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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.

A microlitre, symbol $\mu \mathrm{L}$, is one millionth of a litre, a unit of volume. Since a litre is exactly one cubic decimetre,The prefix micro fixes the factor $10^{-6}$; it does not describe how accurately an instrument can deliver that volume.^[ref]

A $20\mu \mathrm{L}$ sample occupies $0.020\mathrm{mL}$, the same volume as twenty cubes each one millimetre on a side. Delivering $19\mu \mathrm{L}$ instead produces a 5% shortfall: a small absolute volume error can therefore matter greatly in a small-scale chemical reaction.

The microlitre is a volume unit, not a mass unit. To infer delivered volume from a balance measurement, use $V=m/\rho$ with the appropriate liquid density; careful calibration also accounts for effects such as air buoyancy and evaporation. Thus “one microlitre equals one milligram” is an approximation for water under particular conditions, not a unit identity. Gilson's calibration protocol illustrates repeated gravimetric measurements that separate systematic delivery error from scatter between deliveries.^[ref]

A laboratory micropipette is a pipette for transferring small measured quantities of liquid, commonly expressed in microlitres. In piston-operated models, the displacement mechanism matters. An air-displacement pipette leaves an air cushion between piston and sample, whereas a positive-displacement pipette places the piston directly against the liquid.^[ref]

Weighing a delivered sample provides a concrete check. In a simplified calculation that neglects air buoyancy and evaporation, liquid volume follows from mass $m$ and density $\rho$:For an assumed $\rho =1.000mg/\mu \mathrm{L}$, a delivered mass of $99.5\mathrm{mg}$ corresponds to $99.5\mu \mathrm{L}$. Relative to a nominal $100\mu \mathrm{L}$ setting, the deviation is $-0.5\%$. These are illustrative values, not calibration data.

A repeated series can distinguish a displaced mean from wide scatter: measurement accuracy and standard deviation describe different aspects of performance. Actual gravimetric checks account for temperature-dependent water density, air buoyancy and evaporation losses. The manufacturer's method uses a conversion factor and an evaporation correction, showing why a displayed volume alone does not establish the quantity delivered.^[ref]