An end of service life indicator (ESLI) device for a fluid filter is included on the filter and visually indicates when the life of the filter has expired. The ESLI indicates the depletion of the filter by changing the color density on a surface of the device to a color density having a well distinguishable...http://www.google.es/patents/US6979361?utm_source=gb-gplus-sharePatente US6979361 - End of service life indicator for fluid filter
