Method for detecting surface fouling comprising: a) providing a first surface arranged to contact a flow of foulant; b) providing a sensor system comprising at least one surface sensitive sensor unit, wherein a deposition material is deposited on said surface sensitive sensor unit, wherein the deposition material presents a second surface arranged to contact a flow of foulant; wherein the second surface presents all physico-chemical properties of the first surface, wherein said physico-chemical properties of the first surface are responsible for foulant deposition on said first surface; wherein said surface sensitive sensor unit is suitable for generating signals in response to foulant deposited on the second surface in an amount of from 1 ng/cm 2 to 0.1 mg/cm 2; and wherein the sensor system is connected to a data acquisition and monitoring system suitable for monitoring said signals generated by said surface sensitive sensor unit; c) subjecting the second surface to a flow of foulant; d) acquiring and monitoring said signals generated by said surface sensitive sensor unit to detect the amount of foulant deposited on the second surface per area of the surface; wherein the first and second surfaces are inorganic surfaces, or wherein the first and second surfaces comprise an organic polymer; wherein the method comprises a step e) of deeming the amount of foulant deposited on the second surface and detected in step d), to be representative of the amount of foulant deposited on the first surface when said first surface is subjected to a flow of foulant under corresponding conditions; wherein the first surface is larger than the second surface; and wherein the first and second surfaces are independent from each other. a) providing a first surface arranged to contact a flow of foulant; b) providing a sensor system comprising at least one surface sensitive sensor unit, wherein a deposition material is deposited on said surface sensitive sensor unit, wherein the deposition material presents a second surface arranged to contact a flow of foulant; wherein the second surface presents all physico-chemical properties of the first surface, wherein said physico-chemical properties of the first surface are responsible for foulant deposition on said first surface; wherein said surface sensitive sensor unit is suitable for generating signals in response to foulant deposited on the second surface in an amount of from 1 ng/cm 2 to 0.1 mg/cm 2; and wherein the sensor system is connected to a data acquisition and monitoring system suitable for monitoring said signals generated by said surface sensitive sensor unit; wherein the second surface presents all physico-chemical properties of the first surface, wherein said physico-chemical properties of the first surface are responsible for foulant deposition on said first surface; wherein said surface sensitive sensor unit is suitable for generating signals in response to foulant deposited on the second surface in an amount of from 1 ng/cm 2 to 0.1 mg/cm 2; and wherein the sensor system is connected to a data acquisition and monitoring system suitable for monitoring said signals generated by said surface sensitive sensor unit; c) subjecting the second surface to a flow of foulant; d) acquiring and monitoring said signals generated by said surface sensitive sensor unit to detect the amount of foulant deposited on the second surface per area of the surface; wherein the first and second surfaces are inorganic surfaces, or wherein the first and second surfaces comprise an organic polymer; wherein the method comprises a step e) of deeming the amount of foulant deposited on the second surface and detected in step d), to be representative of the amount of foulant deposited on the first surface when said first surface is subjected to a flow of foulant under corresponding conditions; wherein the first surface is larger than the second surface; and wherein the first and second surfaces are independent from each other. wherein the first and second surfaces are inorganic surfaces, or wherein the first and second surfaces comprise an organic polymer; wherein the method comprises a step e) of deeming the amount of foulant deposited on the second surface and detected in step d), to be representative of the amount of foulant deposited on the first surface when said first surface is subjected to a flow of foulant under corresponding conditions; wherein the first surface is larger than the second surface; and wherein the first and second surfaces are independent from each other.