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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.

Diving Safety

The Flexibility of Standard Operating Procedures

Instructor trainer Guy Shockey discusses the purpose, value, and yes, flexibility of standard operating procedures, or SOPs, in diving. Sound like an oxymoron? Shockey explains how SOPs can help offload some of our internal processing and situational awareness, so we can focus on the important part of the dive—having FUN!

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By Guy Shockey

Header Image by Derk Remmers

At first glance, the title reads like a bit of an oxymoron. How can a standard operating procedure (SOP)—which implies a ‘one size fits all’ solution to problem solving—also be flexible? How can flexible also be firm?

One of the things that initially attracted me to Global Underwater Explorers (GUE) was the presence of SOPs. For anyone with a military background, SOPs were our bread and butter. You can create a good SOP while you have the time to think and plan. You can put them into practice, refine them over time, and keep them in place until new or better information comes along to change them. 

For example, airline pilots have a binder full of SOPs for various contingencies. When something comes up, they turn to the correct page and find a list of actions to follow. Pilots understand that these SOPs represent the collective knowledge of many aviators and engineers that have come before them. Many of them have also been revised multiple times, codified, and then even revised after that. Some SOPs require commitment to memory because there may not be a lot of time, and pulling out a three-ring binder or flipping through your iPad to the correct page isn’t the appropriate action. In that case, then those same pilots practice these situations regularly in simulator training. 

One of the primary values of an SOP is that it frees up a lot of situational awareness information processing. You are able to match up “mental models” to the current situation and, rather than processing your information in small bite-sized pieces, you are able to process “chunks” of information that match patterns of something that you know or are familiar with. 

Let me create an analogy that may help make this clearer. If I were to give you a bowl of tomato sauce, some slices of pepperoni, some mushrooms, some cheese, and a piece of baked dough, you could eat them all one at a time and try to figure out what it was you were eating. Or, I can put all those ingredients on that same piece of dough, bake it, and you would instantly know that you were eating pizza. You don’t have to process all the ingredients one at a time. You already have an existing mental model that says “pizza.” We do this when we solve problems. We pattern-match and identify existing mental models all the time, and it’s actually the only way we can actually think as fast as we do. Many problems are actually solved with multiple mental models being applied together. 

Photo by Derk Remmers.

Having an SOP gives you the ability to solve problems more efficiently and effectively because you have a ready-made mental model or solution to a recognized problem. Think of every first aid course you have ever taken and the “ABCs” of first aid. SOPs are incredibly valuable in nearly every environment that includes potential risks. 

If an SOP is shared, it also allows diverse groups to work together. It is no surprise that SOPs from various militaries of the world are often similar, even if they are written in different languages. From personal experience, NATO countries can coordinate and execute complex military operations because they share common SOPs that, if not identical, are very similar and don’t require much adjusting to mesh together. Common expectations and goals can be shared toward a common purpose. 

When in time-sensitive environments, many of these SOPs and the corresponding mental models they help develop can be lifesaving. This doesn’t just apply to the military, but also to law enforcement, paramedics, firefighters, pilots, and any other profession that is often faced with time pressures in making critical decisions. 



Do you share a common operational picture?

There is an interesting term often used in military circles called the “common operational picture” (COP). This is exactly what it sounds like, and is sometimes referred to as “a single source of truth.” Everyone involved in a decision-making cycle needs to be privy to the information that affects their decision. Sharing that information allows us to make informed decisions that often include SOPs. You could argue that we are creating a mental model that lets us apply another mental model!  

Alright, so how exactly does all this apply to diving and GUE diving in particular? I’m pretty sure that many of you have already connected many of the dots. 

In the GUE world, our divers create a COP at the beginning of the dive. We help reiterate this COP with our GUE EDGE pre-dive checklist, which is a great example of an SOP! We review the goals, team roles, our equipment, and the operational parameters of the dive, all in a standardized format that efficiently accommodates teammates from multiple different languages and cultures. I have performed GUE EDGEs in about 10 different languages and I only speak two!  The fact that we were doing this in a standardized fashion meant I could follow along and knew what they were talking about. 

As the dive plan complexity increases, so too does the COP become more complex. Some of our more ambitious exploration projects require even more time spent in planning than actual execution. But because there is a COP, coupled with SOPs (I know that’s a lot of acronyms), these projects usually go off without a hitch. 

Photo by Derk Remmers.

During the dive, there are multiple times that we have team-expected actions that are based on SOPs, and this contributes to and reinforces our COP. It is almost as if we are filling in a PDF form as we go along and confirming the various pieces of information that we need to complete the entire “form” or plan. 

In the case of emergencies, we have ready-to-implement SOPs for just about any equipment malfunction from valve failures to losing your mask. We practice these SOPs so that, in real time, we can employ them in a timely fashion and resolve the problem. These SOPs are just like the ones I mentioned at the beginning of this article and were developed over time and refined with successive reviews and after-action analyses. Finally, they have been codified, and you can now find them in our GUE SOP manual! You will also notice that this manual is of a particular “version,” which tells you that the SOP is constantly being fine-tuned in a dynamic process.

How Can An SOP Be Flexible?

In reality, it isn’t the actual SOP that is flexible, but it is the degree of flexibility it provides to the dive plan itself that is of value. Let me give you an example from the technical diving world. 

Imagine the team is diving on a wreck and experiences a delay on the bottom for whatever reason. It could be that it was done on purpose (discovery of pirate gold!) or maybe it was imposed upon the team as a result of any number of problems, like dealing with an equipment problem or an entanglement, for example. The dive is longer, the decompression obligation is now going to be longer, and there are some decisions to be made. 

Having an SOP here can help provide a solution to the problem with no mess and no fuss. The divers dig into the bag of tricks they learned in GUE technical training, and because of their common operational picture and team-expected actions, they apply the SOP they practice regularly and modify their decompression schedule to suit the new bottom time. What could have been an exciting moment for many divers turns into just another discussion point for their debrief after the dive!

Photo by Alexandra Graziano.

So, while SOPs are usually not flexible in and of themselves, they allow for a great deal of flexibility while diving by freeing up mental processing power and providing ready-made and practiced solutions to potential problems. 

GUE SOPs presuppose the presence of personal diving skills at a high level, and assume that factors such as good buoyancy and trim are second nature. In fact, many of the SOPs state the first step in resolving a problem as “stabilize” or “stop” in all three dimensions. GUE divers see that, as the diving gets more complex, the SOPs also get more complex. For a new GUE Fundamentals diver, demonstrating some of the SOPs required to pass muster as a Tech 2 or CCR 2 diver look more akin to channeling “the force” than anything else. However, like most things, perfect practice produces perfect performance, and so it’s just a matter of putting in the repetitions. 

For me, diving has never been the end but the means to the end. Anything I can do to make those means take up less mental and physical horsepower means that I can devote more of the same to the end goal. And at the end of the day, I am really all about that pirate gold!

Additional Resources:

Note that GUE members or divers taking a GUE course receive access to GUE’s 30-page manual, Standard Operating Procedures.


Guy Shockey is a GUE instructor and trainer who is actively involved in mentoring the next generation of GUE divers. He started diving in 1982 in a cold mountain lake in Alberta, Canada. Since then, he has logged somewhere close to 8,000 dives in most of the oceans of the world. He is a passionate technical diver with a particular interest in deeper ocean wreck diving. He is a former military officer and professional hunter with both bachelor’s and master’s degrees in political science. He is also an entrepreneur with several successful startup companies to his credit.

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