What is the Z factor in pipette calibration?

It converts the weighed mass of dispensed water into a volume, and it bundles together the density of water at its measured temperature, the density of the displaced air and the density of the balance's reference weights. It is not a fixed number — it changes with water temperature, barometric pressure and humidity — which is why a proper gravimetric calibration measures the water temperature at the point of use instead of assuming the room temperature.

Why does evaporation matter when calibrating a pipette?

Because it removes water from the weighing vessel while the balance is settling, and it removes roughly the same amount regardless of how much was dispensed. At 10 microlitres that loss is a meaningful fraction of the delivery, and because it always acts in the same direction it biases the mean rather than widening the scatter. The result is a pipette that appears to under-deliver. Controls include a narrow-necked or lidded vessel, raised humidity inside the balance chamber, a short and consistent weighing time, and measuring the evaporation rate explicitly at small volumes.

How many measurements does a pipette calibration need?

Ten deliveries at each test volume is the usual count, with volumes at the maximum, around the middle and at the minimum of the pipette's range. Ten is enough to give a usable estimate of the scatter, which is the point — a smaller number gives a mean but no reliable measure of random error, and random error is half of what the calibration is for.

Can a pipette be adjusted to correct its error?

Systematic error often can be: a pipette consistently delivering 1.5 % over nominal can usually be adjusted back. Random error cannot be adjusted away, because it is scatter rather than offset — its causes are a worn seal, a damaged tip cone, a sticking piston or inconsistent technique, and the fix is service or training rather than adjustment. This is why the two are reported separately.

How often should pipettes be calibrated?

Common practice runs from quarterly to annually depending on how heavily the pipette is used and what depends on it, with user checks in between. Frequency should follow the consequence of an undetected error and the pipette's own history, not a habit — a pipette used many times a day for a critical dilution earns a shorter interval than one used occasionally for buffer preparation.

Pipette calibration by the gravimetric method, under ISO 8655

Weighing the water a pipette delivers is simple arithmetic wrapped around several corrections people skip — the Z factor, evaporation, and the difference between systematic and random error.

Calibration by Parameter ValiTrac AI editorialUpdated 2026-09-133 min read

ShareLinkedIn X Email

A pipette is calibrated by dispensing water onto a balance and converting the mass to a volume. Everything difficult about it is in the conversion and in the handling, not in the arithmetic.

The method

Equilibrate the pipette, the water and the balance to the room; an hour is not excessive.
Use water of a known, stated purity, and measure its temperature at the point of use rather than assuming the room's.
Pre-wet the tip: several full aspirate-and-dispense cycles before any measurement, because the first delivery from a dry tip is systematically low.
Dispense into a weighing vessel, against the wall, with a consistent technique and a consistent delay.
Ten measurements at each volume is the usual count, at maximum, at about 50 % and at the minimum of the pipette's range.
Record the barometric pressure and the humidity if the Z factor is being taken from a table that uses them.

The Z factor

Mass becomes volume by multiplying by Z, which combines the density of water at its measured temperature, the density of the air displaced, and the density of the balance's reference weights. It is not a constant: over the range of ordinary laboratory temperatures it moves by enough to matter for a tight specification, which is why the water temperature is measured rather than assumed.

What goes into the conversion, and what happens if it is ignored
Input	Why it is there	Effect of ignoring it
Water temperature	Water density varies with temperature	A systematic volume error that tracks the season
Air buoyancy	The delivered water displaces air on the balance	A small consistent bias in the same direction every time
Evaporation	Water leaves the vessel during weighing	Every result biased low, worst at small volumes
Tip pre-wetting	A dry tip retains part of the first delivery	The first reading low, and the mean dragged with it

What goes into the conversion, and what happens if it is ignored

Evaporation is not a rounding error

At 10 microlitres, the water lost to evaporation while the balance settles can be a substantial fraction of the delivery. It biases every result in the same direction, so it does not average out and it does not show up as scatter — it shows up as a pipette that appears to under-deliver. The controls are a lidded or narrow-necked vessel, a trap of water inside the balance chamber to raise local humidity, a consistent and short weighing time, and at the smallest volumes an explicit evaporation measurement subtracted from the result.

