## 4 Electrical Tomography Applications
### Pulp Suspensions Characterisation Using ET: The Authors' Approach  
#### 4.3.1 Overview of the Developed EIT System and of the Experimental Setup  
The authors of this paper have also been working on the improvement of a proprietary EIT system used in data acquisition of two-phase flows (solid/liquid). The reconstructed images aim to validate the CFD developed model [74; 112; 128], particularly when applied to the flow characterisation of pulp suspensions. To obtain improved sensitivity, the developed EIT system determines the differences in both the real and imaginary parts of the impedance over the domain. It is a portable device; consequently, it can be used in any location where 16 or 32 electrode planes have been installed on pipes. To avoid the cost and difficulties inherent in the construction of a current source, a voltage source was projected instead, a choice different from what is typical when designing electrical tomography apparatus. [PERSON] et al. [67], demonstrated that the use of a potential voltage difference instead of current in the domain excitation procedure made it possible to surpass the limitations found when studying more conductive media. The developed EIT system has already been described previously [74]; nevertheless, together with a block diagram of the system architecture and its main modules, presented in Figure 5, a brief description is given here. A set of input/output boards with USB ports (DAQ boards) are used for interfacing the EIT system with a personal computer (PC). Using the analogue input channels of the DAQ boards, the electrical potentials, resulting from each applied stimulus, are read. The developed  
Figure 4: Tomographic images of a liquid with a certain concentration, a liquor, flowing through a packed bed of HDPE pellets at steady flow conditions, obtained using a set of sensing electrode rings located sequentially along a pipe (the planes are numbered from the bottom, starting with number 1). A step reduction in liquid conductivity was made in the flow exiting the down corner. Images display the liquid concentration distribution at steady-state flow ratios of \(Q_{A}\)/\(Q_{R}\) of 0.5, 1.0 and 2.0. A total liquid flux of 75 mL/s was used for \(Q_{A}\)/\(Q_{R}\) of 0.5 and 2.0, and of 50 mL/s for \(Q_{A}\)/\(Q_{R}\) equal to 1.0. In the tomographic diagrams, low conductivity regions are coloured blue and high conductivity regions are coloured red (Reproduced with permission from [PERSON] et al. [126]). EIT system can acquire data at a rate that allows an image reconstruction rate of up to 4000 or 1000 frames per second, depending on the number of electrodes composing the sensing rings, either 16 or 32. To obtain the desired excitation signals, with the desired output amplitude and frequency (up to 25 MHz for this system), a programmable frequency voltage waveform was used. The authors also adopted an excitation strategy based on a voltage signal instead of a current, as has already been mentioned, as the use of this approach resulted in less noisy data [73]. In parallel with the generation of the stimulus signal, two others were also created, both with unitary magnitude, but where one is in phase with the excitation signal and the other is 90 degrees out of phase. To demodulate the read electrical potentials, each of them is multiplied simultaneously by both in phase and out of phase unitary produced signals; the signals outputted by the multipliers go through low pass filters, enabling only the constant terms to be extracted from them, which are the terms needed to calculate the impedance distribution in the domains. A set of analogue bidirectional multiplexers are used for routing of all signals: the excitation is directed to the desired electrode pair, and the read electric potentials to the multipliers. The digital outputs of the DAQ boards are used to address the multiplexers.