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