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