Two techs, one pipette, two different answers

Two technologists on the same bench use the same P200 air-displacement pipette to prepare replicate dilutions from the same stock. One tech's replicates cluster tightly. The other tech's replicates scatter over a wider range, and the mean is measurably lower than the volume the pipette was set to deliver. The pipette itself has not changed. Something about how each person is using it has changed the result.

Pipetting looks simple: press a plunger, draw up liquid, push it out. Underneath that simple motion are several variables, tip fit, immersion depth, aspiration speed, pause time, angle, and liquid properties, that each shift how much liquid actually leaves the tip. A pipette is a piece of test equipment. Like any test equipment, its output has to be checked against a known standard before you trust it for patient work.

You will look at a set of gravimetric replicates, calculate the delivered volume, mean, bias, and coefficient of variation, and use those numbers along with the laboratory's current criteria to decide whether a pipette is performing as expected or needs attention.

A pipette that looks fine and one that is drifting look identical until someone weighs what it actually delivers.