by Rich Walker
Header image original photo by Derk Remmers, edit by Amanda White
For Rules of Thumb-Part One see: Rules of Thumb: The Mysteries of Ratio Deco Revealed
In my last article, I explored some simple strategies you could use to calculate your no-decompression limit (NDL) and how much decompression time you needed if you went beyond that limit. That information applies to dives shallower than 30 m/100 ft. In my own diving, the tools are hugely valuable. I don’t need an expensive dive computer, or a powerful tool for cross-checking a computer if Santa dropped one down my chimney at Christmas.
But, I can hear all of you hardcore technical hipsters now:
My favourite words! But, it’s probably a fair question. Is there a way to work out decompression requirements in the 30-50 m/100-170 ft range? It would be a pretty short article if the answer were no.
Now, when we looked at the shallower strategies, we began by working out the NDL. To be honest, in the 30-50 m/100-170 ft depth range, there is no NDL that makes a dive worthwhile. If you’re happy with a 10 minute bottom time, then fill your boots. It takes me a good half an hour to even work out what day it is every morning, so I see little point in getting all dressed up for 10 minutes of working out where I am, realising that it might be a nice wreck, and then having to start the ascent.
So let’s not bother with NDL in this range. There might be one, but it’s not useful. In my last piece, I shared some tables that were generated from Global Underwater Explorers’ (GUE) DecoPlanner, using gradient factors of 100/100. I’m adopting a similar tactic now, but I’ll be using GUE’s default values of 20/85 gradient factors.
I know. Simon says. I’m not going to get into the merits and detriments of gradient factors. But, if you think that changing 20 GFLo to 30 or 40 is going to make a big difference, then run the tables and see the spectacular 3m/10 ft difference in the first stop depth and the whopping 1-2 minute change in decompression times. And, that all assumes you ascended at exactly 9 m/30 ft/min.
Okay, back to the topic after being so rudely interrupted.
Our depth range is past the point where nitrox has any significant benefit, and that air gas would likely blur my vision, so let’s use a trimix of 21% oxygen and 35% helium, or 21/35. We already know that we’re looking at decompression dives, so we’re also going to add a simple nitrox 50% for the shallower phases of the dive to help with off-gassing. We’ll switch to that gas at 21 m/70 ft.
The table, generated by Deco Planner, shows the decompression time required for each bottom time at each depth. I’ve given you a minute at 21 m/70 ft to get your gas switch done, and I’ve also ignored any decompression deeper than the gas switch. Typically, this is 1 minute or so at 24 m/80 ft for longer dives. This stop is usually done automatically as you slow down your ascent rate (you are ascending at 9 m/30 ft/min right?) as you approach the gas switch.
Anyway, let’s find some patterns in this pile of numbers. I’ve highlighted one interesting point—the 30 minute bottom time at 45 m/150 ft. Here, the bottom time is equal to the decompression time. If we look at the 25 minute dive, we can see that the decompression time is 23 minutes, so if we did the same decompression as the bottom time, then we’d be on the conservative side. The same is true of the 20 minute bottom time. If we did 35 minutes of decompression on the 35 minute dive, we’d be 2 minutes short. The first thing to note is that decompression time does not increase in a straight line—but I never said it did. I’m trying to force a simple straight line rule onto a curve, and that will have limitations. And a man’s got to know his limitations.
Of course. We need to look at what happens at different depths as well. If we start at our original point, 30 minutes at 45 m/150 ft, and go 3 m/10 ft shallower, the decompression time is 4 minutes shorter. Go one step shallower, and we get another 4 minutes shorter. So, maybe a simple and conservative rule might be to say, “If we are 3 m/10 ft shallower than the 45 m/150 ft depth, then we get 5 minutes less decompression than the bottom time.” There are a whole 35 different boxes to evaluate in the above table, so it’s worth laying out that analysis now.
The basic rule we’re trying to test is this: At 45 m/150 ft, total decompression time is equal to the bottom time. For each 3 m/10 ft shallower, we can say that the decompression time is 5 minutes shorter than the bottom time. The table below shows the real decompression requirement, our estimate as well as an error figure to see where the limitations lie.
Each of the boxes corresponds to an individual depth and time, and in each box there are three numbers. The real decompression in the center, the prediction of the simple rule in the bottom right, and the error in the top left. A negative number indicates missing decompression using the rule. A positive number indicates a conservative estimate. I also stopped calculating when the error became more than 5 minutes, hence the bottom right of the table is missing. The limitations are starting to appear.
More Detailed Analysis
Now, the sharper amongst you will notice that in the top left areas of the table, there seems to be a lot of red. This is particularly apparent when the predicted decompression is zero. But, let’s face it; any dive involving a gas switch to 50% is going to involve a slow-down and some sort of “safety stop.” Indeed, if you were to slow the ascent from 21 m/70 ft to a comfortable 3 m/10 ft/min, and then make a 5 minute safety stop at 6 m/20 ft—as recommended by your friendly local dive instructor—then the smallest practical decompression you could make would be 10 minutes. A minute at each stop from 21 m/70 ft to 9 m/30 ft, and then 5 minutes at 6 m/20 ft.
