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Viscosity transducers can reasonably be divided into two categories for clarity:

First generation technologies are those that have their origins in laboratory measurements.

Second generation technologies are those employing new methods and which are purposefully designed for the process environment.

A laboratory instrument is designed to be as close to the theoretical model as practical.

In the translation to a process instrument, design compromises are necessary that affect performance. Different designs achieve a different balance of these compromises.

The very different conditions encountered in a process environment, from those in the laboratory, require  careful appraisal when correlating process and laboratory results. This must account for the influences of the measurement environment.

There are many good references on the Internet to viscometer technologies and so they will only be mentioned here with a simple explanation.

First generation: represented by these principal technologies:

Capillary:

In the laboratory glass capillaries come in a wide variety of styles. Viscosity is usually a function of the flow-through time for a fixed volume of fluid.

In the process environment viscosity is more usually a function of the pressure drop across the capillary at a constant flow rate.

Not all process capillary viscometers for analytical measurement function in the same way. Some use a continuous flow of sample through the system while others draw a fixed volume of sample and cycle it several times through the capillary until a constant result is found.

Most behavioural process viscometers use a continuous flow.

Rotational

In a rotational viscometer a cylindrical bob is rotated within a concentric sleeve.

By varying the diameter of the bob and the gap between the bob and sleeve and by varying the rotational speed, a wide range of viscosities can be measured.

In the process instrument these variations and the height of the bob are exploited to enable a representative flow rate through the measuring cell.

The instrument is well suited to Newtonian and non-Newtonian fluids. By measuring at different rotational speeds the relationship between shear rate and viscosity can be established.

In the process environment a constant flow of fluid through the instrument enables a continuous measurement to be made but with non-Newtonian fluids the added effect of flow shear must be accounted for.

There are many variants on the principal such as rotating disc, cone and plate etc.

These instruments are often well suited to high temperature of operation.

Falling ball

In the Stokes law method the viscosity may be found from the terminal velocity of a ball falling through the fluid.

More usual is simply to measure the average time taken for a ball to fall from one end of a closed tube to the other.

In the process environment there are many  variations of this principal.

Second Generation:

Second generation devices are numerous but here only vibrational devices will be considered in any detail. When appropriate, comment will be added regarding some potentially mainstream technologies such as Ultrasonic.

Vibrational instruments are probably the most dominant new process technology to emerge but there are almost as many different variations as there are manufacturers.

Not all vibrational instruments are the same.

While there are no “good” or “bad” vibrational instruments, the choice of a particular instrument may be good or bad according to the application and the expectations.

In order to evaluate possible designs it is important to understand how they differ.

Vibrational sensors consist of a rod, fork, cylinder plate or tube vibrated at its resonant frequency.

The amplitude of the sensor is damped by the viscous forces of the fluid.

We can categorise vibrational sensors in two ways; by how they move and by how the measurement is derived.:

There are those which displace fluid and those which do not.

Forks and rods usually displace the fluid by their motion (right). These devices may be sensitive to flow effects and to density changes.

Those types which use a cylinder that oscillates, for example, are generally insensitive to direct flow  effects and are insensitive to density changes.

However, because of the flexible shaft, some oscillatory types may be sensitive to flow on the structure.

They may then be classified as those that use amplitude measurement and those that use bandwidth.                




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