A BCD, or buoyancy compensators, are handy and essential bits of equipment that allow a diver to offset their negative buoyancy by adding air from the tank to the BCD while underwater.
BCDs are available in 3 different styles:
Back Inflation: A back-inflate BCD, like the name implies, positions its air cell behind you. This leaves the chest area uncluttered and reduces front-body bulkiness
Jacket Style: The jacket-style scuba BCD was scuba diving’s original buoyancy compensator design and is still very popular among divers. Easy to use, it offers lots of lift capacity and requires minimal adjustment.
Harness System: While not scuba BCDs in the traditional sense, back plate systems perform the same important buoyancy control device tasks. A typical back plate system consists of three basic components: a scuba back plate to hold the tank, a diver’s harness to strap the back plate to the diver, and an air cell, or dive wing, to provide the buoyant lift.