Scared Silly:
Okay so if I am fully understanding Tobin let me take this one step further and work the problem a bit differently.
Rig Weight w/gas is 35lbs
Rig Weight w/o gas is 19lbs (HP100 == 16lbs gas)
Dry Suit (DS) to be neutral the counter weight is 30lbs
So DS counter weight less Rig w/o gas is 11 lbs of additional lead.
If additional 11lbs of lead is on the rig w/gas it is now 46 lbs.
Then add the additional gas for the suit 5 lbs which for the sake of this exercise is also on the rig this brings the total for the rig to 51lbs as well. QED (?)
Now the last part that is not mentioned is that the cylinder weights are when they are fully submerged. If you want to any chance of floating above the water you probably want to have a bit more lift. Say with above one gets a 55 lb bladder. Is the additional 4lbs enough? Seems a little light to me ??????????????????
SS,
We might be on the same page, but I'm not sure I follow your approach.
It really is just as simple as I stated it. Your wing needs to be able to float your rig. The heaviset yoru rig will be is at the start of the dive with full cylinders, so your estimated rig weight should be with full cylinders.
In cold water is is almost never the weight of the rig that dictates the minimum required wing lift, but it's worthwhile to check. In warm water the weight of the rig often does dictate the minimum lift.
Now back to the "other" number. Does a diver need to be negative at the start of the dive, and if so by how much? For doubles I assume manditory deco is likely, so the diver needs to negative by at least the weight of his back gas. It's also nice to be able to add a little gas to the DS for comfort. Thats where I get the 16lbs (backgas) and 5 lbs (DS inflation gas) I used in my example. 16+5 = 21 lbs. The diver needs to be ~21 lbs negative at the start of the dive.
The other requirement is a means of compensating for the initial buoyancy of the exposure suit. Here we are assuming a DS that is 30 lbs positive.
If the diver were to start the dive weighted so that he was neutral at the surface with no gas in his wing and full cylinders, he could descend, but would be unable to maintain depth or deco stops as he breathed down his backgas. when he adds the 21 lbs of ballast determined above, he will need to inflate his wing ~ 21 lbs to stay at the surface.
That how I arrive at 30lbs of initial buoyancy from his suit + 21 lbs for backgas and drysuit inflation.
As previously noted using the full value of the DS initial buoyancy is very conservative, because it's very unlikely one could loose 100% of the buoyancy of their suit.
Regards,
Tobin