The atmospheric pressure has to be considered for an exact calculation of the systematic and random error. Furthermore a calibrated analytical balance used for calibration in combination with an evaporation trap is essential. The evaporation trap is important when measuring small volumes < 10 µL. Each step of the calibration from pre-wetting the pipette tip, over pipetting angle to dispensing speed is critical and needs careful attention. This text guides you through the background of a good calibration and explains all steps to enable you to perform it on your own.
The figure shows the random and systematic measurement deviation for a 500 μl sample. Pipetting results can be imprecise, precise but inaccurate and ideally precise and accurate.