 Air-displacement pipette

An air-displacement pipette uses a piston to change the pressure and volume 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.^[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.^[ref]

For an original repeated-delivery example, compare sets of $100\mu \mathrm{L}$ settings with measured mean deliveries of $98\mu \mathrm{L}$ and $100\mu \mathrm{L}$. 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.