the invention concerns a device for detecting variations in the physical state of the plastic material in a coupling piece used for welding pipes and incorporating a heating resistor , cavities which the softened material can enter and sensors for detecting the state of the material within the cavities . according to the invention , in order to obtain a weld of high quality , it is necessary for the distance between the bottom of the cavities and the part of the resistor nearest this bottom on the one hand , and for the linear dimension in the cross - section of these same cavities near their bottom on the other , to be less than or equal to the thickness of the material which is softened during the heating of the pieces . the invention applies to the control of the welding of sections of piping , particularly for the gas industry .