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M. Kupnik, E. Krasser, M. Gröschl:
"Absolute transit time detection for ultrasonic gas flowmeters based on time and phase domain characteristics\";
Talk: 2007 IEEE Ultrasonics Symposium\, New York/USA; 10-28-2007; in: "Proc. 2007 IEEE Ultrasonics Symposium\", (2007), ISBN: 1-4244-1383-4; 142 - 145.

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-bystep.
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 400°C and a
mass flow range of 163 kg/h, corresponding to a signalto-
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.