Which basic measurement principles of Ultrasonic flowmeters ?

1. Transit-time Technology. One of the most reliable and time-tested measurement principles is Transit-time flow measurement. A typical transit-time ultrasonic flowmeter system utilizes one pair of transducers that function as both the ultrasonic transmitter as well as the receiver. We will explore Transit-time flow measurement in greater detail in the next section.
2. Ultrasonic Doppler Technology. In Doppler-style flowmeters, two ultrasonic transducers are employed in the system. One transmits a continuous ultrasonic wave into the flow. The other transducer receives the ultrasonic wave scattered from suspending particles (or targets). The received wave has a frequency shift compared with the transmitted one. This shift is the so-called Doppler frequency shift, which is directly proportional to the flow velocity. Therefore, by detecting the Doppler frequency, we can derive the flow velocity. The flow rate of the pipe liquid is obtained by computing the product of the velocity and the cross-section area of the pipe.
3. Cross-Correlation Measurement Technology. The Cross-Correlation principle provides accurate and reliable flow measurement. A turbulent flow has a cascade structure of eddies. Along the flow, the characteristics of those structures do not change much within a certain distance, called the Correlation Length. It is by tracking particles along this length, that Cross-Correlation measurement is achieved.
4. Acoustic Doppler Velocity Profiling Technology. Three acoustic transducers are installed as shown in the left figure. The center one is usually a narrow-beam transducer. It transmits a burst of sound pulses with a certain Pulse Repetition Frequency (PRF). The other two are large-angle transducers. They receive the ultrasonic wave scattered from suspending particles (or targets) of the water column notified by both the transmitting and receiving sound beams. By using a pulse-to-pulse algorithm, we can obtain the Doppler frequency shift in two directions, from which a 2D velocity profile can be formed.