Abstract: An air flow velocity controller in a variable air volume control system interlocks a room temperature signal from a thermostat with the velocity set point so that the velocity controller maintains a constantly regulated amount of air into a room in proportion to the room's thermostat demands and independent of variations in the static pressure in the duct.The controller includes a maximum lift cam and a movable spring fulcrum which are both mounted in a common cam housing to permit the maximum velocity setting to be adjusted for varying room requirements while maintaining a constant spring range regardless of the maximum velocity setting.The controller is constructed to provide automatic compensation for control offset produced by changes in the static air pressure in the duct.The controller includes the ability to control the air flow at a minimum velocity so that the air can be economically reheated or a minimum volume of air flow can be maintained for ventilation requirements.