Mass Balance and Volume
Review pipette identity, selected volume, tips, test method, environment, balance, results, uncertainty and decision rule before trusting a dispensed volume.
Direct answer
A calibration check label does not set up that every quantity brought with the aid of a pipette is suitable for every method. Consider the result only after confirming the pipette configuration, decided on test volumes, recommendations, measurement approach, environmental controls, dimension uncertainty and attractiveness requirement.
ISO 8655-2:2022 covers metrological requirements and most permissible errors for particular piston pipettes. ISO 8655-6:2022 specifies a gravimetric reference measurement method for figuring out quantity. The technique applies to the entire device, consisting of the simple equipment and the selected disposable or reusable components involved in transport or uptake.
Health for cause comes first: a pipette can conform to one specification and nevertheless be flawed for a laboratory approach that requires a smaller uncertainty or a extraordinary running quantity.Calibration, routine testing and adjustment
| Activity | Purpose | What it does not prove by itself |
|---|---|---|
| Calibration | Establishes size results and related uncertainty under stated situations | Automatic conformity with every user method |
| Routine test | Checks performance between calibrations using a defined user procedure | Replacement for a complete calibration or an uncertainty evaluation |
| Adjustment | Changes the pipette so its indications or delivered volumes meet a target | The as-found condition before the change |
| Verification | Checks whether specified requirements are met | The measurement relationship unless calibration data support it |
If an adjustment is made, retain as-found results and obtain as-left results. As-found data support any review of laboratory work completed since the previous acceptable check.
Identify the complete pipetting system
The pipette body is only one part of the measurement. Record the configuration used for calibration and for routine work:
- Manufacturer, model, serial number and asset number.
- Air-displacement or positive-displacement design.
- Single-channel or multi-channel configuration.
- Nominal range and each selected test volume.
- Tip type, part number or other essential consumable component.
- Any interchangeable head, channel, adaptor or reusable part.
- Delivery or contained-volume mode relevant to the method.
For a multi-channel pipette, confirm whether each channel was evaluated and how channel-specific results are reported. A result for one channel cannot be assumed to describe all channels.
Confirm the measurement method
In a gravimetric procedure, dispensed liquid is weighed and the observed mass is converted to volume using the controlled calculation defined by the method. The method must address the liquid, balance, environmental observations, evaporation and the factors used in the mass-to-volume conversion.
Ask the calibration provider to identify:
- The standard or controlled procedure used.
- The selected volumes and number of observations.
- The tip pre-wetting and operating sequence required by the method.
- The aspiration and dispensing technique applied.
- The balance and other reference equipment used.
- The environmental quantities recorded.
- The calculation model and uncertainty evaluation.
Do not combine results from different methods as if they are automatically equivalent. ISO 8655 includes gravimetric, alternative and photometric procedures with defined areas of application.
Control the balance, liquid and environment
| Control | Evidence to review | Possible effect |
|---|---|---|
| Balance | Calibration status, resolution, repeatability, capacity and suitability for the dispensed mass | Weak weighing capability increases uncertainty and can hide small volume errors |
| Test liquid | Identity, condition and temperature information required by the method | Mass-to-volume conversion depends on the liquid properties used |
| Air conditions | Temperature, humidity and pressure observations required by the method | Influences evaporation, density corrections and air-displacement behaviour |
| Evaporation control | Method-defined timing, vessel and evaporation assessment | Mass loss can bias a small-volume result |
| Operator technique | Training, consistent timing, immersion, angle and plunger operation | Technique can change both mean delivery and repeatability |
| Tips and seals | Compatible components, seating, condition and leak checks | Air leakage or inconsistent fit can change delivered volume |
These controls are not a replacement for the selected standard. They are the evidence a user should expect the controlled method and uncertainty budget to address where significant.
Read systematic and random effects separately
A set of repeated deliveries supports two different questions:
- Systematic effect: how far the mean delivered volume is from the selected volume.
- Random effect: how much the individual deliveries vary under repeatability conditions.
A common expression for the mean volume error is:
e = Vmean - Vs
where Vmean is the mean measured volume and Vs is the selected volume. The sign convention and reported terms must follow the calibration procedure.
Standard deviation or coefficient of variation can describe dispersion, but the reported statistic, units and calculation basis must be clear. A small mean error does not compensate for poor repeatability, and good repeatability does not establish a small mean error.
Apply the correct acceptance requirement
Before calibration, identify which requirement controls the decision. It may be:
- The applicable maximum permissible errors in ISO 8655-2.
- A manufacturer specification for the exact pipette and configuration.
- A validated laboratory-method tolerance.
- A user performance tolerance based on fitness for intended use.
- A regulatory or customer specification.
ISO 8655-10:2024 provides user guidance, requirements for competence and training, and an approach to performance tolerances and testing for intended use. This helps separate standardized pipette requirements from a user's method-specific suitability decision.
If a conformity statement is required, agree the decision rule and how uncertainty is considered. A measured value, its uncertainty and the acceptance rule belong together.
Review the calibration certificate
- Confirm the pipette identity, type, range and channel configuration.
- Check the tip or consumable configuration used.
- Identify the method and whether volume was delivered or contained.
- Review each selected volume and the individual or summarized observations.
- Check mean volume, systematic error and random-error result.
- Confirm the reported measurement uncertainty and coverage information.
- Identify the acceptance limits and decision rule for any pass or fail statement.
- Determine whether the results are as found, as left or both.
- Check the service location and whether it is covered by the current accredited scope.
Build a useful routine check
A routine check should be short enough to perform consistently and strong enough to detect a change that matters. Define the test volume, tip, liquid, balance, environment, operating sequence, number of observations, control limits and action when a result is outside the limit.
Trend the result over time using the same controlled procedure. Investigate a shift before automatically adjusting the pipette. Possible causes include pipette condition, tip fit, operator technique, liquid temperature, balance performance, evaporation or a change in the test process.
After repair or adjustment, repeat the required calibration or verification and document the new status before returning the pipette to use.
Primary references
- ISO 8655-2:2022, Piston-operated volumetric apparatus, Part 2: Pipettes
- ISO 8655-6:2022, Gravimetric reference measurement procedure for the determination of volume
- ISO 8655-10:2024, User guidance, competence, training and suitability
- JCGM 200:2012, International Vocabulary of Metrology
- ILAC policy documents on metrological traceability and calibration uncertainty
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