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NEDU Deep Stop Summary

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The NEDU stop study remains the most detailed deep stop research done to date. It may well remain this way for some years to come and conceivably even indefinitely. This is because such research is expensive and because the issue appears largely settled in the minds of those with the budgets needed to explore this topic. Other studies are being developed but likely with a much smaller budget and fewer controls. Moreover, the kind of study tech divers would like to see may well fail to identify enough difference between dissolved gas and deep stop profiles to be meaningful. This latter problem is a good place to start our summary of the current research since it also helps contextualize some of the decisions made by NEDU researchers.

In order to be useful a study should demonstrate some difference between the things being measured. All things being equal one might as well stay with the long-used and widely successful dissolved gas models if deep stops and bubble models appear “similar” in outcomes. This means that a study should use a provocative decompression in order to develop some decompression sickness by which to measure a difference in models. In this case, the NEDU study dived US Navy divers without thermal protection on air to a depth of 170 feet/52meters where they conducted work for 30 minutes before ascending over a 144 minute decompression. Divers were often shivering upon surfacing, reducing perfusion and increasing risk of DCS. Some research indicates cold of this sort would be like doubling one’s bottom time when considering the effect of reduced blood flow in cold divers. The NEDU abstract provides a nice overview of the study which can be reviewed in entirety here.

We should first acknowledge this study was a US Navy test designed to evaluate whether there was any benefit to move toward bubble-based models including deep stops. The procedures of tech divers vary considerably from those of most US Navy diving and so it was inevitable that tech divers would find such a study lacking as a useful comparison. These differences complicate evaluation of deep stop in the minds of some tech divers. The main complications relate to 1) the amount of decompression time, 2) the unusual decompression stop arrangement, 3) the breathing gases used, and 4) the temperature of the water. We shall take each of these into consideration in an attempt to outline the reasons for these choices and the primary discontent. However, readers are again encouraged to review in detail these assessments, so they can develop a more informed opinion. 

The amount of decompression and arrangement of stops derive from the US Navy algorithms selected. The total decompression time was based upon the gas content (dissolved gas), VVAL18 Thalmann algorithm which formed the baseline by which to compare a deep stop schedule as generated by the probabilistic BVM(3) bubble model. The bubble model was set to optimize a 174-minute decompression with the lowest possible risk, developing stops that would control bubbles in a way consistent with its model parameters. These aspects have aroused some disagreement in the technical community who argue the total time and associated “deep stops” are longer than reasonable and a far departure from what any tech diver might consider for decompression. However, the dissolved gas model accurately predicted and did result in relatively low incidence of DCS for “shallow stop” protocols. Meanwhile, the argument for deep stops is largely that they should limit supersaturation and bubble formation in a way that provides more benefit than the increase in gas dissolved in slow compartments that result from the stops. This study demonstrates this does not appear to be true, at least within the scope of these profiles. For a variety of reasons, most experts do not believe changing the stop distribution would have a significant effect on this failure of deep stops to work as it was hoped they might. Nonetheless, the additional time when compared to a common deep-stop, gradient approach of 20/85 resulted in 59 minutes of additional “deep stop” minutes, eliciting reasonable discontent among some.

Deep stop proponents also took issue with the use of air diving though most made less of this than the previous discussion revolving around the length and distribution of stops. We don’t have a good reason to believe the value of deep stops should be negated by certain breathing mixes. If deep stops control bubbling in a useful way, they should do so independent of the gasses breathed. Some argue that the value of hyperoxic mixes in concert with deep stops might have an additive value though no evidence appears to exist that supports that contention. 

Finally, some argue that the cold experienced by divers worked in concert with the added time at depth to disadvantage divers on the deep stop profile. The argument is that these divers were ascending while following an unreasonably long, deep-stop schedule and were thus reaching critical parts of their offgassing much later in the dive when they were very cold and where perfusion was greatly reduced. Meanwhile, the argument goes, the shallow-stop divers had finished the bulk of their decompression before they became cold. Some have even argued that this experiment was more about testing thermal issues than deep stops though most experts appear unified in disagreeing with that view. The experts argue that both groups suffered from the same thermal stress and that the low but relevant DCS incidence in the shallow-stop profile support this contention. 

