Adjustable Buoyancy Life Jacket (ABLJ)
Modern sports divers wear a Buoyancy Compensator Device (BCD) that holds the cylinder on the diver’s back and allows the diver to trim his/her buoyancy so as to be totally neutral (ie. they neither rise nor sink down).
The predecessor to the BCD was the ABLJ (Adjustable Buoyancy Life Jacket), also called a horse collar jacket. When wearing an ABLJ a diver also had to have a separate backpack or harness that held the diving cylinder. Buoyancy is manually controlled by the diver who can add air to increase buoyancy, or release air from the ABLJ to sink down. Skilled divers can achieve neutral buoyancy by controlling the volume of air in the ABLJ.
The air can be supplied from the breathing gas cylinder on the diver’s back, from a small inflation cylinder attached to the ABLJ or the diver can blow into the jacket using the mouthpiece and hose attached to the ABLJ.
Fenzy and Spirotechnique were two popular manufacturers of ABLJs.