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