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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.
The SOS Automatic Decompression Meter: Bend-O-Matic or Game Changer?
Introduced in 1959, the Italian SOS Deco Meter—the forerunner of modern dive computers—was the first decompression device used by sports divers that automatically tracked users’ dive profiles. Here former French mine clearance diver, instructor, and historian Stephane Eyme takes us on a deep dive into SOS’s analog technology, compares its decompression prescriptions with those of the US Navy and French air diving tables for single and repetitive dives, and offers his perspective on its impact on the market.
Text by Stephane Eyme. Photos and illustrations courtesy of Stephane Eyme. This story was first published on vintagescubadiving.com.
SOS Automatic Decompression Meter was the first mechanical analog dive computer.
The SOS Automatic Decompression Meter (“DeComPressimetro,” or DCP) was introduced in 1959 by Italians Victor Aldo De Sanctis—a known U/W cinematographer at the time—and engineer Carlo Alinari, both co-founders of Strumenti Ottici Subacquei (SOS), a Torino, Italy-based company specializing in scuba diving instrumentation.
DCP provided a decompression profile to scuba divers during an actual dive.
The device was very simply manufactured. It consisted of a waterproof deformable chamber filled with gas connected to a smaller, rigid chamber through a semi-porous ceramic cartridge.
The rigid chamber was equipped with a bourdon tube to measure the inside pressure. A calibrated indicator interpreted these data to provide divers with a decompression status. The whole mechanism was enclosed in a metal and plastic housing.
Straightforward dive operation mode
During the dive:
Ascent and deco stops:
Quite a hit!
The device was distributed by SOS itself and many notable dive equipment companies. Throughout the US and Europe, Scubapro, Healthway, Beuchat, Sporasub, Nemrod, Barakuda and others, all sold the DCP at some point until the 1970s.
And so, the SOS DCP became the first successful decompression instrument sold at large scale to divers around the world. Scripps Institution of Oceanography reported more than 50,000 units sold.
A mind-blowing concept?
When the SOS DCP came out, the diving world already knew quite a bit about the decompression process.
J.C. Haldane published his perfusion parallel compartments model in 1908, and a boom in decompression research followed..
We knew that human body tissues became saturated from a few minutes to several hours depending on the tissue, that saturation followed a logarithmic curve, and that it was symmetric with the desaturation process.
We knew supersaturation ratios decreased linearly with increased ambient pressure (M-values), and that they were different for each compartment.
Based on this knowledge, divers created and used several sets of deco diving tables, for example; US Navy 1956 and the GERS 65.
While it was true that , the diving community had long been aware that decompression was complex and had to account for a wide variety of factors,even with all those considerations, deco tables remained an approximation—a model that would probably differ slightly from diver to diver.
Still, some questions were bothering me
How could an engineer and a famous U/W photographer imagine a system like the DCP?
And, once they settled on the concept, how did they nail the exact piece of ceramic that rendered the entire human body a piece of clay?
And, furthermore, why did we trust them with our lives?
Below is an extract of the SOS DCP user manual. The device is compared to an “electronic brain.” Remember, this was in the sixties, and this “electronic brain” was at the forefront of technology!
I would have thought that, if SOS’s DCP manual was unmistakably indicating—in 1959—that the DCP was extrapolating decompression data from a piece of ceramic, lots of divers would have said, “WHAT?!” and kept using the dive tables. But then, in 1966, Scubapro essentially said the same in its DCP’s user manual, and still sold countless models!
“The mechanism is a pressure-sensitive sealed bourdon tube in a sealed chamber. The only passageway into and out of the sealed chamber is through a porous ceramic element. The element precisely controls the flow of gas into and out of the chamber.
The gas is contained in a collapsible plastic bag which is protected by the stainless-steel case. An ambient pressure entry port and the strap slots allow for transmission of pressure to the collapsible bag. This differential forces the gas through the flow-controlling porous ceramic element into the sealed chamber.
As the pressure builds up within the sealed chamber, the Bourdon tube response causes the indicating needle to move in a clockwise direction. This movement simulates the nitrogen absorption by the diver’s tissues. Upon ascending, the process is reversed.”
(Scubapro’s DCP Manual user 1966)
The results are here
On the other hand, and very surprisingly, DCP deco procedure profiles were not too terrible. On the first dive of the day, they actually weren’t too far from the profiles given by GERS65 or the NAVY56 table.
The GERS (Groupe d’Études et de Recherche Sous-marine) was created in April, 1945, by Cousteau, Tailliez, and Dumas. GERS was a unit of the French Navy in charge of clearing harbors and coast waters of WWII mines.
In 1965, the GERS expanded its previous dive table span from 45 m/147 ft to 85 m/277 ft. These tables were calculated on a Haldanian model with three and four tissues. They also considered two sets of constant supersaturation coefficients throughout the ascent.
Almost every diver in France until 1990 used these tables. They were the “official dive tables” of the French Federation for recreational diving.
