Study of a Noncontact Flow Transducer Using Semicylindrical Capacitive Sensor

The fluid flow rate through a pipeline is a very important process variable in any process plant that is required to be measured accurately. In this article, a new noncontact semicylindrical capacitive flow sensing technique has been developed and studied. The proposed capacitive sensor consists of two semicylindrical curved copper plates mounted on the outside surface of an insulating pipeline near the turbulence region produced at the junction of a larger diameter pipe and a smaller diameter pipe. Here the insulating pipeline section between the copper plates along with the flowing fluid acts as a composite dielectric material of the sensing capacitor. The semicylindrical curved copper plates along with two lead wires are rigidly mounted without any air gap on the larger diameter pipe using a thin layer of araldite. A mathematical relation between the sensor capacitance and flow rate is derived in this article. This capacitance is found to vary nonlinearly with flow rate and is measured using an op-amp-based DeSauty bridge-type transducer circuit where a similar capacitor with the same fluid at zero flow is used as a dummy capacitor. A prototype unit of the proposed transducer is designed, developed and tested experimentally. The static characteristic curves drawn from the experimental results are found to follow the derived mathematical equation to a very good extent with good repeatability.