US Patent 7524463 Heated volatile dispensing device with dye-based use-up indicator

Heated volatile dispensers are disclosed that are provided with automated dye-based use-up indicators. Multiple migrateable dyes of different colors are positioned adjacent a porous substrate. Heating of the substrate, such as a slab impregnated with an insect repellent, both causes the impregnated chemical to dispense from the slab and the indicator dyes to migrate to one or more visible positions. One dye migrates to a visible position faster than a dye of a different color. Further heating may cause one or both dyes to change color at a visible position. The dye movement imparts information about the degree of use-up of the impregnating chemical.