What Size BC Wing Should I Buy?
The wing is used to control buoyancy throughout the dive; a drysuit (if worn) should only be inflated enough to keep warm (please see Dynamic Instability for more information). At the beginning of the dive add enough gas to the wing to offset the weight of breathing gas in your tank; as you breathe the tank down, gradually vent small amounts of gas from the wing to stay neutral. At the end of the dive, when you are at 10ft, if your tank is nearly empty there should be little or no gas left in the wing.
Buying more lift than you need offers no advantages and may cause other problems. Larger wings tend to cause more drag underwater and it can be more difficult to dump gas while ascending. Additionally, some divers with high capacity wings tend to overweight themselves and then try to compensate by adding a lot of gas to the wing; this makes buoyancy control difficult due to dynamic instability and creates a dangerous situation if the wing fails. Additionally some manufacturers which create large capacity wings put on bungie straps to keep these large wings from going all over the place. Please see the note at the bottom about bungied wings (also known as bondage wings).
Those who use integrated weights, such as the Halcyon ACB weights or various types of tank weights, may need a little extra lift. For example, if you have a single steel tank, steel back plate, canister light, argon system, and 14lb of integrated weight then 36lb lift is probably not enough to keep the rig from sinking if you doff it at the surface. The Halcyon single tank Pioneer wings have a unique expanding panel feature that allows the higher capacity models to have no more drag than the 27lb lift model. Another solution is to just use a standard weight belt and avoid the problems associated with integrated weights.
Finally it should be noted that actual lift is not always equal to the rated lift. Often there are portions of the wing that are "pinched off" when a rig is fully assembled which will reduce its effective lift capacity. Therefore it is strongly recommended to either test a wing with your gear, or ask for references from people that have similar gear configurations before making a final decision.
Let's consider a few practical examples.
We don't need extra wing lift to bring up a heavy object from the bottom; this is dangerous since if you drop the object the extra gas in your wing will send you rocketing toward the surface. It is safer to use a lift bag instead.
Although it is possible to use a double tank wing with a single tank
and adapter this can cause a lot of drag and gas trapping and is not a
recommended practice. Always use the correct gear for the dive.
Bungied WingsThe use of bungied/bondage wings is strongly discouraged. To start with, one of the primary reasons stated for their use is that they streamline your rig. Ironically, they generally do the exact opposite. Hydrodynamics dictates that rough surfaces create increased turbulence which consequently increases drag. The bungies create a very rough surface and thus are adding to drag. Furthermore the bungies have a tendency to trap air which cause both static and dynamic instability issues. However these issues are not the most important reasons to avoid these wings. There are two large reasons that bungied wings are normally avoided. Probably the largest problem with bungied wings is the increased resistance to oral inflation. The bungies will make it significantly more difficult to orally inflate the wing, which can be a serious safety issue. A related issue is that the bungies create a stronger positive-pressure deflate than normally exists. So deflating the wing tends to dump air much faster than a non-bungied wing. This makes proper buoyancy control more difficult. And finally, due to this positive-pressure deflate, it is nearly impossible to use your BC as a "third regulator". This is somewhat of an advanced topic, however your BC can be used as an alternate regulator in very serious conditions. The deflate and inflate buttons are depressed simultaneously to provide air and then the diver breaths out their nose, or removes the BC inflator from their mouth when exhaling. It is important to note that you are not rebreathing the air in the BC, you are effectively breathing it straight from the BC inflator mechanism. Obviously you will want to practice this skill in a pool before trying it "for real"!