The double-threshold method has been widely used in ultrasonic flow measurement to determine time-of-flight (TOF) due to its low cost and ease of implementation. Performance of this method is negatively affected by the cycle-skip phenomenon which occurs frequently under inconstant working conditions, especially varied fluid temperature. This paper proposes a method to suppress the phenomenon to facilitate reliable determination of TOF in ultrasonic flow measurement. First, the double-threshold method is used to generate a feature point to segment the signal. Second, based on the correlation coefficient and signal power, judgement factors of individual signal periods are calculated to determine signal onset. Finally, a valid zero crossing which has a constant lag from the onset is selected to determine the TOF. Thus, the cycle-skip phenomenon is suppressed. Two additional modifications are proposed to eliminate the influence of varied signal frequency and low sampling rate. The proposed method was validated by an experiment based on an ultrasonic water flow sensor. Results showed that the frequently appearing cycle-skip phenomenon can be successfully suppressed by the proposed method.
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