The first PID controller 10 operates on the basis of the set point temperature TSM, controlling the supply of electricity to the heating element(s) 7 acting on the half-clamp 3a, 3b until the temperature T M measured on the same half-clamp 3a, 3b by the relative thermocouple 8 equals the set point value TSM, thereby reaching a predetermined degree of stability.

After reaching the requested stability in the temperature T M measured on the half-clamp 3a, 3b the second PID controller 11 is activated, the purpose of which is to compare the temperature T R measured by the thermocouple 9 located on the reduction element 4a, 4b with the desired working temperature T L and consequently modify the set point temperature TSM