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Rules of Thumb: The Mysteries of Ratio Deco Revealed

Is it a secret algorithm developed by the WKPP to get you out of the water faster sans DCI, or an unsubstantiated decompression speculation promoted by Kool-Aid swilling quacks and charlatans? British tech instructor/instructor evaluator Rich Walker divulges the arcane mysteries behind GUE’s ratio decompression protocols in this first of a two part series.

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By Richard Walker

Header photo: Diver image by Derk Remmers.

Some ideas take on an undeserved aura of mystery. When ideas are inadequately understood (or explained), people have  a tendency to  either regard them as some sort of undeniable truth, or reject them as unmitigated nonsense. 

Ratio decompression (RD) is one such idea. I know people who believe that RD is some kind of magic formula that will protect them from decompression sickness, get them back to the surface faster than anyone else, and leave them feeling more refreshed than they were before the dive. I also know people who consider it to be reckless, unsubstantiated foolhardiness, promoted by quacks and charlatans with less grip on reality than the Flat Earth Society. 

The reality, of course, lies somewhere in the middle. 

In this article, I want to try and explain the ideas, history, and motivation behind using a tool like Ratio Decompression. 

RD aside, there are two ways you can manage a decompression dive. The first is to carry a computer set up to reflect the gas you’re breathing and the conservatism factors you believe are appropriate. You can then jump in, make the dive, and follow the instructions given by the machine. The second method is to plan a series of profiles—using either tables or planning software—and write down a handful of them to cover any realistic variation in depth and bottom time that might happen during the course of the dive. 

You could, of course, do both and carry a range of plans in Wet Notes and then compare these with the instructions from a computer; but, that does seem rather complex. And you are always left with solving the conundrum posed by Confucius in 500 BC: “A man with two watches never really knows the time.” 

But what if this could all be simplified? Most people that dived air back in the day would know the simple rule that their no-stop time at 100 ft/30 m was about 20 minutes. The No Decompression Limit (NDL) at 70 ft/21 m was around 50 minutes. It doesn’t take a Nobel Prize winner to figure that the NDL at 80 ft/24 m would be around half way between those two numbers, or 40 minutes. In the imperial system, this was called the 120 Rule, because NDL + depth = 120. This rule of thumb can’t be directly converted to the metric system unless you’re a math whiz, but then that would defeat the purpose of keeping it simple. A metric version does actually exist, but the way you calculate it is a little different. Amazingly, it works just as well. 

The logical follow-up question is, “What happens if you stray beyond the no-stop time? Do you have to go back to tables, or buy a $1,000 computer?” Amazingly, you can come up with a pretty simple rule for this eventuality as well. Let’s look at a decompression table for air. The 120 Rule is highlighted in green, showing the NDL. Some tables show a few minutes of decompression, but nothing longer than a good old safety stop, right?

Hang time
Hang time! Photo by Sean Romanowski.

Rules of Thumb for Air

Now look at the decompression times, which are to the right of the green areas and are highlighted in red. For these dives, all of the decompression comes in at 6 m/20 ft. 

Table 1. Air Table Source: Deco Planner Buhlmann ZH-16 algorithm with 100/100 gradient factors for purity’s sake.
Table 1. Air Table Source: Deco Planner Buhlmann ZH-16 algorithm with 100/100 gradient factors for purity’s sake.

If you close one eye, and look sideways at the table you should start to see a pattern emerging for the required amount of decompression. Your decompression obligation relates to the number of minutes of bottom time you do longer than the no-stop time. It is approximately one minute of decompression for every minute past the no-stop time. The table below shows the idea.

Table 2. 

Here, the decompression predicted by software is labeled “real deco,” while the result of the 120 Rule is labeled  “ratio.” The “error” label indicates the discrepancy between “real” and “ratio.” For the vast majority of these dives, the 120 Rule calculation is within one minute of the true decompression, if not more conservative. There are a few results where the decompression is insufficient (more than one minute of error) and these are highlighted in red. 

