Theoretical and experimental evaluation of small flow rate ultrasonic flowmeter

In this paper a low-price ultrasonic flowmeter is designed and implemented that measures the time of flight of ultrasonic signal in low flow rates for small diameter pipes with maximum absolute error of 0.12 ml s . The flowmeter operates based on two piezoelectric transducers placed on two ends of the setup in which the fluid flows through the pipe. The flow rate is calculated based on ultrasonic transit time measurement using an electronic board based on a TDC-GP22 converter and an ARM microcontroller. The transmission of the signal across the length of the pipe instead of the classic oblique placement of piezoelectric transducers allows higher accuracy in low flow rate measurement through longer lengths of pipes. In order to measure the flow rate, two setups were designed and fabricated; a simple uniform diameter setup based on measuring the time of flight of ultrasonic wave and a second setup named, reduced diameter pipe, which conducts the fluid across a thin pipe with three different diameters, to study the measurement performance for various diameters. The theoretical relation between flow rate and ultrasonic signal flight time difference in direct and reverse mode for both setups were obtained based on principles of fluid mechanics. Utilizing the collected experimental data, the authenticity of the obtained theoretical relations is confirmed and it is verified that there is a linear relation between flight time difference and laminar flow rates in the setups. After calibration, the final formulas and the results for each setup are compared with the reference flow rates. In the reduced diameter setup, with decrease in the main pipe diameter, the sensitivity of the flowmeter increases. Also, the full scale error and maximum measurable laminar flow rate decrease.