Abstract: A sensor for real time monitoring the quality of welding joints in the plastic film at the production of plastic bags on roll by measuring material changes in the welding joint is provided. The sensor comprises at least two electrode pairs arranged to measure changes in the dielectric properties of the material when the welding joint is passing through the measuring gap of the electrodes and provide a measuring signal proportional to the material change in the welding joint. The measuring signal is evaluated to detect defective welding joints in the plastic film. The electrodes are arranged so that one pair of the electrodes constitutes the signal measuring gap arranged for measuring the welding profile. The form of the measuring electrode conforms to the direction and form of the welding joint. The other pair of electrodes constitutes the reference measuring gap arranged to provide an averaged signal across the surface of the plastic film.