Abstract: A luminescent tube filled with a temperature responsive liquid which causes the tube to glow as the liquid moves up and down the tube is encased within an opaque expansible sleeve which is aligned with graduations on the tube to indicate a critical temperature to be sensed when the tube is mounted on an object, so that if the tube glows above the sleeve the temperature is readily discernable. The tube and sleeve may be permanently mounted on the object or removably mounted by a magnet or clip on the sleeve.