Which positive displacement pump (PD pump) is best for your dispensing needs, linear or rotary? Linear and rotary positive displacement pumps can provide exceptional accuracy and precision. However, each method has their advantages and disadvantages. Positive displacement pumping refers to a pump that retracts in a cavity to generate volume on the suction side and extends to close in the cavity on the discharge. Fluid flows into the suction side during retract and flows out of the discharge during contraction. This is a constant for each cycle.