A method of monitoring the accumulation of scale on a surface in contact with a liquid medium , the method comprising the steps of: providing a refractometer having a photon emission source , a prism through which emitted photons pass, and an optical image detector arranged to detect photons that are refracted through the prism, the prism having at least one measuring surface , positioning the measuring surface in contact with the liquid medium such that photons emitted by the photon emission source will be refracted as a result of the properties of the liquid medium, determining, for a control sample as the liquid medium, a control critical angle of refraction for the medium for photons emitted by the photon emission source, heating the refractometer to correspond to a temperature present at a location within a process system, emitting photons from the photon emission source, measuring a critical angle of refraction of the control sample, determining a cell fouling factor as the percent deviation of the measured critical angle of refraction relative to the control critical angle, and calculating the scale formation based on the measured cell fouling factor, wherein, generally, a critical angle of refraction is the lowest measured angle of reflected photons that pass through the prism's exiting surface for a given sample, wherein the critical angles of refraction are measured, by the optical image detector , by determining a bright-dark boundary between reflected and refracted photons from the measuring surface of the prism, and wherein the refractometer is used to measure the accumulation of temperature dependent scale formation wherein the refractometer is positioned at a location where the control sample is not at a sufficient temperature to form scale. providing a refractometer having a photon emission source , a prism through which emitted photons pass, and an optical image detector arranged to detect photons that are refracted through the prism, the prism having at least one measuring surface , positioning the measuring surface in contact with the liquid medium such that photons emitted by the photon emission source will be refracted as a result of the properties of the liquid medium, determining, for a control sample as the liquid medium, a control critical angle of refraction for the medium for photons emitted by the photon emission source, heating the refractometer to correspond to a temperature present at a location within a process system, emitting photons from the photon emission source, measuring a critical angle of refraction of the control sample, determining a cell fouling factor as the percent deviation of the measured critical angle of refraction relative to the control critical angle, and calculating the scale formation based on the measured cell fouling factor, wherein, generally, a critical angle of refraction is the lowest measured angle of reflected photons that pass through the prism's exiting surface for a given sample, wherein the critical angles of refraction are measured, by the optical image detector , by determining a bright-dark boundary between reflected and refracted photons from the measuring surface of the prism, and wherein the refractometer is used to measure the accumulation of temperature dependent scale formation wherein the refractometer is positioned at a location where the control sample is not at a sufficient temperature to form scale. <EOS>