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