US Patent 7284454 Hand held pipette

A hand-held pipette includes a piston driven by an actuator assembly contained within a housing having first and second mutually spaced non-adjustable stops. The actuator assembly is subdivided into first and second sections respectively provided with first and second contact surfaces. The actuator assembly is arranged to reciprocate between a rest position at which the first contact surface is in contact with the first stop and the second contact surface is spaced from the second stop by a control distance, and an advanced position at which the second contact surface is in contact with the second stop and the first contact surface is spaced from the first stop, with the stroke of the actuator assembly and the stroke of the piston being equal to the control distance. A motor-driven mechanism is arranged to displace one section of the actuator assembly relative to the other section, resulting in a corresponding change to both the overall length of the actuator assembly and the control distance.