Figure 5. Ultrasonic pulse-echo technique for determining the thickness of layers in multilayer materials.

4.3. Foreign body detection

Undesirable foreign materials such as pieces of metal, glass, wood, plastic or other debris may contaminate foods during processing. Many foods are optically opaque and so methods which utilize light cannot be used. If an ultrasonic pulse is propagated into a sample it will be reflected from any boundaries it encounters, providing there is a large enough difference in acoustic impedance between the food and the foreign body, which is usually the case. The distance of the foreign body from the surface of the can is determined by measuring the time-of-flight of ultrasonic pulses reflected from the foreign body and from the can wall (Figure 6): d2=di-t2/ti. By moving an ultrasonic transducer around the sample it is possible to determine the size and location of the foreign body [11]. This technique is a simple example of the imaging techniques used in medicine to determine the health and sex of fetus' in the womb.

4.4. Flow rate measurements

Measurement of the flow rate of materials through pipes during processing is important in many areas of the food industry. There are a number of different types of ultrasonic sensor available which can be used to measure the flow rate of liquids, the three most important being those based on transit time, Doppler and cross-correlation measurements [11]. Ultrasonic flow meters are capable of determining flow rates up to a few meters per second, on systems which have dimensions ranging from less than a millimeter (i.e., blood flow in veins) to greater than a kilometer (i.e., flow of water in rivers).

Transit time. The transit time for a pulse of ultrasound to travel a distance d through a static fluid is given by the equation: t = d/c, where c is the velocity of ultrasound in the fluid. If the fluid is flowing with a velocity V the transit time will be modified. When the ultrasonic wave

Figure 6. Detection of a foreign body in a can using the ultrasonic pulse-echo experiment: di=cti, d2=ct2.

travels in the same direction as the fluid is moving the overall velocity is increased (c+V), and the transit time is reduced t\=d/(c+V). If the ultrasonic wave travels in the opposite direction to the fluid its overall velocity is decreased (c-V), and the transit time is increased t2=d/(c-V). The velocity of the fluid can therefore be determined by rearranging these two equations:

The determination of flow rate is therefore independent of the ultrasonic velocity of the fluid, which is important as this may vary with the composition of the fluid or the temperature. To make measurements upstream and downstream the transducers can be placed across a bend in a pipe, or fixed at an angle to one another [11], In the latter case it is necessary to correct the measurements for the angle.

Doppler. Doppler flow meters measure the frequency shift which occurs when an ultrasonic wave is reflected from a moving object [7]. The frequency shift is related to the velocity of the particles and so their flow rate can be determined [11]. In the food industry these objects may be particles suspended in the fluid or density fluctuations in the liquid due to flow. Cross correlation. Cross correlation flow-meters also rely on the presence of inhomogeneities in the fluid to determine the flow rate. Two transducers are fixed to a pipe at a known distance d apart, and the pulses reflected from the flowing liquid are measured. The signals received from the transducers are compared using a technique called cross-correlation which looks for similar patterns in the two signals corresponding to reflections from the same particle (or set of particles) passing by the different transducers. This permits the time interval t for the inhomogenitity to travel between the transducers to be calculated: V = d/t.

The ultrasonic flow meters described above measure the average flow velocity of the fluid, in practice there will be a flow profile across the pipe. More sophisticated flow meters have been developed which can be used to measure flow profiles [11]. It should also be pointed out that the Doppler and cross-correlation flow meters measure the flow of the particles which may be different from that of the fluid itself.

4.5. Temperature measurements

The ultrasonic properties of materials are sensitive to temperature and so ultrasound can be used to provide information about temperature. Ultrasonic thermometers have been developed which consist of a rod of material with a piece of another material of different acoustic impedance bonded to the end (Figure 7). An ultrasonic pulse propagating along the rod is partly reflected and partly transmitted at the boundary between the two materials. The reflected part travels back to the transducer, whilst the transmitted part propagates through the end-piece before it travels back to the transducer. The difference in time (t) between the two echoes is the time it takes the pulse to travel twice the length (d) of the end-piece. This time depends on the ultrasonic velocity of the end-piece material and its length, both of which vary with temperature. The thermometer is calibrated by measuring t in a series of liquids at known temperature. Careful design of the thermometer is needed to avoid interference from side waD reflections, and reverberations in the end-piece.

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