Connect with us


Diving the SOS: A Practical Discussion

The SOS meter was wildly popular in the 1970s and 80s prior to the advent and broad adoption of electronic diving computers like the EDGE. What’s more, according to avid users, like diving industry pioneer Bret Gilliam, who founded Technical Diving International (TDI) and was the former president of UWATEC, the device actually worked reliably, at least down to 55 m/180 ft. No Bend-O-Matic there! Gilliam, ever the name dropper, is positively bubbling.



Text by Bret Gilliam. Header image courtesy of Stephane Symes, other images courtesy of Bret Gilliam.

Wind the clock back five decades and re-enter an era when “square” dive profiles were the standard, and all divers used tables. Then, in 1959, Strumenti Ottoci Subacquei (SOS)—a small, Italian company led by founders Victor Aldo De Sanctis and Carlo Alinari—quietly introduced a new product. They called it a Decompression Meter or, simply, “Deco-Meter.” The device was distributed by SOS itself but also by a number of dive equipment companies.

However, the Deco-Meter got a major boost when it caught the attention of Scubapro founder Dick Bonin, a Naval officer and diver. It’s not clear exactly when Scubapro took on the actual distribution of the SOS product, but it was likely around 1967, a little less than a decade after its invention.

One of Bonin’s salesmen gave him an overview of the new SOS product while several other staff shared their practical experience with the meter. Jimmy Stewart from Scripps and engineer/designer Dick Anderson also chimed in with their own thoughts on methodology. NOAA underwater geologist Dr. Bob Dill undertook numerous deep, lengthy, repetitive dives and found that the two of us shared a common goal of making our dive exposures more efficient. He reached out to me while I was working on a US Navy project.

I was first introduced to the SOS meter in January 1971, while assigned to a Navy experimental deep diving team filming fast attack submarines in great depths. But, a lot of our standard operational and rigging protocols had us working between 18 m/60 ft and 45 m/150 ft during setups and breathing air. Our longest dive profiles resulted within the two to seven atmosphere range, and we’d already gotten input on alterations to mid-depth profiles from an experimental physiologist in Canada we were working with.

The diving medical officer on our ship was fascinated as I explained what I knew about the SOS meter as well as my conversation with NOAA diving director Dr. Morgan Wells—my first discussion about the concept of “multi-level” dive profiles—a radical departure from square table models and calculations of inert gas uptake and off-gassing. Dr. Wells had  been quite interested in explaining multi-level profile theory to my Naval team in more detail to show how such a calculative departure from the norm could make us more efficient and dive within an improved margin of safety.

Almost every professional diver I knew had embraced the SOS meter and used it exclusively. Virtually every filmmaker and photographer used the SOS meter since it gave them so much more time underwater.

What Works Works

The device was incredibly simple. It was a waterproof deformable flexible chamber with a semi-porous ceramic flow pathway (Bourdon tube) that channeled the pressure through the analog needle indicator to give a display of remaining no-decompression time (or required deco, if applicable). It sounds more complicated than it is. 

The mystery of the device was how the inventors were able to come up with a ceramic porous element that, incredibly, was seemingly able to match the flow rate of inert gas loading and outgassing from the diver’s tissues, and display it on analog display. From an engineering standpoint, to stumble on such a methodology so fundamentally and foundationally accurate with the barest minimum of decompression models was a physiological breakthrough of the first order. 

From the SOS Deco-Meter Manual

This was not anything that Haldane saw coming.

Legendary diving physiologist, Dr. RW “Bill” Hamilton, summed it all up with his precept for decompression tools, “What works is what works.” Bill had studied, led the development of, and created so many decompression models, but the SOS device was unique, with no timing device, no depth gauge, no bottom time display—only an analog needle indicating the diving profile status. Bill used to laugh and say, “There’s no explanation for the SOS meter. It shouldn’t work because it’s based on unknown models. But, somehow it does work… nearly perfectly down to about 55 m/180 ft. And I’ve never seen a case of DCS [decompression sickness] by divers using the SOS meter. It’s kind of crazy.”

