A viscometer (viscosimeter) is an instrument used to measure the viscosity of fluids.
For liquids with viscosities which vary with flow conditions, an instrument called a rheometer is used.
Viscometers only measure under one flow condition.
In general, either the fluid remains stationary and an object moves through it, or the object is stationary and the fluid moves past it.
The drag caused by relative motion of the fluid and a surface is a measure of the viscosity.
The flow conditions must have a sufficiently small value of Reynolds number for there to be laminar flow.
At 20.00 degrees Celsius the viscosity of water is 1.002 mPa·s and its kinematic viscosity (ratio of viscosity to density) is 1.0038 mm2/s.
These values are used for calibrating certain types of viscometer.
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 resonators 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.