So, the first thing to note is that this is a pretty useful concept. And the second, perhaps the most important, is that it has limits that you need to define or understand before using it. 

Those dives where the error in RD is more than one minute only occur on dives that build up more than 30 minutes of decompression, so this becomes our defining limit for the RD tool. To clarify, based upon the table above, the following is true: For dives shallower than 30 m/100 ft, the required decompression time equals the bottom time minus the NDL at that particular depth, as long as the decompression time does not exceed 30 minutes. 



How About For Voodoo Gas?

This approach has been developed assuming that the diver is breathing air. Now, for those of you still diving air, please try and catch up with the rest of the world! [Ed.note: Compressed air is for tires!] For these 30 m/100 ft dives, nitrox 32 is a much better choice of gas. Your NDL is longer, meaning you’ve less need of a RD tool or a $1,000 computer to complete an equivalent dive. But, should you want to go past the NDL on nitrox 32, then the exact same rule can be used (believe it or not). The diver’s required decompression at 6 m/20 ft is equal to bottom time minus the NDL at that depth. Or in other words, one minute of decompression for every minute past the NDL.

Don’t believe me? Let’s repeat the process we’ve just done, but for nitrox 32. First we need a simple way of calculating the NDL. It turns out that a 130 Rule exists for nitrox 32. Your NDL is simply 130 minus your depth in feet. These are shown in green in the table below, just like before, and then the decompression stops needed at bottom times out to 75 minutes. 

Table 3. Source: Deco Planner Buhlmann ZH-16 algorithm with 100/100 gradient factors for purity’s sake.
Table 3. Source: Deco Planner Buhlmann ZH-16 algorithm with 100/100 gradient factors for purity’s sake.

You can see that the 130 Rule works pretty well and—just like its air diving brother—is conservative in shallower water. So what about the decompression? Squinting at the table again, you’ll see that for every minute past the NDL, you need about 30 seconds of decompression. So a 40-minute dive at 30 m/100 ft would need five minutes of decompression, compared to the five minutes required by the table. There’s another advantage of nitrox 32 over air—comparable dives require less decompression. But that’s now two rules, and I like simple things. If you just stick to “One minute past the NDL gives you one minute of decompression,” even I can work that out. If you run the comparison table again you can see how the rule performs. 

Graphical user interface, application, table, Excel

Description automatically generated
Table 4.

You can see pretty quickly that this rule doesn’t become aggressive at all. There are no “red zones” on this table, which is partly due to the conservative interpretation of the calculation (and a little conservatism is no bad thing). Our RD rule for calculating decompression on nitrox 32 dives is then very similar to the one for air. The required decompression time at 6 m/100 ft is equal to bottom time minus the NDL at that depth. Or, in other words, one minute of decompression for every minute past the NDL.

The Virtues of Oxygen

I don’t know about you, but for me, most of the time  decompression is a chore, but it’s also painful when you get it wrong. It doesn’t pay to cut corners despite the allure of a faster decompression. But there is a way to reduce your decompression time, and that’s to use a decompression gas. For dives like this, all of the stop time is at 6 m/20 ft, so oxygen is an ideal choice if you’re qualified to use it. 

Photo by Derk Remmers.

I’m not going to go through the whole process again – at this level of diving, you should prove things to yourself rather than trust some clown on the internet. In short, you can use the same rule as before, but if you switch to oxygen at 6 m/20 ft, observing all proper switch procedures, gas handling protocols, and other things your mother should have taught you, then you can reduce the decompression time by half. 

So, if you’d planned a dive needing 30 minutes of decompression on nitrox 32, using oxygen at 6 m/20 ft would reduce that time to 15 minutes. This is, again, a very conservative implementation, but that’s a good thing in my book. But like I said, don’t take my word for it: get yourself a copy of GUE’s Deco Planner and prove it to yourself. 

In the next installment of this article, I’m going to expand the discussion to deeper dives—which will draw more on the ideas we’ve developed about ratio decompression here—and use them to plan dives in the 30-50 m/100-170 ft range.

