Inside the Ultrasonic Flowmeter, pairs of sensors are fitted across the tube in opposite to each other. Each sensor alternately can transmit and receive ultrasonic signals. Simultaneously the transit time of these signals is measured.
The ultrasonic signals are generated by piezoelectric crystals applying a voltage. Conversely, the piezoelectric crystal creates a voltage when the ultrasonic signals touch the sensor. By increasing the sensors pairs, it is possible to accurately detect and mathematically compensate flow profile distortions.
When there is no flow, the signal transit time and receiving time are the same. When the flow starts in the measuring tube, the ultrasonic signals are accelerated in the direction of flow, de-accelerated in opposite direction.
Now the ultrasonic signals have different transit times. Less time in the direction of flow and more time against the flow. The difference in the transit time measured by the sensors is directly proportional to the flow velocity in the pipe. Together with known pipe cross-section, the actual flow volume can be calculated.
The greater the flow velocity, the greater the measured time difference between the two ultrasonic signals. For ultrasonic flow measurement, the sensor need not have to be fitted inside the pipe wall.
With a clamp-on system, the sensors are fastened directly on to the outside of the pipeline. They can be retrofitted at any time without interrupting the process. With clamp-on sensors, the ultrasonic signals are passed directly on to the pipe wall into the fluid. The signal continues through the fluid is reflected on the opposite pipe wall then measured by the 2nd sensor.
With the clamp-on design, the flow rate can be measured in a very large pipe line up to 4 meters in diameter.
Advantages of Ultrasonic flowmeters:
# Flow measurements in large pipes can be done.
# Flow of both gases and liquids can be measured.
# Ultrasonic flowmeters can measure the flow of nonconductive flows.
# For the measurement of low flow rates, the ultrasonic flowmeter is a better option compared to the Vortex flowmeter. Reynolds’s number doesn’t come into the picture.
# Ultrasonic flowmeters are no moving parts, need not worry about its wear and tear issues.
Limitations of Ultrasonic flowmeters:
# Ultrasonic flowmeters are more expensive than the Doppler version.
# Requires upstream, downstream straight pipeline, 15 pipe diameters.
# Used only in clean gas, or fluid applications.