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Jody wrote;
>I personally think you are meddling much more with the model by adding
>bottom time or depth than by changing the a's.
>
>The safety of the model is based on two things; how accurately the
>compartments reflect what is going on in the body, and how close to the
>"safe" supersaturation limit you are willing to go in each compartment.
>Nearly everyone agrees that it is not prudent to modify the compartments
>without testing. Therefore, nearly all the methods that try to make the
>tables more conservative try to back off the supersaturation limit to
>increase safety. This is a good method because it both directly
>increases safety in a statistical sense and helps compensate for small
>errors in the design of the model itself.
>
>Since this is the goal, the best way to achieve it is through a direct
>method; pulling back the factor a's. This perfectly acheives the goal.
>If you try to add bottom time or depth or %gas to the equation before
>putting it into the model, the results are inconsistant. You may get an
>increase in conservatism in the slow compartments but not the fast ones,
>for example. This approach can also be hazerdous on multi-level dives.
>
>Say, for example, you are adding bottom time. If you add bottom time to
>the shallow end of your dive, the model might think you outgassed from
>some compartments more than you actually did, resulting in a violation
>of the model.
>
> Jody Svendsen
> MiG Technologies
>
I couldn't agree more. I think backing off the maximum compartment pressure
has to be the way to go. Jody is right when he says automatic bottom time
multipliers can lead to problems and it is one of the things I always try
to keep in mind when I am using DRX. (which adds BT)
My current pet theory is that the fast compartment pressures are too
radical, and have been set that way to provide NDL's which are consistent
with observed limits. The problem is that micro bubbling is normal after
a dive that reaches the NDL. That is fine if that is the end of diving
for the day, but if this micro bubbling is allowed to occur right at the
beginning of a two or three hour deco, (where the fast tissues are
controlling), well, it slows down the offgassing. The inert gas goes into
the micro bubbles rather than out the lungs.
Having said that, I should point out that I have *no* evidence, it is just
a theory which fits the facts I have seen.
I'd also like to say that I didn't want to start a flame war against Jody!
As I said before, I've seen other table producers say similar things! If we
are going to have a flame war lets have it against Prof Buhlmann. We all
owe him a great debt, and I for one don't wish to go back to using the USN
tables, but he wasn't perfect. Perhaps a quote from a translated Hans Hass
book might put the Keller (diver/mathematician) and Bulhmann (theoretical
physiologist) team into better perspective:
[after a one hour bottom time dive to 330 ft]
Heberlein recalls: "The dive went off well and when I came on
board I was in the galley and Peter Small [one of the test divers]
was sitting opposite me. Suddenly he began to rub his elbow
like this with his left hand. As I had once had an accident
myself, I immediately asked him, 'Peter, have you got the
bends?' He said he wasn't quite sure. Buhlmann was called and
examined him, and Peter was put in the decompression chamber."
Keller afterwards wrote in a report: "Under normal circumstances
we would not have treated him because such a slight pain is not
at all dangerous and occurs quite frequently after a longish dive.
With a view to the 1000-foot attempt, however, we wanted to treat
him in the pressure chamber so as to be sure he was perfectly
all right. But it's important to make clear that this treatment
has absolutely nothing to do with the fatal accident."
Two days after the bends, Peter Small made a dive to 1000 feet,
supervised by Buhlmann. During the decompression from that dive
Peter Small died, from something, but the details are so muddy that
I have no idea what killed him.
Ok, this means we are diving an alogrithim written by someone who considers
a "slight pain" normal "after a longish dive" and no impedement to doing
a 1000 foot dip in a couple of days.
Does anyone else find this a slightly scary thing?
Cheers Jason O-)
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