Abstract: The apparatus is comprised of two sensors of the thermal type (11a, 11b) made of materials resisting without any alteration to a temperature of 650.degree. C. Probes (15a, 15b) are controlled by a microprocessor (16) to come periodically and momentarily in contact with the sensors in order to calculate (through microprocessor (16)) the thermal time constant of the sensors presenting a deposition layer of environmental polutants. The microprocessor (16) compares the value of the time constant with an order value that commands the closure of a shunt switch (17) of a resistance (18) which, when short-circuited, determines a reinforced heating of the sensors to bring them to a temperature of incineration of organic materials originating from the pollution of the ambient medium and deposited on said sensors. Thus, the sensors are maintained in the cleanliness conditions of their surface, thereby providing for the accuracy of information supplied by the sensors outside cleaning periods.