Sentence:  However , the surface temperature gradient can produce erroneous vapor-pressure measurements for the bulk liquid helium unless precautions are taken to isolate the tube ( which passes through the surface to the <WSD>vapor</WSD> pressure bulb ) from the liquid helium surface . 
Meaning of the word marked with <WSD> tags:  the pressure exerted by a vapor; often understood to mean saturated vapor pressure (the vapor pressure of a vapor in contact with its liquid form)