A viscometer (also called viscosimeter) is an instrument used to measure the viscosity and flow parameters of a fluid. The common Brookfield-type viscometer determines the required force for rotating a disk in the fluid at known speed. Other viscometer types use bubbles, balls or other objects. Viscometers that can measure fluids with high viscosity or molten polymers are usually called rheometer or plastometer.
A newer class of vibrational viscometers operates by measuring the damping of an oscillating electromechanical resonator immersed in the fluid whose viscosity is to be determined. The resonator generally oscillates in torsion or transversely (as a cantilever beam or tuning fork). The higher the viscosity, the larger the damping imposed on the resonator. The resonator's damping may be measured by one of several methods:
- Measuring the power input necessary to keep the oscillator vibrating at a constant amplitude. The higher the viscosity, the more power is needed to maintain the amplitude of oscillation.
- Measuring the decay time of the oscillation once the excitation is switched off. The higher the viscosity, the faster the signal decays.
- Measuring the frequency of the resonator as a function of phase angle between excitation and response waveforms. The higher the viscosity, the larger the frequency change for a given phase change.