A heat transfer surface monitoring (HTSM) cell for use in monitoring scaling, fouling, pitting, and corrosion, of a heat transfer surface, the HTSM cell comprising: a) a heat transfer plate having a first surface on a fluid inlet feed side; b) an edge-lit light guide having an aperture forming a fluid flow channel, the heat transfer plate positioned adjacent to the edge-lit light guide and overlay the aperture, wherein the edge-lit light guide is comprised of a transparent or translucent material to illuminate the first surface of the heat transfer plate; c) an edge illumination light source adjacent to the edge-lit light guide, the edge illumination light source operable to illuminate the edge-lit light guide, thereby providing illumination substantially parallel to the heat transfer plate and allowing an observer to view scaling, fouling, pitting, and corrosion on the first surface of the heat transfer plate; d) a fluid flow channel module having: i) a fluid flow channel module support block, ii) an optical window within the fluid flow channel support block, the optical window operable to view the heat transfer plate, iii) a fluid inlet conduit operable to allow entry of a fluid stream within the fluid flow channel module, iv) a fluid outlet conduit operable to allow exit of the fluid stream away from the fluid flow channel module; and, e) a heating/cooling module capable of transferring heat to the heat transfer plate, the heating/cooling module having: i) a heating/cooling support block, ii) a heating/cooling element, the heating/cooling element capable of being temperature controlled, wherein the heating/cooling support block is capable of conducting heat between the heating/cooling element and the heat transfer plate. a) a heat transfer plate having a first surface on a fluid inlet feed side; b) an edge-lit light guide having an aperture forming a fluid flow channel, the heat transfer plate positioned adjacent to the edge-lit light guide and overlay the aperture, wherein the edge-lit light guide is comprised of a transparent or translucent material to illuminate the first surface of the heat transfer plate; c) an edge illumination light source adjacent to the edge-lit light guide, the edge illumination light source operable to illuminate the edge-lit light guide, thereby providing illumination substantially parallel to the heat transfer plate and allowing an observer to view scaling, fouling, pitting, and corrosion on the first surface of the heat transfer plate; d) a fluid flow channel module having: i) a fluid flow channel module support block, ii) an optical window within the fluid flow channel support block, the optical window operable to view the heat transfer plate, iii) a fluid inlet conduit operable to allow entry of a fluid stream within the fluid flow channel module, iv) a fluid outlet conduit operable to allow exit of the fluid stream away from the fluid flow channel module; and, i) a fluid flow channel module support block, ii) an optical window within the fluid flow channel support block, the optical window operable to view the heat transfer plate, iii) a fluid inlet conduit operable to allow entry of a fluid stream within the fluid flow channel module, iv) a fluid outlet conduit operable to allow exit of the fluid stream away from the fluid flow channel module; and, e) a heating/cooling module capable of transferring heat to the heat transfer plate, the heating/cooling module having: i) a heating/cooling support block, ii) a heating/cooling element, the heating/cooling element capable of being temperature controlled, wherein the heating/cooling support block is capable of conducting heat between the heating/cooling element and the heat transfer plate. i) a heating/cooling support block, ii) a heating/cooling element, the heating/cooling element capable of being temperature controlled,