A method for forecasting batch end conditions through their depiction as a multi-dimensional regions of uncertainty is disclosed. After a lapse of a predetermined time, fifteen symbols are repositioned in symbol display regions. Furthermore, a probe antenna 9a formed from an extension of the inner conductor 9 is positioned so as to encircle the discharge tube 5, and the tip of the inner tube 11 is positioned further inwards than the tip of the probe antenna 9a.