There is no hardware or mechanical apparatus in the pipe at all.
Transit Time Flowmeter
An upstream transducer sends an ultrasonic signal to a downstream transducer. The downstream transducer then sends an ultrasonic signal to the upstream transducer. When there is no flow, the time for a signal to go from the upstream transducer to the downstream transducer is equal to the time for a signal to go from the downstream transducer to the upstream transducer. When flow is present it will speed up the upstream to downstream signal (signal travels with the flow) and slow down the downstream to upstream signal (signal travels against the flow). This change in time (Transit Time) is proportional to the flow. The Flowmeter converts these time changes to flow rate and total.
