# 1.5 Measuring principle

![](https://1338263946-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LrD9o7x4AZT6Lc9ACTJ%2F-LrDMLtjV5IjsrQul1GA%2F-LrDMY4NUUQIR7lq9vl5%2Fsupersonic-flowmeter-principle.png?alt=media\&token=d6312749-843a-4d66-9a84-6a6fc5892cd2)

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.

![](https://1338263946-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LrD9o7x4AZT6Lc9ACTJ%2F-LrDMLtjV5IjsrQul1GA%2F-LrDMjz4SU7oPneYwB9t%2F%E9%87%8F%E6%B8%AC%E5%8E%9F%E7%90%86%E5%85%AC%E5%BC%8F.png?alt=media\&token=61147274-21c4-42d2-9711-11c5480c28b2)

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.