# 1.5 Measuring principle

![](/files/-LvhLpXgUChdDIctoZMb)

Fluid flow rate = V​; sound wave velocity during transmission through the fluid = c​; interior pipe diameter = D,​ interior pipe cross section = A​. When ultrasonic waves are transmitted through water between probes A​ and B​, the forward and reverse flow directions produce a time differential that is proportional to the flow rate.

![](/files/-LvhM0AMDdUykP1aYjSb)

Flow rate is Q = v \* A. If the interior pipe cross section is a circle, then Q= v＊(d² π/ 4). Therefore, the size of the pipe diameter and the flow accuracy have a great relationship.


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