means coupled with said generating means for producing an indication of when said electrical signal reaches {HEAD ~ a preselected value} corresponding to a prescribed accumulation of {TAIL ~ particulates} on said lining 
means coupled with said generating means for producing an indication of when said electrical signal reaches { HEAD ~ a preselected value } corresponding to a prescribed accumulation of { TAIL ~ particulates } on said lining
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH