Below is video tutorial about pressure measurement. DP (Differential Pressure) is widely used principle of all type of fluid flow. The overall energy will remain constant, because while the kinetic energy increasing, the pressure energy is decreasing and while the pressure energy increasing, kinetic energy decreasing.
The crude oil extraction on the sea bay is using this principle. To increase the velocity flow on the pipe, there is a smaller chamber in the middle of the pipe so the liquid can move faster after through the chamber. For further information, watch the video tutorial.