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SCUBA-L  May 2001, Week 1

SCUBA-L May 2001, Week 1

Subject:

Re: Computer/SPG Dilemma

From:

Marv Gozum <[log in to unmask]>

Reply-To:

Scuba diving discussion list <[log in to unmask]>

Date:

Mon, 7 May 2001 19:11:38 -0400

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (104 lines)

While dive computers are reliable, its chances of failure are higher than
the tested and reliable mechanical SPG. Often, devices fail at the wrong
time, and being battery powered opens reliability issues. A mechanical SPG
is less likely to fail or misread as its an exceedingly simple device, and
its mechanism is tested repeatedly during tank fills [pointing from 0-3000
psi, and at each fill the most important range of values are checked before
a dive.]

If a non-air integrated computer failed at depth, one could salvage a dive
with SPG, timer and dive tables. If the rare failure of a computer
included its SPG, it would be harder to determine a safe time interval at
depth, a bigger issue at greater depths.

I'd go for redundancy with a mechanical SPG anyday.

Battery failures often occur on the surface, and computers are designed to
shut-off with power to spare to prevent a dive computer failure at depth.
However, a catastrophe can happen if the 0 ring gives or the computer
floods, particularly with user-servicable batteries [dirt in the 0 ring
makes an imprefect seal, and a users only depth test of a battery change is
during a dive.] This can be avoided by changing batteries with care and
cleanliness... particularly if done in the field.

Many experienced divers I've dove with have 2 of everything, but seldom a
duplicate SPG: computers/depth gauge, timers, even compasses. On riskier
dives, we see pony bottles with separate regulators. Mechanical SPGs seem
to be the most reliable of all dive gear.

In my discussion with DAN about dive computers, Peter Bennett argues
computers are conjectural, since all computer are based on dive table
interpolations, which maybe considered risk tables for DCI [ the higher
your pressure group, the greater your DCI risk, regardless of whose table:
PADI, NAUI, USN, Buhlmann, DCIEM, etc., comparatively the time differences
between all table pressure groups is ~10%] Cochran, Suunto, Uwatec, Abyss,
Aeris etc., and other advanced computers use additional conjectures ...
corrections for recurrent dives, actual ascent rates, RGBM, etc., but per
DAN, while there is data to support such issues to reduce DCI risk, _how_
these hypotheses are manifested as dive algorithms or protocols are
conjectural, and made public only in the execution of their computer [the
formula is not public ... consider Abyss:
http://website.lineone.net/~britannic98/prep/rgbm.htm, yet how is this
discussion actually implemented in their computer?] None take into account
physiological factors of the diver, such as increase risk with age, poor
fitness, obesity, etc., which are more likely to influence DCI risk for a
diver than fine differences among tables or computers.

There has yet to be a definitive study comparing the incidence of DCI
versus different computers.... but anecdotes on safety versus freedom:
http://www.scubadiving.com/gear/computers.shtml

This year, the USN has allowed its divers dive computers
http://www.undercurrent.org/UCnow/UC200105_issue.shtml the delay has been
because of the algorithm ... the USN wants to use their algorithm based on
Navy {NEDU} studies, and it was implemented by Cochran after a competitive
bid. This special computer is not available to the public [yet?]

New developments in preventing DCI will be heared at a medical conference
this summer, which may obviate the confusion in table or computer
algorithms, the story centers around slower ascent rates [ 10 ft/min]
and/or nitrox during a safety stop, regardless of algorithm.

In closing, experienced divers can "feel" the diminishing pressure on the
high pressure hose [it becomes softer and limp as pressure drops], and such
a diver with that much experience, pretty much anything is open to you,
such as an air integrated computer.

The feel of a high pressure hose, as a failsafe backup of an SPG, is an
invaluable skill!

I think you're safest if you understand just what your computer is doing,
use the computer's readout as a guide, rather than relying on it completely
to keep your dive within no-stop limits. It is preferrably user servicable
with easily acquired parts, thus 'low' maintenance. It should be cheap and
simple to use, with a clearly defined algorithm. I use a Genesis Nitrox
Resource, $220, it uses PADI tables [ known also as DSAT Tables} and worse
case, USN air decompression tables. Its fully user maintainable, and
batteries cost $1 in lots of 25 [www.digikey.com] Should it fail, I can
mentally pick up where it left off. I studied Uwatecs/Aladins last year,
they use more flexible Buhlmann tables, which are included with the
computer. Small differences in NSL among many computers are rarely an
issue as few dives are a completely square profile and, given enough air,
you can off gas at a lesser depth per your computer, before descending for
a few more minutes.







On Mon, 7 May 2001 14:30:22 EDT, [log in to unmask] wrote:
>have a degree of concern about relying on a single integrated digital SPG.
>Over the years I have occasionally experienced computer malfunctions, but
>have never had a problem with an analog SPG.
>
>I can purchase a single backup analog SPG w/console for $89 which offers
>redundancy if added to either of the two fully integrated computers, or
>save significant dollars by just purchasing the Aeris 300G and relying on
>it's single analog SPG.
>
>I'd appreciate hearing from the NG on opinions regarding reliance on a
>digital SPG without a backup as well as your thoughts on the three quoted
>computers in general.

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