Has the BC got a pull dump built into the LPI?  Try rolling/orientating
yourself so that your left shoulder [assuming standard gear config] is
at the highest point, and try again.  Do a weight check at end of dive,
not begining as you will be more +ve at end.
If its any consolation I had a crap dive 3 weeks ago, bobbing around
and grabbing rocks to keep down!!!  Know what the problem was?  Shoulder
strap on my wing was trapping air and stopping it going out of the
shoulder dump of my new suit!   Btw I am a trimix instructor with 1000+
divesunder my belt!!!  "Every day is a School day", learning all the
time, be safe.

Dive a lot. Failing that try to spend time in a pool.
If you can master your buoyancy in six feet or less or water you've got it
licked.

For what it's worth, which is not a lot, I wear an additional 4 lbs of lead
for my 3mm wetsuit.  I doubt that the relationship between wetsuit thickness
and weight is linear, but my experience suggests that something around 9 lbs
extra for the wetsuit might be a starting point.

Now that I know you are using a wetsuit, there's an additional factor that
may be contributing to your yo yo experience.  As you know, I hope, the gas
in a wetsuit compresses as you descend and expands as you ascend.  That
accounts for some variation in buoyancy during various portions of the dive.
There is, at least for me, more to it than that.  My wetsuit compresses
normally down to about 15 feet.  Somewhere very close to 15 feet, I get a
noticably larger shift in buoyancy.  Below 15 feet, the shift is, once
again, at a normal rate.  I can't be sure what causes the sudden shift, but
I have an opinion.  While the gas in the wetsuit material compresses
gradually, the walls of the cells containing the gas don't.  They resist
compression until they collapse.  It is that collapse that I think is why I
get a sudden shift.  Once collapsed, the normal compression of gas is, once
again, the primary mechanism.

Whether my theory is correct or not, there's no question that the suddent
shift is real or, in my case, where it occurs.  I'm probably a bit more
sensitive to buoyancy shifts than the average diver.  I most often dive with
an empty wing and no thermal protection (I live in south Florida).  In this
configuration, I have virtually no buoyancy shift during a dive.  At any
rate, the shift at about 15 feet has much the same effect that you describe.
A very small change in depth has a disproportionate effect on my buoyancy,
creating the yo yo effect I think you're talking about.  The solution, for
me, is pretty easy.  I don't stay at 15 feet any longer than necessary.  I
don't plan cold water dives at that depth, I don't linger at that depth
during my descent or ascent, and I have even moved my normal safety and/or
deco stops to 20 feet or more.

If your problem is similar, try watching your depth closely a few times to
see where the sharp change occurs.  It's probably not at the same depth for
your suit that it is for mine, but there probably is a depth where it
occurs.  Avoid that depth.

Fine tuning buoyancy is easiest for those that can dive with the least
volume of compressable gas spaces.  Wetsuits contain a lot of such space
and, as I mentioned, don't always have nice smooth compression curves.
Drysuits have even more space, but normally do have smooth compression
curves.  While they do require more weight, they can be less of a hassel to
deal with.

Somebody else mentioned doing your buoyancy check at the end of the dive.
They're pretty much right.  For most recreational divers, tank pressure
never falls below about 500 psi, give or take a little.  I suggest you
adjust for neutral buoyancy as near to the surface as possible, with 500
psi, or whatever minimum you like better, in the tank.  The critical point
in the dive is your final ascent.  You do not want to be pulled to the
surface, even from your 15, or in my case, 20 foot safety stop.  You'll
necessarily be heavy during the entire dive, but that's the price you pay
for diving in colder waters.

It's not so much educated as it is experienced.  You know that basics, it's
the fine tuning that's giving you problems and that takes time.  It just
takes less time if you are closely assisted by somebody that has done it
themselves and who is good at communicating with others.  If nothing else,
it's nice to have sombody to give and take weight as you seek your own,
personal, best weighting level.

Lee

Captain's log. On StarDate Wed, 29 Jun 2005 04:39:15 +0200 received comm from
Martin Törnsten <martinot@gmail.com> on channel rec.scuba.equipment:

Forgot the link to the page: http://www.halcyon.net/mc/weights.shtml

martin törnsten