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