The transit-time difference method

The transit-time difference method makes use of the fact that the speed of propagation of an ultrasonic signal depends on the flow rate of the fluid in which it is propagating. Just as a swimmer swimming against the current, an ultrasonic signal propagating against the direction of flow of the fluid will be slower as in flow direction.

For the measurement, two ultrasonic pulses are sent through the medium, one in flow direction, and a second one against it. The transducers are alternatively working as emitter and receiver.

The transit time of the ultrasonic signal propagating in flow direction is shorter as the transit time of the signal propagating against flow direction. The transit-time difference Δt is measured and allows the determination of the average flow velocity on the propagation path of the ultrasonic signals. A profile correction is performed to obtain the average flow velocity on the cross-section of the pipe, which is proportional to the volume flow rate.

The use of the transit-time difference clamp-on technology in measuring gas flow presents an exceptional technological challenge.

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