determining concentration of {HEAD ~ carbon dioxide} flowing through {TAIL ~ at least one airflow control device}; and 
flowing through
determining concentration of { HEAD ~ carbon dioxide } flowing through { TAIL ~ at least one airflow control device } ; and
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH