{HEAD ~ The apparatus} comprises {TAIL ~ an airlock} containing a fluid for sealing the vessel and an electrical circuit operatively arranged to pass a current through the fluid to detect passage of bubbles through the airlock 
comprises
{ HEAD ~ The apparatus } comprises { TAIL ~ an airlock } containing a fluid for sealing the vessel and an electrical circuit operatively arranged to pass a current through the fluid to detect passage of bubbles through the airlock
HEAD HEAD HEAD HEAD HEAD HEAD REL 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 OTH OTH OTH OTH OTH