How much extra do you really need?

[RDP]

I have been looking in the archives and have found many threads on "what size wing", "how much wing", and "what is the best for me". What I am interested in is the reasoning behind it all.

My take so far is that the air cell needs to have enough lift to overcome any negative buoyancy on the surface, and compensate the negative buoyancy subsurface to establish neutral buoyancy. (I don't mean that it compensates for being over weighted.)

In my case, with a single AL80 and wetsuit I require 18# (14+4for AL tank) to be properly weighted. This is neutral at the surface with 500psi. I hardly use any air on the surface to become positively buoyant. (One breath manually inflating.) This leads me to believe I only need the 4#ish for the full tank plus a few # more for margin.


Question 1: How much margin is enough once all gear has been considered?


With that in mind, I am changing to a larger tank, steel 120, that is roughly -9# full and neutral when empty. It seems I would now need 9# plus a margin.


Question 2: With 14# as a constant to compensate the positive buoyancy of my gear, what other than the tank, do I need to consider when determining required lift?


I know different gear dictates more lift requirements. And you wouldn't want you gear sinking if you took off the harness. I am just trying to break it down using this example.

Question 3: It seems the common size for single tank wing is 30#-45#. Is this overkill for a diver using the equipment described above?

Please help me better understand this.

 

[cool_hardware52]

I have been looking in the archives and have found many threads on "what size wing", "how much wing", and "what is the best for me". What I am interested in is the reasoning behind it all.

My take so far is that the air cell needs to have enough lift to overcome any negative buoyancy on the surface, and compensate the negative buoyancy subsurface to establish neutral buoyancy. (I don't mean that it compensates for being over weighted.)

In my case, with a single AL80 and wetsuit I require 18# (14+4for AL tank) to be properly weighted. This is neutral at the surface with 500psi. I hardly use any air on the surface to become positively buoyant. (One breath manually inflating.) This leads me to believe I only need the 4#ish for the full tank plus a few # more for margin.


Question 1: How much margin is enough once all gear has been considered?


With that in mind, I am changing to a larger tank, steel 120, that is roughly -9# full and neutral when empty. It seems I would now need 9# plus a margin.


Question 2: With 14# as a constant to compensate the positive buoyancy of my gear, what other than the tank, do I need to consider when determining required lift?


I know different gear dictates more lift requirements. And you wouldn't want you gear sinking if you took off the harness. I am just trying to break it down using this example.

Question 3: It seems the common size for single tank wing is 30#-45#. Is this overkill for a diver using the equipment described above?

Please help me better understand this.

RDP,

My method for calculating required wing lift.

Assumptions

1) You are weighted neutral (eye level) at the surface with no air in your wing.

2) You are diving a wetsuit.

3) You have no air trapping pads etc as part of the BC.

4) Your diving open circuit scuba.

You need enough lift to:

A) Float your rig at the surface without you in it. You can calculate the approximate "in water weight" of your rig by knowing the buoyancy characteristics of your tank, and adding the weight of your Back Plate and regs etc. This seldom greater than 25lbs., often much less. (Example #9 tank #6 BP, #5 reg = #20)

B) Offset the loss of buoyancy your wetsuit looses. Determine how much weight is necessary to sink just your exposure suit, i.e. your suit without you in it. Your wetsuit is just about the only thing that looses buoyancy as you descend. This loss of buoyancy is what your wing or BC or "buoyancy compensator" is compensating for. This seldom more than ~18-25 lbs even for thick suits on large people, here too it's often much less. Conditions that dictate very heavy wetsuits (long, deep etc) usually also include doubles, or a drysuit or both. In these cases the weight of the rig usually governs.

Bear in mind that wetsuits will remain at least slightly buoyant down to ~ 165fsw where they are effectively fully compressed.

Not a bad idea to have a little extra lift as all bc's will take on water as they are used, and this water reduces the available inflation volume.

When does the above method become invalid? When the diver cannot achieve neutral buoyancy at the surface with no air in his wing. If burdened with lots of extra gear, tools, steel stages (bad idea) etc.


Hope this is what you were looking for,



Tobin

 

[simbrooks]

Just to try to clarify, the BC compensates for your wetsuit compression and swing in tank buoyancy during the dive. Lets say for me in my worst combo is 5mm in salt, i wear roughly 12# to take out the buoyancy of the AL80 and wetsuit which is just purely lead, i then have wetsuit compression which could be another 6# and of course my tank which is 2# at the start, which seems to add up to 20#. In my 1mm under the same conditions this only adds up to 10# at most due to less wetsuit compression and less lead being needed to sink the wetsuit. I wouldnt however dive a wing that was only just over the 20# mark, hence the mainstay recreational (non-tropical oriented like the 18# wings) start around 27# and go up from there. I can only imagine in really cold water with lots of lead and wetsuit compression (or loss of drysuit buoyancy if flooded - now neutral) that you could need much more than 30# or so. When you double up the tanks and add on others, then you start calculating up lots of negative weight and so your wing/drysuit needs to have some inflation at the surface just to keep you afloat, most seem to find 50-60# adequate although i have never weighed out such a configuration although i know it is coming one day - but as a thought doubles might be 18-22# just by themselves at the start, hopefully neutral at the end, maybe some lead for your drysuit and of course you might have other tanks and gear. Once you add all this up i would think minimum 40# plus a safety margin - hence most wings starting at 50# for all but the lightest doubles (LP72/AL80 or smaller).