by Reilly Fogarty
Header image: A Navy diver undergoing a thermal experiment at the US Navy Experimental Diving Unit (NEDU) in Panama City, Fl. Photo by Stefan Frink
Heating solutions for divers have come a long way in the past two decades. Not long ago a diver’s only option for active heating underwater was a hot water suit, complete with the logistical limitations of surface or habitat supplied water. Equipment and tenders made the suits difficult to put to use and divers were limited to exploring the seafloor only as far as their hot water hose could reach. These challenges made active heating accessible only to a small subset of the commercial diving industry, and nearly impossible for recreational divers to use except for some notable but extraordinarily rare exceptions.
Limited by thermal exposure on long dives in warm water and short excursions in wintry waters alike, recreational divers began using hot-packs, hot water bottles, and any number of similar solutions to heat their drysuits with typically underwhelming results. The evolution of electrical heating systems however, brought active heating within reach for a huge number of new divers. The first iterations were fickle and expensive, and many home-brew solutions involving heated motorcycle vests and DIY-battery packs were concocted, but the recreational industry adopted the technology in relatively short order. Now divers have a wealth of options for heated undergarments, from minimalist and self-contained systems that wear like a t-shirt and can be used in a wetsuit to full-body undergarments with gloves and booties powered by external battery packs.
Recreational divers have adopted these heated undergarments rather quickly, but the total market share is not yet widespread enough to allow much in the way of statistical analysis. The technology has made diving a winter sport for many, but the proliferation without adequate research has brought some serious concerns about decompression stress to the forefront. While these new tools make it possible to comfortably endure harsher climates and longer exposures, if they are used incorrectly, they can dramatically increase decompression stress. Here’s what you should know as you decide whether to heat your next dive.
Heating’s Double Edged Sword
The crux of the concern surrounding heated undergarment use lies in the effect of temperature on decompression sickness risk. A number of aptly named studies have laid the statistical groundwork for what most of us already believe to be true—temperature has a significant effect on our ability to absorb and eliminate inert gases.
Cold immersion gives us a number of things to contend with. Vasoconstriction, the narrowing of blood vessels, helps shunt blood to the core to maintain core temperature but leaves hands and feet cold and quick to numb. Slow perfusion in these tissues can slow both the uptake and removal of inert gases, the latter of which increases decompression stress. The body will also attempt to eliminate fluid via urination, promoting dehydration, and in some cases breathing rate can be increased via cold water shock or increased metabolic drive to keep the body warm. These factors are familiar to divers, but they all contribute to decompression stress.
Active heating systems—used properly—can address these factors. Keeping a diver warm can minimize vasoconstriction and improve perfusion, improving inert gas elimination in extremities. Warm divers will shunt less fluid to their core, produce less urine and face fewer concerns from cold-induced physiological reactions. It’s the potential to increase total inert gas load via warming throughout a dive, or allowing efficient gas loading and then hampering decompression via the failure or removal of heat on the ascent portion of a dive that can put a diver at serious risk. Not only are these devices prone to failure just by nature of being electrical in an underwater environment, but even properly functioning, their inappropriate application can leave a diver with significantly more decompression stress than they would have faced on a dive without their heating solution of choice.
It’s the severity of these risks in the real world coupled with the conflicting data that makes this a tough topic to tackle for recreational divers. Heated undergarments make an enormous difference on long technical dives, they have the potential to make a dive not only more comfortable but safer, but they can also put divers in needless risk.
Even the best data on thermal status fails to give us more than correlations with DCS symptoms, which makes estimating risk nearly impossible. There is, however, a respectable body of research that indicates divers using hot water suits may experience DCS at a higher rate than their counterparts. One study from 1951 on hot water suit use among surface decompression dives indicated that each 10°C increase in water temperature increased the odds ratio of DCS by 1.96 and that this effect was most pronounced on shorter dives in the study. A later review of that study however, indicated that the probability of some of the DCS symptoms, specifically the Type 2 symptoms could have been “better explained by the dive profile than by the temperature” (Leffler, 2001). Another work by the same author indicates a significant increase in DCS risk among divers who are warm at depth, specifically pointing to vasodilation induced promotion of on-gassing efficiency. This correlation between hot water suit use at depth and DCS risk was also found among divers working on the TWA Flight 800 recovery in 1997.
