Viscosity is a critical measurement parameter for manufacturers and processors in the food and beverage industry.
Unlike many other common process measurements - such as density, boiling point, melting point, pH, conductivity and so on - viscosity is perceived as being particularly complex to measure and is thus not well understood by industry. This is in part because viscosity can be measured in two forms; dynamic or kinematic. Dynamic (or absolute) viscosity is generally measured in milliPascal seconds (mPas) or centi-Poise (cP), whilst kinematic viscosity is measured in centi-Stokes (cSt).
This article provides an introduction to the concept of viscosity and the major technologies available for on-line process viscosity measurement and offers guidance on the strengths and weaknesses of each, as well as practical applications advice.
The concept of viscosity
The concept of viscosity can be simply defined as a fluid's resistance to an applied shear force, ie, its resistance to flow. When this resistance is a constant irrespective of the shear force (under constant temperature and pressure conditions), then this type of fluid is called 'Newtonian' - an example being water. Fluids that change their resistance to flow when having a shear force applied to them are defined as being 'non-Newtonian' - an example being tomato sauce. Understanding the difference between these two is a critical factor in determining the correct viscosity measurement technology for a given application.
Viscosity measurement technologies
There are four common on-line methods for the determination of viscosity; rotational, capillary, time-of-flight and vibrational. Apart from the vibrational method, all of these were originally developed for the analysis of viscosity in the laboratory, and have since been extended to cover on-line applications.
Rotational viscometers
The principle of operation for rotational viscometers relies upon the liquid to be measured being sheared between two surfaces - one being stationary and the other rotating.
Typically, the flow rate of the fluid through the system is neglected for process measurements, however, doing this can cause problems if measurements are made at low shear rates, or the liquid is very shear sensitive. If either of these process conditions exists, the fluid flow rate may contribute significantly to the overall shear rate and thus considerably affect the measured value. This technology is particularly prevalent in applications where lower viscosity liquids are being used for example in the production of sauces and spreads eg, mayonnaise and tomato sauce.
Differential pressure capillary
The principle of operation is that a differential pressure is produced between the ends of a small diameter tube when a liquid flows through it under stable laminar flow conditions. This difference in pressure can be measured using a differential pressure cell, so that when the tube is contained in a temperature-controlled environment and a constant velocity pump is used to produce a constant liquid flow rate, very high accuracies can be achieved.
Such capillary tube meters were originally developed for the oil industry and are suited to low viscosity measurement, such as hydrocarbons. They are usually installed in a small, low flow rate sample loop, rather than directly in the process line, and consequently the system response time is slow when compared to true in-line systems. They also tend to be expensive to install.
Orifice capillary (funnels/cups)
These are gravity-based capillary viscometers with very short capillary lengths (sometimes just holes at the bottom of a container). They are low cost instruments, and as a result, are widely used for providing an indication of viscosity, but cannot provide accurate measurement. These viscometers are not strictly on-line instruments as they rely upon manual samples being taken and measured.
Time of flight
Time of flight viscometers measure the time taken for an element (piston, ball, fluid meniscus) to fall through a known distance. These are usually laboratory instruments as they provide high accuracies on Newtonian fluids.
On-line time of flight viscometers
These units measure the time taken for a solid body (piston, ball, etc) to fall through a fluid. In the piston design, a pneumatic cylinder raises the piston in preparation for the next measurement and pumps a new sample of liquid into the measurement tube at the same time. The piston then falls through the liquid and the time taken is measured. It is a pulsed measurement and is principally designed for clean liquids. It has the potential to be very accurate, however, its dependence upon a fixed piston/wall gap can limit the range over which viscosity can be measured. This principle is inherently less tolerant of particles and product build up compared to other techniques so it is not as commonly used in food processing as other technologies.
Vibrational
The principle of operation of a vibration viscometer is that a vibrating element is maintained in resonance whilst inserted into the liquid. The unit's response to the viscosity of the liquid is then calculated by using a related measurement such as damping or resonance width.
Vibrating fork technology
This technique is unique in the viscosity instrumentation market as both the fluid viscosity and density are accurately and independently measured at the same time. The continuous measurement is performed by determining the bandwidth and frequency of the vibrating fork resonance; the bandwidth gives the viscosity measurement whilst the frequency gives the liquid density. This makes it the only vibrational technology capable of measuring kinematic and dynamic viscosity on-line - which is particularly important in applications such as those found in fish or meat rendering plants where the percentage of solids in the mixture need to be controlled.
