The series A3 is a patent pending microprocessor based controller for positive, negative, and differential pressure as well as velocity and flow. The Series A3 includes (2) 8A SPDT relays and an associated software package that allows full configuration of variables such as setpoints, dead band, engineering units, flow coefficients, and more. The programming is done from a user friendly menu on the LCD display or uploaded using an Opti Linkô programming key. The LCD also displays the exact setpoints or other user defined variables during normal operation. Furthermore, the menu structure can be customized by moving variables to different menu levels to accommodate many applications. Compared to other mechanical products, the Series A3 has a significantly reduced depth, an improved look, and better performance at a lower price.