It’s worth noting that hot water suits and electrical heated undergarments may not be entirely identical systems. Hot water suits have a much greater heating potential and have been shown to cause some fluid loss in divers, primarily from sweat. In real-world applications however, both can be used in a similar enough fashion that many of the lessons learned from research into hot water suit use can be carried over to more modern systems.
The real conflict in data and theory comes in both the application of the heating systems, and the balance of heat needed to maintain dexterity and complete a mission, and decompression risk. Even working from a foundation of data that suggests the following:
- Being warm during a dive increases post-dive bubble scores
- Hot water suits are associated with higher DCS risk
- Post-dive cooling could prolong the period of elevated risk for Type 1 DCS
There is still some room for the safe application of active heating to improve both safety and comfort. A 2007 NEDU study showed a significant decrease in DCS incidence in a group of divers performing a 150fsw/60 minute dive on U.S. Navy Standard Air tables that were kept cold during compression phases (descent and bottom time) and warmed during decompression, compared to a group from a prior study that was kept cold throughout their dive, despite the “cold” group decompressing for nearly 2.5 times as long.
This study was able to create a dataset that included more than 400 dives to a depth of 120 feet of seawater, a standard decompression profile and varying thermal exposures, providing a profile that can be reasonably extrapolated to recreational profiles. The principal of this study was the comparison of DCS incidence odds ratios between these thermal exposures, resulting in a 23.8% DCS odds ratio for a 10C increase in temperature during compression, and decreased DCS occurrence and VGE scores (although postdive VGE scores were only weakly associated with DCS occurrence).
A group of physicians and researchers did take issue with some of the results, hoping to temper recreational divers from extrapolating data directly, but their editorial was not without rebuttal from their colleagues. The study’s authors eventually waded into the academic dispute with their own response clarifying that while the thermal stresses experienced by recreational divers are likely less than found in his experiment, the responses would likely be similar but in lesser magnitude. Because of this, they contend that it would be unwise to ignore the trends they found, and the data could have a profound effect on the larger diving community, remarking that, “We wish to clarify that our study does have implications for recreational and technical divers, implications that should not be ignored.”
Much in the way that the original U.S. Navy Dive Tables were adapted for the recreational market, so too can this data provide valuable lessons to divers who do not necessarily resemble the hyper-fit Navy Dive standard.
With the limited data we have and the considerable academic dispute over the cumulative effects of various heating applications, it seems that the best course of action is to draw from a combination of the NEDU study, community engrained platitudes about thermal status, and a healthy dose of theoretical modeling. Pollock, Clark et. al, and the NEDU all agree at some level that active heating can be applied to improve divers safety. In this application, the NEDU study would seem to indicate that the most appropriate application would be to keep divers cool during the compression phases (during descent and the working portion of the dive), and gently warm them during the ascent to aid in decompression. In situations where the working portion of the dive requires heating at a “minimal level” can likely be safely applied, but excessive heating on ascent should be avoided to prevent dehydration or decompression that is too aggressive.
Logistically, this addresses the worst-case scenario, a diver intending to run active heating throughout a dive who experiences a failure on ascent, leading to a warm compression and cold decompression phase. It also makes it possible to use heating to some extent in the exploration of harsh environments and maintain comfort and dexterity not possible with passive thermal protection. It does not, however, address the fact that decompression models do not account for thermal status, let alone change in thermal status during a dive. Additional conservatism must be applied by the diver, because the addition of an active heating system provides one more variable in amongst the milieu of uncertainty in decompression risk.
