FAQ4 - Capacitance sensors for ECT

If the vessel wall is non-conducting, the electrodes can be located inside the vessel wall, embedded within the wall or located external to the wall as shown in the figures below. However, if the tube wall is a conductor, the electrodes must be located inside the conducting wall and insulated from it, as shown in the first figure (internal electrodes). Back to top

Possible ECT sensor electrode locations

If the vessel wall is non-conducting, internal, embedded or external electrodes can be used. In general, ECT sensors with external electrodes are easier to design and fabricate than sensors with internal electrode sensors and they are also non-invasive, which gives ECT an important advantage over many other imaging techniques. However, if the tube wall is electrically-conducting (usually metallic), the electrodes must be located inside the wall. Back to top

The number of sensor electrodes that can be used depends on the range of values of inter-electrode capacitances and the upper and lower measurement limits of the capacitance measurement circuit. Practical ECT sensors tend to have between 6 and 16 electrodes located around the periphery of the sensor. Back to top

The convention we use to identify electrodes is to number them anticlockwise, starting at the electrode at or just before 3 o'clock. Back to top

As the number of electrodes increases, the electrode surface area per unit axial length decreases and the inter-electrode capacitances also decrease. When the smallest of these capacitances (for opposite electrodes), reaches the lowest value that can be measured reliably by the capacitance circuitry, the number of electrodes, and hence the image resolution, can only be increased further by increasing the axial lengths of the electrodes. However, these lengths cannot be increased indefinitely because the standing capacitances between pairs of adjacent electrodes will also increase and the measurement circuitry will saturate or overload once the highest capacitance measurement threshold is exceeded. See also Q7. Back to top

Axial resolution and overall measurement sensitivity can be improved by the use of driven axial guard electrodes, located either side of the measurement electrodes, as shown in the flexible laminate design illustrated in the figure below. These driven guard electrodes are excited at the same electrical potentials as the associated measurement electrodes and prevent the electric field from being diverted to earth at the ends of the measurement electrodes. For large diameter vessels, axial guard electrodes are normally an essential requirement to ensure that the capacitances between opposing electrodes are measurable.Back to top

Electrode arrangement for a single-plane sensor with driven guard electrodes.

Simple electric field simulations show that the sum of the lengths of the axial guard and the measurement electrodes must equal or exceed the diameter of the sensor to ensure that the electric field across the sensor is reasonably constant and is not diverted to earth in the measurement region and that the capacitances between opposite electrodes remain measurable. Sensor sensitivity can be further improved by increasing the electrode lengths up to twice the sensor diameter, although the axial resolution of the sensor will decrease.Back to top

The capacitance measurement limit equates to measurement electrodes of minimum axial length around 3.5cm for an 8 electrode sensor or 5 cm for a 12 electrode sensor. These dimensions assume that effective driven axial guards are used, as described in the response to Q7. Back to top

The easiest method for constructing ECT electrodes is to use CAD drawing software to produce a master drawing and to use this to fabricate the electrode array from a flexible copper-coated laminate using photolithographic and copper-etching techniques. The electrode foil is then wrapped around the outside of an insulating tube to form the required sensor electrode array. Back to top

Part of a design for an 8-electrode single plane sensor with driven axial guards is illustrated in the figure shown in response to Q6 above and repeated here for convenience. This shows earthed screening tracks between the sets of electrodes (to reduce the adjacent electrode capacitances) together with earthed areas at the ends of the sensor (to allow the screens of the connecting cables to be terminated). Back to top

Electrode arrangement for a single-plane sensor with driven guard electrodes.

The usual method is to us coaxial leads (with a maximum length of 2m to minimise capacitance to ground) to connect the capacitance measurement circuitry to the electrodes. Each electrode must be sceened individually, which means that one coaxial lead must be used for each electrode. Back to top

An earthed screen must be located around the sensor to exclude any external signals and to prevent the signals applied to the source electrodes from interfering with other electronic equipment in the vicinity. Back to top

Discharge resistors (typically 1 MOhm) must be connected between each electrode and the cable screen to ensure that no static charge can build up on the electrodes and connecting leads, otherwise damage may occur when the sensor is connected to the capacitance measurement circuit. The discharge resistors do not affect the capacitance measurement and must be connected permanently to the sensor electrodes. Back to top

These basic techniques can be used to construct either static or sliding sensors with either internal or external electrodes. More complex fabrication techniques must be used to constuct sensors suitable for operation at elevated temperatures and pressures.. Back to top

Last updated 26-05-2002

Copyright 2001 PTL