For illustrating and supplementing Guideline DKD-R 8-1the effect of the major influencing quantities on measurement uncertainty of piston-operated pipettes with air cushion is explained. The thermodynamic description of the pipetting process enables to identify three system- related influencing factors:

1) temperature differences between test liquid, air and pipette,
2) external air humidity,
3) air pressure.

Air pressure dependence and the respective sensitivity coefficient are described by formulae. Empirically determined values are given for the sensitivity coefficients in 1) and 2). Furthermore, instrumental and system-related effects such as angle of inclination, waiting period, pipetting frequency, immersion depth, operational force, reproducibility of piston stroke, counter hysteresis and heat transfer of hand warming are examined. The respective influences are assessed quantitatively.
Additional contributions are the standard deviation of the repeatability and the uncertainty due to gravimetry. A model equation for measurement uncertainty calculation is developed for the identified influencing quantities.