Using heated undergarments in this “cold/warm” fashion seems to be the takeaway for Pollock, the NEDU, and many of their colleagues. The NEDU study goes so far as to say that their group following the “cold/warm” pattern experienced a benefit similar to halving their bottom time, compared to the group that was kept cold throughout the dive and decompressed for 2.5 times as long. The potential for enormous benefit is there, but applied incorrectly it seems likely that the opposite is also true. “Dramatic results demand serious attention” is how Pollock put it, and it’s worth keeping that in mind as you weigh your options, and your wallet, this spring.
1. Effect of ambient temperature on the risk of decompression sickness in surface decompression divers
2. Effect of ambient temperature on the risk of decompression sickness in surface decompression divers
3. Recompression treatments during the recovery of TWA Flight 800
4. Time and temperature effects on body fluid loss during dives with the open hot-water suit
5. Re: Don’t dive cold when you don’t have to (Pollock)
6. The Influence of Thermal Exposure on Diver Susceptibility to Decompression Sickness
7. Don’t Dive Cold When You Don’t Have To (TDI)
8. On diver thermal status and susceptibility to decompression sickness (letter)
9. Thermal stress and diver protection.
Alert Diver: Deep in the Science of Diving: The Navy Experimental Diving Unit by Michael Menduno
Reilly Fogarty is a team leader for risk mitigation initiatives at Divers Alert Network (DAN). When not working on safety programs for DAN, he can be found running technical charters and teaching rebreather diving in Gloucester, MA. Reilly is a USCG licensed captain whose professional background includes surgical and wilderness emergency medicine as well as dive shop management.
When Easy Doesn’t Do It: Dual Rebreathers in Extended-Range Cave Diving
Rebreather technology has enabled cave explorers to extend their underwater envelope significantly deeper and longer. As a result, a few teams are pushing beyond the practical limits of open circuit bailout and so have turned to bailout rebreathers. But they are not without challenges, as Tim Blömeke, who dives into the latest research and field experience, explains.
by Tim Blömeke. Lead image: KUR divers Bob Beckner and Derek Ferguson in the 124m/407 ft deep Mount Doom chamber in Weeki Wachee Spring, Florida, courtesy of Kirill Egorov.
Dual rebreathers are becoming a thing among the elite of extended-range cave diving. Yet the “Blueprint for Survival” for this type of equipment configuration has yet to be written, and practitioners are faced with difficult trade-offs between competing design goals—like fitness for purpose, logistical feasibility, simplicity, reliability, and ease of use, all of which interact with the peculiarities of human nature. A new research paper proposes a pathway for risk assessment.
The introduction of rebreathers has considerably extended the range of exploration in cave diving. This is true especially for deeper dives, where open circuit technology faces the combined challenges of greater required gas volumes and higher required helium content, which make such dives both difficult to execute logistically due to the sheer number of cylinders involved, and prohibitively expensive due to the amount of helium in each of these cylinders.
By conserving the metabolically inert components of the breathing gas (most notably the helium), the use of closed circuit rebreathers (CCR) eliminates a good chunk of this problem, but not all of it: Traditional CCR diving procedures require that each diver have enough open-circuit bailout gas available to safely end the dive in the event of a rebreather failure.
Granted, the amount of bailout gas required for a CCR dive is only a fraction of what would be needed to perform the same dive on open circuit, and if all goes well, the bailout gas will never be breathed by anyone and can be reused for future dives. However, bleeding-edge explorers being who they are and doing what they do, after having used their CCRs to push the range of operations a few miles deeper into the cave systems, they began to encounter an issue very similar to the one that prompted the switch to CCR in the first place: cost and logistics.
As a real-world example, bailing out from a long-distance cave penetration of 7,500 meters at an average diver propulsion vehicle (DPV) travel speed of 40 m/min takes 187 minutes. Assuming a mean ambient pressure of 6 ATA (50 m depth) and a respiratory minute volume (RMV) of 14 l/min, the amount of bailout gas (not including decompression) required to reach the entrance would be 15,708 liters, or more than seven AL80 cylinders filled to 200 bar. This RMV is likely not conservative enough, given the extreme distance and the possibility of a hypercapnic event being the cause of the bailout so, in practice, a safety margin of at about 50% would be added, giving a total of 10-11 AL80 bailout cylinders.
