Low-frequency measurement

The last part, following the method to analyse radioscopy and acoustic emission values, will be to correlate the characteristic values of the radioscopic detection of casting defects with extracted characteristic values of the acoustic emission analysis. The correlation between the time based characteristic values of acoustic emission analysis and the defect characterizing radioscopy values did not come to very satisfactory results referring the low frequency measurements. The reason can be found in the... 

Examples of hydrogen sensors utilizing thermoelectric power and low frequency measurements are discussed below. 

The terms polarizability constant and dielectric constant can be utilized interchangeably in the qualitative discussion of the magnitude of the dielectric constant. The k values obtained utilizing dc and low-frequency measurements are a summation of electronic E, atomic A, dipole P0, and interfacial /, polarizations. Only the contribution by electronic polarizations is evident at high frequencies. The variation of dielectric constant with frequency for a material having interfacial, dipole, atomic, and electronic polarization contributions is shown in Figure 6.1. 

There have been numerous investigations of DNA solution polarization using a variety of techniques of low frequency measurements. The problem is made difficult because of the appreciable ionic conductance of the solutions with or without added salt. The polarization currents 3P/3t, equal to iwP for an A.C. field, make increasingly small contributions to the total current... 

The term is often called the dynamic viscosity. It is an energetic dissipation term related to G"(o>) and has a value approaching that of the steady llow viscosity t] in very low frequency measurements on polymers which are not cross-linked. 

These results show that low frequency measurements on growing real crystal faces require a simultaneous in situ microscopic control of the number of screw dislocations and the surface topography in order to obtain reproducible impedance... 

Both extremes of frequency pose problems low-frequency measurements cannot be made on coatings in rational terms and high-frequency measurements tend to obscure the differences between liquid and solid states of matter. 

Ultrasonic techniques are so numerous that a comprehensive discussion is not possible. Since an ultrasonic wave is an adiabatic pressure wave, in general both a temperature and a pressure perturbation of the system occurs. In most nonaqueous solvents, the temperature perturbation is of primary importance, because chemical equilibria are generally much more sensitive to temperature changes than to pressure changes. In aqueous solutions, however, the pressure perturbation is usually of primary importance, because the thermal-expansion coefficient of water is very small, so that the pressure wave is almost isothermal. A serious disadvantage of ultrasonic methods is that rather large volumes of solution are required for low-frequency measurements and relatively high concentrations (> 10" M) of reactants are required at all frequencies. Recent experimental innovations have alleviated these problems to some extent. The most common ultrasonic... 

Figure 6 (a) Sample cell for high-frequency measurements of liquids (b) sample cell for low-frequency measurements of liquids, solids, and powders. (From Ref 38. With permission from American Institute of Physics.)... 

Figure 60.2 shows the velocity curves of butyl rubber as a function of frequency at different temperatures. The velocity increases very slowly with frequency at high temperatures where the values of the velocity are of the order of 40 m/s. The increase in velocity with frequency is much more rapid as the temperature is lowered, and at 0 °C the velocity is about 300 m/s. The corresponding modulus curves derived from the velocity curves is shown in Fig. 60.2. At all temperatures the modulus was seen to increase with frequency. From ultrasonic experiments on the same polymers, the data obtained at frequencies in the MHz range indicated a continuous rise in the modulus with frequency (measured up to 15 MHz). The dispersion over a limited frequency range can be attributed to a mechanism involving relaxation times of the order of l/w, whereas the entire dispersion range would have to be explained on the assumption of a wide distribution of relaxation times. The relaxation mechanism for these low frequency measurements...

In the studies made by others and by us it has been convenient to use a single frequency with varying the temperature. A low frequency such as 1 Hz is often chosen since secondary transitions and other structural features can be easily detected. Moreover, it is easier to relate the results obtained by using low frequency measurements, to the data obtained from other techniques. 

The time base of available storage oscilloscopes limits low-frequency measurements to about 10 Hz. High-frequency limitations are imposed by effects external to the oscilloscope, principally stray capacitance and transmission line effects in the leads and cell. Measurements can often be made at frequencies above 10 Hz. 

There are two electrode sensor geometries that are common in dielectric analysis. As noted previously, the traditional electrode is a parallel-plate capacitor (Fig. 47). However, this geometry is not well suited for thermoset cure monitoring. In order to obtain high signal levels with the classical parallel-plate technique, thin specimens with large areas are required, and the dielectric properties are an average of the bulk. In addition, low frequency measurements, which can reveal... 

In Figure 2.27, we see that the three impedances are similar at high frequencies, but are very different at very low frequencies. Measuring the... 

On analysis, the Raman spectra provide information equivalent to very low-frequency measurements, even lower than lOcrn Such low-frequency studies provide information on lattice vibrations. 

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