Installation of Rare Metal Ceramic Sheath Thermocouples

The following guidelines, drawn from our many years’ experience of serving the glass and furnace industries, will help achieve the best performance from high temperature thermocouples.

1. 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 inadvertently left in the block from the installation stage.
    
2. Rare metal thermocouples are typically assembled using ceramic sheaths and insulators which are highly susceptible to thermal shock. Preheating and slow insertion into the furnace are recommended. If installing in a running furnace, this can be achieved by laying the thermocouple on the outer shell of the furnace. It is highly recommended to be preheated to 200-300^oC for a number of hours, then the thermocouple can be gradually inserted. A good “rule of thumb” is to insert ceramic protection tubes into “Hot” processes at a rate of 50-100mm per minute to prevent breakage from thermal shock.
    
3. 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.
    
4. Where practical, the head/connection box should be extended 150 to 250mm above the furnace shell or insulation blanket should be installed 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 230°C the compensation factor begin to deteriorate causing reading errors of up to 30°C. It is generally found easier to attach lead wires prior to installing the thermocouple.
    
5. Experience has shown that plug and jack connector terminations, as opposed to a sealed-head, may be plagued with corrosion or poor contact problems.
    
6. Thermocouple assemblies should be fully supported by a stable mounting arrangement. Tripods, flanges, or split stand-offs are effective if properly installed.