The French Navy conducted a statistical evaluation of the safety of the GERS65 tables between 1966 and 1987, using human guinea pigs—err, young, fit, trained, and monitored military divers—and reported a not-insignificant number of accidents following the deco procedures indicated. This was one of a few other motivators for the Navy’s production of new tables—MN90.
I’ll use the GERS65 as a reference alongside the US NAVY56 table to evaluate the DCP deco profiles.
What happened on the first dive?
The following table compares DCP, NAVY56, and GERS65. The time indicated is the maximum bottom time allowed in minutes with no decompression stop on the first dive. GERS65 comes in meters only, rounded to the next meter depth to translate to feet.
Even if the concept of the SOS DCP is really mind-blowing, the results actually are not too far from the tables available at that time.
Looking more closely, DCP was, in fact, more conservative than NAVY56 and GERS65 up to 18 m/60 ft. It was pretty much the same as NAVY56 from 18 m/60 ft to 28 m/90 feet. Less conservative than the NAVY56, but still more conservative than GERS65 from 28 m/90 ft to 37 m/120 ft. Clearly less conservative than both tables after 37 m/120 ft—all for non-decompression dive profiles.
So, saying SOS DCP was not safe… Well, the maths don’t lie. Down to 28 m/90 ft, it was more safe than—or as safe as—the US NAVY56 tables during the first dive. The same happens with GERS65 down to 37 m/120 ft.
It is noticeable during deep dives—37 m/120 ft+—that the DCP became much less conservative than the two other tables. That might be an indicator that the DCP was well-calibrated for long compartments (and long, shallow dives) but not as well-calibrated for quick/medium ones (short, deep dives).
What happened on repetitive dives?
The time indicates the bottom time allowed with no Deco Stop on a consecutive dive.
I won’t get into too many details—I would need much more data to do the experiment justice—but the situation on a consecutive dive is a bit different than on a single dive.
The DCP’s deco profile is almost always located between NAVY56 and GERS65. This means we almost never encounter the situation where DCP is safer than NAVY56. It seems to be less conservative than on the previous first dive scenario for non-deco dive profiles, but it is still safer than GERS65 in any case.
This is possibly why the manufacturer introduced the recommendation to make a deco stop for at least 5 minutes at 3 m/10 ft when diving to more than 45 m/150 ft. This seems a bit like a patch, doesn’t it?
Divers also applied other tricks for repetitive dives: “Move the safe-to-come-up point two ticks to the left for each dive that day.” Of course, the manufacturer did not indicate this rule in its user manual.
Different opinions about the DCP
Amazingly, it is difficult to clearly assess how good or bad the DCP was.
On one hand,
The very device looks like the result of a large experimental attempt to provide safe deco indications. It is not a scientific application based on multi-compartment Haldanian theory.
But DCP forgets serious deco parameters
SOS didn’t consider a few very serious parameters. Not even mentioned, for instance, was water temperature’s impact on deco schedules. Moreover, it approached the problem the wrong way—as cold water increased the viscosity of the gas in the deformable chamber, it diffused slower into the rigid chamber, incorrectly—and hazardously—giving the diver more no-deco bottom time.
Deco surface should differ according to air temperature for the same reasons. Divers sometimes used this factor to decrease surface time by setting the DCP close to the cooling fan of the air compressor!
In addition, different DCPs recommended considerably conflicting decompression schedules for dives with identical depth and time factors. Thus, the DCP’s no-deco limits given by the single dive table fluctuates—sometimes up to seven minutes!
Finally, the DCP’s recommended decompression schedules, in some cases, were more conservative (time-wise) than corresponding US Navy tables. But, in others, the recommendations were far outside the limits of staging according to the tables. Now we know why.
There is no failure warning
The manufacturer provided zero warnings about DCP failure. One potential failure is a needle that does not move or doesn’t start in the blue area, which is easy to check at the beginning of the dive. Another is a malfunctioning device—the needle moved toward the deco-stop zone, but much too slowly.
Don’t forget, this is a mechanical device and, as such, it can’t be expected to be failure-free. But, you had no way to anticipate the problem aside from checking the device right before you dived. Or perhaps attaching it to a line, immersing it to 30 m/100 ft depth for 30 minutes, checking that the needle is about to enter the deco-stop zone, and then waiting six hours to erase its nitrogen memory. Not too practical indeed!
Hence, you could potentially be diving with a malfunctioning device without knowing it, effectively risking your life.
No deco time scheduling
Lastly, the device didn’t provide a time schedule at the deco stop. The DCP only showed that you need to stop, but didn’t tell you for how long. This complicates consumption schedules, which stipulated air. You could easily find yourself with 50 bars in the tank and, without knowing it, beginning a 30 minute deco-stop… breathe shallow!
Scientists say IT IS NOT SAFE!
