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