Abstract: A non-invasive measuring device calculates a temperature of a medium in a container or conduit. The device includes at least one first sensor disposed on an exterior surface of the container or conduit and measures a temperature (To) of the exterior surface. The measuring device also includes at least one second sensor disposed on the exterior surface and measures a heat flux (q?) moving through a wall of the container or conduit. The measuring device further includes a processor operatively coupled to the first sensor and the second sensor. The processor is configured to receive the temperature (To) measurements and the heat flux (q?) measurements from the first sensor and the second sensor and to calculate a temperature of the medium in the container or conduit based on input variables about the container or conduit including radius, thickness and thermal conductivity of the material of the container or conduit.