Abstract: The measuring device comprises two flat-wall heat exchangers (3a, 3b), one to each side of a flat plate (1), two samples (2a, 2b) of similar thickness (E) of the material being tested, placed between the outer surface of the heat exchanger (3a, 3b) and one surface of the central plate (1), a device for positioning and holding the heat exchangers (3a, 3b) and at least two temperature sensors, one of which (T2) consisting of a thermocouple is incorporated in the central plate (1), the other (T1) also consisting of a thermocouple, being incorporated in the outer wall of the plate of one of the heat exchangers (3a). Application in the injection of plastics.