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Process Monitoring of Fluid Systems Based
on Ultrasonic Sensors
P. Hauptmann
Technical University "Otto von Guericke" Magdeburg, Sektion
A utomatisierungstechnik undElektrotechnik, 3010 Magdeburg, GDR
Abstract
The possibilities of the application of ultrasonic sensors for the process monitoring in fluids are described. As an example results of measure- ments during polymerisation processes are shown. The ultrasonic velocity v is the measuring parameter. A pulse travelling method is used. The fundamentals of uhrasound, necessary for the design of ultrasonic devices and the interpretation of the results, are summarised. A smart ultrasonic system .for process monitoring on the basis of velocity and attenuation measurements is explained. The limitations of the ultra-
sonic principle for industrial applications are shown.
1. INTRODUCTION
Various applications for ultrasonics in science, technology, medicine, and other fields have been known for a long time. The fact that considerable advances have been made in the last 10 years in the use of ultrasound, demonstrates its capabilities and usefulness. On the other hand, appropriate studies into the use of ultrasonics for monitoring industrial processes are scarcely known. Papadakis first summarised the most important results in this field in 1976. Although ultrasonic measuring methods were used in several areas of industrial endeavour at that time, they were restricted to characterising piezoelectric and magneto- strictive materials for use as ultrasonic generators and sensors, the investigation of ultrasonic delay lines produced by the electronics industry and for nondestructive testing/evaluation/ inspection. New possibilities for the application of ultrasonic methods for industrial purposes came about with the development of microcomputers. As well as new sensors based on semiconductor materials or fibre optics the so-called classical transducers were regenerated to renaissance as sensors. Ultrasonic sensors are included in the latter category and can work as condition sensors or position-monitoring sensors.
In the following a novel principle of ultrasonic sensor application for process monitoring in liquid homogenous or heterogeneous systems is presented. As an example, results taken during polymerisation processes are shown. The technique applied is illustrated and its limitations are shown. It can be expected that many other applications of the same technique are possible. In connection with other sensors more detailed information about the process investigated will be given along with how process control can be achieved.
2. FUNDAMENTALS
The complete study of elastic travelling waves can be subsumed under two questions: 1. How
fast do they travel?, 2. How fast do they decay? The engineer wants to know how to use these properties. Therefore he has to study the ultrasonic velocity and attenuation rates for different materials.
Ultrasonic velocity and attenuation are defined by the equation:
p = p(x, t) = p0*e^j(wt-kx) (1)
for an ultrasonic wave propagation in the x-direction with a complex wave number kx and an angular frequency o9 = 2nf t is the time and p is the sonic alternating amplitude. The complex wave number kx can be written in the form:
k x = (w/v - m) (2)
where v the phase velocity of the sonic wave and a~the attenuation coefficient. These are the measurable quantities and their values are dependent on the material being used and its structure. Measurements of v and ＆ can provide interesting information about processes or the properties of particular materials. The phase velocity v yields the appropriate elastic modulus M of the mode being propagated. The relationship is: v = (M/a) 1/2 (3)
a is the unperturbed density. For fluids the modulus M is the bulk modulus Kand the waves are longitudinal. For solids, M is an appropriate combination of the elastic moduli of the solid itself. The combination depends on the mode of propagation, and the mode in turn depends on the interaction (or lack of interaction) of the wave with the boundaries of the solid. In isotropic solids as well as a longitudinal wave with：
(4)
a transverse wave also appears with a propagating velocity 
(5)
where G is the shear modulus. The moduli of materials are influenced by many physical phenomena, which may, in turn, be studied by measuring the ultrasonic wave velocities.
The attenuation coefficient yields information about absorption and diffusion processes occuring in the specimen. It depends in a relatively complex manner on the material properties of the transmitting medium.