The rest of the table follows a more predictable pattern, but any estimated decompression longer than 30 minutes tends to get a little inaccurate. If the estimate is 30 minutes or less, then you are within a few minutes of the true figure. I’ve redrawn the table to reflect the “minimum decompression” and removed all dives with longer than 35 minutes of estimated decompression.
Now there are lots more green numbers, and the red numbers are smaller. We’ll get to the deeper depth shortly, but let’s finish up this job first. We’ve developed a simple rule, with some limitations, that allows us to estimate the amount of decompression we need to do for a given depth and bottom time.
What we haven’t done is examine how that decompression is organised in the water column. My method is driven by practicality, as well as a little religion. The way I do it is like this.
I know that the longest stop will need to be done at 6 m/20 ft, and that the rest of the time should be spent distributed across the 21-9 m/70-30 ft stops. I find that the easiest way is to simply divide my estimate of the total decompression by two, and spend that amount of time on the 6 m/20 ft stop.
I then divide the remainder equally across the intermediate stops. So, a 30 minute decompression would be 3 minutes at 21, 18, 15, 12 and 9 meters (/70, 60, 50, 40, and 30 ft) and then 15 minutes at 6 m/20 ft. A 20 minute decompression would be 2 minute stops and then 10 minutes at the final stop, and a 10 minute decompression would be 1 minute stops and a 5 minute final stop.
You’re right, 25 minutes of decompression and just about every other number gets clunky on the divisions, particularly for the intermediate stops. So, I standardise my decompressions to be one of three simple possibilities: 1’s and 5, 2’s and 10, or 3’s and 15. I calculate my decompression estimate based on the bottom time and average depth of the dive and pick the decompression schedule that fits, and round up to the next schedule if I end up in-between. I’m all for an easy life.
Now there’s no wetnotes full of tables. Just three different decompression schedules—pick 10, 20, or 30 minutes of decompression.
I mentioned religion a few lines back, and there is a school of thought that says that dividing the decompression by two to work out the final stop is a bit aggressive, and that it should be more like two thirds on the final stop. Technically, it’s right: and, indeed, on longer and deeper dives, this will work better. But for dives in this range, the simple split works very well.
Extending The Tool A Little Deeper
Now, I promised you a tool that would work to a depth of 51 m/170 ft, and so far we’ve only got to 45 m/150 ft. So, let’s finish off the tool. First, we’re going to switch to a different gas. Trimix 21/35 is a little light on the helium for dives past 45 m/150 ft (END >3.6 ATA), so an 18/45 is a better choice. The nitrox 50% is still a good choice for a decompression gas though.
We can follow a very similar approach to what we did above:
If we try our estimate strategy—but, instead of subtracting 5 minutes of decompression from the bottom time, we add 5 minutes to our estimate for every 3 m/10 ft deeper than 45 m/150 ft—we see that things are actually quite conservative. Here’s the full error table.
Note: The middle number represents predicted total decompression time from Deco Planner; the lower right the estimated decompression using the rule, and the upper left, the difference between the tool (green is greater, red is less)
Green figures all round! I’ve not calculated past 30 minutes of estimated decompression since we’d already established that as a limit.
So, we can finish off our complete rule for estimating decompression requirements for dives in the 33-51 m/110-170ft range.
- For a dive at 45 m/150 ft, the total decompression time is equal to the bottom time.
- For each 3 m/10 ft shallower than 45 m/150 ft, subtract 5 minutes from the bottom time to get the decompression time.
- For each 3 m/10 ft deeper than 45 m/150 ft, add 5 minutes to the bottom time to get the decompression time.
When you’ve worked out your total decompression time, pick one of the three schedules:
These tools are based on 21/35 trimix or 18/45 trimix for the bottom phase and nitrox 50% for the decompression. You can use other gases for sure, but it’s best to run the numbers through decompression planning software and make sure the rules work. And if they don’t, with a bit of work, you’ll find a strategy that does.
For Rules of Thumb-Part One see: Rules of Thumb: The Mysteries of Ratio Deco Revealed
InDepth: Standard Gases: The Simplicity of Everyone Singing the Same Song by Richard Walker
InDepth: Maintaining Unit Cohesion by Richard Walker
InDepth: Decompression, Deep Stops and the Pursuit of Precision in a Complex World by Jarrod Jablonski
InDepth: Part Two: Tech Divers, Deep Stops, and the Coming Apocalypse by Jarrod Jablonski
Rich Walker learned to dive in 1991 in the English Channel, quickly developing a love for wreck diving. The UK coastline has tens of thousands of wrecks to explore, from shallow waters to deep technical dives. He became aware of GUE in the late 1990s as his diving progressed more into the technical realm, and he eventually took cave training with GUE in 2003. His path was then set, and he began teaching for GUE in 2004.
He is an active project diver, and is currently involved with the Mars project (Sweden) and the cave exploration team in Izvor Licanke, Croatia. He is the Chairman and founder of Ghost Fishing UK. He is also a full time technical instructor and instructor evaluator with GUE, providing these services via his company, Wreck and Cave Ltd. He sits on GUE’s Board of Advisors and serves several other industry organizations.
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].”
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