See: Rules of Thumb 2: Further Mysteries of Ratio Deco Revealed  . Are you able to calculate your decompression for a 40-50m/130-165 ft dive using only your gas mix, average depth and bottom time? Here British techmeister Rich Walker further divulges the enigmatic mysteries behind GUE’s ratio decompression protocols in this part two of the series.

Dive Deeper:

InDepth: Standard Gases: The Simplicity of Everyone Singing the Same Song by Richard Walker

InDepth: Maintaining Unit Cohesion by Richard Walker

Wikipedia: Ratio Decompression


Rich Walker learned to dive in 1991 in the English Channel and soon developed a love for wreck diving. The UK coastline has tens of thousands of wrecks to explore, from shallow 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:

Mars project, Sweden; Cave exploration team in Izvor Licanke, Croatia.; Ghost Fishing UK, Chairman and founder. He is also a full time technical instructor and instructor evaluator with GUE, delivering these services via his company, Wreck and Cave Ltd. He sits on GUE’s Board of Advisors and serves several other industry organizations. 

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Risk-Takers, Thrill-Seekers, Sensation-Seekers, and … You?

It’s likely that many in our community no longer think of tech diving as a risky activity, or perhaps even appreciate how important taking risks may be to one’s personal health—let alone that of our species. Fortunately, InDEPTH’s copy editing manager Pat Jablonski dived deep into the origins, meaning, and benefits of regularly taking risks, and even offers a thrill-seeking quiz for your edgy edification. What have you got to lose?

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by Pat Jablonski. Title photo courtesy of Katelyn Compton Escott.

“Life without risk is not worth living.” – Charles Lindbergh

What defines a risk? What is involved in taking a risk?

Difficult questions to answer, because something that feels risky to one person might be yawn-worthy to another. Risk taking, unscientifically, is something you do that gets your blood up, raises your heartbeat, awakens your senses, and makes you hyper-aware of your surroundings.

Surely we can agree that the Covid pandemic has added an unexpected level of risk to everyday life. Add poor drivers, mass shootings, contentious politics, global climate change, and many are left believing that meeting each day is risky enough. But that’s not true for people who identify as risk-takers or thrill-seekers.

“Everyone has a ‘risk muscle’. You keep in shape by trying new things. If you don’t, it atrophies. Make a point of using it once a day.” – Roger Von Tech

There are many activities that go to the trouble of defining the level of risk involved with a specific activity, and while that’s not the purpose of this article, you should know that scuba diving ranks fairly high on the risky behavior scale–higher than skydiving and rappelling. And, cave/wreck diving or freediving isn’t on any risk scale we could locate. We can assume it’s up there—near or at the top.

Fock A. Analysis of recreational closed-circuit rebreather deaths 1998–2010 Diving and Hyperbaric Medicine. 2013 Volume 43, No. 2. With the caveat that they are “best guess numbers,” Fock concluded that rebreather diving is likely 5-10x as risky as open circuit scuba diving, accounting for about 4-5 deaths per 100,000 dives, compared to about 0.4 to 0.5 deaths per 100k dives for open circuit scuba. This makes rebreather diving more risky than skydiving at .99/100k, but far less risky than base-jumping at 43 deaths/100k. The current belief is that rebreather diving has gotten safer.

Divers are a fairly small niche group for many reasons. One of them could involve the degree of danger associated with the sport. Answer this: Do dry land people ever ask you why you would want to take such a chance with your life in order to go where you weren’t meant to go? 

It’s a reasonable question, albeit a hard one to answer.

Photo courtesy of Glen Kwan

“A life without risk is a life unlived, my friend.” – Big Time Rush

Kevin Costner’s Waterworld aside, humans have (yet) to be born with gills or webbed toes. Still, there you are. You’ve spent unmentionable amounts of money. You’ve carved out a whole day, or maybe weeks, away from your to-do list. You’re suited up and look like an alien. You’re on a quest to explore the aquatic world where you’re able to breathe only with a cumbersome apparatus. You’re planning to explore inner space! You’re going to delve into that amazing realm that’s off limits to most people. 

