Abstract: 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. A measurement beam incident upon the reflector tube from externally of the conduit through the window and test material is reflected thereby back through the material and window to detection apparatus. In alternative embodiments, the reflector tube is fixedly mounted coaxially with the conduit to define a fixed window/reflector separation, or is rotatably mounted eccentrically of the conduit axis for adjustment of the window/reflector separation.