Apparatus for measuring a preselected characteristic, such as moisture, of viscous and/or absorption fluids in which an open hollow tube of optically reflective material is disposed within a fluid conduit closely adjacent an optical window to define a test material flow path between the window and reflector....http://www.google.fr/patents/US4115009?utm_source=gb-gplus-shareBrevet US4115009 - Optical sight tube for flowing fluid materials
