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