Viscometer overcomes operational difficulties

Edited by the Engineeringtalk editorial team Apr 11, 2006

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

Viscosity is one of the most useful process control parameters, so it is worth investing in a reliable measurement system.

It is important to remember that most methods of measuring viscosity were originally devised for use in the laboratory, where conditions can be carefully controlled, and are not always suitable for process conditions, where the measurement environment is much more demanding.

The ideal process viscometer combines reliability, repeatability and speed of response with simplicity of installation and ease of maintenance.

Current process viscosity measurement techniques include: the rotational method, where viscosity is related to the angular velocity and torque when the fluid is held between a rotating element and a stationary surface; the pressure-drop or differential pressure method, where viscosity is related to the pressure drop across a small diameter capillary tube through which the fluid is forced to flow; the piston method, where viscosity is related to the time taken by an electromagnetically driven piston to travel a known distance through the fluid; and the tuning fork method, where viscosity is related to the damping effect of the fluid on a two-pronged vibrating element.

All these have disadvantages.

For example, readings can be affected by gas bubbles or solid particles, which may block small diameter tubes completely.

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

Capillary tube viscometers require constant maintenance, and can be awkward and costly to install, as they are generally fitted on a small, low flow-rate sample loop rather than directly into the process line.

Also, the response time of a capillary tube viscometer is too long to be useful for closed-loop control.

Several problems can occur with rotational viscometers: they must be calibrated and used in a vessel of known dimensions; the seals and bearings are potential weak points and wear out over time; any solid particles in the fluid may affect the readings; and they require regular recalibration.

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 XL7.

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.

A side-benefit of the oscillation is that the sensor is largely self-cleaning, and will resist fouling even further if coated with a non-stick material such as PTFE.

As the shear rate is high, the effect of external factors such as vibration or changes in fluid velocity will be negligible.

No compression or sound waves are produced, so the viscometer can be mounted in any orientation in any vessel, regardless of size.

The fit-and-forget unit is easy to install, gives fast, reliable results, and will operate continuously over long periods without requiring calibration or adjustment.