Hi all,
I recently attended a seminar on DCS facilitated by the hyperbaric medicine department of my local general hospital. The director of that program espoused the value of doing your 5 min safety stop @ 15' with up to 70% O2 as an additional means of purging nitrogen and reducing DCS likelihood.
Does anyone do this? If so, what sort of set-up do you run?
I am not trained in the black arts of deco theory and science. But the little research I have done suggests that ad hoc remedies like this may be more dangerous than helpful. There are a lot of factors behind the choice of a deco gas and deco plan, and it is difficult to evaluate a single piece of advice out of context.
As others have noted, 70% EAN has a certain Maximum Operating Depth. Imagine a scenario where you are suffering from Narcosis and suffer an OOG emergency. You might breathe from the deco bottle and that might be bad. Thus, many agencies that train in the use of deco gasses also train in the use of redundant bottom gas supplies such as double tanks.
Also, some investigation into decompression models might be interesting before adopting this suggestion. At one time 80% was a popular deco gas. Why? This was driven by the most popular mathematical model of decompression at the time. When adjustments were made to the model, many divers switched to using 100% at a shallower depth. Likewise, many divers use 50% at a deeper depth. Why? Would that make sense for recreational dives or is it strictly for technical dives?
A final question I would ask is why assume that the deco depth is fixed at a safety stop. Without spending a penny on additional equipment, training, or fills you could adopt a more conservative deco strategy. If you look around, you can discover that some divers adopt various forms of deep stops, be they a minute or so at a deep depth, pyle stops, or a ladder of one minute stops from half of maximum depth.
Do any of these lessen DCS? If so, do they do so more or less than breathing 70% at 15'? I couldn't say, but I would suggest that looking into these things is a good idea before adopting a deco gas. I guess this is a long winded way of saying that we should be suspicious of taking the dive we are already doing and adding a deco bottle. Instead, we should put all aspects of our dive on the table and see what adjustment gives us the greatest safety.
(Speaking anecdotally, I have had the greatest perceived reduction in sub-clinical DCS symptoms after deep recreational dives from two things. First, being really, really disciplined about ascending extremely slowly from my last stop to the surface. Second, adopting one minute ascents. But remember, "The plural of anecdote is not data.")
