Any BC needs to meet two criteria:
1) Be able to float your rig at the surface with full bottles if you ditch it
2) Be able to compensate for the maximum change in buoyancy of your exposure suit.
A couple examples. I make some assumptions here because you did not provide any buoyancy info on your suit.
Single Tank Rig
I will assume that your Dryusuit with minimum gas in it is about 24 lbs positive.
***This may not be the case. You should test your own suit***
Rig
SS plate and harness ~-6 lbs
Reg -2
Full HP 100 -11
Total with a full tank -19 lbs
Total with an empty tank ~-11 lbs
If your suit is +24 and the rig provides 11 of ballast with an empty tank you should need about 13 lbs of weight belt.
If you were to suffer a total failure of your suit, i.e. suit torn open, you could loose all 24 lbs of buoyancy, but no more than that. This is of course unlikely, but it is the worse case scenario.
24 > 19. That means you need a singles wing of greater than 24 lbs. Something in the 26 - 30 lbs range is a reasonable choice.
How buoyant would your suit need to be to require a 40 lbs wing, or a 60?
Doubles rig
SS plate and harness -6
Dual regs -5
Bands and manifold -5
2 x hp 100 full of air or nitrox -21
Can light -2
Total with full tanks -39
Total with empty tanks -23
You need to start the dive negative by the weight of your Backgas 2 x 100 = ~16 lbs of air or nitrox.
You should add another 2-3 lbs to allow for extra suit inflation, no shrink wrap at your shallow stop
That means to stay at the surface at the beginning of the dive with full tanks you will use about 16 + 3 = 19 lbs of the capacity of your wing.
You need in reserve enough capacity to deal with a total failure of your suit.
19 + 24 = 43 lbs.
Your rig will provide about 23 lbs of ballast with empty bottles, and you need 27. (24 lbs of suit buoyancy +3 for extra inflation)
If you add 4 lbs of ballast to your rig, for example a couple soft weight pouches between the wing and back plate, your rig will now be 43 lbs negative at the start of the dive.
Bingo! Suit + gas + 3 = 43 and your rig is 43 lbs negative.
You need a wing that's larger than 43 lbs. Something in the 49 -50 lb range is reasonable.
Imagine how buoyant your suit would need to be, and / or how much gas you would need to carry to require a 94 lbs wing.
You should ask your
LDS to explain how, quantitatively, they arrived at their recommendation.
A couple final points.
Your "personal buoyancy" i.e. do you float or sink in your birthday suit, will impact your total weighting, but it does not impact the size of the wing.
Remember wings are used to COMPENSATE for things change in buoyancy, and your lipids or lack of lipids won't change as you descend.
You need a wing for singles and a different wing for doubles. Lots of goods reasons here including different capacity, different shape, and the likelyhood that you will use different exposure protection with singles vs doubles.
To test your dry suit:
Get a big bag o lead, put on your undies and suit and find some neck deep water.
Jump in and stand up. Vent all the gas you can from the suit. Remove lead from your bag until you just sink when you pick up your feet.
Weight the bag of lead. That represents the buoyancy of your suit with minimum gas in it. If you are of normal body mass Index you are done. If you are aren't add or subtract a few lbs as required.
If you do the test in fresh water and plan to dive in salt you need to add ~1 lbs for every 40 lbs of diver and gear. In this case is almost all diver.
Good luck,
Tobin __________________
http://www.deepseasupply.com
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