Newsgroups: bit.listserv.scuba-l
Date:         Wed, 7 Oct 1992 15:45:00 EDT
From:         "Reef Fish (Large Nassau Grouper)" <RFLNG@CLEMSON.BITNET>
Subject:      Re: To compute or not to compute

For those averse to nitpick/response, please ignore this post.  :-)

Dave "(David Duis Story)" <story@BENT.WPD.SGI.COM>
commented on my comments of Oct 4, 92 1:26 pm.  He seemed to agree with my
main points but disagreed with some details (nits ?), e.g., I said,

>> :...: 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.

I was referring to PADI Tables, and probably should have said so, but the
same is true for ALL Tables, if you take into consideration the accepted
definition of a DIVE (more about that below).

Dave said,

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

That may be the sensible thing to DO, and indeed a NAUI Manual says
something to that effect, " ... a three minute precationary <sic> stop
at a depth of 15 feet be performed at the end of all repetitive dives.
The stop tim <sic> does not count as either bottom time or surface
interval time."

Actually the last sentence is a self-contradictory fudge-factor.  If you
have done a long/deep dive, a 20-minute safety stop (or a 3-minute one)
can't just disappear into thin air (by ignoring it).  It's BOTH bottom
time (at 15') AND surface interval (for outgassing accounting purposes).

But not so for PADI Dive Tables, if you follow their prescriptions.  In
fact, if you SURFACED for 9 minutes and re-entered water to dive, you're
still in the FIRST dive.  That's what prompted my comment

>> Also, Tables do not account the first 10 minutes of
>> your surface interval in repetitive dive-group computations, ...

of which Dave said,

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

Wrong, Dave.

Just look at any PADI or NAUI Surface Interval Time/Credit Table.  The
minimum time starts at 0:10 because of the DEFINITION of a DIVE.

In particular, PADI Diver Manuals have this to say:

"If a surface interval is less than 10 minutes, treat the dives as a single
dive, adding the Bottom Times together and using the deepest depth attained
on either dive to determine the Group Designation."

Can't be any more explicit than that.  Anything less than 10 minutes don't
count.
In other words, the first 10 minutes of surface interval counts ONLY when
you've surfaced AT LEAST 10 minutes.  Now if you want to see a silly
example, here's one:

1.  Descend to 130' and immediately ascend to surface.  (Let's say this
    took 5 minutes.)

2.  Surface for 9 minutes and then dive at 15' for 10 minutes.

3.  Repeat Step 2 10 times (I believe there's plenty of air in a standard
    tank to do it).  :-)

Now what have you (PADI Table User) got?  A 130' dive for 105 minutes!

What have you REALLY done?  Essentially a 1 minute dive at 130' with a
100+ minute safety stop at 15' and 90 minutes of surface interval.  :-)

The real point of this exercise (and other equally silly ones) is that
by definition/convention or discreteness of Tables, or both, one can
easily come up with outrageous examples that Table Users have lost
all senses of reality, whereas computers (their users) at least try to
APPROXIMATE reality (via models and near-continuous updates of the
relevant quantities).

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