Flow measuring system with backflow preventing means

Means for measuring the flow of fluid in a conduit filled with a fluid under pressure comprising a conduit with fluid under pressure to be measured, means for responding to differential pressure at some location in the conduit including a sensing head having a pair of oppositely facing open ended orifices and a separate tube connected respectively to each orifice and extending therefrom through and from the conduit, a pair of normally upwardly oriented closed tubular chambers external of the conduit, each of said chambers having an input passage located near the upper end thereof in communication with a respective one of said tubes and an outlet passage, a supply of a first fluid filling each of said chambers, a U-shaped conduit in each of said chambers, each U-shaped conduit having one end connected to communicate with the input passage thereof and an opposite open end, said U-shaped conduits extending from adjacent to the upper end of the respective chamber downwardly to near the bottom of the chamber and then upwardly therefrom to a location in the chamber adjacent the upper end of said chamber, the outlet passage to each of the chambers including a tubular member having a first open end that extends downwardly to a position in the chamber adjacent the lower end of said chamber and an upper end communicating with the outlet passage, a differential indicating tube having a first end connected to communicate with the outlet passage of one of said chambers and a second end connected to communicate with the outlet passage of the other of said chambers, said differential indicating tube having a downwardly extending loop portion partially filled with an indicating fluid, the remaining portion being filled with the first fluid, the differential pressure sensed at the oppositely facing open ended orifices producing a height differential between the opposite ends of the indicating fluid in the differential indicating tube.