The required amount of bailout gas became too large to be carried on the person of the diver, so that cylinders again needed to be staged in a series of set-up dives. Preparations for extended range exploration dives became ever more involved, and logistics became just as difficult to manage as those of old-school open circuit dives–even more so, arguably, due to the considerably greater distance of the staging points from the cave exit. As happens so often, overcoming one obstacle resulted in the discovery of others further down the road.
New safety concerns started to appear as well: For large-scale exploration projects, bailout cylinders needed to remain in a cave system for months at a time, sustaining severe corrosion damage at the tank neck and tank valve interface in the process due to the galvanic reaction between the chrome-plated brass valve and the aluminum cylinder. This isn’t merely a hypothetical concern: On many occasions, the corrosion was so severe that the integrity of the seal was compromised, and explorers found their previously staged bailout cylinders empty when checking them on their way into the cave. While this can be counteracted by installing a magnesium anode on the cylinder (magnesium is lower in the Galvanic series than aluminum and replaces the latter in the reaction), explorers found that the countermeasure only mitigates the issue but does not eliminate it. Long story short, for extreme extended-range dives, open circuit bailout was becoming ever more impractical and problematic.
Enter The Bailout Rebreather
As a solution to these problems, some explorers began to do away with open circuit bailout altogether and carry a redundant rebreather system—a closed circuit rebreather, or a semi-closed rebreather (SCR) instead. While this practice has gained significant traction recently, the concept itself isn’t new. In his book Into the Unknown, famed Welsh explorer Martyn Farr reports that his German colleague, pioneering cave diver Jochen Hasenmayer, had experimented with a dual unit he dubbed the Speleo-Twin Rebreather (STR-80) as early as 1981.
In 1987, Dr. Bill Stone delivered a proof of concept by spending 24 hours underwater on a dual CCR, he dubbed “Failsafe Rebreather for Exploration Diving” (FRED), during his visionary Wakulla Springs Project 1987. However, it appears that the first person to utilize redundant rebreathers in actual exploration was Olivier Isler from Switzerland. On August 12, 1990, he first used a triple RI2000 semi-closed unit in his crossing of the Emergence du Ressel (Doux de Coly, France), covering a distance of 1850 m/6070 ft at a maximum depth of 81 m/266 ft. The following year, Isler went on to push through the 4000 m/2.5 mi penetration barrier for the first time. More than a decade later, in 2002, Reinhard Buchaly and Michael Waldbrenner pushed the exploration of the Doux de Coly farther using dual RB80s, which were originally designed by Buchaly and continue to be produced to this day by Halcyon.
The decision to replace open-circuit bailout with a rebreather is as obvious as it is bold: Obvious because it replicates the successful solution to a past problem and restores the ability of a diver to carry all the gas they need on their person. Bold because… well. Put yourself in the drysuit boots of a cave diver, hours and hours away from the surface, who just survived an assassination attempt by a complex piece of life support equipment. All technical aspects aside, wouldn’t it be reassuring to fall back on a less complex piece of life support equipment whose proper functioning can be ascertained reliably within a few seconds?
Expressed in numbers, a paper by Andrew Fock, Analysis of recreational closed-circuit rebreather deaths 1998-2010, published in 2013, analyzed dive accident statistics for the period from 1998 to 2010 and found that CCR diving is associated with an increase in the risk of death by a factor of up to ten compared with open circuit diving. That ratio essentially applied to CCR dives, which used open circuit bailout. Rebreather technology and diving practices certainly have improved since the time under investigation, but the fact still remains that the complexity of the equipment adds to the overall risk.
With this in mind, taking a closer look at and trying to define the specific risks and benefits of replacing open-circuit bailout with a redundant SCR or CCR seems a reasonable idea. And this is precisely what a team of authors headed by Derek B. Covington did in a recent (March 2022) research paper, asking the question, “Is more complex safer in the case of bailout rebreathers for extended range cave diving?”
