Sign up for our monthly newsletter so you never miss the latest from InDepth!
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].”
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.
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?”
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.
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.
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
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.