Dielectric Spectroscopy: Choosing the Right Approach - Pharmaceutical Technology

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Dielectric Spectroscopy: Choosing the Right Approach
This tutorial paper is meant to aid in dielectric-sensor selection


Pharmaceutical Technology
Volume 9, Issue 32, pp. 8293

Through-field or fringing-field sensing techniques are typically chosen on the basis of the following: only one sided access to sample is available, significant variations in thickness exist, and the sensor must operate in an electrically noisy environment. To determine whether through-field or fringing-field is desired for an application, access to the material of interest must first be determined. If two-sided access is possible and the sample thickness does not vary significantly, through-field is recommended. If only one-sided access is available or if the sample thickness varies significantly, then a fringing-field measurement is recommended. To obtain an effective measurement with either sensor type, a basic knowledge of the system is important. The nominal thickness of the sample should be known as well as the primary sample constituents and the likelihood of metallic impurities.

A common electrode arrangement for through field imaging is seen in electrical tomography (ET). In this method, the electrodes are distributed around the edges of a cylindrical surface. Individual electrodes are electrically excited and the current or voltage induced on other electrodes is measured. The measured values are used to reconstruct distribution of physical properties in the material by solving the inverse problem. ET is used for biomedical, geophysics, and industrial process control applications (20–22). Compared with other imaging methods such as X-ray tomography and magnetic resonance imaging, ET is fast, noninvasive, and inexpensive in instrumentation. Although ET is useful for measurements in cylindrical surfaces, it is not viable for scenarios in which one-sided measurements are required.

In the case of fringing-field measurements, a common example is referred to as a fringing electric field (FEF) sensor. An FEF sensor can perform three distinct measurement functions depending on how it is configured: defined penetration depth measurements, multiple penetration depth measurements, and one-sided measurements.

In the case of the FEF sensor, the field is nonlinear and often has smaller signal strength than that of the through-field configuration. However, the field is concentrated much closer to the electrodes and therefore has a defined penetration depth. Penetration depth depends primarily upon how close the electrodes are to each other.


Figure 2 (ALL FIGURES ARE COURTESY OF THE AUTHORS.)
Dielectric spectroscopy can be performed with FEF sensors such that only the layer closest to the electrodes is examined. By altering the geometry of the electrodes or by selecting the electrodes that are being excited and those being measured, the penetration of the field can be progressively moved further into the material. By using multiple channel devices, various channels can be used to measure dielectric properties of the material at different penetration depths (see Figure 2).


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