In the end, these are not issues that can resolve through additional debate as evidenced by hundreds of posts and extensive argumentation. However, most divers and especially most experts appear convinced the NEDU study supports an argument that deep stops are actually less efficient because they do not appear to control bubbling enough to overcome the additional gas absorbed by slow tissues during the additional time at depth. The experts argue that all aspects of concern for tech divers i.e. use of air, cold water and extended stop time are not arguments in support of deep stops. Adjustments in these areas through use of shorter deep stops and hyperoxic mixes might reduce the difference but would merely be masking the lack of improved efficiency. 

The NEDU study appears reasonably convincing to most, at least with respect to a lack of compelling value in favor of deep stops, though with some complications as discussed. I will come back to some of these complications but first our review should conclude with the apparent relevance of other studies seeking to establish the value of deep stops. In 2005 a French study evaluated deep stops by measuring venous gas bubbles. We previously discussed the complexity of relying upon such measures though the technique likely remains broadly useful for considering decompression stress across a diving population. The French study suggested that none of the deep models appeared superior in venous bubble control and one was rated inferior. A Ljubkovic Study conducted in 2010 again used venous gas emboli (bubbles) to see how effectively the varying permeability model (VPM) controlled bubbles. They determined it was not particularly effective in this regard although they did not compare its success to bubbles present with other strategies. The Spisni Study in 2017 compared a ratio-deco, rule-based approach to a dissolved-gas, gradient-factor approach and concluded that adding longer and/or deeper stops was not more effective as based upon higher post-dive inflammation associated with deeper stops. 

When all these pieces are considered alongside the more compelling NEDU research, it appears that deep stops are not bringing the long-imagined benefit sought by proponents, at least not in a way that is easily qualified. Anti-inflammatory markers and venous bubbles are both imperfect markers and leave ample room to argue against these studies. Yet, our efforts should be less about resisting developing knowledge and more about learning what we can from accumulated wisdom. To this end, we can merge three of these conclusions into growing sense that deep stops do not appear to be controlling venous bubbles in a pronounced way. This adds an interesting dimension but is it important? We would be hard pressed to argue that increased venous bubbling is a positive development even while acknowledging it is a relatively common occurrence for blood leaving tissues in the process of off-gassing. 

On the one side, deep-stop advocates can argue 1) that venous gas bubbles are not a useful diagnostic measure of DCS, 2) that anti-inflammatory markers show contradictory results in various studies, and 3) that the NEDU study is not representative of technical diving profiles and therefore not an effective indictment of deep stops as commonly used. On the other side, one can argue that 1) deep stops do not appear to effectively control venous bubbles which are very problematic for some forms decompression illness and generally correlated with DCS likelihood across populations of divers, 2) that several studies hint at a weakness in deep stops with the most detailed study to date showing a clearly increased risk of DCS, and 3) no objective study to date appears to support the value of deep stops.

An objective review of the developing science does appear to support the idea that deep stops fail to provide compelling value and may, in fact be less efficient. Some find the evidence compelling, some feel swayed but promote a measured response and some remain entirely unconvinced. Is there anything else we might interpret from the trending science on deep stops? We will return to this subject in part four of our series.

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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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Classical decompression algorithms limit hypothetical tissue gas contents and prescribe decompression schedules with most of the total stop time (TST) allocated to shallow decompression stops. More recent bubble-model-based algorithms limit hypothetical bubble profusion and size and prescribe decompressions with TST skewed toward deeper stops. A large man-trial compared the efficiency of these approaches. Divers wearing swimsuits and tshirts, breathing surface-supplied air via MK 20 UBA, and immersed in 86 °F water were compressed at 57 fsw/min to 170 fsw for a 30 minute bottom time during which they performed 130 watt cycle ergometer work. They were then decompressed at 30 fsw/min with stops prescribed by one of two schedules. The shallow stops schedule, with a first stop at 40 fsw and 174 minutes TST, was prescribed by the, deterministic, gas content, VVAL18 Thalmann Algorithm. The deep stops schedule, with a first stop at 70 fsw, was the optimum distribution of 174 minutes TST according to the probabilistic BVM(3) bubble model. Decompression sickness (DCS) incidence following these schedules was compared. The trial was terminated after the midpoint interim analysis, when the DCS incidence of the deep stops dive profile (11 DCS/198 dives) was significantly higher than that of the shallow stops dive profile (3/192, p=0.030, one-sided Fisher Exact). On review, one deep stops DCS was excluded, but the result remained significant (p=0.047). Most DCS was mild, late onset, Type I, but two cases involved rapidly progressing CNS manifestations. Results indicate that slower tissue gas washout or continued gas uptake offsets the benefits of reduced bubble growth at deep stops.