mainly transducer are those device which change one form of energy into another form with some physical change of its property. but in fan there is no chnge of its any physical quality. i mean no change in dimension, lenth or other things
In ultrasonic flow meter used ultrasound wave which are produced by piezo electric like crystel.
there are one reciver and one transmitter which can placed
1. inside the pine in the direction of flow
2. outer the pipe.
3. inclined to the pipe.