It has made for some interesting discussions over the years. 

Dive tables and square profiles were slowly falling out of favor by many professional divers and numerous sport divers as well. Divers embraced the new SOS device and multi-level diving as an alternative—and safer—approach.

This was over fifty years ago, and many divers had an SOS meter if they worked in the military, commercial, scientific diving communities or with filmmaking, and underwater photography, for example on Hollywood movies, and on documentaries. And so did serious sport divers. The SOS meter was the great liberator for so many divers, since it gave them more freedom and variables.

But How Did It Work?

Since it was an analog instrument with a manual movable needle to indicate allowable no-deco or required deco, there were no batteries required. The only maintenance required was rinsing with fresh water after dives. 

The lower left side of the meter contained the needle—the “blue zone” indicated no inert gas exposure. Once the dive was initiated, the analog needle traveled across the no decompression zone that spelled out SURFACE. If the needle remained within the SURFACE zone, you could immediately surface with no decompression. It was very common for divers to say, “I’m on the C,” or “Are you on the E?”

The face of the SOS Deco-Meter

If you went into the red zone beginning with the “10 Ft.” display, you were going to be there for quite awhile. Probably a good thirty minutes or so. If you hit a “20 Ft.” stop, you were going to be there for about a month or so. And you better have a backup breathing source.

The concept of multi-level dive profiles was an extraordinary enticement to longer periods with no deco obligations. But, a lot of divers still had a hard time getting their heads around the theory and wrestled with understanding the nuance of such deco models.

It was exactly this type of misunderstanding that led some divers to be critical of the SOS meter and call it the “Bend-O-Matic.” Nothing could have been further from reality. The SOS meter had certain ranges that needed to be carefully observed to remain within no-deco limits. But, if used correctly, the SOS device had an amazing safety record. In fact, of the tens of thousands of divers using the meter, none of us were aware of a single case of decompression sickness (DCS) as long as the meters were used within their design range. That’s phenomenal. 

In 1971, I was one of the diving supervisors on a commercial diving project with 126 divers doing 10-12 dives a day in depth ranges between 15 m/50 ft – 43 m/140 ft. Every aspect of the dives was controlled by the SOS meter. This went on for over two months with a perfect safety record.

Gilliam on a film shoot for Skindiver magazine, St. Croix 1978

When we were filming the movie “The Deep” in 1976, the entire cast (including Robert Shaw, Jackie Bissett, and Nick Nolte), Al Giddings, Stan Waterman, location crew, technicians, and divers all controlled their exposures with the SOS meter. It made everything more efficient. The same was true for countless movie, television, and documentary projects—all done on the SOS meter.

The same was true for plenty of explorations, photo shoots, and sport dives. Every diver I knew used the SOS meter with success.

Evolution Was Coming

Though the adoption of the first electronic dive computers, including Orca Industries’ EDGE, which was launched in 1982, grew throughout the 1980s, the SOS meter remained popular. But, interest waned when Dacor released the Micro-Brain in 1988, which featured a decompression algorithm designed by Dr. Albert Buhlmann, and continued to pave the way for dive computers. I later worked with Dr. Buhlmann as a consultant on later versions of the Micro-Brain’s software program.

Even though dive computers changed the sport forever by making it safer and more efficient, the SOS meter laid the foundation for the early breakthrough into modern multi-level diving profiles. That simple, amazing device incorporated the conceptual transition from tables to an alternative protocol for diving. 

It was all done with the barest of technological design—yet it worked efficiently and safely. 

The lateTom Mount in 1969 sporting his SOS Deco-Meter (look above the tanks).

It was not created from a mind-numbing algorithmic deco model. It was crafted from an unknown ceramic element that theoretically represented inert gas flow loading and outgassing in human tissue. When first released, it sold for $18 retail, and it was still attracting buyers over two decades later. 

