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