Patent ID: 6880396

Claim:
A level indicator which is intimately affixed to the outside surface of the outside wall of a container for use in determining the level of the interface between a liquid and the void volume above said liquid within a container; wherein said liquid has a faster rate of heat transfer than said void volume above it within a container; said level indicator being in the form of an elongated strip, vertically extending along the height of a container, wherein said level indicator conforms to the contour of the outside surface of the outside wall of a container such that said level indicator is in intimate heat transfer relationship with the outside wall of a container; wherein said level indicator includes a thermochromic leucodye ink layer, said thermochromic leucodye ink layer having at least one leucodye ink therein; whereby when said level indicator is in a heat transfer relationship with a heat transfer-inducing agent, said at least one leucodye ink in said level indicator, in the region of the interface between the liquid and the void volume above it within the container and having an operating temperature range encompassing the temperature of the heat transfer-inducing agent, will exhibit a profound color change responsive to heat transfer between the heat transfer-inducing agent and the liquid so as to permit the level of the interface to be detected.