Abstract: A control device for an air inlet valve to a second stage regulator for an underwater breathing apparatus which allows a diver, while wearing the apparatus, to adjust the amount of suction pressure required to activate the air inlet to his mouthpiece. The device comprises a small cylindrical structure and a micrometer-type adjustment screw threadedly engageable with an interior wall of the structure. The structure fits on the air inlet to the second stage regulator and is fluidly connected thereto in such a way that any air entering the regulator must pass through a channel formed in the shaft of the screw. The manually adjustable, micrometer-type adjustment screw, when turned, resets biasing means on the normally closed air inlet valve, altering the suction pressure which the user must supply to open the valve. In addition, the air inlet for the control device, when it is fitted to the regulator, is disposed at a 90 degree angle to the air inlet for the regulator alone.