An ultrasonic transit-time gas flowmeter for automotive applications

A new concept for ultrasonic transit-time gas flowmeters, utilizing high temperature resistant capacitance transducers, and results from a preliminary laboratory prototype, are presented. With this concept, measurements of hot (up to 450°C) and pulsating gas flows have been achieved for the first time. Such conditions occur e.g. in the exhaust gas train of a combustion engine. An adaptive pulse repetition frequency (PRF) is used to overcome the problems associated with the range and dynamics of the gas temperature. Large temperature variations prevent a continuous detection of ultrasonic pulse arrival times. Coherent reflections are generated, since the acoustic impedances of the gas and the transducers are mismatched. Depending on the gas temperatures these reflections may overlap with the actual signal. The presented method renders feasible a correct detection over the whole temperature range. Results are presented for the mass flow rate in a 50 mm diameter pipe measured at a PRF up to 5.5 kHz.