Ultrasonic Transducers for Gas and Liquid Flow Metering

Accurate fluid flow metering is essential for modern process control, and ultrasonic flow meters are well established in this area. However, the specific acoustic impedance mismatch for most piezoelectric ultrasonic transducers is different when operating liquids and gases, hence there are problems measuring two-phase flows when using a single type of piezoelectric transducer in a flow meter. This work focuses on the development of a transit-time flow meter using a novel type of capacitive ultrasonic transducer, which can measure the flow in both gases and liquids. A rig for investigating gas and liquid flows has been constructed to enable a thorough study of fluid flow measurement in pipes ranging from 25mm to 100mm in diameter, enabling an analysis of flows from the laminar region through to the fully turbulent. The ultrasonic propagation times in fluid flow have been measured through the use of cross-correlation techniques implemented in MATLAB, which has proved to be a very robust method of measuring the differences between the upstream and downstream signals and thus it was possible to accurately determine the fluid flow rates in the pipe. Further signal processing was used to smooth the output of the flow meter. Different transit-time flow meter configurations were tested to examine the effects of positioning the transducer ports at different angles to the pipe and also, to examine the effects of multiple reflections of ultrasound in the pipe on the output data.