A system and a method for partial stroke testing of a critical valve using a bypass valve includes flowing two streams of gas simultaneously through a bypass valve, opening the critical valve with the gas from the bypass valve, and determining a calibration time interval for closing the opened critical valve to a preset partial stroke testing position. The method includes determining a measured time interval for closing the opened critical valve, creating a tolerance band, and comparing the measured time interval to determine if the critical valve passes the partial stroke testing. The system includes a two stage bypass valve, having four solenoid valves. The system includes a processor with a memory having partial stroke testing logic. Each solenoid valve and the bypass valve include a pressure sensor.