Evolution of Electrical Capacitance Volume Tomography
Electrical Capacitance Tomography (ECT) is a unique instrumentation technique that can quantify and map multi-phase flows in industrial processes by measuring the electrical capacitance between plates distributed around the periphery. Measured quantities include volume fraction, velocity, mass flow rates, and spatial distribution of each phase. They also include imaging of phases, velocimetry mapping, multi-phase decomposition, and flow pattern identification.
ECT has passed through different development stages since its inception, this page highlights the different acronyms used to distinguish each stage:
ECT
This is the basic form of Capacitance Tomography and involves symmetric distribution of capacitive plates around the periphery. In ECT, the plates are typically maintained consistent in length to minimize variation of the electric field in the axial direction. As the 3D variation is inherent in any capacitive plates due to fringing, it is viewed as a source of error in ECT and strategies are followed to mitigate it. Because the electric field in ECT is restricted to 2D variations, the reconstructed images are also two-dimensional.
3D ECT
A 3D tomography technology is typically based on assembling several slices (tomograms) into a 3D formation to represent the imaged body in three-dimensions. Each slice is acquired independently in two-dimensions. In this approach, stacking provides the third required dimension. In ECT, this strategy is followed to form 3D ECT images. However, due to the substantial length of capacitance plates in ECT, stacking results in a very poor resolution in the axial direction in addition to poor coordination between plates from one stack to the other. Stacking in this case can either be performed by mounting ECT sensors as independent rings vertically, or by using one ECT sensor in a dynamic flow scenario where each time frame serves as a new stack.
ECVT solves the poor resolution of 3D ECT by introducing sensors that are designed to emphasize the 3D distribution of the electric field. Unlike ECT, where electric field variation in the axial direction is viewed as a source of noise, ECVT aims to increase the 3D variation of the sensor to utilize it for direct 3D imaging. Direct 3D imaging refers here to reconstruction of capacitance data that has 3D field variations (i.e. ECVT sensors), into a volume image by bypassing the stacking method in 3D ECT. This approach relaxes the symmetry requirement in ECT and enables capacitance plates to be formed in various shapes and in different arrangements. The ECVT sensor flexibility allows the design of sensors to fit virtually any geometry.
Adaptive Electrical Capacitance Volume Tomography
With the introduction of ECVT, the axial resolution of a 3D reconstructed images was greatly improved and a flexibility of design was introduced. However, the overall resolution of the ECVT image was still limited by the maximum number of capacitance plates that can be used in each ECVT sensor. Because each capacitance plate is required to have a substantial surface area to maintain signal quality, the maximum number of capacitance plates that can cover the periphery of the column is also limited. AECVT was introduced to overcome this limitation by forming synthetic plates from small capacitive segments. The surface area of each synthetic plate is large enough to guarantee a good signal quality. Additionally, synthetic plates can be formed in different shapes and can be electronically shifted in small increments. Capacitive segments that form each synthetic plate can also be activated by different voltage levels. The combination of all those factors enables the acquisition of measurements that are orders of magnitude higher than conventional ECVT. By relaxing the limitation on number of measurements inherent in ECT and ECVT, AECVT is able to develop into a high spatial resolution tomography technology similar to MRI and CAT scans. Moreover, the ability to activate different capacitive segments with different voltage levels allows real-time zooming into different regions in the imaging domain.