Open some small holes according to a certain rule on the meeting area of flowing on the outside wall of the round pipe or diamond tube perpendicular to the flow in order to measure the total pressure of the fluid( dynamic pressure and static pressure). Open a small hole on the back flow, in order to measure the static pressure of the fluid. Total pressure and static pressure are respectively led to the high and low pressure chamber of differential pressure transmitter; the output of the differential pressure transmitter is the flow of the fluid.