Detailed explanation of the measurement principle of ultrasonic flowmeter

 

The measurement principle of ultrasonic flowmeter

 

When an ultrasonic beam propagates in a liquid, the flow of the liquid will cause a slight change in the propagation time, and the change in the propagation time is proportional to the flow rate of the liquid. The relationship conforms to the following expression:

 

in

 

θ is the angle between the sound beam and the liquid flow direction

 

M is the number of times the sound beam propagates in the liquid in a straight line

 

D is the inner diameter of the pipe

 

Tup is the propagation time of the sound beam in the positive direction

 

Tdown is the propagation time of the sound beam in the reverse direction

 

ΔT=Tup –Tdown

 

Assume that the speed of sound in a static fluid is c, the speed of fluid flow is u, and the propagation distance is L. When the sound wave is in the same direction as the fluid flow (i.e. downstream), its propagation speed is c+u; otherwise, the propagation speed is cu. Two sets of ultrasonic generators and receivers (T1, R1) and (T2, R2) are placed at two locations with a distance of L. When T1 emits ultrasonic waves in the downstream direction and T2 in the reverse direction, the time required for the ultrasonic waves to reach the receivers R1 and R2 respectively is t1 and t2, then

 

t1=L/(c+u) t2=L/(cu)

 

Since the velocity of the fluid in industrial pipelines is much smaller than the speed of sound, that is, c>>u, the time difference between the two is ▽t=t2-t1=2Lu/cc. Therefore, when the propagation speed c of the sound wave in the fluid is known, the velocity u can be calculated by measuring the time difference ▽t, and then the flow rate Q can be calculated. The method of flow measurement using this principle is called the time difference method. In addition, the phase difference method, frequency difference method, etc. can also be used.