Abstract: An indicating device is provided, employing a plunger or piston movable along a casing, through a viscous liquid that provides an opposing force to the force provided by a spring that urges the piston in its movement. The viscous liquid is substantially uniform with temperature changes. A rod carried by the piston is initially in engagement with the casing, but releasable upon reaching a predetermined temperature, for substantially uniform movement of the piston through the fluid, with the rod being re-engageable with the casing upon solidification of a eutectic metal type connection upon reduction of the temperature, whereupon piston movement is halted. The device is used to determine not only if a product at the interior of a container has reached the desired temperature, but the length of time that the interior of the container remained at the desired temperature, and to this end, calibrations on the device may be provided.