Low-pressure and low-temperature pump assembly

The invention discloses a low-pressure and low-temperature pump assembly. The low-pressure and low-temperature pump assembly is provided with components for defining the displacement and dead volume of a pump; all the components are manufactured with manufacturing tolerance; when the manufacturing tolerance is overlaid, a dead volume not surpassing 20 percent of displacement is obtained together. The pump assembly comprises a pump piston, and the pump piston can move between a full retraction position and a full extension position in a reciprocating way in order to define the displacement. The dead volume is defined by the volume of fluid left inside a piston cylinder when the pump piston is at the full extension position.