Measuring fluid flow through a pipe is a common requirement in process industry. This can be achieved by using different instruments.
The simpliest device is the so called orifice plate. When such an orifice plate is located correctly into a pipe, a differential pressure between the inlet and the outlet of this element can be observed while the fluid is flowing. Under these circumstances, the flow is proportional to the pressure difference. This makes it possible to measure fluid flow by measuring this differential pressure.
Apart from fluid and pipe conditions, there are there are three basic parameters which let us size our system correctly: orifice diameter, differential pressure and fluid flow.
This app offers you to choose freely two of them and calculate the other one.