Electrical impedance, transducer measurements

Piezoelectric Transducers Certain ciystals produce a potential difference between their surfaces when stressed in appropriate directions. Piezoelectric pressure transducers generate a potential difference proportional to a pressure-generated stress. Because of the extremely high electrical impedance of piezoelectric crystals at low frequency, these transducers are usually not suitable for measurement of static process pressures. 

Similarly, we can observe a fundamental vibration of about 500 kHz (about 2 ms) and a beat vibration of about 100 kHz (about 10 ms) for the conventional ultrasonic transducer in Figure 33.17. It was considered that this beat vibration was based on the difference (about 105 kHz) between the two peak frequencies at about 471 kHz and 576 kHz shown by the black arrows in Figure 33.16. Figure 33.18 shows the measurement results of the electric impedance of the aerogel ultrasonic transducer. The results of acoustic measurements in Figures 33.15 and 33.16 and the electrical measurement results in Figure 33.18 were compared. 

The electrical impedance method, in which complex voltage/current ratios are measured without the need to determine absolute values of force, displacement, or velocity, can also be applied to soft solids the Fitzgerald transducer apparatus, - which uses shear sandwich geometry, is an example which provides considerable versatility and precision. 

In another type of measurement, the parallel between mechanical and electrical networks can be exploited by using variable capacitors and resistors to balance the impedance of the transducer circuit. These electrical measurements readily lend themselves to computer interfacing for data acquisition and analysis. 

Electrical Variables. Included here are those variables which are measured as the product of a process, as in the case of measuring die current and voltage of a generator, and also as part of an instrumentation system. Numerous transducers, of course, yield electrical signals that represent by inference some other variable quantity, such as a temperature or pressure. Variables in this class include electromotive force, electric current, resistance, conductance, inductance, capacitance, and impedance. 

The change of the electrical properties, related to the surrounding atmosphere, can be transduced as a change of resistance, impedance or work function. The easiest measurable parameter is the sensor resistance in DC conditions. It may be measured by a voltamperometric technique at constant bias but, in commercial chemical sensors, the sensing film is usually inserted inside a voltage divider. 

The definition of the power of ultrasonic equipment is rarely obvious to the chemist. Often, it can be described as the consumption from the electricity supply. However, there can be significant differences between the power supplied from the mains and that delivered into the reactor. Most chemists accept that a satisfactory value is the acoustic power introduced within the reacting medium, which requires physical or chemical methods for its measurement. A relative measurement (from the transducer assembly into the medium) can be linked to the displacement amplitude of the emitter. However, as referred to above, if the amplitude is maintained at the same level during a reaction, a modification of the physical system, e.g., viscosity, temperature, etc., changes the acoustic impedance and the power is not the same. Constant power into the system can only be obtained when the amplitude can be varied, making this parameter unsuitable for power measurements. 

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