Pipette calibration means proving that a pipette delivers the volume it is set to, and that it does so consistently. The accepted way to do it is gravimetric: weigh the water the pipette dispenses and convert the weight to volume. It is the method in ISO 8655, the international standard for piston pipettes. Here is how it works, and where an in-house check ends and a professional calibration begins.
What you need
- An analytical balance with readability suited to the volume. Smaller volumes need finer readability, down to 0.001 mg for the smallest pipettes
- An evaporation trap or a covered weighing vessel, because small volumes evaporate fast enough to skew the result
- Distilled or deionized water, left to reach room temperature
- A thermometer, barometer and hygrometer to record conditions
- New tips of the type the pipette normally uses
The procedure
- Let the pipette, tips and water sit in the room for at least two hours so everything is at the same temperature.
- Record the water temperature and air pressure.
- Set the pipette to the first test volume. ISO 8655 tests at the nominal (maximum) volume, 50% and 10%.
- Pre-wet the tip by aspirating and dispensing a few times.
- Dispense into the weighing vessel and record the weight. Repeat ten times at each volume.
- Convert each weight to volume using the Z-factor for your water temperature and air pressure, which corrects for water density and air buoyancy.
- Repeat for every test volume, and for every channel of a multichannel pipette.
The calculation
For each test volume, work out the mean volume and the standard deviation of the ten readings. From those come the two numbers that matter:
- Systematic error (accuracy): how far the mean volume is from the set volume, as a percentage
- Random error (precision): the coefficient of variation, the standard deviation divided by the mean, as a percentage
Compare both against the limits in ISO 8655 for that pipette size, or the manufacturer's tighter limits. If either is out, the pipette needs adjustment or repair, and then a full recheck.
In-house check or professional calibration?
Many labs run a short version of this between calibrations, with fewer replicates on their own balance, to catch a failing pipette early. That is good practice. A full calibration adds what surveyors look for: a balance and reference standards that are themselves calibrated and traceable to NIST, controlled conditions, adjustment and repair when needed, and a certificate with as-found and as-left results.
Our pipette calibration service handles the full procedure on-site or by mail-in. For intervals, read how often pipettes should be calibrated.