## 3 Electrical Tomography Principles and Applications  
### Electrical Tomography  
Due to its low cost, portability and handling safety (because no harmful radiation is used), Electrical Impedance Tomography exhibits strong potential for use in several academic/industrial areas. It has a simple setup, and offers quick and easy operation, making it robust enough to be used in many industrial situations. As has already been mentioned, its main drawback is its low spatial resolution, which is usually confined to between 6% and 20% of the pipe radius [53]. However, with the development of more advanced algorithms for inverse problem resolution, there have been significant improvements with respect to increasing the achievable spatial resolution. With Electrical Tomography, both quantitative and qualitative data regarding multiphase systems can be obtained for modelling purposes. Non-invasive collection of data by electrical tomography enables the reconstruction of cross-section images, providing material distribution profiles in a suspension flowing through a pipe, or information about possible transient events taking place. The resulting conclusions can then be used in processes redesign or control. In some cases of the real-time imaging of industrial processes, electrical tomography is the most appealing technique, due to its simplicity and low cost. Electrical tomography can be divided in two procedures: Electrical Impedance Tomography, EIT (note that Electrical Resistance Tomography, ERT, can be seen has a particular case of EIT), and Electrical Capacitance Tomography, ECT. They both allow the generation of images that reflect the variation of the conductivity/resistivity or permittivity along the under-observation domain; due to its simplicity of implementation, ERT, which is ideal for pure resistive domains, is the most used [57]. When systems are heterogeneous, and composed of materials possessing diverse electrical characteristics, EIT and ECT can be used in the observation of changing processes; pneumatic and hydraulic transportation, mixing, cyclones, and fluidised beds are only some examples. The number of images generated by time interval is variable: it depends mainly on the electronics and structure of the acquisition system, on the measurement protocol used to gather the data, and on the reconstruction algorithm used for image building. Typically, increasing the data gathering speed would make it possible to obtain data about faster process, but consequently with a higher noise level and reduced image quality of the distribution profiles. Electrical tomography can be used on-line or off-line. For on-line measurements, poor spatial resolution images are usually obtained, consequence of the need to use either fast acquisition procedures or data processing algorithms that only require a short time. Conversely, for off-line measurements, the acquisition times can be customised to adapt to the changes taking place in the flow process, while the processing algorithms can be optimised to the highest spatial resolution possible, because the processing time can be longer. In addition to the reconstruction algorithm used, other features determine the quality of the obtained images, mainly physical ones, such as the number of electrodes, electrode diameter and material, spacing between them, etc. Because in some applications the aim is to obtain information about average quantities, such as void fraction or mean velocity, the developed model's validation requires images with high spatial resolution. In the following sections, the authors start by addressing the basic principles and operation of Electrical Impedance Tomography, followed by a short review of the use of Electrical Tomography for industrial/process monitoring purposes, with an emphasis on the pulp and paper area.