You may look all matter-of-fact, cool as a cucumber, another day at the office, but it’s a thrill, isn’t it? Inside, you’re a kid with butterflies in your tummy who’s getting away with something big and exciting. Okay, it’s true–you and your team are highly trained, your equipment is top-notch, every box is checked off, and you are behaving responsibly. However, you’d have to be in a coma to not realize that what you’re about to do is taking a risk. Who doesn’t know that people have died doing what you’re doing? Answer honestly: How much more exhilarating is the experience when you know it’s not a walk in the park? Our own Michael Menduno admitted that “the feeling of being more alive lasted for days” after a dive.

So, you’re a diver. Does that mean you’re a risk taker? A thrill seeker? A sensation seeker?

Let’s dive into that subject, first by taking a little quiz, shall we? 

Photo courtesy of SJ Alice Bennett

From A Death Wish to Life Is Precious

In the past, too many mental health professionals treated risky behavior like a disease in need of a cure, focusing on the negative side of risk, even using government funding to address risky behavior and stamp it out. 

Before that, Sigmund Freud might have even believed that thrill seekers had a death wish; in fact, it’s what was believed for many years. 

Modern-day science doesn’t support either theory.

“Only those who will risk going too far can possibly find out how far one can go.” – TS Elliot

For our purposes, we’re focusing on the positive aspects of taking chances, pushing boundaries, and seeking experiences that make life feel . . . more alive. Richer. Fuller. We want to examine what goes into the psyche of a person (like you?) who is enthusiastically willing to engage in an activity already identified as dangerous, possibly even by the people who are engaging in it, and hear what some experts on the subject have to say about such people.

Photo courtesy of Jen MacKinnon

The University of Michigan’s Daniel Kruger proposes that taking chances is a fundamental part of human nature going all the way back to our ancient ancestors—prehistoric humans who had to constantly put their safety on the line in their fight for survival. Think fighting off a wooly mammoth with a stick. Kruger believes we have consequently retained many of those same instincts today, and he believes that it’s a good thing. 

This writer, who is related to a major risk-taker, has always believed that heart-quickening experiences are essential for a well-lived life. I’m convinced and have long proposed that those pulse-pounding moments are often accompanied by a deepened understanding of and appreciation for one’s life—perhaps all life. And I’m happy to report that current science confirms that belief.

“If you are not willing to risk the unusual, you will have to settle for the ordinary.” – Jim Rohn

Dr. Kruger is one of the scientists who proposes that taking risks means “seeking that moment when life feels most precious.

This should not be news for you diving adventurers out there.

Nature vs. Nurture: Born That Way or Learned To Love Adventure

Another scientist, Marvin Zuckerberg, affirms the theory that risk taking is in our DNA. “Certain people have high sensation-seeking personalities that demand challenges and seek out environments that most people’s brains are geared to avoid.” I’ll go out on a limb and say that underwater caves or shipwrecks would qualify as environments most would avoid.

Dr. Cynthia Thompson, the researcher behind a 2014 study from the University of British Columbia, was early to look at the genetic factors that might make a person predisposed to participating in extreme sports, ones that are typically defined as activities where death is a real possibility. The results of her study revealed that risk-takers shared a similar genetic constitution, a genetic variant that influences how powerful feelings are during intense situations.

Photo courtesy of Steve Boisvert

Most scientists agree that personality is a complicated mix of genetic and environmental influences. The “nature vs. nurture” dilemma is alive and well. Dr. Thompson concluded that people who engaged in so-called high-risk sports were not impulsive at all, not reckless either. Instead, “they’re highly skilled masters of their discipline who take a very thoughtful approach to their sports.”

A study conducted in 2019 examined human boundaries, people who pushed them to their limits and beyond, and what made those people tick. Zuckerman labeled such people “sensation seekers” and defined them as “people who chase novel, complex, and intense sensations, who love experience for its own sake, and who may take risks to pursue those experiences.” Is that you?

