Abstract: A fluid flow sensor, comprising a tubular element having a greater diameter than and vertically mounted on a pipe for serially interconnecting two portions of this pipe. One portion is connected to the upper end of the tubular element while the other portion is connected to its lower end. A magnetic piston is slidably mounted within the tubular element and is therefore free to move along it. A by-pass conduit interconnects the lower portion of the pipe with the upper portion of the pipe. The piston moves upwardly in the tubular element when the fluid flows. Fluid flows from the portion of the pipe connected at the lower end of the tubular element to the one connected at its upper end through the by-pass. The piston moves downwardly by gravity to the lower end of the tubular element when the fluid stops flowing. A coil wound around a portion of the tubular element produces in electrical signal when the piston moves in the tubular element.