Using a qualitative approach, the authors discuss the reasoning behind bailout rebreather use, its history, different configurations and the various advantages and disadvantages and, finally, the additional potential for human error created by increasing the complexity of the equipment.
Bailout SCR vs. Dual CCRs
In terms of configurations, there are two main choices for a bailout rebreather: SCR or CCR. With an SCR, the diver still has to carry bailout gas. However, an SCR (such as the side-mounted Halcyon RBK) extends the use of this gas by a factor somewhere between four and ten, thereby drastically reducing the number of cylinders needed while being only the size of a single AL80 cylinder itself. Other advantages of a bailout SCR are that its relative simplicity and lack of sensors or other electronics make it much easier to set up, maintain, and use than a secondary CCR.
These advantages, however, do not come without downsides. With an SCR, the diver does not have the option of adding oxygen into the loop, and the actual oxygen content of the gas breathed is always somewhat lower than the oxygen content of the gas in the cylinders carried. How much lower exactly depends on the portion of the gas vented into the water on each operating cycle of the unit—or the rate of fresh gas supply into the unit—as well as the metabolic needs of the diver.
Therein lies the crux: For normal operation, the amount of oxygen consumed by the diver, and thus the resulting effective composition of the breathing gas, can be calculated quite reliably. In a bailout scenario, however, it isn’t unlikely for the metabolic needs of the diver to be increased due to higher workload. Without sensors to measure PO2, the precise composition of the breathing gas in the SCR loop becomes unknown, creating a risk of hypoxia, with all the potential consequences that come with it. This risk is unique to SCRs and not present when diving open circuit (where the cylinder sticker tells us what we’re breathing) or while on a CCR (where sensors tell us what we’re breathing).
The other approach is to go for a redundant CCR, as Stone envisioned back in 1987. While seemingly the “purest” in concept—replacing like with like—and optimizing redundancy, the added complexity is significant. Everybody who owns a CCR (especially an eCCR) knows that these machines need lots of love to remain in good working condition. Now multiply that by two: twice the number of sensors, two scrubbers, two sets of primary electronics, two sets of secondary electronics … and that’s just out of the water.
To have the redundant system available to them at all times during the dive, divers now need to manage the contents of two breathing loops instead of one. Furthermore, in order to be able to provide assistance in the event of a problem, divers working in a team need to be aware of the failure modes of and emergency procedures for not only their own units, but also the units used by their teammates. Unless everybody on the team is using the same machines for primary and bailout, this considerably adds to the training requirements, as well as to the complexity of the decision-making tree in an emergency situation. Nevertheless, by maximally reducing the required amount of gas to be carried by each diver, a redundant CCR theoretically provides the greatest degree of independence and offers the greatest potential range of exploration.
Approaches to Risk Assessment
To date, the use of dual rebreathers is still too rare for a quantitative, empirical assessment of its safety to be practical, and there is no systematic process in place for collecting data on dual-rebreather dives. “It’s really almost impossible to put a number on it,” said researcher and explorer Andy Pitkin, who co-authored the study. “I think there are only a small number of divers in the world who really need a bailout rebreather, but there are probably quite a few who use them because the idea appeals to them more than using OC bailout. Of course, there is no hard dividing line between the two groups. Where does logistical difficulty become impossible? That’s a very subjective judgment.”
The diversity of configurations and procedures used is another obstacle to objective study. “Are we using identical primary and bailout rebreathers, or is one unit specifically designed as a backup? If the latter, should the bailout unit be another CCR or an SCR? If the former, what are the diving procedures? Does the diver switch between loops at regular intervals, analogous to the procedures for independent doubles or sidemount diving? This would arguably add to task loading. Do the units have separate DSVs or a single, shared one, like that used by Richard Harris and Craig Challen of the Wet Mules? If the diver doesn’t alternate between units, then what other procedures are in place to ensure that both loops remain breathable at all times, especially during depth changes? If using dual CCRs, then what is the approach to ensuring redundancy of the diluent and oxygen supplies?”
