Rotationsviskosimeter

Rotationsviskosimeter, or rotational viscometer, is an instrument used to measure the viscosity of a fluid by applying controlled rotational motion and monitoring the resulting resistance. The sample is placed between a rotating element (spindle, cone, or cylinder) and a stationary element. By varying the rotational speed and recording the torque required to maintain it, the device derives the fluid’s resistance to flow in terms of viscosity.

Viscosity in rotational viscometry is defined as the ratio of shear stress to shear rate. In practice, the shear rate depends on geometry and speed, and viscosity is obtained from torque, speed, and calibration constants. Most devices report dynamic viscosity in Pascal-seconds (Pa·s) or centipoise (cP); temperature is tightly controlled because viscosity is strongly temperature dependent. Many instruments support multiple geometries to span wide ranges.

Common geometries include concentric cylinders (Couette), cone-and-plate, and parallel-plate configurations. Instrument calibration with standard fluids and routines per international standards (ISO, ASTM) ensure repeatability and comparability of results. Suitable sample volumes vary by geometry and viscosity.

Applications cover many industries, including lubricants, paints and coatings, food products, cosmetics, and polymers. The technique is capable of characterizing Newtonian fluids as well as non-Newtonian, shear-thinning or shear-thickening behavior, and can produce flow curves by recording viscosity at different shear rates. Limitations include thixotropy, volatility, and sensitivity to temperature and air entrainment.