Capacitance measurements bridge method

In the voltage method the normal voltage is applied across the capacitor terminals and line current, /, is measured. The value of capacitance, C, can be determined from the equations provided in Section 23.5.2. In the bridge method... 

A key factor in the suitabihty of cokes for graphite production is their isotropy as determined by the coefficient of thermal expansion. After the calcined coke was manufactured into graphite, the axial CTE values of the graphite test bars were determined using a capacitance bridge method over a temperature range of 25 to 100°C. The results are summarized in Table 24. Also included in the table are bulk density measurement of calcined cokes and the resistivity values of their graphites. 

The ionic conductivity of a solvent is of critical importance in its selection for an electrochemical application. There are a variety of DC and AC methods available for the measurement of ionic conductivity. In the case of ionic liquids, however, the vast majority of data in the literature have been collected by one of two AC techniques the impedance bridge method or the complex impedance method [40]. Both of these methods employ simple two-electrode cells to measure the impedance of the ionic liquid (Z). This impedance arises from resistive (R) and capacitive contributions (C), and can be described by Equation (3.6-1) ... 

Admittance measurements were used extensively prior to the widespread use of impedance spectroscopy in the 1980s. Capacitance bridge methods are typically used, though this limits the lower bound on the measured frequency to several hundred hertz. Corrosion processes, whose time constants are normally measured at or below 1 Hz, cannot be directly interrogated with this method. 

The application of dielectric constant techniques to thermophysical measurement of solids has been used for a number of years (114, 115). The early uses of the technique involved isothermal measurements employing bridge methods. Recently, techniques have been developed that permit the measurement of the dielectric constant of a solid as a function of temperature, in a manner similar to other TA techniques. Chiu (116) used the term dynamic electrothermal analysis (ETA) to describe the measurement of both the capacitance and the dissipation factor of polymeric samples. Nottenburget al (117) developed an automated technique that permitted the rapid determination of the dielectric properties of a substance over a wide range of temperature and frequencies. This technique, which was called dynamic dielectric analysis (DDA), was modified to measure concurrently the DTA curve of the sample as well (117, 118). This new technique was called dynamic dielectric analysis-differential thermal analysis, DDA-DTA,... 

The specimen may be a sheet of any size convenient to test, but should have uniform thickness. The test may be run at standard room temperature and humidity, or in special sets of conditions as desired. In any case, the specimens should be preconditioned to the set of conditions used. Electrodes are applied to opposite faces of the test specimen. The capacitance and dielectric loss are then measured by comparison or substitution methods in an electric bridge circuit. From these measurements and the dimensions of the specimen, dielectric constant and loss factor are computed. 

The differential capacity can be measured primarily with a capacity bridge, as originally proposed by W. Wien (see Section 5.5.3). The first precise experiments with this method were carried out by M. Proskurnin and A. N. Frumkin. D. C. Grahame perfected the apparatus, which employed a dropping mercury electrode located inside a spherical screen of platinized platinum. This platinum electrode has a high capacitance compared to a mercury drop and thus does not affect the meaurement, as the two capacitances are in series. The capacity component is measured for this system. As the flow rate of mercury is known, then the surface of the electrode A (square centimetres) is known at each instant ... 

The conductivity of a solution is measured using an AC bridge with a two-elec-trode conductance cell on one arm (Fig. 5.40(a)) a balance is sought, manually or automatically, by adjusting the variable resistance and capacitance in another arm of the bridge. Usually AC voltage of a few volts and 1 kHz is applied to the cell. The impedance caused by the double-layer capacity at the electrodes does not affect the measured values of conductivity. In some cases, the conductance is measured with a four-electrode cell, as shown in Fig. 5.40(b). For practical methods of measurement, see the reviews in Ref. [25],... 

A common method of level measurement is to use a capacitance bridge. A typical arrangement is shown in Fig. 6.31a in which the sensor consists of two concentric metal cylinders. In the case of a circular tank, the wall of the tank can be employed as the outer cylinder of the sensor. The capacitance of the sensor is ... 

There are, in general, two types of measurements. In the older and perhaps simpler method the capacitance of a condenser is determined when it is empty (or when it contains a standard substance) and when it is filled with the gas, liquid, solution, or solid. The various states require different sizes and types of condenser cells and measuring circuits, and errors arise from difficulties of maintaining uniform field on the condenser plates, balancing the bridge circuit and, especially for solids, filling the condenser uniformly. H bonding materials, however, present no unusual problems. This technique (most frequently... 

The methods used to plot impedance data began with plots of effective resistance and capacitance, reflecting the use of bridges for measurement. These plots gave... 

This operation determines the values of R and C that, in series, behave as the cell does at the measurement frequency. The impedance is measured as a function of the frequency of the ac source. The technique where the cell or electrode impedance is plotted V5. frequency is called electrochemical impedance spectroscopy (EIS). In modem practice, the impedance is usually measured with lock-in amplifiers or frequency-response analyzers, which are faster and more convenient than impedance bridges. Such approaches are introduced in Section 10.8. The job of theory is to interpret the equivalent resistance and capacitance values in terms of interfacial phenomena. The mean potential of the working electrode (the dc potential ) is simply the equilibrium potential determined by the ratio of oxidized and reduced forms of the couple. Measurements can be made at other potentials by preparing additional solutions with different concentration ratios. The faradaic impedance method, including EIS, is capable of high precision and is frequently used for the evaluation of heterogeneous charge-transfer parameters and for studies of double-layer structure. 

The test specimen, whose dielectric constant and loss factor are to be measured, is cut or molded to a suitable shape and thickness determined by the material specifications or the test method. The thickness of the specimen must be accurately measured. The electrodes are selected, based on convenience and whether or not the specimen must be conditioned at high temperature and high relative humidity. The test specimen with its attached electrodes is placed in a suitable measuring cell and its capacitance and a-c loss are measured using a suitable bridge. For routine work when either the highest accuracy is not required, or when neither terminal (of the specimen) is grounded, it is not necessary to place the solid specimen in a test cell. 

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