3D-1 Absolute Transit Time Detection for Ultrasonic Gas Flowmeters Based on Time and Phase Domain Characteristics

We present an absolute transit time detection algorithm for ultrasonic gas flowmeters (UFMs). The major objective is a reliable and accurate detection, even when the received signals experience a change and degradation of their shape. This can be due to parasitic effects, such as high gas temperatures and pressure fluctuations. We employ a time and phase domain based detection algorithm that determines the absolute transit times independently for the upstream and downstream channel. The Hilbert transform is applied to calculate the wrapped phase signal; each section of this phase signal is analyzed step-by- step. The algorithm was tested on real measurement data obtained from a double-path UFM (wetted configuration using capacitive ultrasonic transducers) installed at the end of an exhaust gas train of an automotive combustion engine. Over a gas temperature range of 400degC and a mass flow range of 163 kg/h, corresponding to a signal- to-noise ratio (SNR) range from 18 to 8 dB, all transit times were detected correctly, i.e. without any cycle skip. Further, our results show that the algorithm outperforms cross-correlation methods in terms of the absolute transit time detection.