= Air-displacement pipette
An <air-displacement pipette> uses a piston to change the <pressure> and <volume (thermodynamics)> of an air cushion above a liquid sample. On piston withdrawal, reduced pressure allows ambient pressure to drive liquid into the tip; subsequent piston motion expels it. The piston itself remains separated from the sample.<Gilson Guide to Pipetting>[second edition, pp. 8–11, air-cushion mechanism and aspiration/dispensing sequence]{ref}
The air cushion explains why a mechanical setting alone does not establish the delivered volume. Its response can depend on <temperature> and sample properties, including volatility and <density>. A <positive-displacement pipette> uses direct piston contact instead of that intervening gas volume.<Gilson Guide to Pipetting>[pp. 9–12, contrast between mechanisms and influence of sample properties]{ref}
For an original repeated-delivery example, compare sets of $100\,\mathrm{\mu L}$ settings with measured mean deliveries of $98\,\mathrm{\mu L}$ and $100\,\mathrm{\mu L}$. Knowing those means alone does not identify which set has less scatter. Both the mean error and a dispersion measure such as <standard deviation> are needed to describe performance. A <microlitre> is an exact unit relationship; an individual <micropipette> delivery is an experimental measurement.
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