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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.

Cave

Shedding Light On Darkness: Our Plunge Into GUE’s Cave 1 Course

Remember the first time you stuck your head in an underwater cave? Recreational diving instructor Justin Barbour sure does. After his first cavern foray at Ginnie Springs last fall, he swallowed his trepidation, and he and his GUE dive buddies signed up for a Cave 1 course in Mexico. In addition to learning to manage an escalating series of valve, manifold, regulator, and light failures, here’s what went down.

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by Justin Barbour
Header photo by Justin Barbour

There is always one style of diving that most divers are quick to avoid: Cave Diving. It has developed a reputation as being one of the most dangerous styles of diving in the world, and the divers who do it regularly tend to be categorized as insane. This was definitely my thoughts when I initially signed up for my cave diving class. I was constantly told that cave divers were tempting mother nature to flick them into the afterlife at high speed. 

So why sign up for a class that you’re scared of? I was looking for a new challenge. After one of my teammates returned from Mexico from his first cave class it was all he could talk about. I joined him in Florida at the GUE Conference last year and decided to test the waters out in Ginnie Springs at The Ballroom. The moment I passed through the opening and saw the whole room open up I was hooked. The feeling of being able to overcome the fear of the unknown was something I truly never thought I would be able to do and it was incredible. Swimming around the room and looking at the rock work, exploring the cracks and crevasses, experiencing a new kind of darkness, it can easily be seen how addicting this could be. The next day my teammate Fred Espino and I signed up for our Cave 1 class in Mexico.

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Kian Farin descending into the Ballroom at Ginnie Springs, FL.

We trained for months diving as often as we could both during the day and night to simulate the feeling of being in a cave. Running line around rocks, a seemingly endless number of valve drills and s-drills, and fine tuning our positioning kicks to make sure we were ready to hit the ground running when we arrived. After every dive I caught myself thinking “are we really about to fly to Mexico and hop into a cave?” I asked myself that question even right up to the point of throwing my carbon fiber backplate into my pelican case the day before my flight.

Once in Cancun, it’s about an hour-long drive to Zero Gravity (ZG). At times the roads can feel a bit like Mario Kart brought to life, but it only added to the sense of adventure we were embarking on. Once we arrived at Zero Gravity, we were immediately introduced to Christopher Le Maillot and Fred Devos, the two owners of Zero Gravity. Between the two of them they have an uncanny ability to make you feel immediately at home.

There are early mornings at ZG. Before the sun rose on our first day we were up and working on our gear. We had dives planned that day to get acclimated to diving in this new environment and some last-minute rehearsals on skills. We arrived at the Dream Gate cenote and were in awe. We briefed and got our gear together with our guide as fast as possible and climbed down the ladders to get inside of the cave. Once in the water, trying to take in this wild moment in our lives, I realized that any notions of fear that I had were completely gone. It was only the giddy feeling you feel as a kid coming down the stairs on Christmas morning.  

As we descended into the cavern, we quickly realized how people can get hurt. Caves are the darkest places on Earth, and yet inspire a sense of wonder. As our can lights cut through the darkness, they begin to reveal the true beauty of the cave. The speleothems, or decorations, are some of the most magnificent parts inside of the caves in Mexico. These stalactites and stalagmites are remnants from when the cave was dry before the last ice age. As we swam through the cave, we realized that we are truly swimming through a frozen moment in time.  Looking at the walls you can see the different types of rock layers as they were built up over tens of thousands of years. 

A close up of a cave

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Stalactites and stalagmites in the Nai Tucha cave.

The next morning, we got to meet our instructor Laszlo Cseh. We spent most of the first day in the classroom going over theory, the history of cave diving, and the basics for why the rules are set in stone. Laszlo seemed to have an endless supply of examples for these rules in real life scenarios which seemed to be calming rather than alarming to us. That afternoon we drove to our first cenote Xtabay where we got to run through safety drills and such to make sure our fundamentals were down before running our first line into the cave. After a few practice runs in the open water portion it was time to try our first line run into the cave. 

The cave initially opened up into a massive room with an enormous triangular rock in the middle of it from the last time the roof collapsed. Once we got to the top of the rock, we were able to tie into the main line and do our initial checks of gas, time, and a quick flow check. Laszlo however had other plans for us. Running up and down the line practicing our kicks to make sure they were perfected for our first official cave dive the following morning. Even that little rush of running the line from open water to the main line was amazing.

The following morning, as with every morning, we were up and downstairs bright and early analyzing our tanks and making sure our gear was good to go so we could get to the cenote first. Upon arrival we quickly set our gear up, listened to the briefing by Laszlo, and walked back into Xtabay for our first official cave dive. Fred ran the line in this time and made quick work up to the main line. From there we were able to proceed into the cave.

One of the first things we realized when in the cave is the vastness of it. Shining our lights ahead we couldn’t see where the end of the beam was, even with crystal clear water. Once we left the entrance, we immediately came to what is known as a Halocline. This is where the saltwater in the bottom of the cave meets with the freshwater creating a false ceiling. As you pass over it, it looks like you’re swimming over a pool inside of the cave. This is something that can play tricks on the mind particularly to a disorientated diver on heading out of the cave. 