Scientists conducted very significant studies far beyond what a simple diver like me can understand. Their conclusions included:
“The meter’s performance is compared with the US Navy’s no decompression limits. It is concluded that use of the meter by recreational divers should be discouraged.” S. Howard, H. Bradner, K. Schmitt, Scripps Institution of Oceanography, La Jolla, Calif. 92093, USA Medical and Biological Engineering, September 1976
“Certainly, these techniques will make diving more complex for ‘fools’—but anyone who dives to depths in excess of 30 m/100 ft and thinks all is rosy when following a DCM is a fool. Deep diving in a hostile environment requires careful planning and thoughtful techniques, and no mechanical mechanism exists which can always reliably predict decompression schedules for divers at various depths for variable periods. Surely, it is safer to err conservatively and stick to the ‘deepest depth X longest time’ method. There are many ex-patients who can recommend this practice from personal experience with DCMs which failed.” Carl Edmonds, Automatic decompression meters. SPUMS J . 1973; 3: 9
On the other hand,
There is a cadre of probably tens—if not hundreds—of thousands of dives using the DCP with no decompression incidents at all [See companion story by Bret Gilliam]. A huge number of divers can testify to using this device for many years with no problem. I probably used mine on a couple hundred dives. I’m still here to tell the tale!
Why? Let’s travel back in time to the 60s—when sex was safe, and diving was dangerous…
Equipment was emerging
The scuba equipment industry was in a very embryonic stage. Double hose scuba regulators were introduced no more than 15 years earlier. The first prototype of Maurice Fenzy ABLJ was developed in 1961—so until then you were diving on your legs—and Georges Beuchat introduced its Tarzan wetsuit in 1963 and the Jet Fins in 1964.
Equipment in the 60s was, indeed, still very much emerging and would take time to penetrate the market; as a result, there were diving mandates to be in good physical shape, and divers are fit. In comparison, today’s equipment is far easier to use and even, sometimes, gives us the false impression that diving doesn’t require good physical condition.
Another kind of diver
In the 60s, divers simply weren’t the same as they are today! Scuba diving was still quite new and enjoyed by a very limited number of divers. A lot of them were former Navy—they were trained divers, fit and very experienced. This is one of the reasons why almost all national diving federations used a military-like training plan for new divers in the beginning.
This is mainly because instructors were former Navy divers, and it was the only way they knew. The long swims, the hard training sessions, the 5 minutes lifting a weight belt over your head while paddling… It came from the Navy.
As a result, the profile of the average diver back in the 60s was probably much closer to the military divers who were using the GERS65 tables than it is today. I guess if today’s divers were using the SOS DCP instead of electronic dive computers, results would be far more disastrous.
A large number of dives were done across Europe in federal clubs, with depth limits based on certification levels.
Another consideration is the dive profile itself: SOS’s DCP deco profile, when used shallower than 28 m/90 ft, was safer than (or as safe as) the US NAVY tables. In federal clubs, we did most of our recreational dives in this range due to certification level limits. Consequently, DCP guided numerous club divers with no problematic decompression outcomes during dives to less than a 30 m/100 ft.
What were the key benefits supporting its success?
The DCP was a piece of cake to operate. No need to understand the table, thoroughly plan your dive, or remember your deco parameters. It was freedom. You just had to follow the guide and enjoy the dive!
Even the DCP’s user guide was only a seven-page booklet, of which three were useless for operational purposes! With just a quick read, you could strap it on and dive right away. It was extremely intuitive.
The DCP was very practical for calculating desaturation during surface-time between consecutive dives. DCP was doing everything for you. No nitrogen factor to calculate, no additional minutes on bottom time. You just dived, and DCP would do the rest for you.
Most importantly, the DCP was following your dive profile! That was quite a revolution in a square-dive-profile-world. Suddenly, you could dive much longer by slowly ascending a cliff and get more time to enjoy the dive! That was a true difference compared to table-based diving (at least for multilevel dives).
Was the DCP a bendomatic, or a game changer?
I think it is fair to say that the SOS DCP was a game changer in this emerging scuba world. The DCP would eventually bring a new perspective to diving. It was a brilliant idea, though probably a bit incomplete on the development side.
Yet, the device laid out a genius concept—that we could design a device to do the math for us and change the way we dive. This probably inspired equipment manufacturers to look into electronic dive computers, the very same ones that appeared on the market during the 80s, but this time with a far more advanced scientific basis.
See Companion story: Diving the SOS: A Practical Discussion by Bret Gilliam
Eyme’s website offers a wealth of historical resources and tools and tips: VintageScubaDiving.com
InDepth: Oh Deco, Oh Doppler, O’Dive: Assessing the World’s First Personal Deco Safety Tool by Michael Menduno
As a former clearance diver with the French Navy, Stephane Eyme’s scuba diving experience includes running his own dive centre in the Canary Islands, supervising underwater archaeological excavations, and working for the largest dive shop in Paris. He has more than 30 years’ experience as instructor for the French Scuba Diving Federation (FFESSM) and is a PADI Master Instructor in teaching status. He runs the website VintageScubaDiving.com and often organises vintage try-dive events and participates in vintage equipment gatherings to share his passion with the diving community. He lives in Valencia on the Spanish Mediterranean coast, and lives by a motto: “What matters is being under [water].”