The SF series is a paddle flow switch product line with field adjustable paddle length for a variety of flow conditions and pipe sizes. The SF paddle flow switch utilizes the force of the fluid flow in a pipe to move the paddle. As the fluid flow moves the paddle an internal spring is compressed, which moves a magnetic element. With no flow, or with an empty pipe condition, the paddle flow switch spring is fully expanded and the magnetically activated reed switch contact is open, indicating that a no-flow or empty pipe condition exists. As flow increases, the spring is compressed and the magnetic element moves toward the reed switch. When the flow moves the paddle in an upward angle of at least approximately 20⁰~30⁰ (or more) the magnet moves into proximity of the reed switch and activates it and the contact closes to indicate that a fluid flow condition exists.