Systematic and random error are different problems

ISO 8655 sets limits on both, and they are not interchangeable. Systematic error — the mean delivery differing from the nominal volume — is usually a calibration or adjustment matter, and a pipette that is consistently 1.5 % high can often be adjusted. Random error, the scatter between deliveries, is usually technique, a worn seal, a damaged tip cone or a sticking piston, and no adjustment fixes it.

A report that gives only a mean has hidden the random error entirely. A pipette whose mean is perfect and whose scatter is twice the permitted limit will pass that report and fail in use.

Technique is part of the instrument

Pipetting results depend on the operator to a degree that surprises people coming from other parameters: the angle of the tip, the depth of immersion, the speed of the plunger, the pause before withdrawal, whether the technique is forward or reverse. A calibration measures the pipette in the hands of whoever calibrated it. That is an argument for calibrating with the technique the laboratory actually uses, and for treating a large between-operator difference as a training finding rather than an instrument finding.

Frequently asked questions

What is the Z factor in pipette calibration?: It converts the weighed mass of dispensed water into a volume, and it bundles together the density of water at its measured temperature, the density of the displaced air and the density of the balance's reference weights. It is not a fixed number — it changes with water temperature, barometric pressure and humidity — which is why a proper gravimetric calibration measures the water temperature at the point of use instead of assuming the room temperature.
Why does evaporation matter when calibrating a pipette?: Because it removes water from the weighing vessel while the balance is settling, and it removes roughly the same amount regardless of how much was dispensed. At 10 microlitres that loss is a meaningful fraction of the delivery, and because it always acts in the same direction it biases the mean rather than widening the scatter. The result is a pipette that appears to under-deliver. Controls include a narrow-necked or lidded vessel, raised humidity inside the balance chamber, a short and consistent weighing time, and measuring the evaporation rate explicitly at small volumes.
How many measurements does a pipette calibration need?: Ten deliveries at each test volume is the usual count, with volumes at the maximum, around the middle and at the minimum of the pipette's range. Ten is enough to give a usable estimate of the scatter, which is the point — a smaller number gives a mean but no reliable measure of random error, and random error is half of what the calibration is for.
Can a pipette be adjusted to correct its error?: Systematic error often can be: a pipette consistently delivering 1.5 % over nominal can usually be adjusted back. Random error cannot be adjusted away, because it is scatter rather than offset — its causes are a worn seal, a damaged tip cone, a sticking piston or inconsistent technique, and the fix is service or training rather than adjustment. This is why the two are reported separately.
How often should pipettes be calibrated?: Common practice runs from quarterly to annually depending on how heavily the pipette is used and what depends on it, with user checks in between. Frequency should follow the consequence of an undetected error and the pipette's own history, not a habit — a pipette used many times a day for a critical dilution earns a shorter interval than one used occasionally for buffer preparation.

References

[1]ISO 8655 — Piston-operated volumetric apparatus (Parts 1 to 7)
[2]EURAMET cg-19 — Guidelines on the Determination of Uncertainty in Gravimetric Volume Calibration
[3]ISO 4787:2021 — Laboratory glassware: Volumetric instruments, methods for testing of capacity and for use
[4]ISO/IEC 17025:2017 — General requirements for the competence of testing and calibration laboratories
[5]JCGM 100:2008 — Evaluation of measurement data: Guide to the expression of uncertainty in measurement (GUM)

General technical guidance written against the cited sources. It is not regulatory or legal advice and does not replace the applicable standard, guideline or a qualified reviewer's judgement.

Have a question on this topic?

Ask ValiTrac AI and see the evidence and calculation behind the answer.

Ask ValiTrac AI

Free calculator: Calibration interval

Paste an instrument's as-found history and see how fast it is actually drifting, how much of its tolerance that consumes, and how long the evidence supports trusting it. The strongest input to an interval is the instrument's own record — this reads it.

Open the calculator

Calibration by Parameter

Balance and scale calibration: eccentricity, repeatability and minimum weight

What a proper weighing-instrument calibration covers beyond a few test weights — off-centre loading, repeatability at low load, and the minimum weight that decides what you may weigh at all.

2026-09-13 · 4 min read

Calibration

How to set calibration intervals

There is no universal interval. Set it from risk, manufacturer data and — above all — the instrument's own as-found history.

2026-09-13 · 3 min read

Metrology

Repeatability vs reproducibility

Repeatability holds everything constant except time; reproducibility changes operators, instruments, locations or days. The gap between them reveals hidden influences.