Ultrasonic flow meter is designed by the principle of transit time method. The transit time type ultrasonic flow meter is equipped with two transducers that are clamped on the outside of a closed pipe at a specific distance.
The two transducers which function as both ultrasonic transmitters and receivers. The flow meters operate by alternately transmitting and receiving a frequency modulated burst of sound energy between the two transducers. The burst is first transmitted in the direction of fluid flow and then against fluid flow. Since sound energy in a moving liquid is carried faster when it travels in the direction of fluid flow(downstream) than it does when it travels against fluid flow(upstream), a differential in the times of flight(ΔT) will occur.
The difference in the transit time measured is directly and exactly related to the velocity of the liquid in the pipe, as shown following,
Where
Θ is the include angle to the flow direction
M is the travel times of the ultrasonic beam
D is the pipe diameter
Tup is the time for the beam from upstream transducer to the downstream one
Tdown is the time for the beam from downstream transducer to the upstream one
ΔT=Tup-Tdown