Newsgroups: bit.listserv.scuba-l
Date:         Sun, 4 Oct 1992 13:26:00 EDT
From:         "Reef Fish (Large Nassau Grouper)" <RFLNG@CLEMSON.BITNET>
Subject:      Re: To compute or not to compute

In response to 21329KAD@MSU.BITNET (Kim.Dyer) on

>>      ...       what are the pros and cons of diving with a computer?

"(Craig Campbell)" <craig@CATTNTS.CANADA.NCR.COM> writes

> The pros:  Multilevel dives calculated automatically and at a frequency no
> :   diver could match.

I think that's the chief advantage, BY FAR, for MULTILEVEL diving.

A corollary to that is that the computer credits (adds remaining BT) your
BT as you ascend from deep to shallow; and does so in a near-continuous
manner.  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.  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.

> ::  There is no chance of making a mistake reading tables.

On the other hand, as several other discussants (in SCUBA-L and rec-scuba)
indicated, SOME computers make BAD mistakes when the battery is low;  and
I've seen a few divers who didn't know how to read their computers.  :-(

> ::  Decompression is automatically calculated, and you are told how
> :   close you are to decomp, so you can get some idea of how non
> :   optional a given safety stop should be.

Also, it updates your remaining BT at the current depth, even if you're
nowhere close to a decompression dive.

> ::  Records your dive, for later  transcription into your log book.

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.

> The cons:  If it fails mid dive, you must fall back to tables.  This means
> :   being thrust onto a square profile.  This will significantly
> :   increase your likelyhood of now being in a decompression dive.  :...:

Technically true perhaps;  but not true in practice for those who consult
their computers in sufficient frequency so that if they discover the
computer failure (due to leak or dead battery -- probably the most frequent
causes) while they are in the midst of a non-decompression dive, all they
have to do is to do a normal ascent and complete the dive in NDL mode.

More importantly, if you started your dive deep and gradually work your
way up in a MULTILEVEL dive (as nearly all my dives are),  after a certain
time interval, no fall back to tables is possible!

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.

>::  A computer failure necessitates doing all your table calculations
>:   under water.

Not true.  See above.

>::  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).  :-)

>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.

Me:  I dive with an Edge AND a Skinny Dipper (identical algorithm, one
backing up the other).  I not only love them, but CAN'T DIVE without them.
Dives in the (100-130 fsw)/(45-60 min.) range are fairly TYPICAL on my
*multilevel* dives.  Both have failed (leak or low battery) on multi-
day dive trips, but never at the same time.

One interesting thing I've noted was that when my SD died on low-battery
on the 4th-day of a 5-dives per day trip, and I re-started it on new
batteries, its loss of memory of the first 4 days hardly affected the
available BT profiles (compared to the EDGE which had been running
continuously).  The ONLY notable differences were the available BT at
the shallowest depths (30 - 50 ft) before each dive (corresponding to
the slowest compartments) and the "time to fly" of a couple of hours
less than the 19-hrs on the EDGE at the end of all dives for the trip.
Thus, battery failures are not as disastrous as it may seem (although
computer manuals suggest not diving for 24 hours before re-starting).
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.

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.

Unless you're a perennial square-profile diver, the answer to the question
in the "Subject" should be obvious in this day and age.

-- Bob.
Comments: Gated by NETNEWS@AUVM.AMERICAN.EDU