Fluid flows through the sensor housing. Because the blades of the impeller have a certain angle with the flow direction, the impulse of the fluid makes the blades produce rotating torque. After overcoming the friction torque and fluid resistance, the blade rotates and the speed is stable after torque balance. Under certain conditions, the speed is proportional to the flow. Because the blade has magnetic conductivity, it is in the magnetic field of the signal detector (composed of permanent magnet steel and coil), and the rotating blade cuts the magnetic field lines and periodically changes the magnetic flux of the coil. Therefore, electric pulse signals are induced at both ends of the coil, which are amplified and shaped by the amplifier to form a continuous rectangular pulse wave with a certain amplitude, which can be transmitted to a display instrument to display the instantaneous flow or total amount of fluid.