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Crown Thermocouples

Overheating of the crown in a glass melter promotes accelerated refractory corrosion and wear. Operating a crown at too low a temperature, on the other hand, may reduce melting efficiency and increase fuel consumption. Crown thermocouples provide important data on the operating temperature of the melter, regenerator and refiner.

Determination and control of the melter hot spot, essential for effective glass circulation, can be readily achieved by placing a number of crown thermocouples along the center of the arch. A reliable sensor will also provide the necessary data to prevent potentially destructive overtemperature conditions to occur.

In all gas or oil fired glass melters the crown thermocouple must withstand the highest tank operating temperature.Temperatures of 2800°F to 2900°F are typical. In high alumina or silica glasses, temperatures may exceed 2950°F at the hot spot. For thermocouples to perform dependably great care must be exercised in the selection of materials, particularly with regard to purity, and in their fabrication.The precious metal thermocouple models featured here provide a variety of designs for all crown applications.

Application Notes and Installation

The following guidelines, drawn from our many years' experience of serving the glass industry, will help achieve the best performance from crown thermocouples.

Whether crown thermocouples are immersed into the atmosphere or mounted in dead end blocks, the following recommendations apply:

  • Blind holes should be cleaned out to ensure that no foreign matter is present. Some early failures have been noted due to attack by nuts and bolts; excess batch; or low grade ceramic cement being inadvertantly left in the block from the installation stage.
  • Pack the entrance hole through which the thermocouple is mounted to prevent hot gas "sting out." Hot gas cutting of the ceramic and overheating of the head are thereby prevented. Thermocouples mounted in dead-end blocks should still be packed in preparation for eventual block wear or cracking which will subject the assembly to the "chimney effect."
  • Atmosphere thermocouples should not be immersed more than 2 inches. The rate of attack by batch dust carry-over and resulting corrosion increases significantly at greater depths.
  • The head/connection box should be extended 6 to 10 inches above the crown or insulation blanket to insure that reasonable head temperatures are maintained at the connection point of the extension lead wire. Overheating of the terminals is a major cause of open-circuit signals as well as of erratic readings. In addition, when the temperature of the connection point between the thermocouple and extension wire exceeds 450°F the compensation factor begin to deteriorate causing reading errors of up to 50°F. It is generally found easier to attach lead wires prior to installing the thermocouple.
  • Experience has shown that plug and jack connector terminations, as opposed to a sealed-head, may be plagued with corrosion or poor contact problems.
  • Thermocouple assemblies should be fully supported by a stable mounting arrangement. Tripods, flanges, or split stand-offs are effective if properly installed.


All BASF Catalysts produced thermocouples for glass melters are fabricated with high purity alumina insulation. The alumina content is at a minimum of 99.7%. While it is commonly known that high purity alumina is needed for stable performance of precious metal thermocouples used at high temperatures, it must be realized that type and nature of the remaining impurities are equally important. BASF Catalysts choses its high purity alumina with great emphasis on eliminating known deleterious contaminants such as Fe, Ni, Cr, Sb, among others. Equally as important in the selection process are the concentrations of certain glass formers and combinations of them such as B2O3, SiO2, Na2O, etc.

Thermocouple Wire

The wire used in all thermocouples is matched and calibrated to comply with ISA, ANSI, or IEC reference tables to within +/-1/4% of temperature from 400°C to 1450°C. All thermocouples supplied on a single order are fabricated from a single batch of wire insuring that all thermocouples provice equivalent data. Lot calibration, as well as specific unit calibrations data can be supplied at an additional cost.

Fibro Platinum

Some thermocouple designs use Fibro Platinum with types R and S. Fibro is a proprietary wire making process which imparts an elongated grain structure to the metal. This structure in addition to strengthening the wire imposes barriers to the migration of contamination and subsequent weakening of the material. This material provides additional stability of calibration and increases the thermocouples effective life.

Scrap metal recovery

Precious metal thermocouples always have value - even when they are no longer usable. The metal content of bottom thermocouples can be recycled into new replacements; or monitary credit be given for their intrinsic value; or provide the user with book credit for use at a later date. All reclamations are made on a weight basis. For the most accurate and beneficial credit, spent thermocouples should be returned intact for disassembly at the plant so that proper material and alloy separation can be performed. Contact the customer service office for specific details on how to return spent thermocouples.