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Date:         Wed, 7 Oct 1992 10:06:42 PDT
From:         "(David Duis Story)" <story@BENT.WPD.SGI.COM>
Subject:      Re: To compute or not to compute
              92 1:26 pm

Bob Ling writes:
>
> [...] Note that Tables not only credit you NOTHING when you do your
> safety stop (when you're outgassing) you treat such time as if you were
> still diving at your maximum depth for that dive -- a gross distortion
> of BT accounting.

Actually, most table designers offer the diver the option of either
counting the safety stop as bottom time, or simply ignoring it.

> Also, Tables do not account the first 10 minutes of
> your surface interval in repetitive dive-group computations, and treat
> greatly different surface interval times in the same "groups" in huge
> discrete chunks of time.

The first half of this is not true: tables certainly count all of your
surface interval as surface interval.

I agree with the second half: tables can't afford to be as
fine-grained as computers, else they'd be HUGE.

> "(Craig Campbell)" <craig@CATTNTS.CANADA.NCR.COM> writes
> > ::  There is no chance of making a mistake reading tables.

While they may be easy to read, the diving public is provably unable
to use them for simple repetitive diving.  I recently posted the
abstract of an interesting study to rec.scuba, in which the authors
conclude that well OVER HALF of the certified divers in the USA
CANNOT use dive tables to correctly compute repetitive dive profiles.

> The number of dives logged and the details in each log depend on the
> computer.  That is one criterion some potential-purchasers may consider
> as important.

Good point.  And why don't any computers record surface intervals?  If
they did, I wouldn't need to log anything until the end of the day.

> Here're a couple of my recent computer-recorded max. depth (fsw) and BT
> (minutes) in NO-decompression dives:  (131/49 for 1st of three dives on
> the same day) and (129/49 for the 1st of 5 dives on the same day).  I
> should add that on those dives, at no time (except for the first few
> minutes at or near max depth) was I even close (within 20 minutes say)
> to a decomp dive, so that IF my computer should fail after I had been
> diving for 35-40 minutes say (now probably at less than 50 fsw), all I
> would have had to do was to surface.  I don't see HOW one could/should
> fall back on 140/40 Table at that time.

I posted what I thought was an impossible set of profiles from a
previous trip to Saba, and Nick Simicich managed to show me how I
could fit such a dive onto the PADI Wheel.  I was suprised:

Nick Simicich writes: (Date: 16 Feb 91 02:25:12 GMT)
||Dave Duis writes:
||E.g., when I visited Saba last August, I dove several profiles such as
||130'/90 minutes.  That profile just won't fit on a table, not even on
||a Wheel.  Combine this kind of profile with repetitive diving, 4 dives
||a day for 5 days....  Now what do I do when my batteries finally die
||and I find out all my spares were stolen out of my luggage?
|
|130'/10, 80'/10, 35'/70 fits easily on the wheel, as does 130/10,
|80/10, 50/20, 35'/55.  Leaves you a Z.

But it does leave you a Z, and the kind of repetitive diving Bob and I
are talking about doing just doesn't work, even on a PADI Wheel.

> >::  If you rely on your computer for a while, you may forget how to
> >:   do table calculations.  Then, if it fails and you are forced to
> >:   fall back to tables.....
>
> Like riding a bike, once learned, I find it IMPOSSIBLE to forget how to
> do table calc.  (as clumsy and non-sensical such calculations are for
> multilevel dives).  :-)

See above study about >>50% of all certified divers not being as
capable as you.  I should note that ~30% of the INSTRUCTORS who
voluntarily took the exam also failed!

> >Me:  I dive with a computer and love it.  I also dread the day it fails
> >     and I have to fall back to tables.
>
> Don't dread it.  Buy and use a SECOND one.

Amen!

> Thus, battery failures are not as disastrous as it may seem (although
> computer manuals suggest not diving for 24 hours before re-starting).

Unless you're diving profiles which stress the slow compartments
(which can happen deeper than 30fsw, BTW.)  If you reset your slow
compartments by restarting your computer and then dive along the edge
of the slow compartments, you may be at much higher risk of DCS.

> That also suggests that a computer with a battery that YOU can change
> (as opposed to one that can only be factory-installed) has some
> practical advantages.

Agreed.  And a computer which retains its compartment loading memory
across battery changes, like the Delphi/Phoenix, is even better.

> In summary, for the kind of diving I do -- frequent, multilevel, deep-long
> (for max. depth/total BT) but no-decomp, a computer is absolutely
> indispensible.  I use (and recommend using) TWO, for more or less the
> same reason you carry a redundant air source, I suppose, as a backup.
> Having two computers eliminates most of the CONS (errors, failure, etc.)
> that a one-computer diver may encounter.

I agree wholeheartedly.

Cheers,

David Story                        NAUI AI Z9588, PADI DM 43922, EMT
story@bent.wpd.sgi.com		  Every dive is a decompression dive.
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