The SOS meter quickly went into the collectors’ cabinets and is remembered fondly as a pioneering piece of gear that launched a new technology closely followed by another revolutionary innovation: nitrox.

Time moves on, but somehow manages to get it right sometimes.

See companion story: The SOS Automatic Decompression Meter: Bendomatic or Game Changer? by Stephane Eyme

Dive Deeper:

Shearwater blog: On The EDGE by Michael Menduno

BluTime History: DECO BRAIN HANS HASS, the first dive computer by Andrea Campedelli

Dacor’s Micro-Brain Pro Plus Manual

Bret Gilliam is president of the consulting corporation OCEAN TECH, following his entrepreneurial successes in the diving and maritime industries. Bret is also a prolific writer and photographer on various subjects related to diving. He has been published in virtually every diving magazine internationally as well as National Geographic, Sports Illustrated, Outside, Time, Playboy, and many others. Author of nearly 1500 published articles, his photography has graced over 100 magazine covers, and he has been primary author, editor, or contributor to 72 books and training manuals on diving and other topics such as hyperbaric medicine, emergency treatment of diving patients in remote locations, scientific papers, maritime operations and engineering, and decompression models and algorithmic adaptations for diving computers.


What is Undeserved in “Undeserved Decompression Sickness”?

Divers still seek comfort in the notion of the “underserved” hit to explain unexpected incidents of decompression sickness. “Hey, my computer said I was fine.” NOT. Here diving physiologist Dr. Neal Pollock exposes the fault in this notion. While decompression algorithms take into account the divers’ profiles, i.e., time and pressure, there are a multitude of factors that can potentially impact divers’ decompressions, as the author explains. Once divers’ reject the escapism that accompanies the ‘undeserved’ label, they can get on with the important business of diving and giving adequate consideration in their deco planning.




by Neal W. Pollock, PhD

Photo by SJ Alice Bennett

Spoiler Alert: the most undeserved element in the title is the word “undeserved.”

Describing cases of decompression sickness as “undeserved” generally speaks more from an emotional perspective than a rational one. The driving factors are typically faith in imperfect tools and a desire (conscious or unconscious) to shift responsibility. 

Decompression algorithms rely almost exclusively on pressure and time data to predict effects. Enticing pictures can be painted on the authority of any algorithm, but the reality is that all rely on limited input to interpret complex situations for people who are not uniform. Modern decompression models are important constructs that can help us to dive safely, but the products are rudimentary from a physiological perspective, without sufficient sophistication to deserve unquestioned trust. 

The dive profile is almost certainly the most important determinant of gas uptake and elimination, but the truth is that we do not yet have sufficient data to quantify the impact of many of the variables that can influence outcomes (Pollock 2016). Instead, algorithms rely on simple measures and mathematical bracketing with the hope of covering the contributing factors. The problem is not in doing this; the problem is in being surprised when the outcome is not what was expected. 

Decompression safety is influenced by a multitude of factors, variably related to the dive and the diver.

Decompression Factors

Accepting that the dive profile is the most important determinant of decompression risk, there are additional factors that can also have dramatic effects. Exercise is one of these. Pre-dive exercise may have complicated effects on the subsequent diving exposure. Exercise during the descent and bottom phase will increase inert gas uptake and the resulting decompression stress. Mild exercise during the ascent and stop phase can promote inert gas elimination and decrease the resulting decompression stress, but excessive exercise can promote bubble formation and increase decompression stress. Post-dive exercise is likely to increase decompression stress in all cases. Practically, while the concepts are clear, the definition of meaningful thresholds for “mild” and “excessive” exercise is difficult at best, and quantifying real-time effects far exceeds current capabilities. 

