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A rheometer imposes a controlled stress or deformation and measures the complementary response. In a rotational instrument, a sample fills a gap between surfaces such as concentric cylinders, parallel disks or a cone and plate. The directly controlled or measured quantities include torque, angular displacement and angular velocity; the geometry converts these into shear stress, shear strain and shear rate.^[ref]

For example, a small-angle cone-and-plate rheometer produces approximately uniform shear rate $\dot{\gamma }=\Omega /\alpha$ and shear stress $\tau =3\mathcal{T}/(2\pi R^3)$, with cone angle $\alpha$, radius $R$, rotation rate $\Omega$ and measured torque $\mathcal{T}$. A parallel-plate rheometer instead has a radius-dependent shear rate. The conversion therefore requires the appropriate flow model. Wall slip, edge failure, temperature drift and the instrument's own inertia can mimic a material response, especially for very weak or rapidly varying signals.