Actual Decompression Planning: navigating around accelerated vs optimized deco

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Taken from a presentation first made March 13, 2001, updated April 2008

“Smile, breathe and go slowly”   Thich Nhat Hanh, Vietnamese Buddhist Monk, Teacher
All dives and certainly every staged decompression dive carries a very real risk that the people making it will suffer some sort of decompression stress… either sub-clinical or full-blown. The likelihood of getting bent depends on the variables; Who, When and How more than constants such as the type of brand of decompression model used. And of course, depending on variables produces variable results.

Once we accept that getting bent is a real probability – albeit with a variable risk – we can begin to manage decompression diving in a realistic way and work at cutting the unpredictability to an acceptable level. None of us wants to take a ride in a recompression chamber – touch wood, I’ve managed to avoid it so far – but it’s not something we can ignore or be frightened by.

So what do we need to know as divers… decompression divers… to keep us healthy? Decompression from a dive – every dive – consists of two phases that can be broadly defined as ascent beyond the off-gassing ceiling and surface off-gassing… or, less formally, in-water decompression followed by a surface interval. Whether formal or not, we cannot say a dive has been successful until both phases are completed and there are no complaints of joint pain, paralysis, skin rash or any other signs or symptoms caused by decompression stress. Doppler testing has shown bubbles persisting in divers for as long as several days, so the waiting period is probably longer than many of us admit to. This of course should have each of you thinking to yourselves: “I still want to conduct successful staged-decompression dives… but it looks like I need help to make that happen!”

When we discussed the Alchemy of Decompression [previous chapter], we saw that being successful is a broad combination of several factors including education, scepticism, conservatism, adequate health and fitness, adopting and following diving practices that conform to an accepted norm, and, perhaps most importantly, some luck.

Unfortunately, luck will always be an influence in the outcome of your diving. Your job is going to be concentrating some considerable effort on the other factors so that the percentage of luck in your personal equations is kept to an absolute minimum.

At some point, and it may as well be right now, you should make some sort of determination on how hard you are prepared to work at this job. You have a straightforward choice to exert some control over what happens to your body… or not. You will never get the luck quotient to zero but I feel it’s well worth trying. I hope you do too.

While you work through that… and I expect an answer before we go diving together… consider that many divers make their first staged decompression dives under the auspices of an experienced decompression instructor. The chances are that their instructor will suggest various strategies to draw up ascent schedules that keep “everyone safe.” These may include wearing a personal dive computer (PDC), using existing “hard” tables – such as those from Buhlmann, DCIEM, BSAC, US Navy – or cutting “custom” tables using decompression software.

Some instructors may employ combination strategies that typically consist of custom tables backed up by individual dive computers or a PDC backed up by custom tables. The hope is that the student leaves the class with an understanding that there are several workable strategies to manage decompression stress. They also leave with a handful for dives to supply empirical evidence of what worked for them. This empirical stuff – data from actual dives – that’s the gold standard for them and their future dives but few realize it; but more on this later.

During a decompression course, students are guided towards making desicions that conform to some acceptable norm arrived at by their certifying agency, their instructor or an amalgam of both. Regardless, course dives are inherently conservative – or should be. This strategy is partly due to the standards published by the agency for the courses themselves. Typically exposures are limited to a specific maximum total ascent time or a ratio of decompression time to bottom time or limited by the flavor and volume of gas participants are “allowed” to carry. The reasoning behind this is pretty simple: instructors make a living teaching not riding in a chamber or waiting for their customers to finish a table six recompression. If your instructor has to teach another similar course starting the day after he or she finishes up with your course, my guess is course dives will not feature aggressive profiles.

Oh, a quick definition. An aggressive profile is one where decompression – either in-water or on the surface or both – are accelerated. In other words, at least one factor pushing for a conservative approach has been swept aside or ignored. An optimal profile on the other hand is one where every practical opportunity to make the diver’s probability of getting decompression sickness close to 0 is taken. We will contrast these two approaches to deco planning a little later.

When a diver graduates a decompression diving course and begins to plan his or her own dives, they will typically follow pretty much the same general format that was presented during their course. By dive eight or nine post class, it’s normal for some slight changes to have taken place. The Human Factor usually pushes the needle closer to one as time passes. In rebreather diving we talk about the “Death Zone,” a time when the diver’s wariness of new technology begins to wear off and their experience as open-circuit divers begins to over-rule caution. Complacency causes small leaks to appear in the dyke. Water starts to trickle in. The hope is that they notice something is going on and smarten up before a full-on breach washes them away. Well the same effect is true of decompression diving. Over time, some level of complacency almost always sets in. If we could see probability of getting bent (PGB) as a straight line with 0 (no chance) at one end and 1 (100 percent certain) at the other, we might see what happens a little clearer.

Zero probability of getting bent means not diving. So, the probabilistic pointer is obviously somewhere to the right of that. On their last course dive executed with an instructor – assuming they did not end up on oxygen, clear fluids and being driven to a chamber – the pointer was closer to 0 than to 1. For the sake of illustration, let’s say that for every 100 times that profile is dove, between three and four people get bent. That’s a 0.035 PGB, and that’s high but no higher than for many tables accepted in the diving community.

OK, so now they plan their first post course dive the following weekend and most divers will try to do everything exactly as shown… very little is different (apart from not having an experienced decompression diver watching them like a mother hen). However, over time, small changes take place. Perhaps during a stop that was supposed to be at 9 metres for three minutes, they made a mistake and only stayed for two minutes and 30 seconds. “No biggy,” they think. Perhaps they were holding onto an ascent line and it was moving around. “That’s OK, I got away with it,” they say. Perhaps their decompression gas was delivering a partial pressure of 1.3 bar when the tables they used assumed 1.4 bar. “I did not notice much difference,” they explain. Maybe because they were not on a course, they drank a second glass of wine at supper the night before… Whatever it is and how trivial it may seem, something was changed and usually that change means their probability pointer is shifted closer to 1. But they get away with it – for a while – and this begins to insulate them from the lessons presented to them by their instructor. It can happen to any one of us. So what can we do to fight it?

I’m a strong advocate for taking notes after every decompression dive and allowing this to become a habit. The notes can be short but should be contemporaneous, and contain at least the ascent profile followed – including ANY and ALL deviations – gases used, overall feeling post-dive compared to pre-dive, and notes about any other factors that may have influenced the outcome – was the diver well-hydrated, were they rested, did they work during the dive, were they warm or cold during the dive, did the decompression go smoothly, were gas switches slick and so on. By listening to what his body tells him after a dive, a diver builds his own probabilistic dive table which can be refereed to again and again.

Look at it this way. The first time we follow a custom decompression table or a PDC profile, we are trusting it to keep us safe and whole but there is no guarantee it’ll work. The PGB is somewhere between 0 and 1 but we really cannot be sure where exactly. It’s a crap shoot. Our profile might carry a PDC of 0.01 or 0.10… who knows. If we dove the same profile last week or last month and felt fine after it, and today start the dive in better shape – let’s say better rested and better hydrated – there are still no sure bets but at least we know the odds are in our favor and the good money is on the PGB being close enough to 0 for the dive to be doable.

So take notes.

OK. Let’s go back to what is meant by accelerated vs optimized decompression.

(to be continued…)


One thought on “Actual Decompression Planning: navigating around accelerated vs optimized deco

  1. I listened to you speak at the NACD last November and I would like train with you some. I am Full Cave (NACD)and I feel that mine training is incomplete. What classes are you teaching and when?

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