{HEAD ~ An isolation valve} 140 selectively isolates {TAIL ~ calibration circuit} 144 from the purge line 154 
selectively isolates
{ HEAD ~ An isolation valve } 140 selectively isolates { TAIL ~ calibration circuit } 144 from the purge line 154
HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH