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> This is rather more than a little bit hard to swallow: are you trying to imply
> that as a software vendor you have no responsibility for any results of using
> your product??!! So I suppose then that you advertise your product as simply
> being an electronic Buhlman table, and not a product which generates
> decompression schedules? NOT!
Our software is a calculator, a tool. The user generates decompression
scheduals, not the software. It's made pretty clear in the MiG Plan user's
guide.
> o Buhlman has never claimed that his research was to be used the way tech
> divers currently use it: as a straight algorithm to generate deco schedules
> for real-world diving. Somehow, I get the feeling that you do not properly
> relate this to your clients...
Sure he has. He presents his algorithm, explains how it can be used to
make tables, and gives you a set of air tables to both save you the hassle
of making them yourself, and to give you a standard to check your math
with. I've never heard him quoted as saying "look, making tables like
this is unsafe."
> o The straight Buhlman algorithm lacks conservativeness, and has been
> responsible for many cases of DCI. If you somehow add conservatism to the
> algorithm, you are then no longer producing even a version of the Buhlman
> algorithm, but rather a one-off deco algorithm which is completely untested
> and unproven.
As far as I know, all the popular models (with the possible exception of
the DCIEM model, anyone who knows otherwise please jump in here) have
been responsible for many cases of DCI. There is a statistical risk of
DCI on any resonable exposure dive, and even very "conservative" tables
would not eliminate it.
I don't think that adding small amounts of extra conservatism helps you
very much. I don't hear people running around saying "boy, with the new
shorter no-dec limits on the RDP we have only 10% the DCI we had on Navy
Tables." Once you get to low risk, you get dimishing returns with
increased conservatism. It may be that Buhlmann tables are not low risk
when they are used for dives with hours of decompression, and in that
case adding some conservatism would be prudent (says so in the MiG
user's manual, in-fact.)
> o The entire notion of "adding conservatism" is a rather interesting area to
> explore: how are you sure that the modifications you made to the Buhlman
> algorithm actually do increase conservatism (and therefore safety)? Did you
> test them? Is more decompression time always more conservative? Can you
> prove this? Oh wait, I forgot: you are not responsible for the outcome of
> using your software!
Adding or lengthening decompression stops without a mathmatical basis can
produce unpredictable results and is therefore a BAD IDEA. Pulling back
the maximum super-saturation gradients (M-Values or factor a's) is a
mathmatically sound approach. Does it increase reliability? Maybe. It
is the best we can do without a totally new model.
> o How do you know that the output of your program is actually faithfull to the
> Buhlman algorithm? Can you prove your implementation of the Buhlman
> mathematics rigorously? Do you test your software with the naieve approach of
> running some number of known dives, or do you actually do something more
> sophisticated? Remember, people are trusting their life to your software --
> but of course, you deserve neither credit nor blame for the reliability of the
> output, right!! (NOT!)
We did extensive testing against Buhlmann's published tables, ensuring
that our numbers agreed EXACTLY. There is little to prove in the basic
math we used; we use exactly the same equations as Buhlmann uses for the
most part. We are not using any fancy iterative equasion solving methods
that could yield unexpected results under some conditions, so the risk of
the program giving correct answers for one set of numbers but not another
is near zero.
> o How would you feel about this situation: Boeing produces a new, computer
> controlled plane, but they use the fly-by-wire algorithms developed by Airbus.
> Rather than subject the plane to rigorous testing, they simply claim that
> their implementation is faithful to the original algorithm development done by
> Airbus, so Boeing bears no responsibility. Would you fly in that plane?
> Would you insure it?
Fly-by-wire is a control system. Control systems are based on feedback
loops, in the case of fly-by-wire many many feedback loops. This type of
system is prone to develping osillations and sudden violent control
deflections until the interacations between each of the systems is fully
optimized. This type of system must be given rigerous testing because
its actions under all conditions cannot be fully predicted in the lab.
Despite such testing, there have been several accidents caused by
fly-by-wire systems in Airbus's, largely attributable to that most
critical interaction between the control systems of the airplane and the
pilot.
A program such as decompression table generator does not use feedback
loops, and there are no possible "strange" interactions due to the
influence of other variables such as wind gusts in fly-by-wire systems.
> o Adaptations of the Buhlman algorithm for Trimix diving are hacks which have
> never been supported by Buhlman.
There are accepted methods for using models to make mixed gas tables. I
have never heard them disputed. If you do not accept the view that these
methods are safe, then you should limit yourself to air diving because all
other tables, from NOAA Nitrox to US Navy Heliox tables are based on them.
> o The Buhlman *tables* are just that: tables. The *tables* have built-in
> rounding which makes them inherently different than the Buhlman *algorithm*.
> Unless your software is simply doing table lookup, it does *not* generate the
> Buhlman *tables*.
Not true at all. The Buhlmann tables match the algorithm exactly. There
are no "fudges" to the tables, I assure you. There are a couple things
in Buhlmann's book that many people overlook when designing software to
generated his tables, resulting in small descripancies that many people
pass off as "rounding errors".
Jody Svendsen
MiG Technologies
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