Actual Decompression Planning… (part two)


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Taken from a presentation first made March 13, 2001, updated April 2008
OK. Let’s go back to what is meant by accelerated vs optimized decompression and look at how two quite different approaches to the same dive, illustrate some core differences.

First we need a dive to use as an example and we’ll take something very simple: an ocean dive to 35 metres (about 120 feet) for 25 minutes. I’ve chosen this dive because there’s nothing about it that’s mind-blowing. The depth is within the range of an experienced advanced diver – certainly a diver interested in learning about technical diving – and the volume of gas needed to complete the dive – a topic we’ll discuss at length when we talk about gas management – can be carried by an open-circuit diver without surface support More germane for our current purposes, the total ascent time – including any required stops – will be moderate (about the same as the bottom time). In short, this is an entry-level technical dive and a good place to begin our discussions.

Now, what remains is to choose gases that will work for this dive and a set of tables or an algorithm to give us our ascent schedule. The dive could easily be conducted using air, but from the perspective of decompression management, a 30 percent nitrox would be a better choice. There are other options too but let’s keep this uncomplicated. EAN30 will deliver an oxygen partial pressure of 1.35 bar at depth. This is perfectly acceptable. Now for decompression gas, our diver – who we will call Jennifer, a fairly new decompression diver – has 100 percent oxygen.

One final decision is what algorithm are we going to have Jennifer and her buddy use on this dive. We’re spoiled for choices, but I want to suggest using something that should be familiar to most experienced sport divers – hard tables. We could say: “Just follow what your PDC tells you to do, but that’s not much of a learning experience. We could also have them generate custom tables with one of many available decompression software packages. But these applications do ALL the work and at this point in Jennifer’s development as a decompression diver, I think she should work through the process longhand and make informed decisions about things that might affect her well-being.

No dive tables put the PGB at zero but the DCIEM serial tables are considered a lot better at putting it close by the “experts.” I like them for this sort of dive planning even though they’ve been designed around a premise that low-grade post-dive bubbling is acceptable for some profiles, and they’re a popular choice for people in that transition between experienced sport diver and neophyte technical diver.

These tables, for the record, were developed by a diving research team working for the Canadian military starting in about 1962. The name of the model comes from the research facility in North Toronto, Ontario, where it was originally developed (the Defense and Civil Institute of Environmental Medicine, now renamed Defense Research Development Canada by the way). Part of the development process involved setting up what has become the most comprehensive decompression database in the world, containing recorded details of all research dives at DCIEM and DRDC since 1964.

The Sport Diving Tables are considered one of the most conservative available, and particularly applicable to cold water dives since testing was carried out with working dives in cold water. The methodologies employed at DCIEM and DRDC included extensive Doppler testing of subjects pre- and post-dive, and a circumspect interpretation of the data from those tests. The other unique thing about the DCIEM table – and something that sets it apart from neo-Haldanian cousins – is its construct as a serial model. In the DCIEM model there are four compartments each with about a 21 minute half-time with only the first compartment exposed to ambient pressure. As gas tension builds in the first compartment it bleeds into the next and so on. The Haldanian and neo-Haldanian models have all compartments exposed to ambient pressure. What difference does this make? Intuitively it seems to be a model that better reflects what may actually be happening in a diver’s body, and it results in dive profiles that are better suited to recreational diving – both sport and technical – than say the US Navy tables.

So, given all this, we will draw Jennifer’s ascent information from the version of DCIEM Sport Diving Table published in 1995.

Now a quick explanation about the choices of bottom mix and deco gas. We should already accept that using nitrox on the bottom and switching to a richer nitrox during ascent is a perfectly acceptable practice because it seems to lessen decompression stress. The EAN30 certainly gives a little edge compared to diving air. Its equivalent air depth at 35 metres is about 30 metres ( ([FN2 X (Depth + 10)] /0.79) – 10). Not much but every little helps. As stated before, the partial pressure of oxygen at depth with this mix is about 1.35 bar which Jennifer sensibly rounds up to 1.4. That gives a single dive limit of 150 minutes and a daily limit of 180 minutes. There’s an acceptable buffer for CNS loading for this type of dive.

The oxygen is an efficient finishing gas. There is no nitrogen in the mix and therefore by switching to oxygen at its maximum operating depth (MOD) for a resting decompression, which is about 6 metres or 20 feet at 1.6 bar PO2, will create the maximum theoretical increase in vacant partial pressure for the nitrogen dissolved in the Jennifer’s body. The result should be efficient gas elimination and some experts suggest that stop times derived from air tables can be cut by about one third when breathing oxygen. Of course, there is a downside because the CNS loading at 1.6 bar is high… about 2.3 percent per minute We will look at some options to manage this in a while.

Let’s look at what ascent time the DCIEM tables give for Jennifer’s dive. It is an ascent of 15 metres a minute plus or minus three metres and a straight 10 minute stop at six metres and another 10 minutes at three metres. These figures are based on air as a breathing medium.

An aggressive approach to her decompression obligation would have Jennifer ascend at 15 metres per minute, to arrive at her six metre stop ready to start breathing from her decompression gas. Since it is oxygen, she has decided to maintain the five-minute stop but when she ascends to three metres she cuts that stop from the suggested ten minutes to five. Her logic is that the tables called for two stops totalling15 minutes. She is using oxygen and therefore feels she can eliminate one third of that time. The choice to keep the six metre time unaltered and to cut the three metre stop in half was based on nothing more than her understanding that a five-minute safety stop at six metres is required regardless of what gas she might be breathing. Strikes me this is a good rule to follow… especially in this case.

Her decompression schedule then is two minutes to ascend from 35 metres to six metres, five minutes there and then an additional five at three metres and from there to the surface for a total of a little more than 12 minutes ascent time. Accelerated decompression. Short as possible and as aggressive as hell.

An optimal approach considers the variables on the day of the dive. Perhaps the diver is not as well hydrated as she should be, the water is cool and is moving, and there is another dive planned for later in the day, the moon is full, whatever it is, it’s accounted for. To push the odds in her favor with regard to off-gassing, Jennifer plans her ascent a little differently.

Firstly, she ignores the advantage given her by her nitrox bottom mix. She will use no EAD and therefore her decompression stops would be 10 and 10. Secondly, she slows her ascent to the slowest allowed by the DCIEM table… that’s 12 metres per minute. Lastly, the oxygen time. Although she could abbreviate both 10 minute stops to about seven each, she cuts only the six metre stop and it by only two minutes. The last stop at three metres she leaves intact.

Her ascent time then is two and a bit minutes travel time from 35 metres to six metres, eight minutes waiting there breathing oxygen and then an additional 10 minutes at three metres. She also makes the last stage of her ascent to the surface very slowly… perhaps taking a full minute between finishing her three metre stop and breaking the surface for a total of 21 to 22 minutes. Optimal decompression. She has take full advantage of every tool available to her to help eliminate as much inspired inert gas as is practical. Her aim is to surface with minimal bubbling rather than as quickly as possible.

And so, we now have some base from which to work at planning our own forays into staged decompression diving. You must ask yourself if you are interested in accelerating decompression or optimizing it.

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