An apparatus for treating a workpiece with a granular media includes a hopper for storing media and discharging the media into a flow path. A source of compressed air mixes with the media, which is forced through a nozzle for directing media onto the workpiece. A flow sensor includes a beam extending into the flow path and is responsive to flow of media in the flow path to increase and decrease deflection of the beam in response to increasing or decreasing flow of media through said flow path. A proximity sensing device measures deflection of the beam and generates an electrical signal which varies in response to deflection of the beam. The flow path is defined by a conduit having a branch in which the beam is mounted. One end of the beam is mounted on a base in the branch. The base is mounted on a cap closing an open end of the branch, permitting the beam to be easily removed and changed when necessary.