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