For a simple, homegenous liquid the following equation applies:
(6)
where 1/s is the she~ir viscosity, k is the thermal conductivity and ;( is the ratio of the specific heats at i.e. at constant volume and constant pressure. Relax appears in the case of relaxation processes and this contribution can be much higher than the viscous part (determined by r/s) and the thermal part (determined by k). If relaxational effects are absent it is obvious that the attenuation increases with the square of the frequency. This behaviour is very important for many technological applications and has to be taken into consideration in industrial equip- ment. Because in liquids the ratio X is nearly 1 the second term in equation plays an unimportant role. Very often heterogeneous systems occur in technical processes. They are complicated multiphase or multicomponent systems and can appear in the form of suspensions or emulsions and sometimes contain partially dissolved gases. The sonic parameters depend, in a complex way, on the material parameters of the different phases or components. When there are diffusion centres in the system (i.e~ such as solid spheres or gas bubbles) then the following equation can be used to determine the attenuation.
(7)
R is the radius of the solid spheres or gas bubbles. A very high attenuation may occur at higher frequencies (for particles in the Ixm-range this happens at frequencies higher than 10MHz). Under such conditions measurement under industrial conditions can be impossible. In gases, the thermal effects (second term in equation (6)) are not negligible and a very high attenuation appears in the ultrasonic frequency range. This can be very troublesome for measurements in heterogeneous systems. In solids, except for many polymers, the attenuation is in general relatively small. But, if there are diffusing centres or boundary surfaces, a considerable attenuation which depends on frequency in a complete way can occur. The previously described behaviour of the different phases can enable conclusions to be made about the most useful frequency range for the technological applications of ultrasound. For solids and liquids this range is from 500kHz to 5MHz, for gases it is from I kHz to 100kHz.
The acoustic impedance, defined as av, plays a dominant role in the description of sound propagation in different materials and the sound behaviour at the specimen/bond/ transducer interface. In the case of a perpendicular incident of a sound wave at the boundary of two materials with Z~ = ~r I v t and Z2 = a2v2 the following equations can be used to calculate the intensity reflexion coefficient R and the intensity transmission coefficient T 
A knowledge of this behaviour is necessary for all methods when working in the echo mode. Travelling waves are generally introduced into the specimen by piezoelectric transducers which effectively are the sensor elements. At this time ferroelectric ceramics are most common but sometimes, for special applications the classical material quartz is used. In new development polymer foils from PVDF or PVF 2 plan an interesting role and appropriate sensor construction can be realised with these polymers. The most important characteristics of the ultrasonic sensors so described are their resonant frequency, band width and the efficiency of the conversion of the electrical signal into the mechanical displacement and amplitude of the sound wave. The last property can be influenced, not only by the sensor material used but also by different attenuation materials on the reverse side of an ultrasonic sensor. At frequencies lower than 1 MHz the structure of the sound field has to be taken into consideration.
3. INDUSTRIAL APPLICATIONS
Measuring methods
For process monitoring pulse methods are mostly applied. An electrical signal interacts with the transduction devices (the ultrasonic sensor) to produce a mechanical displacement. This mechanical wave is introduced into the specimen or the process medium either immediately or after some delay which can be encountered in an intermediate propagation medium. Echoes from the specimen may be received by the same transducer for the pulse-echo operating method, or the signal passing through the specimen may be received by another transducer in the through-transmission operating mode. Using equations (1) and (2) the following is valid: 
x is the path length travelled by the ultrasonic wave in the medium and t the corresponding time for it so to pass. Uo and U are the voltage amplitudes sensed at positions x = 0 (Uo) and x (U) and are proportional to the sonic alternating pressure p.
The velocity measurement is reduced to a pure time measurement if the fixed distance of the transducers is known. Such a time measurement can relatively easily be obtained with high accuracy (up to 10-5). A digital signal can be obtained and on-line measurement is possible.The determination of the ultrasonic attenuation during processes is more difficult when a method with fixed transducer distance is used, as is common in industrial applications. Then changes of the impedance of the medium influence the amplitude of the receiver signal. If these changes are unknown the attenuation investigation gives incorrect values. Therefore, only inaccurate relative conclusions can be made. Very often methods with variable transducer distances are applied in laboratories and then the problem does not exist but the precision of measurement is not great. It is for this reason that velocity measurement is used in most cases for technological applications.
