For sustained accuracy and consistent and repeatable operation, pipettes should be calibrated at periodic intervals. These intervals vary depending on several factors:
The skill and training of the operators. Skilled operators tend to operate the instrument more correctly and make fewer accuracy-robbing mistakes.
The liquid dispensed by the pipette. Corrosive and volatile liquids tend to emit vapors which ascend into the pipette shaft even under proper operating conditions and may corrode the metal piston and springs, or the seals and o-rings that provide an air-tight seal between the piston and the surrounding sleeve.
Proper and careful handling. Pipettes that are frequently dropped, are subjected to careless handling or horseplay, or that are not properly stored in a vertical position, will tend to degrade in accuracy over time.
The accuracy required by the instrument. Applications requiring maximum accuracy also demand more frequent calibration. Instruments used for purely research applications or in educational settings generally require less frequent calibration.
Under average conditions, most pipettes can be calibrated semi-annually (every six months) and provide satisfactory performance. Institutions that are regulated by the Food and Drug Administration‘s GMP/GLP regulations generally benefit from quarterly calibration, or every three months. Critical applications may require monthly service, while research and educational institutions may need only annual service. These are general guidelines and any decision on the appropriate calibration interval should be made carefully and include considerations of the pipette in question (some are more reliable than others), the conditions under which the pipette is used, and the operators who use it.