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M. Kupnik, A. Schröder, P. O´Leary, E. Benes, M. Gröschl:
"Adaptive Pulse Repetition Frequency Technique for an Ultrasonic Transit-Time Gas Flowmeter for Hot Pulsating Gases";
IEEE Sensors Journal, 6 (2006), 4; 906 - 915.

A technique of using an adaptive pulse repetition frequency (PRF) to operate an ultrasonic contrapropagation transit-time gas flowmeter (UFM) is introduced. This adaptive PRF technique allows transient measurements of hot (up to 450 °C) and pulsating (up to 1.5 kHz) gas flows. Such conditions occur in the exhaust gas of a combustion engine. Here, a UFM with the widely used fixed PRF technique is not applicable, because the large gas temperature variations would prevent a reliable detection of ultrasonic pulse arrival times. Coherently reflected waves are generated within the gas because of the unavoidable acoustic impedance mismatch between the gas and the transducers, and, depending on the gas temperatures, these echoes overlap with the main signal. The adaptive PRF technique overcomes this problem and allows correct pulse detection over the whole temperature range required. The UFM utilizes special high-temperatureresistant capacitance ultrasonic transducers (CUTs) to meet the requirements in terms of operating temperature range and dynamic response. Results, which are obtained with a preliminary laboratory prototype, are presented for the exhaust gas mass flow rate in a Ø 50-mm pipe measured at gas temperatures of up to 450 °C and at PRFs of up to 5.5 kHz, which is an increase in frequency response of one order of magnitude in comparison to existing measurement systems.