Kinematic viscosity (cSt) = dynamic viscosity (mPas)/density (g/cc).
The addition of a temperature element in this system allows the measurement of referred density, referred viscosity and viscosity gravity gradients.
Oscillating sphere
Oscillating sphere technology uses a stainless steel sphere that oscillates about its polar axis with a precisely controlled amplitude. The viscosity is then calculated according to the power required to maintain this predetermined amplitude of oscillation; the higher the viscosity, the larger the power consumption. While it is attractively simple in design, the oscillating sphere viscometer measures a function of viscosity that is density dependent (so it can only measure dynamic viscosity).
Vibrating rod
These viscometers determine the dynamic viscosity by measuring the damping of a resonator that is excited at its natural frequency in a torsion (or twisting) vibration. A constant power source vibrates the rod, and the amplitude variations are measured to determine the viscosity. Like the oscillating sphere, the vibrating rod cannot measure kinematic viscosity and also can be susceptible to noise problems, especially at high viscosities.
Making the choice
There is no panacea technology for viscosity measurement, but by answering the following questions, it is possible to determine the optimum solution for most applications.
Newtonian or non-Newtonian?
Orifice capillary viscometers offer a simple, low cost solution for Newtonian fluids, but they are not accurate. Oscillating sphere and vibrating fork/rod instruments can be used where continuous measurement of Newtonian fluids is required. Non-Newtonian, shear sensitive fluids are accurately measured using rotational instruments, although vibrational sensors can be used when only repeatability is required.
Consider exactly what measurement is required. What accuracy/repeatability is needed for the application? While accuracy of measurement is always desirable, repeatability is more important in process control applications. This can be achieved in most applications with most of the technologies described above.
However, in many process measurements the kinematic viscosity is a more useful measurement to determine the product quality as it relies upon both the (dynamic) viscosity and density of the fluid. Most in-line process viscometer technologies on the market measure the dynamic viscosity (or a function of it) except notably the vibrating fork that additionally measures the liquid density.
Capital cost versus maintenance issues and cost of ownership
Although the capital cost of a rotational viscometer is not excessive, the maintenance costs can be high. Vibrational viscometers can be more expensive to buy but they have no moving parts and are virtually maintenance-free.
What are the response time requirements?
Capillary tube viscometers have been used for process control applications however their system response time is usually much slower than other solutions. Vibrational and rotational viscometers that can be installed directly in the process line give a fast response.
Are solids present and/or are the fluids corrosive?
For fluids that contain particles, avoid on-line time of flight and capillary tube viscometers as they are not tolerant to product build-up. Vibrational viscometers are designed to self-clean when inserted into the process line. When using an optional PTFE coating, these instruments can further resist the effects of sensor fouling when solids are present.
Corrosion effects can also be reduced by the correct selection of sensor material. Typically, vibrational viscometers are offered in a choice of different materials.
Is it a hygienic application?
Strict regulations govern the use of instrumentation in the food and beverage industries. Hygienic standards such as 3A in the US and the European Hygienic Engineering & Design Group (EHEDG) standards are usually required.
Process conditions - pressure, temperature, flow rate
Most on-line viscometers have installation criteria that should be followed in order to obtain the best performance. One of the most neglected factors concerning process viscosity measurement is that of temperature. Many applications fail to insulate the viscometer thermally and its surrounding pipe work, with significant measurement errors resulting. Viscometers with integral temperature measurement provide a significant benefit when applied in process applications, as they allow greater measurement capability than just measuring the line viscosity alone.
Viscosity range
Care should be taken when assessing the process viscometer design as well as the instrument's maximum range. Capillary viscometers typically operate over a narrow viscosity range, and therefore a number of sensor tubes will be required for larger viscosity span. Vibrational and rotational viscometers tend to have at least a 10:1 turndown ratio on their measured viscosity range.
It has historically been difficult to measure process viscosity, and technologies that worked in one application have proven to be inconsistent in others. With the advent of the latest designs in vibrational fork viscometer technology, which allow the direct on-line measurement of both viscosity and density, the measurement of viscosity and the consistency from application to application is improving.
As with other parameters, the successful measurement of viscosity depends upon choosing the right technology for the job, installing it to the manufacturer's recommendations and following the correct operating procedures.
Although this article describes most of the traditional viscometers, other sensor technologies exist that have not been covered.
For more information contact Dave Rosser, DLM, 011 457 0500, [email protected],
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