E-Zine July 2009
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Transit time ultrasonic flowmeters (sometimes called time-of-flight ultrasonic flowmeters) alternately transmit ultrasonic energy into the fluid in the direction and against the direction of flow.
At no flow conditions, it takes the same amount of time to travel upstream and downstream between the sensors.
Under flowing conditions, the upstream ultrasonic energy will travel slower and take more time than energy traveling downstream.
When the fluid moves faster, there is an increase in the difference between the times required for the ultrasonic energy to travel upstream and downstream between the sensors.
The electronic transmitter measures the upstream and downstream times to determine the flow rate.
The following equation expresses this relationship when the transmitting and receiving devices are flush with the pipe wall.
It should be noted that some suppliers simplify this equation by substituting the square of the transit time for (Tu * Td) in the denominator.
In some designs, sensors may not be mounted flush with the inside pipe wall, so part of the ultrasonic energy path may travel where there is no net flow.
Because Tu and Td appear as a product in this equation, the measured transit times should be corrected to account for the time to travel through the no flow distance.
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Excerpted from The Consumer Guide to Ultrasonic and Correlation Flowmeters