The present disclosure generally relates to a method and apparatus for determining a metric related to erosion of a ring assembly used in an etching within a plasma processing chamber. In one example, the apparatus is configured to obtain a metric indicative of erosion on an edge ring disposed on a substrate support assembly in a plasma processing chamber. A sensor obtains the metric for the edge ring. The metric correlates to the quantity of erosion in the edge ring. In another example, the ring sensor may be arranged outside of a periphery of a substrate support assembly. The metric may be acquired by the ring sensor through a plasma screen.