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Based on our previously developed tuning procedure for fractional order proportional integral controller (FO-PI), we present in this paper an extensive comparative experimental study on coupled-tank liquid level controls. Our experimental study consists of four steps, they are mathematical modeling of the plant, identification of plant parameters, water-level controller design and comparisons in Simulink [software (s/w) mode] and final experimental verification and comparisons in real-time [hardware (h/w) mode]. The FO-PI controller is compared with Ziegler NicholÂ¿s (ZN) and Modified Ziegler NicholÂ¿s (MZN) conventional integer order PI controllers in terms of load disturbance rejection, changes in plant dynamics and setpoint tracking. Experimental results confirmed that FO-PI controller is a promising controller in terms of percentage overshoot and system response in liquid-level control in face of nonlinearities introduced by pumps, valves and sensors.