Pipette Calibration and Performance Checks Explained

Pipette calibration measures how much liquid a pipette actually delivers compared with its set volume, usually by weighing dispensed water and converting mass to volume. Performance checks are shorter in-house tests between calibrations that confirm the pipette is still within its acceptance limits.

Pipettes sit behind almost every result in PCR, ELISA, microbiology and chemistry. A drifting pipette can produce errors that are small, consistent and hidden inside normal-looking results, and both ISO 15189:2022 and ISO/IEC 17025:2017 expect equipment that affects results to be calibrated or checked with traceability.

What ISO 8655 covers

ISO 8655 is the international standard series for piston-operated volumetric apparatus: air-displacement and positive-displacement pipettes, burettes, dilutors and dispensers. The series was revised in 2022, so check that you are working to the current edition.

Its parts cover terminology and general requirements, requirements for piston pipettes including maximum permissible errors, a gravimetric reference procedure and alternative measurement procedures. Manufacturers also publish their own specifications, which may be tighter. Your laboratory decides which limits apply and documents that decision.

The gravimetric method in principle

The gravimetric method weighs water dispensed by the pipette and converts the mass into volume. In outline:

1. Let the pipette, tips and water equilibrate to room temperature in a draught-free area.
2. Use distilled or deionised water of suitable quality.
3. Use a balance with resolution suited to the volume; small volumes need a semi-micro or micro balance and evaporation control.
4. Record water temperature, air pressure and, ideally, humidity.
5. Dispense and weigh replicates at defined test volumes, for adjustable pipettes typically nominal, 50% and 10% of nominal.
6. Convert each mass to volume with the Z-factor, then calculate the mean, systematic error and random error.

The Z-factor (in µL/mg) corrects for the density of water at the measured temperature and for air buoyancy at the measured pressure. At 20 °C and standard atmospheric pressure it is about 1.0029 µL/mg, rising slightly with temperature. Take values from the table in ISO 8655 or validated software rather than using one fixed number all year.

Accuracy vs precision: systematic and random error

A pipette can be precise but inaccurate: every dispense is 9.7 µL instead of 10 µL. That usually points to mis-adjustment or wear. Poor precision more often points to technique, a damaged tip cone, an air leak or poorly fitting tips.

In-house checks vs accredited calibration

Accredited calibration is carried out by a calibration laboratory accredited to ISO/IEC 17025 for pipette calibration. The certificate reports results with measurement uncertainty and traceability, and the provider can usually adjust or repair the pipette.

In-house performance checks are shorter gravimetric checks by trained staff, for example a few replicates at one or two volumes. They do not replace calibration unless your procedure meets the gravimetric requirements and your balance, thermometer and barometer are themselves calibrated. Most laboratories use both.

These frequencies are examples, not requirements. Set your own and justify them.

Setting frequency based on use and risk

A risk-based interval considers:

• Impact of the volume: dispensing template into a quantitative assay matters more than adding wash buffer.
• Frequency of use and number of users
• Volume range: small volumes are less forgiving of wear and technique
• Liquid type: volatile, viscous or corrosive liquids stress seals
• History: previous drift, failures and repairs

Review intervals using your own results. Pipettes that consistently pass may justify longer intervals; any that drift need shorter ones, and the rationale should be recorded.

Technique errors that look like pipette faults

• Angle: hold the pipette close to vertical when aspirating; tilting changes the volume drawn.
• Pre-wetting: aspirate and dispense a few times before the first measured volume to equilibrate the air cushion.
• Immersion depth: just below the surface; too deep coats the tip outside, too shallow draws air.
• Tip fit: use tips recommended for the pipette, and calibrate with the tip type you actually use.
• Rhythm: smooth, consistent plunger movement with a brief pause after aspiration.
• Temperature: warm hands and cold reagents both change delivered volume.
• Viscous or foaming liquids: consider reverse pipetting where your method allows.

Before blaming the pipette, ask a second trained person to repeat the check with fresh tips.

Worked example: a drifting 10 µL pipette in a Dubai PCR lab

A molecular laboratory in Dubai checks its small-volume pipettes quarterly. An analyst dispenses ten replicates at 10 µL from a pipette used to add template to a quantitative viral load assay. The room is at 22 °C, where the Z-factor is about 1.0033 µL/mg.

The mean mass is 9.680 mg, giving a mean volume of 9.712 µL. The systematic error is (9.712 - 10) / 10 × 100 = -2.9%. The SD is 0.105 µL, so the CV is about 1.1%. Last quarter's check showed -0.3%. A second analyst repeats the check and gets -2.8%, so technique is ruled out.

The result is outside the laboratory's acceptance limit, which is based on the manufacturer's specification. The pipette is labelled out of service. QA reviews internal QC since the last satisfactory check: controls were within limits with no shift, so no results are recalled, and the impact assessment is documented.

A service engineer finds a worn piston seal. After repair, an accredited provider calibrates the pipette and it returns to service. As a corrective action, the lab moves pipettes used in quantitative assays to monthly checks.

Common mistakes

• Treating a calibration sticker as proof the pipette is fine today
• Ignoring temperature and pressure, or using one Z-factor all year
• Using a balance with too little resolution for small volumes
• Checking only the nominal volume of adjustable pipettes
• Calibrating with different tips from those used in routine work
• Returning a dropped or repaired pipette to use without calibration
• Failing to assess the impact on results after a failed check

Key takeaway: Calibrate gravimetrically to ISO 8655, check in-house between calibrations at a risk-based frequency, and treat every failure as a trigger to assess the impact on results.

Frequently Asked Questions

How often should pipettes be calibrated?

There is no single interval that fits every laboratory. Many laboratories calibrate every 6 to 12 months and carry out in-house checks more often, with the interval based on how critical the pipette is, how heavily it is used and its history. Document the rationale and confirm expectations with your accreditation body.

What is the Z-factor in pipette calibration?

The Z-factor converts the mass of dispensed water into volume, correcting for water density at the test temperature and for air buoyancy at the measured pressure. It is expressed in µL/mg and is about 1.0029 µL/mg at 20 °C and standard atmospheric pressure. Values come from tables in ISO 8655.

What is the difference between pipette accuracy and precision?

Accuracy, or systematic error, is how close the mean delivered volume is to the set volume. Precision, or random error, is how closely repeated dispenses agree, usually expressed as SD or CV%. A pipette can be precise but inaccurate, which usually points to adjustment or wear rather than technique.

Can pipettes be calibrated in-house?

Yes, provided the procedure follows the gravimetric principles of ISO 8655, staff are trained, and the balance, thermometer and barometer are calibrated with traceability. You also need to estimate measurement uncertainty. Many laboratories do routine checks in-house and use an accredited provider for full calibration, adjustment and repair.

What water is used for gravimetric pipette calibration?

Distilled or deionised water of suitable laboratory grade is used, equilibrated to room temperature along with the pipette and tips. Its temperature must be measured because water density, and therefore the Z-factor, changes with temperature. Check the current edition of ISO 8655 for the specified water quality.

Next steps

To build calibration and equipment control into a wider quality system, see our ISO/IEC 17025 Advanced: Laboratory Competence & Accreditation course, or PCR & Molecular Diagnostics Fundamentals if you work at the molecular bench. For related reading, see calibration vs verification in laboratories, measurement uncertainty in testing labs and internal quality control in medical labs. For help choosing, speak to a counsellor.