As we swam through it the fresh and saltwater began to mix clouding the visibility to near zero. Once we staggered on either side of the line however we were able to maintain visual contact with each other, the line, and were able to enjoy the 86ºF/30ºC saltwater that we just dipped into. 

The colors in the cave were incredible. The blues and greens were striking as our lights slowly moved about the cave. The rock walls were white in the saltwater, and a tan color in the freshwater.

A close up of an animal

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Tannic acid stained stalactites.

As we moved room to room, I glanced at my computer and realized we’d been swimming for 30 minutes. While that’s not an unusual time for an open water dive, I rapidly came to the realization of “wait, if something goes wrong, we’re going to have to turn around and swim at least another 30 minutes to get back out.” It was this sobering thought that helped me understand the severity of what we were doing. Nothing a few glances around the cave couldn’t snap me out of the thought and continue on with the dive. 

As we continued to swim, we came upon a bed plane section of the cave. This is where the cave narrows, but not too small, vertically and can stretch for hundreds of feet horizontally. As we swam forward, we truly started to feel like cave divers. Alas only a few minutes into the dive we hit our turn pressure and had to exit the cave.

As we surfaced Fred and I immediately popped our regulators out of our mouths and exploded with excitement. It seemed like months of hard work and uncertainty finally paid off in that moment, and it was only day two of the class. Laszlo, in his usual Laszlo calm and collected nature, reigned us in with a subtle, “Meh it was ok, let’s see how you do in the cave.” Little did we know the gauntlet that was ahead of us.

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Divers Keith Chu (left) and Fred Espino (right).

Throughout the class we would arrive first thing in the morning, set up our gear, run through dry drills, gear up, and do three to four dives in a day. Upon finishing those we would drive back to ZG and would be in theory for a few more hours before finally being sprung loose for tacos at Marre Marre across the street. They were very long days to say the least, yet every morning we would wake up before our alarms and couldn’t wait to see what the next set of dives had in store for us. 

The next few days were filled with every failure that Laszlo and his trusty bubble gun sidekick could think of. From light failures (primary and both back up lights), to regulator failures, we drilled until it became second nature. On one particular dive we were particularly tested. As we swam through the cave just as we were passing through a thermocline I had a simulated primary light failure. After going through the procedures for stowing, deploying backups, etc. Fred and I turned the dive and started our exit. Suddenly he also experienced a simulated primary light failure and had to go through that process. One thing after another happened honing in our focus to the point where we found ourselves in total darkness. As we stayed in touch-contact we moved along the line as a unit. This shows a level of trust in not only your training, but your teammate as well.

Though as the days went on our confidence began to grow swimming around in the caves. Our awareness began to expand to allow us not only to focus on the team, but also begin to actually see the cave. Looking for the jump lines off the main line, seeing more of the details in the room, genuinely being able to take in the vastness and natural beauty that helped shape these caves over millennia.

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One of the massive passageways through Nai Tucha.

On the final day we were tasked with two of the most important skills a new cave diver can learn: lost line drill while blindfolded, and gas share exit while blind. These are skills that hopefully a diver never needs to use, however we are going into a hostile world every time we enter a cave, so it’s important to train for every scenario to be prepared for whatever is thrown our way. In a strange way having the blindfold on elicited a sense of calm like I’d never felt. One hand on the line, the other out in front feeling our way out, with Fred hanging onto my elbow, we moved as a unit through the cave, navigating each station (tie off on a rock) as we moved. It felt like a new puzzle each time we came to a station as we moved our way out. 

By the end of the day, Laszlo decided that we’d been through enough and we officially were Cave 1 divers. The feeling was hard to describe for a number of reasons. Chiefly both Fred and I had finally reached the moment we had been training for, studying for, and fearful about. We passed. We were officially apart of that group of divers most people think have a screw loose. 

Looking back on what we learned and experienced, it’s clear to see that there are really only a few hard-fast rules to follow, similarly and readily transferable to open water dives, which minimize the risk we take on in the caves. First, plan your dive and dive your plan. This includes careful gas planning, and everyone taking responsibility for their own gear, gas, and navigation. Having a continuous guideline starting in open water, so that even in zero visibility you can always find your way out. Maintain your equipment making sure all regulators, valves, and lights are in prime shape to avoid potential problems in the caves. And lastly, ALWAYS be aware of the mainline. With these in mind one can go around the world swimming through the history books in caves and mines exploring what once was and embracing the beauty of what is.

Additional Resources:

GUE Cave 1 Description
Cave 1 Video


Diving since 1998 from the Maldives to the Midwest, Justin is a PADI Master Scuba Diver Trainer and GUE Diver who brings a courageous mentality and humorous demeanor to the water. From 2015-2020 he spent countless hours volunteering at the Aquarium of the Pacific applying his expertise to educate children on sustainable ocean interaction. 
As a boy he would read constantly about marine biology, wanting to see it for himself he dove right in and has hardly been up for air since. Most intriguing to him is exploring and learning the history of shipwrecks. Justin’s life above sea level includes a career path that has taken him from medical sales to tech startups, every endeavor inspired by the tenacity of being a former collegiate athlete. His goal is to never stop pushing himself, allowing the water to continually reveal character and to share those experiences with others.  

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