Sentence:  However , the surface temperature gradient can produce erroneous vapor-pressure measurements for the bulk liquid helium unless precautions are taken to isolate the <WSD>tube</WSD> ( which passes through the surface to the vapor pressure bulb ) from the liquid helium surface . 
Meaning of the word marked with <WSD> tags:  conduit consisting of a long hollow object (usually cylindrical) used to hold and conduct objects or liquids or gases