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When the sensor vibrates at its natural frequency, the the viscous damping of the fluid affects the response.

There are two primary methods used to derive the viscosity from this behaviour:

Amplitude techniques

Bandwidth Techniques

This first chart illustrates how amplitude varies with frequency and shows the amplitude at resonance.

Notice how the amplitude drops with an increase in viscosity.

There are two ways in which this can be used.

Some devices work by increasing the energy supplied to the resonator to maintain a constant amplitude at resonant frequency.

The viscosity is a function of this power.

Other technologies simply measure the amplitude change at resonant frequency.

Notice, however, that as the viscosity increases, the bandwidth increases. An increasing number of technologies are using bandwidth to determine viscosity.

Bandwidth devices can be said to be digital because they measure the frequency at the ½ power points, and not the amplitude which is analogue.

There are some valuable benefits from bandwidth measurement where some applications are concerned.

All these sensors measure the dynamic viscosity.

Notice that those which displace the fluid are sensitive to changes in the fluid density. Far from being a disadvantage, this is a very favourable point. By monitoring the change in resonant frequency with density, the density can be determined and hence kinematic viscosity can be calculated.

Other features:

Vibrational viscometers can vary from each other very significantly.

These factors are all important when evaluating a particular design for an application. One of the first obvious parameters is price. Vibrational viscometers can vary very significantly in price according to their capability and method of operation.

But to determine suitability these factors should be considered:

Calibration is important.

Some devices feature very basic algorithms to convert the amplitude or bandwidth to viscosity.

Others are more sophisticated and account for such factors as velocity of sound effects, flow effects, the non-linearity of the response, temperature effect on density (if measured).

The simplest of technologies are suitable for basic behavioural applications. At the other end of the spectrum are those that are used for analytical measurements where accuracy at all conditions is vital. Here it is not just the sensor and its calibration that is important but the additional algorithms necessary to support any of a number of different base viscosity methods.

Viscosity Technologies (cont.): Back

Tuning fork Vibrating element density/viscosity  process transmitter

LEMIS DC 52 vibrating cylinder viscometer - inline format.