154433Frequency and Data Acquisition

When using alternating current to measure the resistivity, the determination of the frequency is of great importance for accurately obtaining the resistivity of the specimen. If the frequency is too high, it will

FIGURE 15.15 The electrodeless resistivity apparatus.

lead to electrical radiation and some adverse effects such as Maxwell-Wager effect, viscous conduction effect, and their combined effect. These will influence the measurement of resistivity [29]. On the other hand, if the frequence is too low, there will be a very short period of direct current at that moment of the upper quarter and down quarter in sinusoidal current. Banthea et al. [30] and Lakshminarayanan et al. [31] recommended that 1 to 7 kHz was suitable for measuring the resistivity of high-resistance materials such as cementitious materials. In theory, the resistivity may be frequency dependent, but in a range of around 1 to 2 kHz the resistivity may be regarded as frequency independent. As a result, 1 kHz was employed as the measuring frequency of the electrodeless resistance apparatus.

The signals received were converted by an A/D-transient recorder card and stored on the hard disk of a computer. The whole electrodeless resistivity apparatus is illustrated in Figure 15.15.

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