The welding together of pipes of weldable polymer material through the use of a muff, a cylindrical pipe connector of weldable material that comprises a heating element, is known. The ends of the pipes are introduced into the muff and form a weld area together with the muff. The heating element, which is normally a coil, is supplied with current that heats the ends of the pipes and the muff in the weld area to such a high temperature that the ends of the pipes and the muff are welded together. Heat must be supplied during a sufficiently long period for the temperature in the weld area to become sufficiently high for a mechanically strong and sealing weld of high quality to be formed. If insufficient heat is supplied, parts of the weld area will not reach the desired temperature and the expected melting will not occur, and this will result in the weld being of lower quality. In contrast to this, if too much heat is supplied it is the case that there is a risk that the material in the ends of the pipes and the muff will be damaged through deformation of the pipe. It is therefore important to control carefully the supply of heat such that sufficient heat is supplied during a certain period of time, such that a weld of high quality is formed between the ends of the pipe and the muff.
In order to solve this problem, the placing of a thermoelement in the weld area takes place. The temperature can in this way be measured and, based on this, it can be determined how the temperature in the weld is developing and when the welding operation is to be ended. It has, however, proved to be the case that thermoelements are difficult to place in a satisfactory manner in the weld area and, furthermore, that thermoelements indicate the temperature at only a single point. There is also a risk that electrical contact will arise between the thermoelement and the heating element, leading to measurement errors. Furthermore, external measuring instruments may be needed in order to control the temperature development, which leads to more extensive equipment and higher costs.
A second existing method to control the heat supply is to use an empirical method, prediction, through, for example, ending the welding operation after a certain pre-determined period of time, which is intended to correspond to a certain temperature in the weld area. It is, however, uncertain whether the strength and quality of the weld that is formed correspond to the stringent requirements, since external factors will always directly influence the result.