Essentially two types of probe of the abovementioned type are known, namely those which are fixed and those which can be retracted through the wall of the furnace. Those which are fixed and cannot move above the charging surface have the disadvantage of disrupting the fall trajectory of the charging material and of being exposed to impacts with the latter, which results in very rapid wear. Those which can be displaced radially are introduced into the furnace between two charging cycles and are extracted again for the following charging cycle. These probes do not have the disadvantages of the fixed probes, but they do require means for displacing them, and means for sealing their passage through the wall of the furnace. Moreover, the handling they require constitutes a loss of time.
U.S. Pat. No. 4,061,036 describes an improved probe which pivots. The arm of this probe is fixed to a vertical rod passing through the oblique part of the wall of the head of the furnace and which comprises means for pivoting it about its vertical axis. Under the effect of this movement, the horizontal probe arm affects a sweeping movement over the charging surface. This probe has the advantage of being able to take measurements over its entire sweeping surface, whereas the other probes can take measurements only in a radial line. Furthermore, the probe arm can be displaced quickly into an averted position close to the wall of the furnace in which it does not disrupt the charging and is not exposed to the fall trajectory of the charging material However, the upkeep of the probe, in particular the replacing of the thermocouples and the unplugging of the tubes for taking gas samples requires the whole probe to be disassembled, this disassembly being made all the more difficult by the fact that it must be performed vertically through the opening in the oblique part of the wall of the furnace.
All the probes known hitherto have the further disadvantage of not being able to be adapted to the charging profile, which can vary from one furnace to another. The consequence of this is that the heights at which the samples and heat measurements are taken vary from one measuring orifice to another, which can cause errors in interpreting the measurements.