Mailer: Elm [revision: 70.30]
Newsgroups: bit.listserv.scuba-l
Date:         Mon, 12 Oct 1992 11:54:24 MDT
From:         "(Steven G. Lanaghen)" <sgl@SPATIAL.COM>
Subject:      Re: To compute or not to compute

> >  Every instructional book on tables that I have seen states that the
> >  bottom time of a dive begins when you leave the surface heading
> >  for the bottom and ends when leave the bottom and head for the
> >  surface. Safety stops are considered part of the ascent and do not
> >  need to be counted as part of the bottom time.
>
> Interestingly enough, at least one interpretation of the 'direct
> ascent to the surface rule' indicates that the direct ascent to the
> surface starts at the *end* of the safeyty stop.

        Do you have a reference on this interpretation?  I have always read,
        been taught and learned that the definition of bottom time was from
        beginning of descent to beginning of direct ascent to the surface.
        A safety stop is considered a precautionary decompression stop and
        should be considered in calculations as such. Safety stops are
        considered part of ascent time, not bottom time.  What does this
        interpretation you reference consider a decompression stop to be?  Is
        that bottom time as well?

> Leaving the bottom
> is subject to strict interpretation, for the US Navy tables, and

        "Leaving the bottom" is pretty black and white, but is not the
        key factor - it is the beginning of the direct ascent to the
        surface.  As long as you are ascending (or holding a calculated
        safety/decompression stop) you are off-gassing and this should not
        be considered bottom time (on-gassing).  If you ascend at a
        ridiculously slow rate or hold for much longer than the calculated
        decompression time, you may stop off-gassing, but even then you
        would not be on-gassing either (you would be at equilibrium).

> ascent rates that deviate from 60 FPM have to be adjusted for.

        Too-rapid ascent rates off-gas too rapidly - potentially to the
        extreme of embolism.  Slow ascent rates off-gas slower but still
        do not add to bottom time (on-gassing).

> Personally, I prefer a slower ascent rate, and an interesting theory
> was presented on relative fast compartment overpressures that seemed
> to me to be a good argument for a slow ascent rate (and safety stop)
> when discussing ascent rates.

        I agree 100%.  I instruct and perform as a Public Safety Diver for
        a dive rescue team.  I teach an ascent rate of 30-40 FPM.  With
        the additional adrenalin and stress of performing rescues, it is
        that much more important to protect the diver from embolism.  I
        also happen to err on the side of safety in this area in my own
        recreational diving as well.  I may not maximize my bottom time,
        but I'm confident I will be back down...

        I do not use the wheel.  I've used lots of tables, and have settled
        on the Navy tables under Doppler limits.  Because of our response
        area, my team happens to use the 1986 Beuhlman tables (for altitude).
        We work under a physician advisor and work in conjunction with a
        local hyperbaric doctor and Dive Rescue International.  Computers are
        great, and many are in fact based on the Beuhlman calculations (if
        your computer is altitude adjustable).  I'm all for a computer to
        eliminate my table lookup mistakes - but I believe that dive planning
        is critical, and divers can become too complacent relying soley on
        computers.  Computer feedback and backup is great.