Abstract: Apparatus for externally determining the level of a mass of flowable material such as liquid or granular material contained in a closed or sealed container having walls of magnetic material includes an elongate strip of thin magnetized metal sheet material coated with a thermochromic substance which varies chromatically with variations in temperature. The elongated strip is applied to the outer surface of the wall of the container in heat conducting relationship thereto by magnetic attraction and extends along the height of the container. The container is exposed to a temperature change either natural, such as that which occurs at sunset and sunrise, or artificially induced. Because of the difference in the rate of heat conduction of the void volume and the filled space of the container, the container wall experiences a temperature gradient which is most pronounced at the interface of the contents and the void space. The temperature of the container wall changes abruptly at the surface of the contents.