Abstract: A temperature sensor has a flexible nylon tube connected by a fluid-tight coupling to a metal bellows within a metal housing. The interior of the tube and bellows is filled with a pressurized liquid whose critical temperature corresponds to a threshold temperature to be sensed. The pressurized liquid holds the bellows extended, holding an actuating piston back against the action of a spring. When the threshold temperature is reached, the liquid in the tube must boil and ruptures the tube, causing the bellows to collapse and the actuating piston to move. A projecting actuator then mechanically initiates a response such as the closing of a damper in an air-conditioning duct.