If you're using a high capacity single cylinder, you can use a recreational BCD, providing the bladder has enough capacity to give you positive buoyancy at the surface while wearing a heavier tank. A regular BCD has the disadvantage of not being particularly streamlined, especially when it comes to wreck penetration. Also if you're using double tanks, you need something a bit more robust.
More common for use in technical diving is a wing, backplate and harness. The backplate and harness take the strain of the heavier tanks, while the wing provides additional lift for positive buoyancy on the surface. This configuration also has the advantage of being much less cluttered and more streamlined than a normal BCD, and usually has plenty of metal D-rings to clip on accessories.
There are a number of manufacturers of wings, harnesses and backplates just as there are many different makes of recreational BCDs. Amongst them are OMS, Dive-Rite and Halcyon.
First
of all, the size / capacity of the wing should be chosen carefully, depending
on what type of tanks you are using. The wing should provide enough positive
buoyancy at the surface to offset the weight of the tanks. Obviously the
heavier the tanks, the more lift the wing needs to have.
You also have the option of a single or double bladder
wing. (Most recreational BCs are single bladder).
A
double bladder wing will have 2 inflator hoses, one for each bladder.
The second bladder and inflator hose are redundant, and are used only
for backup in case the primary bladder / inflator hose fails. Very important
if you are using heavier tanks.
The wing can have 'bungees' (elastic ties) which keep
the material of the wing compact when not fully inflated (underwater).
This provides additional streamlining by stopping the deflated wing from
flapping around and becoming entangled on a dive.
Dump valves are also a useful feature so you can maintain your position in the water.
If you can, choose a wing which gives you more than 1 choice of how high / low to attach your rig to the tanks. A wing with 6 holes for the tank bands to attach to will give you 3 different height settings.
A backplate is usually made out of aluminium or steel, with the harness attached. The material of a harness is a combination of rugged nylon and ballistic-type cloth that can withstand the weight of double tanks. Permanently attached metal D-rings on the shoulder straps and the bottom of the harness provide hook-ups for accessories, such as lights, line reels, and stage cylinders.
Bolting the backplate to the tanks creates a 'hard mount' that presents less drag while swimming, and less wobbling / shifting of the rig if the diver changes position in the water.
A more advanced harness may have several different features to a more basic model: padding for comfort, extra D-rings for accessories, quick release clips, pockets attached etc
