High-frequency transducers and correlation method to enhance ultrasonic gas flow metering

Research activities carried out in past years have shown that in gas networks some constraints exist on the frequency at which ultrasonic transducers can be operated. The study of the transmission and attenuation of the signal and noise mechanisms gives us a defined suitable frequency range (K.R. Wild, A European collaboration to evaluate the application of multi-path ultrasonic gas flow meters, paper presented at 4th International Symposium on Fluid Flow Measurement, Denver, CO, 1999). A working frequency of 500 kHz proves to be the most suitable to avoid noise effects. This frequency is above the noise level detected in gas pipelines. Moreover, the signal loss due to attenuation of ultrasound in gas is still negligible.In parallel, the use of this frequency allows the application of efficient numerical techniques such as the cross-correlation method for signal processing. An initial process based on this method has been developed for gas flowmeters. It provides low uncertainty for the parameters involved in the flow measurement process.A single-path flowmeter equipped with this system has been tested on the Gaz de France test facilities. Its accuracy is better than 1.5% from 250 to 1000 m³/h without initial adjustment. An auto-calibration process also forms part of the system, using a systematic comparison between ultrasonic measurement of sound velocity and a theoretical approach. The signal-to-noise ratio remains large enough to perform the measurements correctly, even with a control valve installed close to the flowmeter.