Thermal state is another potentially dramatic factor (Gerth et al. 2007). Being warm during the descent and bottom phase can substantially increase blood flow and delivery of inert gas to the periphery and increase the subsequent decompression stress. Being cool during the descent and bottom phase can decrease inert gas uptake and decrease the subsequent decompression stress. Being cool during the ascent and stop phase will inhibit inert gas elimination and increase the subsequent decompression stress. Being moderately warm during the ascent and stop phase can promote blood circulation to the periphery and increase inert gas elimination, but excessive heating of peripheral tissues in this same phase can promote bubble formation as heating decreases the solubility of inert gas, effectively increasing the decompression stress.

Again, as with exercise, it is extremely difficult to identify meaningful thresholds for thermal state at different points in a dive, and quantifying real-time effects is not within current capabilities. It is certainly clear that the ambient temperature measured by a dive computer can have little correlation to the thermal status of the diver, and any thought that this information informs decompression models in a meaningful way is misplaced. 

The wild card of individual (“predisposition”) factors further highlights the challenges unmet in current decompression models. Not only are these parameters not measured, it is unclear how the information could practically guide the risk assessment at this time if available. While the importance of these factors is hard to assess, it is also noteworthy that some, most often dehydration, may be used as scapegoats to explain away decompression sickness (DCS). 

A state of dehydration can adversely affect circulation, potentially impeding inert gas elimination, but this almost certainly has much less impact than the dive profile, exercise, or thermal state in many cases. The impact is also not as straightforward as making it a blame agent might imply. For example, if a state of dehydration impairs inert gas elimination during the ascent and stop phase to increase decompression stress, might it not also decrease inert gas uptake during the descent and bottom phase to reduce the decompression stress?

Sound levels of hydration are good for general health and probably for decompression safety, but a state of dehydration in no way guarantees an outcome of DCS, just like a good level of hydration in no way guarantees an outcome of no DCS. The blame directed to dehydration is probably related to the observation that DCS can be accompanied by clinically important fluid shifts. This, though, is more a consequence of the disease than a cause. 

Photo by SJ Alice Bennett

The rest of the predisposition factors offer similar challenges. Physical fitness appears to confer some protection against decompression stress, but the quantification of such effects is not yet possible. A history of DCS can go either way, with persons prioritizing blame shifting over understanding or behavioral changes having a higher risk of repeat events, and persons improving understanding and moderating risk factors having a lower risk of repeat events. Increasing age is a risk factor, but the partitioning of chronological vs physiological age still needs to be worked out, as does the interaction between age, physical fitness, and biological health.

Women might have a slightly higher physiological risk, particularly during the first half of their menstrual cycle, but this is likely largely (or more than) mitigated by sex-based differences in risk tolerance and practices. The norms and practices of a buddy can affect individual risk either positively or negatively. Circulation issues include state of hydration, the presence of a patent foramen ovale (PFO), and possibly old injury sites that disrupt circulatory pathways. The presence of a PFO is likely only able to become important in decompression stress if bubbles are present, which will depend on a host of other factors. Biological health elements are likely to offer interesting insights in the future, but the ability to assess and understand them exceeds current capabilities. 

The major point here is that there are a lot of unknowns and half-knowns that make it important to not expect any decompression algorithm to describe truth. They offer a first order approximation of risk. They provide what might be reasonable guidance within a wide swath of possible error. Staying within guidelines does not guarantee safety. The goal should be to plan for the possibility of suboptimal elements, perhaps several of them, that could influence outcomes.

Photo by Sean Romanowski

Conservative Settings

Divers often address the recognized shortcomings of dive-computer-based decompression models by altering conservative settings and/or practice. Those who feel they are bends-resistant may push the limits; those who prefer greater peace of mind may add buffers. One of the additional challenges is that not all practices put forward to enhance conservatism will actually act in that way. The best example of this is probably deep stops. The concept of stopping deep to minimize bubble formation was enticing, but flawed. Stopping too deep will certainly minimize the possibility of bubble formation at that point, but at a point when it would never reasonably be expected for bubble formation to occur. The problem is that the time at the deep stop depth allows any tissue that is not fully saturated to take up more inert gas. The additional uptake creates increased decompression stress as the diver ascends. The concept was well intended, but the impact was counterproductive. 

