Newbie BC Problem

"Jim" wrote

> My new BC a rear-inflation weight integrated system. I had a lot of
> problems dumping all the air from this BC . . .

Rear inflation BCs are not known for trapping gas. You do, however, have to
get the vent to the highest point.

.. . . which caused me to over-weight myself and hence yo-yo.

It may have caused you to overweight, but it did not cause you to yo-yo.
You did that on your own. You'll get better with practice.

> Also I was front-heavy with an aluminum 80 tank and 24 or 26 lbs. in the
> pouches.

No surprise there. All you weight was in front. This is pretty common with
newer divers. Many, in fact, go out of their way to arrange things this
way. As they gain experience, they move weight around and/or change to
different, more balanced equipment.

I presume you're using significant thermal protection, either a drysuit or a
thick wetsuit. If not, then you're carrying way too much weight.

> One thing I think I see is that I need to redo the waist strap because it
> is a little too
> loose - thought that might be contributing to trapping air in the BC.
> Also I wonder about having some weight permanently mounted on the back
> of the BC.

Scott Koplan used to make a weighted single tank adapter for plate and wing
setups. If a STA will fit your kit, you can do it yourself if you're even a
little handy or check with Scott to see if he, or anybody else are still
making them.

Since you're clear across the country from me, the best advice I can give is
get together with somebody from the shop that sold you that BCD and get them
to help you sort it out.

Lee

I was wearing a 7 for thermal protection. I think I was maybe 2 lbs
over on the weight to get under with some trapped air. Since I was
never neutrally bouyant without air trapped in the BC and/or I had
trouble releasing air from the BC, the yo-yo was more than I had ever
seen before.

My plan is to get educated before I go back to the dive shop so I'll
know if they're telling me the right thing - after all they shouldn't
be surprised at what happened.

Thanks for replying - any other ideas our there?

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.


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Jim wrote:
> I was wearing a 7 for thermal protection. I think I was maybe 2 lbs
> over on the weight to get under with some trapped air. Since I was
> never neutrally bouyant without air trapped in the BC and/or I had
> trouble releasing air from the BC, the yo-yo was more than I had ever
> seen before.
>
> My plan is to get educated before I go back to the dive shop so I'll
> know if they're telling me the right thing - after all they shouldn't
> be surprised at what happened.
>
> Thanks for replying - any other ideas our there?

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.

"Jim" wrote

>I was wearing a 7 for thermal protection. I think I was maybe 2 lbs
> over on the weight to get under with some trapped air.

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.

> My plan is to get educated before I go back to the dive shop so I'll
> know if they're telling me the right thing - after all they shouldn't
> be surprised at what happened.

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

Thanks everyone! I've talked with the shop. We're going to address
trim by adding a few lbs. of bullets to the back and weight pouches
there too so I can balance front/back. I'm almost certain the suit
compression/decompression mentioned above accounts for some of the
yo-yo. Since my wife and I were both wringing out new equipment we
stayed in the 20 -25 range alot and when I went up even a little I
picked up boyancy very suddenly. I wonder if a brand new suit might be
even more susceptible to this issue. I'm also looking at the shoulder
strap issue wrt to trapping air.

I'm also looking at the shoulder
> strap issue wrt to trapping air.


Forgot to say, bit obvious really, I dive in a Dry Suit. IMHO trapped
air in a wetsuit is not an issue. BTW new wet suits or semi dry suits
are more buoyant because the gas bubbles have not been constantly
crushed and released back to normal.




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Captain's log. On StarDate Fri, 24 Jun 2005 20:15:55 GMT received comm from "Lee
Bell" <leebell@ix.remove.netcom.com> on channel rec.scuba.equipment:

: > Also I wonder about having some weight permanently mounted on the back
: > of the BC.
:
: Scott Koplan used to make a weighted single tank adapter for plate and wing
: setups. If a STA will fit your kit, you can do it yourself if you're even a
: little handy or check with Scott to see if he, or anybody else are still
: making them.

What about the weighted STA at the bottom of this page?

martin törnsten

--
http://martin.tornsten.com/

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:

: Captain's log. On StarDate Fri, 24 Jun 2005 20:15:55 GMT received comm from "Lee
: Bell" <leebell@ix.remove.netcom.com> on channel rec.scuba.equipment:
:
: : > Also I wonder about having some weight permanently mounted on the back
: : > of the BC.
: :
: : Scott Koplan used to make a weighted single tank adapter for plate and wing
: : setups. If a STA will fit your kit, you can do it yourself if you're even a
: : little handy or check with Scott to see if he, or anybody else are still
: : making them.
:
: What about the weighted STA at the bottom of this page?

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

martin törnsten

--
http://martin.tornsten.com/

"Martin Törnsten" wrote

> > Scott Koplan used to make a weighted single tank adapter for plate and
> > wing
> > setups. If a STA will fit your kit, you can do it yourself if you're
> > even a
> > little handy or check with Scott to see if he, or anybody else are still
> > making them.

> What about the weighted STA at the bottom of this page?
> http://www.halcyon.net/mc/weights.shtml

Other than the fact that it's a blatant rip off of Scott's design, there's
nothing wrong with it. I'm a bit surprised that Halcyon, who Scott used to
work for, who was more than ready to sue others for copyright infringement,
who lost their suit, is now marketing something so obviously somebody else's
design.

Thanks for the link. I'll give Scott a call and share it with him. I'm
sure he'll have something appropriate to say.

Lee