Known Industrial Applications of Ultrasound
There are two major fields of industrial application of ultrasound: (a) non-destructive testing/evaluation/inspection, and (b) process on line monitoring. On line process control allows the making of continuous corrections if the monitored parameter shows deviations from the desired value. The parameter of interest may not be measured directly; often another parameter functionally related to it allows more convenient measurement, i.e. measurement of ultrasonic propagation velocity or attenuation. In many industrial processes a knowledge of the flow velocity is important. For this aim ultrasonic Doppler Flowmeters are very often applied (Figure l(a)) and they are reliable and accurate devices.19] They have also found wide application in medicine. In the last few years more and more correlation techniques have been introduced for industrial applications. Ultrasound plays an important role as a sensor in these cores (Figure l(d)). On the basis of the cross correlation function the flow velocity in liquids can be determined. ~l~ Two other types, the vortex shedding type (Figure I(c)) and the sing-around version (wave transit-time differences) (Figure 1 (b)) use also ultrasonics to detect flow dependant phenomena. A new ultrasonic sensor system for high sensitive flow measurement is described in it31 Ultrasonic transducers on the basis ofinterdigital structures and a new double-looked-loop principle for the detection are applied. Very often ultrasonic sensors are applied for liquid level measurements in tanks. These methods are applicable not only for liquids.ill] Several methods are known in which the concentration in liquid binary systems are determined on line. This is possible when the ultrasonic velocity is a strong function of concentration in liquid mixtures, emulsions or solutions. The chemical, food or pharmaceutical industry can use this principle. In gaseous systems ultrasonic sensors are applied as distance sensors for robotics or as temperature sensors for combustion processes. As passive sensors ultrasonic transducers act by acoustic emission. Many other examples could be cited. A summary is given in[12]
Ultrasonics for Polymerisation Monitoring
Polymerising systems are complicated and can be heterogeneous multicomponent or multiphase. The ultrasonic parameters give overall information about the state of such systems. Although the velocity reacts very sensitively to material or phase changes this information is unspecific, If one wants to get a quantitative statement on an interesting value, for instance the conversion, the components of the system have to be investigated in detail in order to understand their influence or to find empirical relationship. Detailed results from investigations during several polymerisations are given in Ir ~61. To demonstrate the ability of ultrasonic sensors an example of the Polyvinylacetate-polymerisation will be shown in the following.
If Figure 2 the results of different kinds of VinylAcetate (VAc)-polymerisations are shown. Every state of polymerisation is characterised by a definite c-value. The different slope of the curves is determined by the influence of the components responsible for the polymerisation, Although it seems that no unambiguous conclusions can be made the experiments have shown that this might be possible. If the influences of the major components are known quantitatively, then predictions are possible. For semicontinuous polymerisation as shown in Figure 2 the following relation between c and the components of the polymerising system is valid.
v o is the velocity of the dispersion medium, c L is the limit concentration of the solvated monomer, c~ is the total used monomer concentration, Cp is the initiator concentration, c s is the stabiliser concentration and cE is the concentration of added electrolytes./x v/Z~c are the corresponding concentration coefficients. If the concentration coefficients, especially the
temperature dependence of the different components are known, Cp or cMcan be determined immediately by computer. For other kinds of VAc-polymerisations or other systems, such as Vinylchloride, Styrene-Butydiene, modified algorithms or empirical relationships have
been obtained.
In Figure 3 a schematic diagram of the laboratory equipment used is shown. Two piezoceramic transducers were mounted at the bottom of a laboratory reactor at a distance of 4.5cm. The velocity was measured using pulse travelling technique at a frequency of I MHz and with a precision of 1 part in 104. The pulse width was lps. In addition to the travelling time the temperature of the medium was also measured. The ultrasonic velocity corrected by the temperature coefficient was calculated using an 8 bit-microcomputer. From this value the corresponding process parameters as the conversion cp or others can be determined on the basis of an available algorithm for the medium investigated. This information is obtained immediately which is the great advantage of this principle. In this way the process can be monitored and controlled. In the equipment used, shown in Figure 3, the microcomputer controls the temperature and the dosage of additional to the reaction.
