Sentence:  However , the surface temperature gradient can produce erroneous vapor-pressure measurements for the <WSD>bulk</WSD> liquid helium unless precautions are taken to isolate the tube ( which passes through the surface to the vapor pressure bulb ) from the liquid helium surface . 
Meaning of the word marked with <WSD> tags:  the property resulting from being or relating to the greater in number of two parts; the main part