up diving as she was not given the opportunity to tell her side of the story.

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Photo by Gareth Lock.

Observations on potential contributory factors and error-producing conditions:

  • Deviation of standards on the part of the instructor/dive-center owner taking OW divers to 24 m/78 ft, maybe driven because of the need to generate revenue and offer something unique.
  • Authority gradient between the instructor and AOW diver meant that the AOW diver felt they couldn’t end the dive before they even got in the water or once in the water.
  • Inferred peer pressure to help out when they weren’t qualified or experienced enough to act in a supervisory role.
  • Poor technical skills on the part of the OW divers and the AOW limited their situation awareness to be aware of hazards and risks.
  • Limited awareness on the part of the instructor regarding the location of all the divers during the dive.
  • Positive note – good decision on the part of the instructor to ascend with the five OW divers in poor conditions and not keep them on the bottom or get them to ascend on their own.

A full account of the second event can be found here where you can also download a guide which contains more detail than the video covers and also gives you details on how to run a learning event at your dive center or in your own classes.

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We can see that the learning opportunities have increased in the second stories. They allow certain issues to be identified like time pressures, financial pressures, peer-pressure, authority gradient, teamwork, leadership, decision-making and situation awareness. These aspects are rarely captured or recounted in the narratives we see online or in incident reports. There are a number of reasons: 

  • They are often considered ‘common sense’, 
  • Our brains are constantly looking for simple answers to complicated or complex problems, and one of the easiest ways to do this is to find an individual or piece of equipment to ‘blame’ rather than look wider.
  • Those involved don’t consider these factors to be important so they don’t write them down.
  • Those involved don’t know about these error-producing conditions or human factors so they don’t know to include them.
  • There is no formalised and structured investigation process for diving incidents by diving organisations to facilitate the capture, analysis and sharing of second stories.

Telling second stories isn’t enough to create learning though. We have to work out how to change our own behaviors, and that is where the free materials and courses which The Human Diver provides come in. They help develop these non-technical skills in divers, instructors, instructor trainers, and dive center managers/owners to help them make better decisions, communicate more clearly and lead/teach more effectively. Ultimately, it is about having more fun on the dive, and ending each dive with the goal described at the start of this article intact and creating learning in the process.


Since 2011, Gareth has been on a mission to take the human factors and crew resource management lessons learned from his 25 year military aviation career and apply it to diving. In 2016, he formed The Human Diver with the goal to bring human factors, non-technical skills and a Just Culture to the diving industry via a number of different online and face-to-face programmes. Since then, he has trained more than 350 divers from across the globe in face-to-face programmes and nearly 1500 people are subscribed to his online micro-class. In March 2019, he published ‘Under Pressure: Diving Deeper with Human Factors’ which has sold more than 4000 copies and on 20 May 2020, the documentary ‘If Only…’ was released which tells the story of a tragic diving accident through the lens of human factors and a Just Culture. He has presented around the globe at dive shows and conferences to share his passion and knowledge. He has also acted as a subject matter expert on a number of military diving incidents and accidents focusing on the role of human factors.

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