On Wed, 1 Mar 2000 14:16:36 -0800 (PST), Michael J. Black wrote:

>Sean T. Stevenson wrote:
>
>> Perhaps the most important issue which seems to have been overlooked
>> is that electronics can and will fail at any time, and if you are
>> relying upon a computer for your decompression profile you are up the
>> proverbial creek without a paddle.
>
>Unless you have a backup computer, of course (besides your head).

The redundancy of a backup computer seems to be a sub-optimal solution
to a self-imposed problem, especially when you consider the cost of
these devices.  Also, did you read JJ's Baker's dozen?  He raises some
very valid points.


>> Since you really need to carry the tables (or understand deco well
>> enough to calculate on the fly) anyway, the computer becomes an
>> unnecessary redundancy.
>
>I don't know too many divers whose brains work more quickly and
>accurately than computers, but if you really can do all your deco
>calculations in your head then your logic stands.  Seems like the
>credibility meter is getting a little jumpy again.  MJB

Computers, while being extremely fast at calculating anything
pertaining to their programmed algorithm, are damned poor at adaptation
to adverse circumstances.  I certainly don't claim to be able to
calculate tables as precisely (note: I didn't say accurately) as a
computer, but in the event of an electronics failure (depth and bottom
time), I can make a pretty good estimation of a bailout table that will
get me out of the water, and can adapt this based on the loss of one or
more decompression gases, or a necessitated abbreviation due to
emergency circumstances.  The problem is that computers are only as
accurate as their programmed algorithm, and there are just too many
variables (many of which are unquantifiable) to create an appropriate
algorithm.

The base algorithms that are programmed into the computers, or table
generation programs, such as the compartment based Haldanean models
(Buhlman's ZLH 12 and 16, et al) or Bruce Weinke's RGBM model, are just
statistical predictions, of a generally logarithmic relationship.  The
truth is that we really have no way to model exactly what is happening
in the human body with respect to decompression.  Decompression
computers typically apply conservatism factors to these models, with no
regard to the variables that necessitate the conservatism.  Also, these
models themselves make predictions which seem to be in contradiction to
real world evidence.  By diving with tables (typically created using
these same models, to start), I can incorporate deep stops, modify the
planned decompression stop depths, alter the conservatism based on how
cold I am, if I am well hydrated, how I feel generally, etc., interrupt
the decompression to deal with an emergency and either resume it or
commence IWR.  Remember "Plan the dive, dive the plan." ?  Is, was and
will always be excellent advice, and if you have a complete dive
profile planned (and the tables or knowledge to make modifications if
necessary), then the computer becomes a source of extranneous
information, rather than a necessary piece of life support equipment.

-Sean


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