= Positive-displacement pipette
A <positive-displacement pipette> transfers liquid with a piston that contacts the sample directly, avoiding the intermediate air cushion of an <air-displacement pipette>. In the handheld designs described by the manufacturer, the capillary and piston form a disposable assembly. The mechanism can reduce effects associated with the expansion or contraction of trapped air.<Gilson Guide to Pipetting>[second edition, pp. 8–11, positive-displacement construction and operation]{ref}
This distinction is useful when handling liquids whose <viscosity>, <density> or volatility differs substantially from water. The instrument still measures a <volume (thermodynamics)>: a selected $25\,\mathrm{\mu L}$ does not imply a fixed <mass> for every liquid. For an illustrative <density> of $1.20\,\mathrm{mg/\mu L}$, the corresponding mass is $30\,\mathrm{mg}$ by $m=\rho V$.
A gravimetric check therefore needs the appropriate <density> conversion. Direct displacement also does not make every delivery perfect: instrument condition, the fitted assembly and the test procedure remain relevant. Repeated measurements distinguish a systematic displacement of the mean from scatter in individual deliveries.<Gilson Guide to Pipetting>[pp. 46–49, performance assessment and gravimetric procedure]{ref}
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