Abstract: A highly accurate pumping device for aspirating and dispensing variable volumes of fluid is presented. The pumping device comprises a housing with a slotted optical sensor, a stepper motor with a fine pitch lead screw, a coupling linking the stepper motor shaft and the lead screw, an anti-backlash nut, an anti-rotation arm linking the plunger with the anti-backlash nut, an end of travel sensor feature integrated in the arm, and a top which incorporates a chamber and a lead screw bearing. A seal, located in the top, seals against the plunger and the chamber. The plunger is attached to the anti-rotation arm and eccentrically aligned to the lead screw. The pumping device further includes one or more ports through which fluid may be introduced into or out of the chamber. The pumping device may be easily changed to accommodate different full-scale volume requirements due to the self-aligning features.