A couple of general comment. Forgive me if I am repeating points that others have made.
1) Wings need to be sized based on the negative buoyancy of dive gear *in the water* The dry weight might matter to who ever has to hump the gear across the parking lot, but dry weight plays no part in wing sizing.
2) My definition of minimum ballast is the amount of total ballast required to hold a shallow stop with an empty cylinder. (actually near empty cylinder, if your cylinder is empty you are likely done "stopping"
)
3) Exposure suits cannot lose more buoyancy than they start with at the surface. This true for both wetsuits and drysuits tested with minimum gas in them.
4) Personal buoyancy does not change with respect to depth if the diver is breathing compressed gas. (In other words fat does not compress. ) Personal buoyancy will impact total ballast requirements but usually does not impact required wing capacity. Most divers are pretty close to neutral in their swimsuit, humans are after all largely salt water.
5) Any BC needs to meet two criteria:
A. Offer enough lift to offset, i.e. "float" the divers rig at the surface with a full cylinder if the diver and their buoyant exposure suit is separated from the rig, as in ditching it or donning the rig in the water.
B. Offer enough lift to be able to compensate for the maximum possible change in buoyancy of the diver's exposure suit.
6) If a diver starts a dive with a full cylinder and weighted so that they are eyelevel at the surface with *NO* gas in their wing they will not become more negative than the initial buoyancy of their exposure suit. In other words a diver wearing a wetsuit that is +22 lbs at the surface can only become -22 lbs if they manage to fully compress their suit *if* they started the dive as described above.
Having Fitted 1000's of divers over the past 14+ years a few things jump out.
In cold water it is almost always the buoyancy of the diver's exposure suit that will dictate the minimum required wing capacity. This should make sense when one observes that most cold water divers need a weight belt in addition to the ballast their rig provides. Thick wetsuits or drysuits with warmer undies tend to require considerable ballast, often more than a stainless plate, harness, reg, and empty steel tank can provide.
In warm water floating the rig almost always dictates the minimum required wing capacity. This should make sense given the thin (3mm) wetsuits typical of warm water diving.
What inputs are needed to size a wing? The buoyancy of the exposure suit, which often requires the diver test their suit, and the max negative buoyancy of the divers rig. Estimating the max negative for a rig is pretty straight forward if one know the buoyancy numbers for the cylinders they are using.
With this info the rest is simple math.
Tobin
