【正文】 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 105). A digital signal can be obtained and online measurement is determination of the ultrasonic attenuation during processes is more difficult when a method with fixed transducer distance is used, as is mon 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 UltrasoundThere are two major fields of industrial application of ultrasound: (a) nondestructive 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, . 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 ] 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 singaround version (wave transittime 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 doublelookedloop 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 ] 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 bustion 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 MonitoringPolymerising systems are plicated and can be heterogeneous multiponent 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 ponents 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 Polyvinylacetatepolymerisation 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 cvalue. The different slope of the curves is determined by the influence of the ponents 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 ponents are known quantitatively, then predictions are possible. For semicontinuous polymerisation as shown in Figure 2 the following relation between c and the ponents 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 ponents are known, Cp or cMcan be determined immediately by puter. For other kinds of VAcpolymerisations or other systems, such as Vinylchloride, StyreneButydiene, 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 . 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 bitmicroputer. From this value the corresponding process parameters as the conversion cp or others can be determined on the basis of an