FAQ1 ECT TECHNOLOGY

Electrical Capacitance Tomography (ECT) is a measurement technique which allows information about the spatial distribution of a mixture of dielectric materials inside a vessel to be obtained by measuring the electrical capacitances between sets of electrodes placed around its periphery and converting these measurements into an image showing the distribution of permittivity. Back to top

ECT can be used with non-conducting materials such as plastics, hydrocarbons, sand or glass and is often used with mixtures of two different dielectric materials, as the permittivity distribution corresponds to the concentration distribution in this case.

We are often asked whether it is possible to use ECT with water. There are two basic difficulties when using water. The first arises because although pure water is an insulator, only small amounts of impurities present in the water cause it to become conducting. The second problem results from the very high relative permittivity (80) of water, which causes major distortion of the electric field, lines inside the capacitance sensor. To summarise, special precautions must be observed if ECT is to be used successfully with water. Back to top

Where the mixture is flowing along the vessel, measurements of the concentration distributions at two separate axial planes allow the velocity profile and the overall flow rate of the fluid to be found.

Outline details about how ECT can be used to measure the flow of materials in 2-phase systems can be found by clicking here.

Background information on ECT can be found in papers by Huang (1989), Reinecke (1996), Loser (2001) and Byars (2001). Full details of these papers are given below.

Huang S.M. et al., (1989), Tomographic imaging of two-component flow using capacitance sensors, J. Phys. E: Sci. Instrum. 22, pp 173-177.

Reinecke N. and Mewes D., (1996), Recent Developments and industrial/research applications of capacitance tomography, Meas. Sci. Technol. 7 pp 233-246

Loser T., Wajman R. and Mewes D., (2001), New Reconstruction Algorithm for Electrical Capacitance Tomography, Proceedings of the 2nd World Congress on Industrial Process Tomography, Hannover, Germany, pp 20-28.

Byars M., (2001), Developments in Electrical Capacitance Tomography, Proceedings of the 2nd World Congress on Industrial Process Tomography, Hannover, Germany, pp 542-549.

You can view a copy of this last paper by clicking here.

A list of further papers on ECT technology can be viewed by clicking here.

An ECT system consists of a capacitance sensor, capacitance measurement circuitry and a control computer, as shown below, together with some suitable control software. Back to top

A Basic ECT System

For imaging a single vessel type with a fixed cross-section and with a fixed electrode configuration, the measurement circuitry can be integrated into the sensor and the measurement circuits can be connected directly to the sensor electrodes. This simplifies the measurement of inter-electrode capacitances and is potentially a good design solution for standardised industrial sensors. However, most current applications for ECT are in the research sector, where it is preferable to have a standard capacitance measuring unit which can be used with a wide range of sensors. In this case, screened cables connect the sensor to the measurement circuitry. Back to top

Yes, but most work to-date has used circular geometries. Back to top

The image resolution achievable depends on the number of independent capacitance measurements, but is generally low. To a first approximation, the angular resolution is equal to the number of electrodes located around the sensor periphery and the radial resolution is equal to the number of independent measurements divided by the angular resolution.

For example, for a 12-electrode sensor, the angular resolution will be around 30 degrees. As there are 66 possible independent capacitance measurements, the radial resolution will be around 20% of the sensor radius, or 10% of its diameter. Back to top

Images can be generated at high frame rates depending on the number of electrodes on the sensor. For example 100fps is achievable using a 12-electrode sensor, increasing to 140 fps for an 8-electrode sensor. Back to top

Last updated 23-05-2002

Copyright 2001 PTL