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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! 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.

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. 

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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

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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. 

Additional Resources:

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. 

Diving Safety

The Flexibility of Standard Operating Procedures

Instructor trainer Guy Shockey discusses the purpose, value, and yes, flexibility of standard operating procedures, or SOPs, in diving. Sound like an oxymoron? Shockey explains how SOPs can help offload some of our internal processing and situational awareness, so we can focus on the important part of the dive—having FUN!

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By Guy Shockey

Header Image by Derk Remmers

At first glance, the title reads like a bit of an oxymoron. How can a standard operating procedure (SOP)—which implies a ‘one size fits all’ solution to problem solving—also be flexible? How can flexible also be firm?

One of the things that initially attracted me to Global Underwater Explorers (GUE) was the presence of SOPs. For anyone with a military background, SOPs were our bread and butter. You can create a good SOP while you have the time to think and plan. You can put them into practice, refine them over time, and keep them in place until new or better information comes along to change them. 

For example, airline pilots have a binder full of SOPs for various contingencies. When something comes up, they turn to the correct page and find a list of actions to follow. Pilots understand that these SOPs represent the collective knowledge of many aviators and engineers that have come before them. Many of them have also been revised multiple times, codified, and then even revised after that. Some SOPs require commitment to memory because there may not be a lot of time, and pulling out a three-ring binder or flipping through your iPad to the correct page isn’t the appropriate action. In that case, then those same pilots practice these situations regularly in simulator training. 

One of the primary values of an SOP is that it frees up a lot of situational awareness information processing. You are able to match up “mental models” to the current situation and, rather than processing your information in small bite-sized pieces, you are able to process “chunks” of information that match patterns of something that you know or are familiar with. 

Let me create an analogy that may help make this clearer. If I were to give you a bowl of tomato sauce, some slices of pepperoni, some mushrooms, some cheese, and a piece of baked dough, you could eat them all one at a time and try to figure out what it was you were eating. Or, I can put all those ingredients on that same piece of dough, bake it, and you would instantly know that you were eating pizza. You don’t have to process all the ingredients one at a time. You already have an existing mental model that says “pizza.” We do this when we solve problems. We pattern-match and identify existing mental models all the time, and it’s actually the only way we can actually think as fast as we do. Many problems are actually solved with multiple mental models being applied together. 

Photo by Derk Remmers.

Having an SOP gives you the ability to solve problems more efficiently and effectively because you have a ready-made mental model or solution to a recognized problem. Think of every first aid course you have ever taken and the “ABCs” of first aid. SOPs are incredibly valuable in nearly every environment that includes potential risks. 

If an SOP is shared, it also allows diverse groups to work together. It is no surprise that SOPs from various militaries of the world are often similar, even if they are written in different languages. From personal experience, NATO countries can coordinate and execute complex military operations because they share common SOPs that, if not identical, are very similar and don’t require much adjusting to mesh together. Common expectations and goals can be shared toward a common purpose. 

When in time-sensitive environments, many of these SOPs and the corresponding mental models they help develop can be lifesaving. This doesn’t just apply to the military, but also to law enforcement, paramedics, firefighters, pilots, and any other profession that is often faced with time pressures in making critical decisions. 



Do you share a common operational picture?

There is an interesting term often used in military circles called the “common operational picture” (COP). This is exactly what it sounds like, and is sometimes referred to as “a single source of truth.” Everyone involved in a decision-making cycle needs to be privy to the information that affects their decision. Sharing that information allows us to make informed decisions that often include SOPs. You could argue that we are creating a mental model that lets us apply another mental model!  

Alright, so how exactly does all this apply to diving and GUE diving in particular? I’m pretty sure that many of you have already connected many of the dots. 

In the GUE world, our divers create a COP at the beginning of the dive. We help reiterate this COP with our GUE EDGE pre-dive checklist, which is a great example of an SOP! We review the goals, team roles, our equipment, and the operational parameters of the dive, all in a standardized format that efficiently accommodates teammates from multiple different languages and cultures. I have performed GUE EDGEs in about 10 different languages and I only speak two!  The fact that we were doing this in a standardized fashion meant I could follow along and knew what they were talking about. 

As the dive plan complexity increases, so too does the COP become more complex. Some of our more ambitious exploration projects require even more time spent in planning than actual execution. But because there is a COP, coupled with SOPs (I know that’s a lot of acronyms), these projects usually go off without a hitch. 

Photo by Derk Remmers.

During the dive, there are multiple times that we have team-expected actions that are based on SOPs, and this contributes to and reinforces our COP. It is almost as if we are filling in a PDF form as we go along and confirming the various pieces of information that we need to complete the entire “form” or plan. 

In the case of emergencies, we have ready-to-implement SOPs for just about any equipment malfunction from valve failures to losing your mask. We practice these SOPs so that, in real time, we can employ them in a timely fashion and resolve the problem. These SOPs are just like the ones I mentioned at the beginning of this article and were developed over time and refined with successive reviews and after-action analyses. Finally, they have been codified, and you can now find them in our GUE SOP manual! You will also notice that this manual is of a particular “version,” which tells you that the SOP is constantly being fine-tuned in a dynamic process.

How Can An SOP Be Flexible?

In reality, it isn’t the actual SOP that is flexible, but it is the degree of flexibility it provides to the dive plan itself that is of value. Let me give you an example from the technical diving world. 

Imagine the team is diving on a wreck and experiences a delay on the bottom for whatever reason. It could be that it was done on purpose (discovery of pirate gold!) or maybe it was imposed upon the team as a result of any number of problems, like dealing with an equipment problem or an entanglement, for example. The dive is longer, the decompression obligation is now going to be longer, and there are some decisions to be made. 

Having an SOP here can help provide a solution to the problem with no mess and no fuss. The divers dig into the bag of tricks they learned in GUE technical training, and because of their common operational picture and team-expected actions, they apply the SOP they practice regularly and modify their decompression schedule to suit the new bottom time. What could have been an exciting moment for many divers turns into just another discussion point for their debrief after the dive!

Photo by Alexandra Graziano.

So, while SOPs are usually not flexible in and of themselves, they allow for a great deal of flexibility while diving by freeing up mental processing power and providing ready-made and practiced solutions to potential problems. 

GUE SOPs presuppose the presence of personal diving skills at a high level, and assume that factors such as good buoyancy and trim are second nature. In fact, many of the SOPs state the first step in resolving a problem as “stabilize” or “stop” in all three dimensions. GUE divers see that, as the diving gets more complex, the SOPs also get more complex. For a new GUE Fundamentals diver, demonstrating some of the SOPs required to pass muster as a Tech 2 or CCR 2 diver look more akin to channeling “the force” than anything else. However, like most things, perfect practice produces perfect performance, and so it’s just a matter of putting in the repetitions. 

For me, diving has never been the end but the means to the end. Anything I can do to make those means take up less mental and physical horsepower means that I can devote more of the same to the end goal. And at the end of the day, I am really all about that pirate gold!

Additional Resources:

Note that GUE members or divers taking a GUE course receive access to GUE’s 30-page manual, Standard Operating Procedures.


Guy Shockey is a GUE instructor and trainer who is actively involved in mentoring the next generation of GUE divers. He started diving in 1982 in a cold mountain lake in Alberta, Canada. Since then, he has logged somewhere close to 8,000 dives in most of the oceans of the world. He is a passionate technical diver with a particular interest in deeper ocean wreck diving. He is a former military officer and professional hunter with both bachelor’s and master’s degrees in political science. He is also an entrepreneur with several successful startup companies to his credit.

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