[Sensor Sub-Component] Water Level Sensor Housing

This is a two-piece Thing: Top Tube and Bottom Tube.

This Thing combines with my [Sharp GP2Y0A21 IR Sensor Housing](http://www.thingiverse.com/thing:808262) and [IR Sensor Puck](http://www.thingiverse.com/thing:819267) to form an infrared water level measurement sensor.

- The two pieces fit together in a friction / pressure fit. The cylinder is perforated to provide visual indication of the puck, water level, and any obstructing debris.

- The top three threads are 4mm ANSI M Metric threads at 17mm deep, and join the IR Sensor Housing to the tube structure.

If I were to re-mix this part, I would remove the side perforations, and replace them with a vertical "window", either completely plastic-free, or with a transparent material. This would eliminate ripples in the body of water from affecting the puck and therefore the sensor reading, by extending the tube under the water's surface.

I might add an "intermediate" tube section, so users can extend the housing, up to