Abstract: An apparatus is provided for removing a coating from a substrate, comprising a nozzle having an outlet and adapted to direct a particle stream therethrough at a predetermined flow rate, a signal source for emitting a signal capable of traversing the particle stream, and a signal sensor positioned to detect the signal emitted by the signal source once the signal has passed through the particle stream. The particle stream is directed from the outlet of the nozzle toward a coating on a substrate to remove the coating from the substrate. Since the signal emitted by the signal source traverses the particle stream before being detected, the intensity of the signal detected by the signal sensor corresponds to a flow rate of the particle stream such that a subsequent change in the intensity of the signal that is detected by the signal sensor indicates a change in the flow rate of the particle stream. A method of monitoring a particle flow in an apparatus used for removing a coating from a substrate is also provided.