The number of open questions and the range of possible, viable answers seem endless. Similar to the situation in the early days of cave diving, the book on bailout rebreathers has yet to be written. While many of the timeless principles from Sheck Exley’s famous booklet, Basic Cave Diving: A Blueprint for Survival continue to apply accordingly, there is no broad consensus yet on best practices, no SOP Manual, no standardized configuration, no published training standards for dual rebreather diving by any training agency. People are still working things out for themselves or their teams.
In consideration of these difficulties, and as a starting point for a discussion, the authors of Is more complex safer… propose a generalized approach to assessing the risks of dual-rebreather diving. Rather than delving into the minutiae of the failure modes of each individual diver’s equipment setup and diving procedures, they outline a method for identifying potential error-producing conditions (i.e., opportunities for human operators to make mistakes) based on a theoretical model originating in risk assessment for nuclear power plants: the WITH/TWIN model (Table 1). The acronym WITH stands for Workplace Design, Individual Capabilities, Task Design, Human Nature. TWIN refers to the same items (Task, Workplace, Individual, Nature).
The underlying idea of this approach is to move beyond merely looking at “human error” prima facie—oh, the diver failed to pack his scrubber properly? How could they! They neglected to monitor their PO2? Pay more attention!—and instead, analyze the conditions that are conducive to such errors. For the purposes of the model, a diver’s equipment configuration is part of their Workplace, their training and fitness belong in Individual Capabilities, the mission, including not only managing one’s gear but also navigation, linework, photography/videography, and surveying fall under Task.
All these aspects interact with Human Nature. We get stressed when things get exciting, we get complacent when things go smoothly. We are prone to false assumptions, we are terrible at intuitive probability assessment, and our ability to pay attention falls off rapidly once the number of items that need our attention increases significantly beyond the number of voices in our heads. Much like running a nuclear power plant, excellence in cave diving isn’t achieved by sporadic strokes of genius but instead by consistently avoiding mistakes, and an important aspect of the design of equipment and procedures for either is to compensate for the inherent weaknesses of the human mind.
In the words of the study’s authors:
“Divers and explorers need to consider not just the technical aspects of operating the dual CCR as an equipment-based system, but also the socio-technical aspects and error-producing conditions that adding additional complicated equipment has to the wider system, especially when it comes to training for and executing abnormal operations when workout levels will be high and awareness will be reduced. Nonetheless, as the use of this configuration grows, the risks and benefits will become clearer to investigators and divers alike.”
It will be exciting to observe the future development of dual-rebreather diving as it matures and see where the consensus for best practices will end up… stay tuned and stay safe!
Diving and Hyperbaric Medicine: Is more complex safer in the case of bail-out rebreathers for extended range cave diving? Derek B Covington, Charlotte Sadler, Anthony Bielawski, Gareth Lock, Andrew Pitkin
Fock AW. Analysis of recreational closed-circuit rebreather deaths 1998-2010. Diving Hyperb Med. 2013;43(2):78-85.
NSS-CDS (free download): Basic Cave Diving: A Blueprint for Survival by Sheck Exley
InDEPTH: The RB80 Semi-closed Rebreather: A Successful Exploration Tool by David Rhea
Halcyon: Using The RB80 As A Side-mounted Bailout Rebreather by Andy Pitkin, Karst Underwater Research (2018)I
InDepth: Rebreather Holiday Shopping Guide (2020)
aquaCORPS Pioneer Interviews: Stoned: Interview With Dr. William Stone (1994) by Michael Menduno
InDEPTH: Diving Beyond 250 Meters: The Deepest Cave Dives Today Compared to the Nineties by Michael Menduno and Nuno Gomes
Deep Tech: Victory At Last (1998) by John Simenon: Olivier Isler is setting penetration records with a triple-redundant semi-closed rebreather
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