## 5 Conclusions  
Theoretical models developed to describe multiphase flows have been developed that have different assumptions requiring experimental verification for validation. For example, reliable data on velocity profiles in concentrated pulp suspensions during pipe flow is scarce in the literature. A multitude of techniques is now available, but a condition of non-disturbance of the flows is required in view of the complexity of multiphase systems. Non-invasiveness is therefore beneficial and is characteristic of some of these techniques. Laser Doppler Anemometry (LDA), Magnetic Resonance Imaging (MRI), Ultrasonic Velocity Profiling (UVP) and Electrical Tomography were reviewed and compared for their energy requirements, spatial and temporal resolution, duration of observation times, spatial and temporal coherence and stability, cost of equipment, and field and scale of applications. The compromise between the advantages and limitations of these techniques led us to focus on the Electric Tomographic techniques, the principles behind them, and their application in the visualisation of pulp suspension flows. Our efforts were directed towards designing proprietary, low-cost, portable Electric Impedance Tomographic (EIT) hardware. To circumvent the difficulties and costs of designing a current source that suits the goals of the EIT system being designed, the choice was made to move from traditional EIT systems and, instead, to design a voltage source. The EIT system depicted here can be used with 16-electrode or 32-electrode rings. Examples of _Eucalyptus_ and _Pine_ pulp suspension flows are reported here, demonstrating the influence of fibre concentration and type, as well as flow velocity, on the distribution of the fibres in the cross-sectional area of a horizontal pipe. The developed Electrical Impedance Tomography system was used to characterise the behaviour of pine and eucalyptus fibre suspension flow through a pipe at different fibre concentrations--1%, 1.5% and 2.35% (w/w)--and at varying flow velocities (between 0.5 and 3 m/s) based on the cross-section images obtained for totally developed flows. Especially for the pine fibre suspensions, it was clearly possible to identify a plug formation, which was probably due to the geometry of the pine fibres, which were longer than the eucalyptus ones. With respect to their usage for the estimation of suspension velocity, eucalyptus fibre suspensions were used in three different concentrations--1%, 2% and 3% (w/w)--and at velocities of between 0.25 and 1 m/s. The real suspension velocity measured by a flowmeter was quite reasonably estimated by the cross correlation of EIT images obtained in two consecutive electrode rings separated from one another by 300 mm. The developed EIT system allows 2D image reconstruction rates of up to 4000 or 1000 frames per second, depending on the number of electrodes used in the acquisition (16 or 32). With respect to the resolution of the obtained cross-section images, it is much higher than previously reported in the literature (which is evident by comparing Figure 3[53] and Figure 8). Thus far, the authors have used the developed equipment not only in the monitoring of fibre suspension flow and the estimation of fibre flow velocity, but also with great success in other diverse two-phase systems (solid-liquid[72], liquid-liquid[111], just to mention two). In the next step, the objective will be to upgrade the overall system (signal acquisition and image reconstruction algorithms) to visualise three-phase systems. Considering the results presented here, the usefulness of this technique and equipment is demonstrated and clearly justifies its use at an industrial scale, despite its lower spatial resolution when compared with other tomographic procedures. All authors contributed equally to this work. All authors have read and agreed to the published version of the manuscript.