“History is full of risk-takers. In fact, you could say that risk-takers are the ones who get to make history.” – Daniel Kruger

Other experts posit an alternate theory—one proposing that modern society in the age of seatbelts, guardrails, child-proof caps, safety precautions, laws, rules, and regulations has dulled the sense of survival. In other words, life has flattened out and no longer feels exciting, or risky. So, is one of the reasons we seek excitement because of boredom? 

Maslow’s Theory of Self-actualization

I don’t honestly know who was the first proponent of risk-taking being a positive thing, but the work of Abraham Maslow, the founder of humanistic psychology, was one of the first. Maslow became one of the most influential psychologists of the twentieth century, and he developed a theory of human motivation that advocates for “peak experiences.” Peak experiences are not attained without risk.

“One can choose to go back toward safety or forward toward growth. Growth must be chosen again and again; fear must be overcome again and again” – A Maslow

He proposed that, in addition to meeting basic needs, all humans from birth seek fulfillment in terms of what he called self-actualization—finding their purpose/being authentic. Self-actualization involves peak experiences—those life-altering moments that take us outside ourselves, make us feel one with nature, and allow us to experience a sense of wonder and awe. Maslow also believed that those who were able to have such peak experiences tended to seek them out rather than waiting for the next random occurrence. Hence the anticipation of the next dive?

“Do one thing every day that scares you.” – Anonymous

Photo courtesy of Adam Haydock

Out of Your Comfort Zone Into A World of Wonder

Psychologist Eric Brymer from Queenstown University of Technology in Brisbane, Australia, has spent years studying extreme athletes and has this to say: “They’re actually extremely well-prepared, careful, intelligent, and thoughtful athletes with high levels of self-awareness and a deep knowledge of the environment and of the activity.”

Recent research backs up what some extreme sports athletes have been saying for years, even if only to themselves.

“What participants get from extreme sports is deeply transformational, a sense of connecting with a deep sense of self and being authentic, a powerful relationship with the natural world, a sense of freedom,” says Brymer. “They get a strong sense of living life to its fullest as if touching their full potential.”

Brymer’s comments mirror what Maslow, the founder of humanistic psychology, said back in the 1940s.

We’re not advocating for taking stupid chances (such as diving without proper training, or necessary precautions) and we don’t believe anyone reading this article does that. We simply intended to focus on the scientific evidence that supports adventurers—people who get a thrill from an activity that offers—as a bonus; a chance to feel awakened from the mundane and thrust into a world of wonder. 

Risk-takers and sensation- or thrill-seekers chase unique experiences. Often, those experiences bring awareness of important issues or increase essential knowledge about the planet we share. Many people overanalyze and dither when faced with an unfamiliar situation; they shy away from unsettling circumstances. Risk-takers face the unknown and trust themselves to prevail. Learning to scuba dive, for example, pushes people out of their comfort zone, takes them into a realm foreign and mysterious. Diving forces divers to pay complete attention to a task, to focus with laser-like precision in order to conquer misgivings, and to attain a skill that few others have. Confidence comes with accomplishment. Leadership emerges. Fear is overcome. 

Sensation-seekers see potential stressors as challenges to be met rather than threats that might defeat them. With action, resilience develops. High sensation-seekers report lower perceived stress, more positive emotions, and greater life satisfaction. Engaging in extreme activities brings them peace. 

What does it bring you?

Dive Deeper

Bandolier: Risk of dying and sporting activities

National Geographic: What Makes Risk Takers Tempt Fate? Recent research suggests that genetic, environmental, and personal factors can make people take on risky—even potentially fatal—challenges.

Healthday: Taking Risks By Chris Woolston HealthDay Reporter


Pat Jablonski heads up the copy edit team for InDEPTH. She is a blogger, a writer of stories, a retired tutor, English writing teacher, and therapist. She’s a friend, a wife, a proud mother and grandmother. She is also a native of Florida, having spent most of her life in Palm Beach County. She has a B.A. in English from FAU in Boca Raton and an M.S.W. from Barry University in Miami. She learned to swim in the ocean, a place she thinks of as home, but she doesn’t dive.

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