• The flow meter consists of a crystal transducer mounted outside the pipe in which flow is to be measured. This transducer is a piezoelectric crystal which emits an ultrasonic wave and the wave is projected at an angle through the pipe wall into the liquid.
• As the liquid flows through the pipe the particles and bubbles in the liquid also move and these particles act as reflectors for the ultrasonic wave.
• Part of the ultrasonic wave is reflected by the particles and bubbles in the liquid and is returned through the pipe wall to the transducer.
• The particles and bubbles move with the velocity of the fluid, so the frequency of the reflected wave is shifted to particle velocity, according to Doppler principle.