Many forehearths utilize fibre optic pyrometers as zone temperature measurement
instruments. When using these instruments it is important to understand the field of view and their positioning and alignment. The filed of view increases as the pyrometer head is moved away from the glass surface. It is important to ensure the field of view comprises the glass surface only. If the sighting tube or aligning facility is incorrectly positioned the filed of view may “see” some of the roof block refractory. The measured temperature willtherefore be derived from a portion of the glass temperature and a proportion of the refractory temperature. Consequently the measured temperature will be lower than the actual glass temperature. In this case the zone may be at the required set point but the temperature measuring system does not reflect this.