Transducing techniques

The potential of an electrode is related directly to the activities, and thus indirectly to the concentrations, of the chemical species involved in the equilibria that establish the potential. The main virtues of potentiometry are simplicity, very low power requirements, and the possibly small size of the sensors. The main drawback is that an unwanted reaction may enter into determination of the potential and sensitivity may be poor. Potentio-metric sensing, as a transducing technique, can be coupled with an infinite... 

Pandit and King (1982) and Bathe et al. (1984) presented measurements using transducer techniques, which are somewhat different from the accepted values of Kiefte et al. (1985). The reason for the discrepancy of the sonic velocity values from those in Table 2.8 and above is not fully understood. It should be noted that compressional velocity values can vary significantly depending on the hydrate composition and occupancy. This has been demonstrated by lattice-dynamics calculations, which showed that the adiabatic elastic moduli of methane hydrate is larger than that of a hypothetical empty hydrate lattice (Shpakov et al., 1998). 

A wide range of physical parameters are suitable for chemical sensing applications, consequently, there is a very wide variety of different transducers. Some examples of frequent transducing techniques are metal oxide semiconductor devices (MOS diodes and field effect transistors) relying e.g. on changes in electrical fields or opt(r)odes concerning optical phenomena such as absorbance and fluorescence, but also miniaturised capacities [1]. Mass-sensitive, or acoustic, devices constitute another very popular class of transducers. Within this chapter we will focus on this transducing technique and introduce its abihties and properties in combination with selective artificial interaction materials. 

One of the first biosensors using amperometry as the transducer technique was proposed by Clark [32] and has been commercialized by YeUow Springs Instruments. Here the receptor contains the enzyme glucose oxidase (GOD) providing the reaction ... 

Fiber-optic sensors are attractive options (but more expensive) for acquiring measurements in harsh environments such as high temperature or pressure. The transducing technique used by these sensors is optical and does not involve electrical signals, so they are immune to electromagnetic interference. Raman spectroscopy uses fiber-optics and involves pulsed light scattering by molecules. It has a wide variety of applications in process control (Dakin and Culshaw, 1997). 

The technique presented above has been extensively evaluated experimentally using ultrasonic data acquired from a test block made of cast stainless steel with cotirse material structure. Here we briefly present selected results obtained using two pressure wave transducers, with refraction angles of 45° and 0°. The -lOdB frequency ranges of the transducers were 1.4-2.8 MHz and 0.7-1.4 MHz, respectively. The ultrasonic response signals were sampled at a rate of 40 MHz, with a resolution of 8 bits, prior to computer processing. 

By employing this technique, the frequency range best suited for a particular material can be automatically estimated and utilized for inspection, without the need to employ a tailor-made transducer. Consequently, a single wide-band transducer can be used to get near-optimal inspection results for a wide range of materials. 

In order to ensure perpendicular beam incidence on the cylindrical specimen, the circular B-scan profiles were acquired by high frequency (narrow beam) transducers in a synthetic circular aperture array. From these profiles two-dimensional reflection tomograms were reconstructed using a filtered backprojection technique. Straight line propagation was assumed. Several artificial discontinuity types in a cylindrical Plexiglas (Perspex) specimen were compared with similar artificial discontinuities in a cylindrical A/Si-alloy [2]. Furthermore, examples of real discontinuities (an inclusion and a feed head) in the cylindrical AlSi-alloy are presented. 

Some discontinuities may be identified by a conventional two-dimensional ultrasonic technique, from which the well-known C-scan image is the most popular. The C-scan technique is relatively easy to implement and the results from several NDE studies have been very encouraging [1]. In the case of cylindrical specimens, a circular C-scan image is convenient to show discontinuity information. The circular C-scan image shows the peak amplitude of a back-scattered pulse received in the circular array. The axial scan direction is shown as a function of transducer position in the circular array. The circular C-scan image serves also as an initial step for choosing circular B-scan profiles. The latter provides a mapping between distance to the discontinuity and transducer position in the circular array. 

As reported before [Ref. 1], there are some essential parameters that influence the results of the testing, such as the thickness of the expired specimen, the quality and coarse grain of the built-in concrete, and the properties of the specimen-surface for the transducer s coupling. At the onset of testings none of tlrese parameters were available. As a result, we had to carry out preliminary investigations in order to prove the applicability of our testing-technique "in situ". 

Fig. 5, also an A-scan, shows the possibility of the echo-technique for concrete. The interface and backwall-echo of a 20 cm thick concrete specimen are displayed (RF-display). A HILL-SCAN 3041NF board and a broadband transducer (40mm element 0) are used which enable optimal pulse parameters in a range of 50 to 150 kHz. Remarkable for concrete inspections is the high signal-to-noise ratio of about 18 dB. 

Ultrasonic techniques are an obvious choice for measuring the wall thickness. In the pulse-echo method times between echoes from the outer and inner surface of the tube can be measured and the wall thickness may be calculated, when the ultrasonic velocity of the material is known. In the prototype a computer should capture the measuring data as well as calculate and pre.sent the results. First some fundamental questions was considered and verified by experiments concerning ultrasonic technique (Table I), equipment, transducers and demands for guidance of the tube. 

A variation on the transit time method is the frequency-difference or sing-around method. In this technique, pulses are transmitted between two pairs of diagonally mounted transducers. The receipt of a pulse is used to trigger the next pulse. Alternatively this can be done using one pair of transducers where each acts alternately as transmitter and receiver. The frequency of pulses in each loop is given by... 

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