BlueDevil:
I haven't seen the Undercurrent article but it sure sounds interesting. The Scapa Flow square profile divers having the highest incidence of the bends leads me to speculate a bit. At around 25m NDL times are around 20 - 25 min (give or take a bit), and obviously are getting progressively shorter as you go deeper. For example, if we were to take a 30m dive the Buhlmann NDL is 17 min. Now not many people want to go to all the trouble of getting prepared for a dive to 30m and then spend any less time than 17 min on the bottom. So if you are doing a square profile dive to 30m (ie there is nothing to look at between the surface and the bottom) there will be a strong tendency to stay for the full NDL time and then surface. Therefore these square profile dives, in depths over about 25 min, will often be conducted to the full extent of the NDLs (by table or computer - makes no difference). ie you are pushing right to limits of the allowed times.
Now contrast this with a multilevel dive, say on a coral wall or bommie with a 30m bottom. typically divers will head straight down to their max depth and spend a few minutes there and than start ascending very slowly. As they gradually work their way up the computer is giving them credit for the fact they are spending time in shallower water. Often on coral reefs the life on the top of the reef in the 5- 10m zone is stunningly good (often a greater density of life than near the bottom). So divers may well spend a large proportion of their dive in shallower water. As they slowly ascend they are in effect doing deep safety stops along the way, and often the top of the reef is in about 5-6m (perfect safety stop depth) where they may easily spend 15-20 min enjoying the miriads of fish around them. From a deco point of view it would be hard to get a safer profile than that! And conceivably you may not have come even close to the NDLs on your computer for this multi-level dive.
To me this could be a reasonable explanation as to why square profiles carried the highest risk. The stat that does surprise me is the low rate of DCS amongst liveaboard divers. I was under the impression that this was a potentially high risk activity, since it is not uncommon for liveaboards to offer 4 or 5 dives per day, for several consecutive days without a break. Did the article give any rationale for this anomally?
No, the article let the data speak for itself. The article did advise further research explaining such differences would probably be helpful. The subscription and accessory on-line subscription to Undercurrent isn't a whole lot of dough, and I don't want to go too far on the article without treading on copyrights - for example, I'm not going to copy and paste it anywhere, nor keypunch in the article. The article does say this data came from the DAN Project Dive Exploration - maybe some DAN members can shed more light, such as the folks posting DAN statistics earlier in this thread?
My limited third-hand knowledge of UK area diving is seawater dives in that area of the world are often dives requiring planned deco obligation in cold, low vis, high current potential waters - not something I have any personal experience in, nor do I care to acquire such experience. If someone with such experience here can shed more light, I'd welcome such info as well. My imagination is these square profile dives are done with ascents and descents along a mooring line fixed to the wrecks, so if someone is doing an unreasonably fast ascent while using a line in current, they're in better physical shape than their time perception should allow. I don't think it's plausible to presume the higher DCI data per 10,000 dives in the Scapa Flow divers is primarily attributable to rapid ascents, but again I don't have first hand knowledge. The article did specify the Scapa Flow dives were square profiles - not multilevel profiles. This was the only group in the article specifically designated as square profiles.
Liveaboard divers were in general charachterized as "Gorilla Divers" doing multiple dives per 24 hour period for a 7 to 10 day consecutive period. If the Scapa Flow divers are doing that many dives in that time period in cold water, they're really tough hombres (and muchachas) - I doubt they do so. The liveaboard diver stats had about three times the number of dives per diver than the shore and day boat divers, suggesting a more intensive dive schedule for that group, but still nowhere near the intensity of the Cozumel dive guides in that same metric. It would be interesting if DAN presented the utilization of dive computers for liveaboard divers vs. shore and day boat divers. intuitively, I would expect dive computer utilization to be higher among the liveaboard divers than general shore and day boat divers, as a basic part of the typical liveaboard diver experience, but the article doesn't present such data one way or the other. It would be very useful data if presented.
Shore and day boat divers were charachterized as having more novice divers in this group than the other three groups presented. If one is assuming a greater tendency for rapid ascents for any group, the group with the greater population distribution of novice divers would be where I would assign that factor lacking other hard data personally.
Cozumel dive guides to me are folks that have to be in pretty decent physical fitness just due to the nature of the job and location, so I would personally rank the subjects in the other groups as perhaps less physically fit on average than the Cozumel guides, with the exception of the Scapa Flow divers - that bunch I think must be more physically fit than the average diver just due to the nature of the dive they are doing. And, I doubt these folks are doing excessive rapid ascents - I threw in the qualifier "excessive" as they may have more issues chasing down divers who don't have much drift dive experience or dive experience than typical dive guides at other locations, considering the location diver population. They may also have to do more 'bounce' diving when folks don't heed the lecture on downwellings during their dive briefings relative to other typical dive tourist locations. We certainly didn't observe any rapid ascents performed by the guides on the dives we did there in 2001.
I disagree with the assertion that the NDL limits profile of a wreck dive such as presented for Scapa Flow or a multilevel dive on a coral reef would be the same using tables or a computer for "the limits" as the NDL tables I'm familiar with give zero credit for any off-gassing during the ascent from the dive to the safety stop - unless you're referring only to tools such as the PADI Wheel. On the other hand, I don't know of a single dive computer that doesn't give any credit for off-gassing during the ascent from the dive to the safety stop. The credit for such off-gassing would move the "limit" further out, and the same profile would actually be further from the "limit" than the same profile by using tables, because the very definition of "limit" is different in the two contexts. Your presentation of the two different profiles does not account for no credit for the more gradual ascent profile inherent in square profile tables vs. algorithms used by dive computers. The depth averaging etc. as Snowbear has posted is again a different approach to "limits" than square profile tables.
Regarding ascent rates, if someone is doing slower ascents when not using a mooring line than they are performing when using a mooring line, that individual needs something to really help them out. Going hand over hand up a line vs. without can really let you pace yourself if you haven't tried this to see the difference. Some folks as they have posted in this thread are well skilled for either approach, but ascents without a reference line in physical contact are generally charachterized as more challenging to properly execute than those which make use of such a line. This may be a factor in liveaboard diver statistics vs. shore and day boat diver statistics, but it's not presented in the article.
There was no presentation of dive computer use for any of the statistics in the article. However, considering the data sample sizes presented, it would appear most such divers would be using dive computers based on other DAN statistics quoted earlier in this thread.
Maybe DAN or some other organization can perform more analysis. If the data base itself was made accessible for public read-only access, I'm sure the results and presentations available by mining such data would be prolific. However, it would also potentially allow for many presentations to be made by folks without scrupulous use of statistical analysis, and could potentially be worse in having a situation where bad information is worse than no information.
