a hand held self - contained automated pipette for portable operation having an electrically operated digital linear actuator . the actuator preferably includes a stepper motor driving a rotor . a threaded screw is coaxially positioned within the rotor and is connected to an actuator shaft having elongate grooves slidable in a guide for preventing shaft rotation so that precise linear motion is imparted to the shaft . a pipetting displacement assembly having one of various sizes is removably attached for actuation by a common actuator including programmed movement of a displacing piston in a displacement cylinder to optimize air interface volume , neutralize variations in vacuum pipette effects , and provide an accommodated stroke and readout for improved accuracy while pipetting and / or titrating different ranges of volumes . upon calibration the piston undertakes immediate excursion to an end of travel limit and after motor slippage is retracted to a home position . this home position is chosen for optimum preservation of an air interface volume between drawn liquid and the piston tailored with particularity to the displacement assembly being used . multiple precision modes , including pipetting , multiple dispensing , titration , dilution , and measurement , are provided .