The spatial filtering effect of capacitance transducer electrodes (flow measurement)

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Abstract
Capacitance sensors are finding increased application in the measurement of two-component flow. An important parameter in these measurements is the frequency bandwidth of the ring and parallel plate types of sensing electrodes. In this paper the theoretical response of these electrodes to permittivity impulse signals is derived and compared with experimental data. A machine is used to drive a plastic bead through both types of electrode to obtain the impulse response for comparison with theoretical predictions. Experimental results are presented for beads travelling along the central axis of the sensor and near to its wall. For electrodes with an active length greater than 40 mm theory and experiment agree, for shorter lengths an electrode length coefficient is introduced. This gives the minimum effective length of this type of electrode as 30 mm, which is attributed to field fringing effects.

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