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, <EOS>