Today there are many different ultrasonic sensors to measure flow, liquid level and concentrations or viscosity in several processes. One of the biggest problems by doing the measurement using ultrasonic sensors is the appearance of bubbles at the measurement place caused by chemical reactions, high temperature or fast flow. These bubbles falsify the received signal and the measurement becomes erroneous. Therefore, it is useful to know if there are gas bubbles or not. It is important to obtain the information about bubbles in real-time, how ever the detection of the signal amplitude is not reliable to detect gas bubbles. The amplitude is influenced by solid particles, rapid changes of temperature and liquid droplets of other densities in the measured liquid. These arguments are the motivation to find new possibilities to detect the appearance of gas bubbles in liquids independent of the transparency and rheological properties of liquid.
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