The present invention relates a method for the welding of pipes (1) of a weldable polymer material, where the pipe has ends (1a, 1 b), and a muff (7) of a weldable polymer material, where the method comprises that the muff (7) is placed with an overlap over the ends (1a, 1b) of the pipes, that the muff (7) is welded to the ends (1a, 1 b) of the pipes through the placing of an electrically conducting band (L, 9) that is permeable to molten plastic between the pipe (1) and the muff (7), and that a power supply (PU-DC, PU-AC) is connected to the band (L, 9) through feed cables (M) and that an electrical current is supplied in order to heat the band (L, 9) and the surrounding polymer material such that they melt together around the band (L, 9) in order to form a weld, where the method comprises that the initial ambient temperature T0 of the weld is measured before the band is heated, that a voltage UL across the band is measured, that the electrical current lL supplied to the band is measured, that the initial resistance R0 of the band is calculated before the band is heated, that the electrical current supplied to the band is caused to increase, that the resistance R of the band is calculated based on the voltage and the measured current, that the change ?R in resistance is calculated, that the increase ?T in temperature of the band is calculated based on the change in resistance and in that the temperature Tw of the weld is calculated through adding the initial ambient temperature T0 of the weld and the increase ?T in temperature of the band. The invention relates also to an device for the welding of a pipe (1) of a weldable polymer material.