Robustness of the controller is vital in many process industries. The robust controller will not only compensate for disturbance but also for the parametric variation in the process itself. This paper proposes the design of such a robust propositional-integral (PI) controller for a non-linear level control process, like hemi-spherical tank system, using volumetric observer. In a level control system, the volume has a linear relation to the differences in inflow and outflow, whereas the rate change in height is non-linearly related to the flow difference. The robustness of the proposed controller was enhanced by using volume error instead of level error. The volume error is derived by using a volumetric observer, which observes the tank volume and a volume estimator, which estimate the desired tank volume. This enables a linear controller like PI controller to be more robust over the entire span of tank height. The robustness of the proposed controller was compared with the conventional controller and its performance is validated by using the averaged performance indices like averaged-ISE, ITAE and IAE. Simulation results illustrates that the proposed controller has faster response and lesser oscillations. © Research India Publications.