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