Newb Equipment List - I have seen the light (horror?)

[GillScales]

I'd go analog for the backup gauges,keep that redundancy as failproof as possible,and study the dive tables more. The rest of the gear choices look good to me,nice price too esp for Egypt iirc

 

[Colliam7]

Split open !!! One of the tourists on the intro dive trip was in the water when the second incident happened, and would not dive again. The first incident was with instructor gear.

Wow, that would have certainly gotten my attention, too! Fortunately, it is not only impressive, but quite uncommon - both in frequency and in presentation (hoses will usually start to bubble / leak as an initial indicator of deterioration, and a catastrophic failure is not something you see very often. But, if I was a new diver, it would have made an impression on me as well.

However, being an engineer, it gives one pause that maintenance practices, replacement of parts, QA/QC of hoses, etc may not be quite right. I understand a resort DS having to balance revenue and costs, but two incidences in one day is a good indication something may be wrong.

Not necessarily, unless it happened repeatedly - every day, or every other day, at least once every week, etc. What you may have seen was simply a random concordance of unrelated, but similar, events. (Or, it was possibly a really crappy dive operation, with 'crappy' being a technical term.)

1) I assume the different algorithms were developed from different sets of data that included physical/environmental i.e. depth, density hence pressure, temperature, exposure time, ascent rate, etc of a different population of divers and medical data before, during and after testing. Is this correct.

YES.

2)However, as you guys have pointed out there are differences in the algorithms. Would you call those differences substantial?

For recreational diving, NO. For decompression diving, the answer depends on what you consider to be 'substantial'. For example, last week, I did several modestly shallow (170 ft) decompression dives on the same wreck, over 2 days. I wore two computers (Suunto HelO2, and Liquivision X-1), on the same arm. The Suunto consistently showed a longer decompression obligation, compared to the X-1. The difference was approximately 10 (extra) minutes, across an overall run time of 73 minutes. Is that substantial? Possibly, possibly not. The first time, I followed the HelO2, and stayed at 20 feet for the additional 10 minutes, even though my X-1 had cleared. The second time, I surfaced when the X-1 cleared, and put up with the annoying beeping of my HelO2 until I could pull the back cover off, take out the battery to reset it, and put the battery back in. I had no symptoms of DCI, although I am sure I microbubbled both times - that a physiologic certainty, irrespective of what algorithm you use. Which algortihm was 'right'? Heck if I know.

3) If my logic so far is correct (and please correct me if I am wrong) then the difference in the algorithms implies different control groups of divers behaved differently physically and medically to the same test conditions.

YES, . . . but. There are probably indentifiable subsets of divers (and there is quite a bit of interesting work going on at DAN right now to try to characterize, however coarsely, some of those subsets). The bigger / more important / more signficant point to keep in mind is that there is simply considerable inter-individual variability.

4) If point 3 is indeed the case, how do you know which algorithm is best suited for you? If you use one algorithm, are you rolling the dice? Extreme and unlikely because certainly the algorithms are conservative. If you have two algorithms, your chances of finding the right algorithm for you and your body are greater. It may be one or the other or something in between, but you have some direction.

You really don't know, except by empirical observation; you are not rolling the dice, and it never hurts to be conservative (although that is not a guarantee of safety, either).

Dive computer algortihms are mathematical models, developed on the basis of actual data, to attempt to explain the observed data. (I suspect that you, as an engineer, can easily relate to this). Much of the work is scientifically quite impressive, in fact. However, even though attempts may be made in developing the models to account for physiology, the models are nonethless only mathematical, not physiological, and therefore imprecise - the error functions are quite sizeable. The degree of precision is generally not sufficient to allow for individuals to determine which algorithim is best for them, even if you attempt to match your individual physiological characteristics to the experimental cohort, although expressed beliefs that one or another algorithm is more 'correct' often appear to have the same intensity as religious beliefs. (I find it 'amusing' to read some of the often heated discussions about different dive computer algorithms, and which one is 'right', 'more accurate', etc. The time would be better spent servicing gear, or speculating on where the universe goes.)​

---------- Post Merged at 07:02 AM ---------- Previous Post was at 06:43 AM ----------

If you have two different algorithms on your computer, how will you know which one is wrong, if one is not working properly?

Actually neither computer is wrong or more likely both computers are wrong.

This reminds me of a discussion that Lynne has probably heard before. If you have two sets of lab values for a patient, drawn within a similar time frame, and the values for one (or more) parameters are discordant, which is right? What action should you take? Trick question - the common practice is to draw a third set, which either confirms the results of one of the previous draws, or provides a third set of discordant numbers, leaving you more confused than before. Of course, in that case, all three sets are ignored, or the one that fits what you expected to begin with is selected as the 'correct' one.

So, if you have two dive computers giving you different information, pick the one that is a) more conservative, b) more convenient (e.g. stay at the bottom longer, or surface earlier), or c) consistent with whatever you wanted to believe, to begin with.

 

[Agility]

As an engineer I am sure you will enjoy reading Deco for Divers by Mark Powell which will probably answer most of your algorithm related questions :cool2:

 

[wrusty33]

Thanks so much for the detailed response Colliam7. Interesting discussion and learning experience. May the force be with you for all of your dives. Yep, saw your icon, but from your writing you don't need the force.

Russell

---------- Post Merged at 07:49 AM ---------- Previous Post was at 07:48 AM ----------

Ordered the book...... Thanks !!!

 

[TSandM]

The book will clarify a lot of things.

Surprisingly little is known about the actual kinetics of inert gas within the body, simply because it's a hard thing to measure. In recent years, we've had Doppler technology to detect bubbles . . . but even that is confusing, because some people with lots of bubbles have no symptoms, and some people with fewer bubbles have symptoms.

The models that are used to generate decompression tables require assumptions -- some of them are as simple as whether the loading of various parts of the body with inert gas occurs serially or in parallel, and some are as complex as trying to figure the critical radius of a bubble, below which it collapses from surface tension, and above which it inevitably expands. The no-stop times at various depths that are generated by different models can vary quite significantly -- I've posted before about a dive where five divers, using three different algorithms, ended up with no deco, or 20 minutes of deco, from a dive that was identical except for about a 60 second 10 foot descent on the part of two divers -- one of whom ended up with the 20 minutes, and the other of whom was home free.

Models also vary a great deal in how much experimental validation is done of them before they are published; I think you'd be surprised to find how little has been done with some.

An article I read by Dr. David Sawatzky several years ago pointed out, correctly, that the biggest reduction in DCS incidence occurred with Haldane's original insights, and everything we have done since has been fiddling around at the edges. Mr. Powell's book will help make that clear, and I also recommend, for anyone who has a lot of curiosity about this, the GUE DVD, "The Mysterious Malady", which is a series of interviews with the folks who are on the cutting edge (as much as there is one) in decompression research.