Electrical Capacitance Tomography (ECT)

Electrical capacitance tomography is known as the other extreme of electrical impedance tomography, whereby it is the measurement of the dielectric constant. This was one of the techniques which were first developed for process tomography and the capacitance sensors are now widely used for industrial two-component flow measurement. It is a low cost, high speed, robust and non-intrusive type of tomography technique (Yang, 1996).

The basic technique of ECT can be described by considering a parallel plate capacitor. The capacitance between these plates is dependent upon the dielectric permittivity, the area of the plates and the distance between those plates (Xie et al., 1992). Normally, the sensors are in the form of electrodes which are symmetrically mounted on the outside of an insulating pipe or vessel. As shown in the equation below, if the area of the plate is known, as well as the distance between them, so effectively by measuring the capacitance; we are effectively measuring the dielectric constant.

Electrical Capacitance Tomography obtains information about the spatial distribution of a mixture of dielectric materials inside a vessel, by measuring the electrical capacitances between sets of electrodes placed around its periphery and converting them into an image showing the distribution of permittivity (Malcolm Byars, 2001). The measured values or data will be manipulated to reconstruct the cross sectional image of the pipeline by computer programming.

There are two types of measurement circuits suitable for ECT. There are the charge/discharge circuit and the AC based circuit, due to their immunity to stray capacitance. As shown by Yang (2003), the main difference between these two types of circuits lies in the positions of the demodulators. 

The sensing electronics then takes measurements for all possible combination pairs of electrodes. Specifically for capacitance tomography measurements, the surface of the electrode needs to be sufficiently large to provide sufficient signal.

ECT systems can be used to obtain images of the distribution of permittivity inside ECT sensors for any arbitrary mixture of different dielectric materials. It has been developed to image industrial processes containing dielectric materials, for example, gas/oil flows in oil pipelines, gas/solid flows in pneumatic conveyors and fluidization processes in fluidized beds. ECT is most successful when applied to materials such as oils, plastics, dry powders and under favorable circumstances pure water, all of which have low electrical conductivity.