Fouling of heat exchangers is one of the main causes of thermal efficiency decrease during their use. In spite of the efforts displayed these last decades, the existing detection and fouling monitoring devices are still far from taking into account the heat transfer and the depositphenomena involved.So, this study is devoted to the elaboration of a thermal sensor representative of industrial conditions in order to set up an efficient cleaning programme. Particularly, it deals with the problem of gas side particulate fouling of cross flow tubular heat exchangers. An original localised and technologically simple sensor associated with a data processing related to a 2D inverse method was developed. It allows to obtain accurate informations from temperature measurements inside the wall which separates the liquid flow from the air flow.The sensor enables the accurate estimation of the convective heat transfer angular distribution as well as the deposit thickness from a 2D steady state inverse method. The probe was tested on an industrial device, and the fouling thickness distributions were validated by laser measurements.