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