A viscosity sensor is a device to measure the viscosity of a fluid. For liquids with viscosities which vary with flow conditions, an instrument called a rheometer is used. A viscometer only measure under one flow condition. Generally speaking, to measure the the viscosity, the flow conditions must have a sufficiently small value of Reynolds number for there to be laminar flow.
Standard laboratory viscometers for liquids include U-tube viscometers and Falling sphere viscometers. Commercially available viscometers include Falling Piston Viscometer, Oscillating Piston Viscometer, Vibrational viscometers, Rotational viscometers (Stabinger viscometer and Stormer viscometer), and Bubble viscometer.
Common types of shear rheometer include Pipe or Capillary, Rotational cylinder, Cone and plate, and Linear Shear. Commercially-available extensional rheometers are Rheotens, CaBER, FiSER, Sentmanat, Acoustic, Falling Plate, and Capillary/Contraction Flow.