Modern approaches to viscosity measurement

Edited by the Engineeringtalk editorial team Feb 6, 2006

Techniques for measuring the viscosity of fluid samples in the field have been available for many years - but many methods are awkward, time-consuming and unreliable.

Techniques for measuring the viscosity of fluid samples in the field have been available for many years.

Despite their longevity and widespread use, however, many of them are awkward, time-consuming and unreliable.

Newer technology is now providing technicians, researchers and operators with fast, reliable viscosity measurement that does away with the clumsy methods of the past.

Among the common "traditional" techniques are the "efflux cup", where viscosity is related to the time for a known volume of fluid to drain out of an accurately-sized hole at the base of a specially shaped container.

Alternatively, using the "falling ball" method, viscosity is related to the time required for a steel ball to fall a given distance through the fluid.

Clearly, this method cannot be used if the fluid is opaque.

In the "glass tube" capillary method, viscosity is related to the time for a given volume of fluid to flow from a reservoir through a specially shaped tube, such as the "Ubbelohde".

Although widely used, these "time of flight" methods can be seriously unreliable, especially at low viscosities, because of timing difficulties.

Accurate temperature control, essential for reliable results, is often lacking.

In their crudest forms, these methods depend very much on the skill and experience of the operator, which can make repeatability hard to achieve.

A further consideration is the practical inconvenience of having to ensure that the equipment is properly cleaned after use.

Other techniques include the rotational method and the "tuning fork" method.

In the first,viscosity is related to the angular velocity and torque when the fluid is held between a rotating element and a stationary surface.

In the second viscosity is related to the damping effect of the fluid on a two-pronged vibrating element.

Both these methods have drawbacks.

The "tuning fork" method is prone to errors resulting from reflection of the sound waves from nearby surfaces.

Rotational viscometers present a number of problems: they must be calibrated and used in a vessel of known dimensions; they require regular recalibration; they require a range of accessories, some of which are delicate and expensive; and the seals and bearings need periodic replacing.

As a result, the maintenance costs of rotational viscometers can be relatively high.

All the difficulties highlighted above can be overcome by using a torsional resonance instrument such as the Hydramotion Viscolite.

In this type of viscometer, shear stress is imparted to the fluid by a stainless steel rod oscillating at its natural resonant frequency with a twisting movement.

The drag force on the sensor dampens the oscillation, and the resulting energy loss gives a measure of viscosity.

As no sound field is produced the Viscolite can be used in any container, regardless of size or dimensions.

The smooth monolithic sensor can be wiped clean in seconds, and with no moving parts the Viscolite is virtually maintenance free.