Explicating the Characteristics of Thermocouples

Characteristics of Thermocouples

  • Temperature Reading Tolerances: Tolerance refers to the max allowable value that is gathered by subtracting the temp-reading or hot junction temperature from the standard –temp converted from the applicable EMF temperature table.  
  • Max Operating Temperature: The operating temp limit refers to the upper temp, at which, the thermocouples could be utilized constantly in the air. The max limit refers to the upper-temp, at which, the thermocouples could be utilized temporarily for short time duration due to the inevitable conditions. There are thermocouple charts, which should be used for guidance only, & should not be used for full guarantee. The most vital factors affecting the service life of thermocouples are as under: 
    • Temperature: The service life of thermocouples diminishes by approximately fifty percent whenever there is a temperature raise by 50 degree Celsius. 
    • Diameter: When the wire’s diameter is doubled, it augments the service life by two to three times. 
    • Thermal-cycling: When a thermocouple is thermally cycled above room temperature to 500 degree Celsius, its service life is reduced by approximately fifty percent in comparison to the thermocouples that are continually operated at the same temperature. 
    • Protection: When a thermocouple is covered with a protective sheath & placed in ceramic insulator(s), its service life is amplified greatly. 
  • Reaction Times of Thermocouples: The reaction times of thermocouples are largely described as the time, which is required for the thermal voltage (i.e. the output) reaching 63 percent of the maximum range for the step-change temperature. This depends on different factors, which include the dimensions of the thermocouple, its design & construction, the tip configuration, as well as the nature of that medium wherein the sensors are located. If the thermocouples are immersed in environments with higher heat capacities, wherein, heat transfers are fast, then the most effective reactive time would be practically the same to that of the thermocouples themselves (which is the internal reaction time). But, if the thermal properties of that medium are poor (for example, in static air), then the reaction time would be hundred times higher. 
  • The Length of Immersion: The assemblies of thermocouple are state-of-the-art ‘tip’ sensing devices, that are suitable for both surface as well as immersion applications, as per their designs. But, the immersion type should be utilized with care for avoiding any process-induced stress conduction errors that can lead to higher or lower values. A thumb rule is to soak in an environment at least four times of the outer diameter of the sheath; quantitative data is not applicable, but care must be taken to get logical results. 
  • Measurement of the Surface Temperature: Though the assemblies of thermocouples are mainly tip-sensing devices, still using shielding tubes makes surface detection impractical. Physically though, the probe doesn’t fit the surface & vapor conduction causes reading errors. If thermocouples have to be utilized in a reliable way for surface sensing, then they should either be exposed or welded junctions, with very low thermal mass, or placed in a design that allows for actual surface contact while attached to some surface. The thermocouples could be attached to the surfaces in a number of ways, like an adhesive pad, washer & stud, ferrous magnet, & pipe clips.

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