There is no hardware or mechanical apparatus in the pipe at all.
Doppler Flowmeter
A sending transducer sends an ultrasonic signal into the liquid. The ultrasonic signal is reflected off solids, bubbles, or discontinuities in the liquid and returned to a receiving transducer. If the solids, bubbles, or discontinuities are not moving the frequency of the returned signal is identical to the sent signal. If the solids, bubbles, or discontinuities are moving the frequency of the returned signal is changed. This change in frequency is proportional to the flow (as discovered by Mr. Christian Doppler). The Flowmeter converts these frequency changes to flow rate and total.
