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