To ensure optimal measurement results, the following points should be observed during the installation and use of temperature sensors:
1. Errors caused by improper installation
Examples include thermocouple installation locations or insertion depths that fail to reflect the true furnace temperature; specifically, thermocouples should not be installed too close to doors or heating elements, and the insertion depth should be at least 8 to 10 times the diameter of the protective tube. Failure to fill the gap between the thermocouple protective tube and the furnace wall with insulating material can lead to heat leakage or cold air infiltration; therefore, this gap should be sealed with insulating materials such as refractory mud or asbestos rope to prevent air convection from affecting measurement accuracy. Errors also arise if the thermocouple's cold junction is positioned too close to the furnace body, causing the temperature to exceed 100°C. Installation should avoid strong magnetic and electric fields; thermocouples and power cables should not be routed through the same conduit to prevent interference-induced errors. Thermocouples should not be installed in areas with stagnant flow of the measured medium; when measuring gas temperature inside a pipe, the thermocouple must be installed facing against the flow direction to ensure full contact with the gas.
2. Errors caused by degraded insulation
Insulation issues-such as excessive dirt or salt deposits on the protective tube and terminal block-can lead to poor insulation between the thermocouple elements and the furnace wall. This problem is exacerbated at high temperatures, causing not only a loss of thermoelectric potential but also the introduction of interference; the resulting errors can sometimes reach over a hundred degrees.
3. Errors caused by thermal inertia
Thermal inertia causes the instrument's reading to lag behind changes in the actual temperature, an effect that is particularly pronounced during rapid measurements. Therefore, thermocouples with thinner thermoelements and smaller protective tube diameters should be used whenever possible; if the environment permits, the protective tube may even be removed. Due to this measurement lag, the amplitude of temperature fluctuations detected by the thermocouple is smaller than that of the actual furnace temperature fluctuations. The greater the measurement lag, the smaller the amplitude of the thermocouple's fluctuation readings and the greater the discrepancy from the actual furnace temperature. When using a thermocouple with a large time constant for temperature measurement or control, the temperature displayed by the instrument may show minimal fluctuation, while the actual furnace temperature could fluctuate significantly. To ensure accurate temperature measurement, a thermocouple with a small time constant should be selected. The time constant is inversely proportional to the heat transfer coefficient and directly proportional to the diameter of the thermocouple's hot junction, as well as the material's density and specific heat. To reduce the time constant-aside from increasing the heat transfer coefficient-the most effective method is to minimize the dimensions of the hot junction. In practice, protective sheaths made of materials with high thermal conductivity, featuring thin walls and small inner diameters, are commonly used. For high-precision temperature measurements, bare-wire thermocouples without protective sheaths are employed; however, as these are prone to damage, they require timely calibration and replacement.
4. Thermal Resistance Error
At high temperatures, if a layer of coal ash or dust accumulates on the protective sheath, thermal resistance increases and hinders heat conduction, causing the indicated temperature to read lower than the true temperature. Therefore, the exterior of the thermocouple's protective sheath should be kept clean to minimize this error.
