A method of adjusting the relative thickness of an electrode assembly (10) in a plasma processing system (6) capable of supporting a plasma (20, 120) in a reactor chamber (16). The electrode assembly is arranged in the reactor chamber and includes at least one electrode having a lower surface that may have defined by at least one sacrificial protective plate (100). The electrode has a nonuniform thickness resulting from a plasma processing operation performed in the reactor chamber. The method includes a step of forming a plasma (120) designed to selectively etch the at least one electrode at the lower surface, followed by a step of etching the electrode with the aid of the plasma to reduce the nonuniformity in thickness (T(X,Z)) of the at least one electrode. The thickness of the electrode may be measured in situ using an acoustic transducer (210) during the processing of workpieces as well as during the restorative plasma etching of the electrode.