# 1.5 Measuring principle

![](https://2386771058-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LvhKtBvsv-DVMTySk5A%2F-LvhKzqf5Qv_CHADnJ_l%2F-LvhLpXgUChdDIctoZMb%2Fsupersonic-flowmeter-principle.png?alt=media\&token=b96752e7-bd6d-419d-85b3-b5592c7f2be5)

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://2386771058-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LvhKtBvsv-DVMTySk5A%2F-LvhKzqf5Qv_CHADnJ_l%2F-LvhM0AMDdUykP1aYjSb%2F%E9%87%8F%E6%B8%AC%E5%8E%9F%E7%90%86%E5%85%AC%E5%BC%8F.png?alt=media\&token=e83cb40e-7bc4-4ce7-a0c9-6031efa1b0f7)

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.