Abstract: A buoyancy compensation apparatus for underwater use by a SCUBA diver, includes a pressurized air source, a buoyancy vest including air chambers and a pressure regulating unit. The pressure regulating unit includes a chamber in communication with the vest air chambers and the pressurized air source. The pressure regulating unit also includes a normally closed air injection valve and a normally closed relief valve. The wall of the pressure regulating unit chamber includes a flexible portion that flexes inwardly when outside water pressure exceeds the pressure in the vest air chambers, which, in turn, opens the air injection valve. The relief valve opens when air pressure in the vest air chambers exceeds outside water pressure. The air injection and relief valves respond faster than the walls of the buoyancy vest air chambers so that the volume of the vest air chambers remain generally constant even as the diver changes depth.