Ultrasonic flow measurement

General Measuring principle

The transit-time difference method

The transit-time difference method exploits the fact that the transmission speed of an ultrasonic signal depends on the flow velocity of the carrier medium; similar to a swimmer swimming against the current, an ultrasonic signal moves slower against the flow direction of the medium than when in flow direction.

The measuring principle

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 measuring principle

The transit time of the ultrasonic signal propagating in flow direction is shorter than 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.

Since ultrasounds propagate in solids, the transducers can be mounted onto the pipe. The measurement is therefore non-invasive, and no cutting of pipes is necessary for the installation of the transducers.