Ultrasonic flowmeters operate using the transit-time differential method. The Transit-time differential measurement is based on a simple physical fact. Imagine two canoes crossing a river on the same diagonal line, one with the flow and the other against the flow. The canoe moving with the flow needs much less time to reach the opposite bank.
Ultrasonic waves behave exactly the same way. A sound wave travelling in the direction of flow of the product is propagated at a faster rate than one travelling against the flow (vAB > vBA).
Transit times tAB and tBA are measured continuously. The difference (tBA – tAB) in time travelled by the two ultrasonic waves is directly proportional to the mean flow velocity (vm).
- tAB is Time required for ultrasonic wave to travel from Sensor A to B sensor
- tBA is Time required for ultrasonic wave to travel from Sensor B to A sensor