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What you need is really double the gas(es) required, at the worst possible time. So for a simple no deco air or nitrox dive in an overhead environment (such as a cave), the worst point is the furthest point of the dive. Now since it will take you presumably the same time and gas to swim out as to swim in, this reduces down mathematically to one third in, one third out, one third reserve. On say an air or nitrox dives with around 20 minutes deco, plus minor wreck penetration, you need to plan and monitor your gas so that at any time, you could lose 50% of your gas, and still safely exit the wreck, fail to find the anchor line, blow a bag, ascend, perform my now drastically extended deco (due to extended bottom tim), surface, and either surface swim (in very calm situations), or get a line tow. On my ID AL80s, with my consumption rates and speeds at which I perform these actions, basically I have to start leaving the inside of the wreck at 1500psi. I am routinely on the boat at 1100psi. No credit for partial fills - all just part of the cost of diving. (Oh, and now that they have an idea of what to charge, Kings County is much cheaper.) Yeah, I did some spreadsheets a few years back. Analyzed the heck out of everything. I have not retained them. So again, the general rule (of which the rule of thirds is a special case): you need a fully redundant rig, and double the amount gas(es) to terminate the dive safely, at the worst possible point (the furthest penetration/the longest planned deco/the deepest depth/...), and assuming that other things have also started to go wrong. If you dive in buddy dependent mode, then this reads: you need the amount gas(es) YOU need PLUS the amount your buddy needs, OR double the amount you need, if you breathe more heavily, to terminate the dive safely, at the worst possible point (the furthest penetration/the longest planned deco/the deepest depth/...), and assuming that other things have also started to go wrong. There are plenty of special cases, but the general idea is that any given point, you need two ways out. Ronald D Thompson wrote: > > Has anyone resolved the fact that the Rule of Thirds is often (or seems to > be) inadequate for decompression diving? I specifically refer to the > observation that in decompression diving, "going" from the starting point > to the turn-around point of a dive often (usually?) requires far less time > and gas than "returning" from the turn-around point to the starting point > of the dive (because of the decompression obligation). This means, for > example, that if one of two identically breathing and kitted dive buddies > has a total gas failure at the turn-around point, then not both divers can > return safely to the starting point of the dive if they are diving the > Rule-of-Thirds! > > Does anyone have a standard "fix" or "modification" for the Rule of Thirds > (in the context of decompression diving) that he/she uses? > > Thanks in advance for your responses, and apologies if this has been > discussed earlier (I couldn't find a reference) in the Archives). > > Ronald > > P.S. The only obvious way around this seeming inadequacy with the Rule of > Thirds that I can see is for me to complete a formal (and time-consuming) > gas-matching optimization computation using a set of obvious constraints. > I do this as a matter of course. However, I was wondering if someone had > an approach that was more ad hoc (and much quicker) and less brute force > than mine. > > P.P.S. This problem plagues me whenever I begin planning for the > loss-of-deco-gas scenario while planning an accelerated deco air wreck > dive. > > > -- > Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. > Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'. -- Wrolf Wrolf's Wreck: http://wrolf.net -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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