Abstract: The present invention is embodied in a tensiometer apparatus and process that employs novel techniques for performing the required routine service operation of refilling a sealable measurement chamber of the apparatus with liquid from a reservoir using a novel, piston-type, “jet-action” plunger mechanism. These novel techniques result in a tensiometer which is easier to use, requires no tools for assembly or replacement of parts, and is easier and lower cost to manufacture and maintain. These novel techniques also result in a tensiometer which has improved measurement sensitivity and accuracy, higher reliability and a longer operating lifetime than conventional tensiometers.