From: sdodd@OES.ORST.EDU (Stephen M Dodd)
Subject: Re: Boyancy control with drysuits
Organization: Oregon Extension Service, Oregon State University, Corvallis, Or.
Date: Sat, 15 Aug 1992 16:24:16 GMT

In article <1992Aug12.191716.11186@bmerh85.bnr.ca> scribler@bnr.ca writes:
>Goodday,
>
>Can any of you drysuit experts explain how to handle
>boyancy control with drysuits? I have only 25 dives
>with my (shell) drysuit and have no problem with descent
>nor with hovering but when I follow an upwards slope I
>have to contort myself to release the air every once
>in a while but I float upwards during this thus I need
>to swim down to continue along the bottom. My exhaust
>valve is at my left shoulder. When I reach about 10'
>water I generally hit the surface before I can dump out
>all my air. Well, suggestions??????
>
>Thanx
>
>The Mad Scribbler

    Your experiences are common with dry suits.

    You have to "fly" your drysuit a little further ahead than without
    one.  This means you have to anticipate bouyancy changes and
    pre-correct for them.  Many start out with too much weight and then
    offset it with air.  The air then changes volume a lot on a dive
    while you change depth and generates a lot of need for continual
    adjustments.  Keeping air in the suit to a minimum, and the weight
    to offset it as low as possible will help reduce the occilations.

    A rule of thumb around here is that your BC should be empty,
    bouyancy is controlled in the suit.  I suppose it helps a lot to
    have only one valve to manage.  Many of our dry suit dives are below
    60 feet around here.  Thus your suit will experience a volume change
    of about 200% for every pound above neutral. A truely extra pound at
    depth will then result in the addition of enough air to radically
    alter your near surface bouyancy.

    As it is imperative that you not experience a rapid uncontrolled
    ascent it is good to insure that you are negative, and ascending
    from finning when moving up.  Most of the pressure change happens in
    the last fifteen feet or so, and as the dive progresses, the
    bouyancy changes there tend to surprise the returning diver.  If you
    dive to say sixty feet and then ascend to the surface, the air in
    your suit will change volume radically.  You will have probably
    added a little air before reaching that point to offset squeeze.
    This "arms" the bomb.  So it makes sense to begin the bouyancy
    adjustment BEFORE the ascent, planning to safety stop at fifteen
    feet in any case.  Once securely stoped at fifteen feet, you can
    resume your ascent under better control.  I usually like to finn all
    the way to the top, while negative, and only use the BC on the
    surface.  Even at that you may find your dry suit puffing out a bit
    in the last three feet!

    Once again ... plan to ascend to a full stop at fifteen feet.
    Regard passing that as a failure.  After your safety stop resume the
    ascent through finning.  You may even need to dump air at fifteen
    feet just before rising the rest of the way to the surface.  This
    may indicate that you have too much weight and are thus forced to
    compensate with too much air.  But be sure to straighten this all
    out at fifteen feet.

    Steve



-- 
- Stephen M. Dodd, System Analyst, Oregon Extension Service
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
sdodd@oes.orst.edu          Internet          Oregon State University
503-737-0699                                  Corvallis, OR 97331