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