<p>A method for detecting particles in a fluid involves passing any ultrasonic signal through the fluid. A Fast Fourier Transform analysis is performed on the received signal, to determine a characteristic frequency of the fluid where the amplitude is greatest. Both the amplitude of FFT spectrum at the characteristic frequency and the time-domain amplitude are monitored. A drop either in the amplitude of the FFT spectrum at the characteristic frequency or the time-domain amplitude indicates the appearance of particles in the fluid.</p>
