Embodiments include systems, devices, and methods for monitoring etch or deposition rates, or controlling an operation of a wafer fabrication process. In an embodiment, a processing tool includes a processing chamber having a liner wall around a chamber volume, and a monitoring device having a sensor exposed to the chamber volume through a hole in the liner wall. The sensor is capable of measuring, in real-time, material deposition and removal rates occurring within the chamber volume during the wafer fabrication process. The monitoring device can be moved relative to the hole in the liner wall to selectively expose either the sensor or a blank area to the chamber volume through the hole. Accordingly, the wafer fabrication process being performed in the chamber volume may be monitored by the sensor, and the sensor may be sealed off from the chamber volume during an in-situ chamber cleaning process. Other embodiments are also described and claimed.