Abstract: An assembly for accurately and easily positioning and aligning a sensor device relative to a fluid flow stream in a pipe, incorporates a threaded housing which is engageable with a standard pipe tee-fitting to provide a platform for linearly positioning the sensor relative to the axis of flow. A sensor unit subassembly is rotatably supported on the platform so that it may be rotated into proper radial alignment with the flow axis while it is supported on the platform at the proper linear position relative to the flow axis. A paddlewheel flowmeter having a single leg supporting the paddlewheel axle in cantilever fashion forms the sensing unit which exploits the improved alignment accuracy of this assembly to avoid build-up of sludge and debris which might affect free rotation of the paddlewheel. Separate and independent lock-nuts fix the housing in position relative to the tee-fitting, and fix the sensor sub-assembly in position relative to the housing.