 Welcome to ultrasonic flowmeters. When measuring the flow of dirty liquids, particles may corrode a sensor placed in the fluid, or the particles may build up and alter the operation, resulting in an inaccurate measurement. An ultrasonic flowmeter is a non-invasive sensor, meaning that it is outside the pipe and not in contact with the fluid. This flowmeter propagates sound waves through the pipe wall into the flowing liquid. When the sound waves are reflected by moving particles, the frequency is shifted due to a phenomenon known as the Doppler effect. The emission of sound waves originates in an oscillator, a device that sends pulses of current through a piezoelectric device made of a crystal. The pulse causes the piezo device to physically vibrate and move to create sound waves to form and pass through the fluid. These waves are at a higher ultrasonic frequency than what can be heard by humans. The pulses are transmitted upstream into the flowing liquid. Each ultrasonic wave that is sent upstream gets reflected back to the receiving element from particles or gas bubbles flowing along with the liquid. The receiving element consists of a piezoelectric device that detects pressure fluctuations created by the reflected pulsating sound waves. This transducer converts the sound into electronic pulses that are processed by the sensor's electronics. The measurements made by this device are based on the frequency of the reflected pulses. Because the fluid is moving toward the receiver, the frequency of the reflected pulses is higher than the frequency of the pulses that are transmitted. The difference in frequency of the received pulses compared to the frequency of the transmitted pulses is proportional to the velocity of the fluid. As the velocity increases, the received frequency increases to create larger differences relative to the fixed frequency of the transmitter. Watch as we increase the velocity. By measuring the frequency difference between the electrical pulses of the oscillator and the pulses at the receiver, flow rate can be determined. To illustrate why the frequency of the received sound waves is higher than the frequency of the transmitted waves, first examine what happens if the fluid is not flowing. Watch as we launch the sound waves. When the wave is reflected off the particle, it takes the same amount of time to return as it did to launch. When a series of sound waves is launched, the time duration between the crests of the launched sound wave and the crest of the reflected waves is the same. However, if the particle is flowing toward the sensor, the time duration between the crests of the reflected waves decreases. Watch as we show no movement, slow movement, and fast movement of the sound waves. Suppose that the frequency of the transmitted pulses is 100 kHz. The time duration between the pulses is 10 microseconds. When the reflected pulses are returned to the sensor from a stationary particle, the pulse duration is 10 microseconds, which is converted to a frequency of 100 kHz. This is the frequency formula. The frequency of 100 kHz at the receiver is compared electronically by the sensor to the transmitted frequency of 100 kHz. The ultrasonic flow meter indicates that the flow rate is zero because both frequencies are the same. Suppose that the flow rate is the same speed as the sound waves launched by the transmitter. The particles collide with the wave crests every 5 microseconds. Therefore, the frequency of the reflected sound waves measured by the receiving sensor is 200 kHz. This is the frequency equation. This frequency of 200 kHz is compared to the transmitted frequency of 100 kHz. The difference of 100 kHz indicates the flow rate of the liquid. The flow rate of the liquid being measured in an industrial application is not likely to be the same rate as the transmitted sound waves, so the measured frequency will not be twice as high as the transmitted frequency. A more realistic measured frequency will be a relatively small percentage over what is transmitted. Now, let's check your understanding. Ultrasonic sensors are used to measure the flow rate of dirty liquids. As the speed of the liquid increases, the time between crests of sound waves that reflect back to the sensor decreases. The ultrasonic flow meter uses both the transmitted frequency of sound waves and the reflected frequency of sound waves to measure the flow rate. Congratulations! You have completed this activity, ultrasonic flow meters.