RavenC once bubbled...
Jason, Are you diving a STA? As mentioned above, a single tank adaptor can and will make up weight difference. For instance, when I dive salt water, I use a heavy STA. When I dive fresh, I use a lightweight. Right now, I am just right with BP and STAs. You don't need ditchable weight if you are neutral. You can swim it up. Trim is affected by where you wear your weight too. That's why weighting is important. The BP setup is designed to put the weight in the proper places and not spread it out. I am having custom weight pockets made to thread onto my webbing for increasing the mil on my suit for colder water diving - with more mil I have to add weight. But the idea is not for ditchable. BTW my BP is a custom short heavy so I start with 8 lbs.
Most of us are neutral in the water without any gear on. We
need to add weight to compensate for the exposure suit that
we wear.
How much weight we need to add in the form of negative
buoyancy tanks, backplates, STA, trim weights, weight belts,
vweights, etc is all dependent on the exposure suit.
When diving there are two things that change our buoyancy
thruout the course of the dive.
1. The weight of the gas in our tanks. A typical single tank
will shift about 6lbs during the course of dive as the gas
is consumed. At the end of the dive your system will
be 6lbs more positive then it was at the begining.
2. If diving a wetsuit or a neoprene drysuit we have the
added complication that the positive buoyancy of the suit is
due to closed-cell air bubbles in the neoprene. Just like any
air space these air bubbles are subject to Boyles Law and as
the pressure increases with depth the bubbles will get
smaller. As the bubbles get smaller the suit is less buoyant.
In determining how much weight we need to add there are
two critical points during the dive
First at the end of the dive we need to be neutral at 10 feet so
that we can make our deco (or safety stops).
Second at the beginning of the dive, at depth, we need to be
able to swim our system to the surface in the event of a wing
failure.
An example - say we need to have 20lbs of weight to
compensate for our wetsuit and to be able to make the deco
stop.
At the beginning of the dive we will be 6lbs negative at the
surface because we will have the weight of the air to contend
with. So we have
20lbs positive (wetsuit)
- 20 lbs (weights - backplate, lead, etc)
- 6 lbs (full tank of air or nitrox)
Total -6 lbs
Say we make a dive to 100 feet. The pressure is 4 ATA.
(1 ATA at the surface, 1 ATA for every 33 feet of depth)
The buoyancy of the wetsuit is reduced to 1/4 of what is
was at the surface. 20 divided by 4 is 5 lbs of positive
buoyancy. So at 100 feet we have
5 lbs positive (wetsuit)
-20 lbs (weights)
-6 lbs (full tank)
Total -21 lbs.
We compensate for this 21 lbs of negative bouyancy by
adding 21 lbs of positive buoyancy to the wing. A 27 lb
wing is going to pretty full at this point. (Lots of drag)
If the wing fails and you have no ditchable weight you
will need to swim 21 lbs directly to the surface.
Worst case you will not be able to swim up a slope or
hand over hand up an anchor or mooring line.
I can't swim 21 lbs to the surface. I can manage 10lbs
but I am not a happy camper particularly if I have
any sort of surface swim to the boat or beach.
So some of the weight needs to be converted to ditchable.
More realistically off San Diego I would do this dive in a
Trilaminate Dry Suit where the suit itself has no positive bouyancy.
There is an air space between the suit and body that has to be
maintained but its a seperate system from my wing. If my suit
and the trapped air space contributes 20 lbs of buoyancy at
the surface it will contribute the same at depth. So if properly
weighted I am never more then 6 lbs negatively buoyant unless
I have 2 failures - my wing fails and my drysuit floods.
Take care
Marc Hall
march@gue.com
www.enjoythedive.com