One of the conservative settings that is intuitively simple is gradient factors. The M-value describes a theoretical limit of supersaturation that a tissue can tolerate before problematic levels of decompression stress develop. This limit is another first order approximation of risk, with no guarantees of safety if staying within the limit. Gradient factors (GF) simply tailor limits to a different percentage of the M-value. GFs are typically presented as two values, GFlow and GFhigh, presented as GFlow/GFhigh. Those who believe in deep stops may choose a GFlow less than or equal to 20%. Those who do not believe in deep stops will likely choose a GFlow equal to or greater than 30%. Those who feel confident in their overall ability to tolerate decompression stress might choose a GFhigh in the 85% range. Those who want to add more buffers to protect against unknowns and surprises might choose a GFhigh less than or equal to 70%. 

Determining whether a case of DCS should be considered “deserved” or “undeserved” is problematic when prescribed limits are based on incomplete data and when they can be altered by a variety of settings. The argument, “My computer said it was okay!” holds little if any weight. A better approach is to focus on the fundamentals. The first fundamental is to consider when DCS is a possibility. As a rough rule of thumb, any dive within the traditional recreational range (40 m/132 ft) that is approaching half the US Navy no-decompression limit carries a non-zero risk of DCS. Similarly, pretty much any dive deeper than the traditional range carries a non-zero risk. “Non-zero risk” repudiates the claim of “undeserved.”

Photo by SJ Alice Bennett

Once the possibility of DCS has been accepted, the most productive deliberation includes an honest assessment of all of the risk factors that may have contributed to the outcome. While trying to pin the blame on one modifiable risk factor can be comforting, it probably does much less to ensure future safety. There are many effects that cannot yet be quantified, but the risk potential can be recognized. Focusing on any one variable can discourage a more honest appraisal of the possibilities.

Assessing the Risk

DCS symptoms may develop due to frank violations of accepted practice, but many cases are shrouded in ambiguity. An honest and objective assessment will almost certainly improve understanding and future outcomes more than claiming an “undeserved hit” will. It is unlikely that any two exposures will truly be identical, either for two divers sharing one dive or one diver repeating a given dive. Subtle differences can accumulate to have a meaningful impact. These differences coexist with the probabilistic nature of decompression stress, effectively that a safe outcome experienced once or several times may not guarantee the same for all future exposures. 

Once all reasonable contributing factors have been considered, some room should be left for doubt. Appreciating the knowns, the unknowns, and the complexity of interactions can promote thoughtful practice without frustration. Practices can be optimized without guarantees. The first step is getting rid of the escapism that accompanies the “undeserved” label.


Gerth WA, Ruterbusch VL, Long ET. The influence of thermal exposure on diver susceptibility to decompression sickness. NEDU Report TR 06-07. November, 2007; 70 pp.

Pollock NW. Factors in decompression stress. In: Pollock NW, Sellers SH, Godfrey JM, eds. Rebreathers and Scientific Diving. Proceedings of NPS/NOAA/DAN/AAUS Workshop. Wrigley Marine Science Center, Catalina Island, CA; 2016; 145-56.

InDEPTH: In Hot Water: Do Active Heating Systems Increase The Risk of DCI? by Reilly Fogarty

Neal Pollock holds a Research Chair in Hyperbaric and Diving Medicine and is an Associate Professor in Kinesiology at Université Laval in Québec, Canada. He was previously Research Director at Divers Alert Network (DAN) in Durham, North Carolina. His academic training is in zoology, exercise physiology, and environmental physiology. His research interests focus on human health and safety in extreme environments.

Subscribe for free
Continue Reading

Thank You to Our Sponsors

  • Fathom


Education, Conservation, and Exploration articles for the diving obsessed. Subscribe to our monthly blog and get our latest stories and content delivered to your inbox every Thursday.

Latest Features